Application Date: Oct. 15, 1943. No. 17006/43.
Complete Specification Left: Jan. 10, 1945.
Complete Specification Accepted: Nov. 19, 1946.
Improvements in or relating to Pulse Signal Selecting and Indicating Systems
We, FREDERIC CALLAND WILLIAMS, of 1, The Lynches, Albert Road South, Great Malvern, Worcestershire, ERIC LAWRENCE CASLING WHITE, of Watchfield, 167, Sutton Court Road, Chiswick, London, W.4., and DOREEN BLUMLEIN, of Lanherene, Lescudjack, Penzance, Cornwall, legal representative of ALAN DOWER BLUMLEIN, deceased, late of 37, The Ridings, Ealing, London, W.5., all British subjects, do hereby declare the nature of this invention to be as follows:-
This invention relates to signal selecting and indicating systems and has particularly although not exclusive reference to the visual indicating arrangements for radio location systems of the kind which utilise a cathode ray tube to provide an indication of echo signals. Such radio locating systems have become known as Radar systems, the term Radar being a coined word derived from the words radio direction and range.
In typical radio location systems a carrier wave modulated by signal pulses of short duration is radiated usually in a directional manner and echoes of the radiated signals reflected from any objects within the zone and range of radiation are demodulated and applied to a cathode ray tube which, in the absence of means to suppress echoes which it is not desired to observe, will render such echoes visible without discrimination. The echoes may, for example, be due to direct reflection from the ground, the sea, trees of buildings and they may be rendered visible as deflections in a continually visible trace of any desired from produced on the luminescent screen of the cathode ray tube, such trace being circular or linear and bearing any desired relationship to an indicating scale of measurement applied to the screen. Again, the trace may normally be invisible and only rendered visible on reception of echo signals. Unless suppressing or discriminating means are provided, however, a confusing number of echoes are likely to appear and tender observation of a particular echo or class of echoes extremely difficult.
In the case of radio location equipment installed in an aircraft for example, and intended to facilitate interception of other aircraft, it is important that a clear indication of echoes received from a particular reflecting aircraft should appear clear of possibly misleading echoes in order that the position of the aircraft under observation may readily be ascertained.
The object of the invention is to provide apparatus which will enable a substantial degree of automatic discrimination to be effected between signals applied to visual indicating equipment, particularly of the kind employed for radio location purposes, in order that substantially only those signals which it is desired to observe shall be rendered visible.
According to the present invention, a circuit arrangement including means for providing an indication of a selected re-current input signal applied to said circuit, includes automatically operating means for generating local signals the timing relation between which varies progressively, the arrangement being such that on the generation of a local signal coinciding in timing with said selected signal, said indicating means are energized and said selected signal is rendered evident.
In applying the invention to radio location equipment in which echoes of a pulse modulated carrier wave are rendered visible on the luminescent screen of a cathode ray tube, a thermionic valve pulse generating circuit is employed to generate local signals under the control of the radiated pulses and the timing of the local signals is varied in cyclic fashion between two extreme values by automatic progressive variation of the cathode potential of valve included in said circuit. The local signal generator is such that on receipt of an echo signal coincident in timing at the instant of reception with a local signal, the cycle of variation in timing of the local signals is arrested.
In a particular form of apparatus embodying the invention, the cathode potential of said valve in said pulse generating circuit is controlled by the current passed by a further valve connected in series with the first mentioned valve and said further valve serves to effect arrest of said timing.
According to a further feature of the invention, a plurality of recurrent local signals which may be of differing timing and duration are generated in order to facilitate rejection of undesired signals or to provide measuring signals or to serve both these functions. One such local signal is arranged to be of such duration that it may be utilised to co-operate with a desired echo signal to produce a visual indication and another local signal of longer duration is utilised to co-operate with an undesired signal in such a manner that no visual indication is produced and the timing cycle of said local signals, which is arrested on the coincidence of one of said local signals of longer duration and said undesired signal, is recommenced.
In a particular form of radio locating system embodying the invention and designed to permit observation of the motion of a single echo reflecting object relative to locating equipment, a group of two or more local signal pulses having fixed durations and time relationships with each other is generated at such time delay after each transmitted pulse as is appropriate to the range of echoes being sought, this time delay being caused to vary periodically so as to search between predetermined limits of range until such time as an echo is encountered between said limits, when the searching is caused to stop and the group of local pulses to follow thereafter any variations in range of the echo, under the control of one or more signal rectifiers or amplifiers which are only switched into operation during appropriate local pulses.
If the reflecting object is, for example, a friendly aircraft, or a ground beacon station, it may repeat the return signal at greater strength than the true echo and may alter its characteristics, for example, by sending it back at a different frequency, as in beacon stations, or by increased its width, or by adopting both these expedients. Variation of the returned signal pulse width may be effected in accordance with a code and in such a case a rectifier for d-coding the signal may be employed capable of discriminating between two different widths.
The echo signals may be picked up by separate aerials composing a directive array and the energy fed sequentially to the deflecting plates of a cathode ray tube which is operated to produce a spot indication of the position of a particular reflecting object. The dimensions or shape of the spot indication may be modified in a manner which will provide an indication of the distance of the reflecting object. Thus, for example, the spot indication may be expanded into a line of light preferably still retaining a bright spot at its centre and preferably having its ends well defined, the length of the line being an indication of range. The length may, for example, vary approximately inversely proportionally to range, thus giving an appearance of the apparent size of the reflecting object. In such a case the cathode ray tube may constitute an aircraft pilots indicator and an auxiliary tube may be provided for inspection and operation by an observer who is thus enabled to make preliminary selection of any desired reflecting aircraft and to bring the pilots indicator into operation only when desired. The local signals generated and used in the various ways outlined, will be referred to in this specification as strobing pulses.
In order that the nature of the invention may be more clearly understood one form of radio-location equipment embodying the invention will now be described in greater detail by way of example with reference to the accompanying drawings in which:
Fig. 1 is a block diagram showing the main connections between the component parts of a radio-locator transmitter and receiver equipment for an aircraft.
Fig. 2 is a circuit arrangement including the strobe pulse generating, timing and associated control valves,
Fig. 3 is a circuit arrangement including valves which apply ground echo rejecting and range indicating signals,
Fig. 4 is a circuit arrangement including the receiver output valve, a noise limiting valve, a gain control valve, and a signal suppressing valve, and
Fig. 5 is a circuit arrangement including a signal measuring valve, an amplifier and the connections of an output switch to a cathode ray indicating tube.
Referring to Fig. 1 of the drawings a transmitting dipole aerial TA is mounted at the forward end of the fuselage of an aircraft carrying the equipment and is designed to radiate exploring signals over a wide forward angle of search. A pulse modulated carrier wave is fed to the aerial from a transmitter T comprising an oscillator which generates a carrier frequency of 200 megacycles per second. The oscillator, the details of which are not shown, comprises a pair of valves having their anodes coupled by a Lecher tuned circuit including a capacity coupled sliding bridge which is ganged for simultaneous tuning with the cathode circuits of the valves which are also Lecher tuned. The aerial feeder is variably coupled with the anode conductors by a sliding capacity bridge similar to the tuning bridges. The control grids of the oscillator valves are connected directly together and to a reflecting delay network which simulates a 2000 ohms leak resistance and, due to reflection effects, serves rapidly and effectively to damp the generated oscillations at the termination of each modulating pulse.
The modulator circuit represented by M, the details of which are now shown, comprises a multi-vibrator which supplies pulses to the parallel input circuits of four valves, the outputs from which are fed into a delay network from which modulating pulses of two micro-seconds duration separated by intervals of 1500 micro-seconds are fed through choke coils to the oscillator anode circuit. A synchronising pulse is taken from the cathode circuits of the four parallel connected valves and is fed through the lead A to an observers indicating cathode ray tube S, and another synchronising pulse provided by the multi-vibrator is fed through lead B to trigger a timing circuit in the receiving equipment as described in detail later. The power supply unit for the modulator includes a relay fed from a tapping in an inductance in the anode circuits of the four parallel connected valves, the relay being energized to open the circuit of the supply to the primary winding of the rectifier transformer if the transmitter valves cease to take the modulator load.
The receiving aerial RA comprises two pairs of dipoles. Those comprising one pair are mounted spaced apart along the wing of the aircraft at equal distances from the fore and aft axis and discriminate between reflecting objects spaced in azimuth. The dipoles comprising the other pair are mounted one above and one below the wing of the aircraft and the echo signals picked up by these aerials provide an indication of the elevation of the reflecting object with respect to the aircraft.
The signals picked up by the aerials RA are fed through a rotary switch AS in sequence to a radio frequency amplifier RFA from which they are passed to a frequency changer FC, thence to an intermediate frequency amplifier and second detector IFD followed by a low frequency amplifying stage LF and the final receiver output stage V611. The stage V611 and certain of the remaining unit represented by block diagrams in Fig. 1 will be described in greater detail later with reference to Figs. 2, 3, 4 and 5 of the drawings, but brief reference will first be made to their separate functions.
In order to prevent the receiver amplifier from being overloaded by the outgoing signals, the local oscillator in the frequency changer FC is suppressed for a period prior to the transmitted pulse and lasting until just after the end of the transmitted pulse by a negative pulse applied to its control grid. This pulse is derived from the synchronising pulse provided by the modulator M fed to a pilots control unit PS through a lead B to a delay network DN 600 and thence to suppressor unit V 601 and lead G. Negative suppressing pulses are simultaneously fed from unit V 601 through lead H to the screening grids of the radio and intermediate frequency amplifying valves.
A further pulse derived from the delay network DN 600 is fed by a further lead marked B to a valve circuit V 6t02, the output from which assists in controlling the timing of strobing pulses generated by a unit V 604/5, such timing being also controlled by an output from the unit V 615/6/7 which will be referred to as a timing drift control unit. The pulse generator unit V 604/5 includes a delay network into which the generated pulses are fed and from which strobing pulses having selected duration and timing separation between them are taken. One such pulse which will be referred to a pulse E, is fed by a lead E to a signal measuring rectifier V 606 and services when it coincides with and covers an echo signal, to pass rectified voltages representative of the signal picked up from the directional receiving aerials through an amplifier V 614 and an output switch S 601 which rotates synchronously with the aerial input switch AS, to the deflecting plates of a cathode ray tube in the indicating unit V.I.
Another strobing pulse which will be referred to as pulse F, of longer duration than pulse E, is fed by lead F to a unit V 608/9 which serves to prevent ground reflections from showing on the indicating tube and also to initiate a new timing drift cycle. A third strobing pulse, thereafter referred to as pulse D and fed by lead D to a rectifier valve V 607 which when applied through lead L to the unit V 615/6/7 "locks" the strobing pulse D to the echo signal. The drift circuit V 615/6/7 is coupled by a lead N to a unit V 619/20/21 which, in combination with the automatic gain control voltage taken from a noise limiter valve V 610 by lead 0 supplies a distance indicating voltage to the spot on the indicator tube. A further connection by lead P from the stage V 615/6/7 controls a valve B612 which removes a biassing potential from the control grid of the cathode ray tube VI normally suppressing the electron beam and allows the selected echo signal to appear so long as the strobing pulse and signal are locked together.
Signals from the receiver output stage V 611 pass through three channels, one represented by lead I feeding the signal measuring rectifier V606; another represented by lead J feeding the ground echo rejecter unit V 608/9 and the third represented by lead K feeding the valve V 607 which will hereafter be referred to as the drift stop valve. The operation of these three last-mentioned units depends upon the dual control or coincidence of an echo signal and a strobing pulse.
The echo signals appearing on the auxiliary indicator tube S are presented against a linear time base which is synchronised by the pulses derived from the modulator M and supplied by the lead A previously referred to. This auxiliary tube is fed direct with signals from the receiver output stage V 611 by lead R, and also receives a strobing pulse from the unit V 604/5 through lead E. The auxiliary tube S can be used either for testing the main equipment or for preliminary echo searching when the aircraft carries an observer as well as a pilot. In the latter case the cycle of search by the automatically controlled strobing pulses is stopped by means of a suitable switch not shown, and an incoming echo signal is identified and selected by manual control of the strobe pulse generating valves by the observer. The switch is then operated to restore automatic operation so that the subsequent movement so the selected target are continuously shown on the screen of the pilots tube.
Certain of the units shown in Fig. 1 will now be described in greater detail and reference will first be made to the strobe pulse generating and timing circuits shown in Fig. 2 in which the valve V 602 corresponds with the unit V 602 in Fig. 1. Synchronising pulses of the same repetition frequency as the outgoing signals are fed to terminal t1 from the delay network DN 600 (Fig. 4), through a condenser C.620 and resistance R.607 to the control grid of the valve V 602. The valve V 602 is normally biassed to beyond anode current cut off and the applied pulse brings the grid of the valve V 602 to its cathode potential and causes the anode voltage of that valve to drop substantially to cathode level, this being followed by a steady rise at a rate which is determined in part by the time constant of the network comprising resistances R 609, R 610, and R 612 and condensers C 622, C 623 in the anode and screen grid circuits and in part by the prevailing cathode potential.
If the cathode potential is assumed to be held fixed the anode voltage will after a certain internal of time reach a critical value at which it triggers the control grid of the first of two inter-coupled valves V 604, V 605, the anode of valve V 602 being connected directly to the control grid of valve V 604 through a resistances R 620. The valve V 605 is thus caused to feed an impulse into a time delay network DN 601 at the tapping point p1, the delay network being short circuited at one end and terminated at the other by an impedance R 627 equal to the characteristic impedance of the delay network. The applied pulse travels along the network and is reflected back in opposite sense from the short circuited end, the difference between incident and reflected pulses being taken from spaced tappings p.2 p.3, p.4 the pulses so derived constituting the strobing pulses having different durations and being thereafter referred to as pulses E, D and F. Such strobing pulses are produced at instants determined by the prevailing potential of the cathode of the valve V 602 and in applying the present invention, provision is made to vary the phase or timing of the strobing pulses over what may be termed a searching period during which the potential of the cathode of the valve V 602 is automatically and cyclically varied between two predetermined limits. Such variation had the effect of gradually increased the time interval between the signal radiated by the transmitter and the production of the strobing pulses until one of the strobing pulses coincides with an echo signal, as is inevitable if an object producing echoes is within range of the equipment during any cycle of drift or the cathode potential from one extreme to the other. It will be seen from the subsequent description that when an echo signal coincides with a strobing pulse, the normal progress of the drift cycle is arrested, the echo signal and the strobing pulse become "locked" together and an indication appears on the cathode ray tube in the unit V.I.
The voltage applied to the cathode of the valve V 602 is varied is saw tooth fashion by the current which passes through a valve V 615, the anode of which is connected to the cathode of valve V 602 through a small resistance R 683 and then to the high tension supply through resistances R 605 and R 606.
The anode of the valve V 615 and hence the cathode of the valve V 602 is caused to fall steadily in potential owing to positive bias potential applied through the resistance R 681 to the grid of the valve V 615 and the negative feed back from the anode to the grid through the condenser C 683. Owing to this feed back the anode potential can only fall at such a rate that the discharge current through the condenser, which is equal to its capacity multiplied by the rate of change of the voltage across it, this voltage occurring almost entirely on the anode owing to the large voltage gain of the pentode type valve preferably constituting the valve V 615, is substantially equal to the current flowing through the resistances R 681 from the source of the applied positive potential.
When the cathode of the valve V 602 reaches a potential of about 25 volts positive, the diode having cathode 4 of the valve V 609 (Fig. 3) starts conducting owing to the connection from terminal t2 (Fig. 2) to terminal t3 (Fig. 3) and the resistance R 711. A large negative bias potential is applied through resistances R 728, R 737 and R 710, connected in series so that when the potential on terminals t2 and t3 reaches 25 volts positive, the connection between resistance R 711 and R 710 and the cathode 4 of the diode just reaches zero potential. Further lowering of the potential of the cathode of valve V 602 then results in the said diode becoming conducting and in consequence lowering its anode potential. This lower potential is effective, by a connection between terminals t9 (Fig. 3) and t10 (Fig. 2), upon the grid potential of V 617, causing the anode potential of V 617 to rise. Owing to the DC coupling through the resistances R 693 and R 689 from the anode of the valve V 617 to the control grid of the valve V 616, the negative bias potential applied to this grid is removed, the anode potential falls and the potential of the suppressor grid of the valve V 615 also falls with consequent anode current cut off and rapid rise in the anode potential. The condenser C 683 is thus charged up from the high tension supply through resistances R 605, R 606, and R 683 and through the grid current in valve V 615. The rapid rise of cathode potential of the valve V 602 is differentiated by the condenser C 685 and the resistance R 689 and holds the grid of the valve V 616 at zero potential even though, as soon as the cathode of the valve V 602 rises above 25 volts positive, the original effect initiating the return stroke through the valve V 609 and V 617 is removed.
A valve V 618 is provided to supply a stable voltage of 125 volts positive and as soon as the cathode of valve V 602 reaches this voltage the diode anode 5 of the valve V 618 conducts and prevents further rise. Hence the differentiation by the condenser C 685 and the resistance R 689 ceases, current through the valve V 616 is cut off once more, the potential of the suppressor grid of the valve V 615, which is prevented from going appreciably positive by the resistances R 685 and the diode anode 5 of the valve V 617, returns to normal, and the downward travel of the potential of the anode of the valve V 615 and of the cathode of the valve V 602, starts again.
During each saw tooth cycle the three strobing pulses D, E, F are separately combined with the incoming echo signals which are tapped off from two parallel resistance/capacity networks in the cathode circuit of the amplifier V 611, shown in detail in Fig. 4. One network includes a condenser C 664 and resistance R 657, the value of these components being selected to respond with discrimination to the prolonged ground reflections, and the second network includes condensers C 661/2/3 and resistances R 653/4/5, this combination having a short time constant such that it will accept the relatively short target echoes. A third cathode connection leading to a terminal t4 serves to supply signals to the auxiliary indicator tube S. this connection corresponds with the lead R in Fig. 1.
The pulse F, which is of comparatively long duration, and starts slightly after pulses D and E, is fed from terminal t5 on delay network D 601 to terminal t6 in Fig. 3 connected to an input grid of a valve V 608 in that Figure. A second input grid of the same valve is coupled to the network C 664, R 657, in the cathode circuit of the valve V 611, by connecting terminal t7 in Fig. 3 to terminal t8 in Fig. 4, and has applied to it the comparatively lone ground echoes. When in the course of searching, there is coincidence between the pulse F and a ground echo, V 608 is rendered conducting and condensers C 657 and C 658 discharge through the limiting resistance R 644 and V 608 for a sufficient time to lower the potential of the cathode of V 609 which is connected to the common point of C 657 and C 658 to a potential below that of its associated anode 5 which is normally held at about 40 volts positive by a leak resistance R 696. The diode anode 5 then takes current and its potential and that of the condenser C 688 is lowered. After a succession of, for example, ten ground echoes coinciding with the pulse F, the rate of search being sufficiently slow to permit this with ease, the diode anode 5 becomes sufficiently negative to cause anode current cut off in the valve V 617, the diode anode 5 being connected through terminals t9 and t10 to the control grid of the valve V 617 and the search return stroke is initiated as already described.
The drift cycle is of course, only completed in the absence of echo signals from a reflecting object or from the ground as the incidence of such echoes would cause the drift cycle to be arrested. In the absence of such echoes each drift cycle causes the strobing pulses to search from a minimum distance range of a few hundred feet to a maximum distance of about 34,000 ft. As explained subsequently, the operation is such that during the coincidence of a pulse F and a ground echo no energy is passed to the indicating cathode ray tube and indication appears.
The pulse D which is initially of the same duration as the pulse F, thought it starts a little earlier, is fed from tapping p.3 to the control grid of a valve V 607 and the resulting pulse in a delay network DN 602 in the screen grid circuit is fed back through a condenser C 653 to the outer control grid and reduces the effective duration of the pulse D to approximately 1 micro-second. The anode of the valve V 607 is coupled through a condenser C 648 to terminal t11 and through terminal t12 (Fig. 4) to a point between the condenser C 662 and resistance R 653 in the short time constant network in the cathode circuit of the valve V 611. When an echo signal of significant amplitude coincides with the pulse D, the dual input to the valve V 607 causes that valve to pass sufficient current to effect arrest of the normal drift cycle and thereafter to "lock" the pulse D to the echo signal. The overlap of the echo signal and strobing pulse D first adjusts itself until the rectified output of valve V 607 applied through resistances R 731, R 638 to the control grid of the valve V 615 equals the fixed current flowing through R 681 owing to the positive bias. The steady rate of change of potential of the anode of V 615 is thus stopped and the search cycle arrested. As the echo pulse moves in one direction or the other the overlap automatically readjusts itself and the cathode potential of the valve V 602 steadily follows the variation. The cessation of the drift cycle is accompanied by automatic operation of a valve V 612 which supplies an output to the cathode ray indicator tube as explained subsequently.
The pilot can reject a particular echo signal which may have been locked with a strobing pulse in favour of another echo by pressing a button-switch, no shown, but which may be mounted on his control unit P.S. thus energising a relay, not shown, which earths through a terminal t16 a resistance R 692 in the high tension supply and so applies a negative pulse to the outer control grid of the valve V 607 breaking the existing lock and allowing the strobe pulse to move forward in search of another signal.
The third strobe pulse E is of about 3 micro-seconds duration and starts simultaneously with the pulse D. it serves as a signal-measuring pulse in the sense that it allow rectified voltages of peak amplitudes representing those induces by the incoming signals in the different elements of the directional receiving aerials to pass through to the indicator tube as soon as the latter has been made receptive by the pulse D. the spot then takes up a position of the screen which indicates the orientation of the target relatively to the line of flight of the observing craft. The pulse E is fed from terminal t13 (Fig. 2) to terminal t14 (Fig. 5) which is connected through condenser C 698 and resistance R 630 to the control grid of a valve V 606, the anode of which is also coupled to the network C 662, R 653 in the cathode circuit of the output amplifier V 611 by a connection from terminal t15 (Fig. 5) to terminal t12 (Fig. 4). Once the pulse D has been locked to an echo signal, the echo and the pulse E will be coincident. During the pulse, the valve V 606 then acts towards the signal as a diode rectifier, and as the signal is positive it gives a negative output representative of the peak value of the signal pulse. This rectified D.C. output is smoothed by the filter comprising resistances R 631, R 634 and condensers C 647 and C673 and is amplified by the valve V 614. The output from the valve V 614, representing the peak values of the incoming signals from the various aerial sin turn is fed through a condenser C 676 and resistance R 679 to the moving contact making elements S1, S2 of the rotary output switch S 601, Fig. 1 which are driven synchronously with the aerial input switch AS, so that each signal is fed in proper sequence to the four deflector plates of the cathode ray indicator tube V.I.
Voltages representative of the signals received by the two elevation aerials are fed through contacts C1, C2 of the rotary output switch to the plates Y1, Y2 of the tube and voltages representative of signals from the two azimuth aerials re fed through contacts C3, C4 to the plates X1, X2. The valve V606 supplies a marker pulse for the observers indicator tube S (Fig. 1) through a cathode lead and a terminal t31, for indicating the position of the strobe pulses relative to the signals.
An auxiliary voltage is simultaneously applied to the deflecting plates of the indicating tube so that in addition to showing the orientation of the target the spot also indicates its relative distance from the observer. For this purpose a valve V 621 (Fig. 3) having its anode and control grid coupled through a transformer T602, generates a rectangular wave, during the positive half-cycle of which the cathode voltage, due to the load resistance R 725, exceeds that of the diode electrodes 4 and 5. During the negative half cycle, a pulse is supplied through a condenser C 693 to the control grid of the valve V 620, the pulse being of sufficient amplitude to cut off current in the valve completely. The anode current wave form of the valve V 620 consists of substantially symmetric square pulses and these are distorted by the anode load circuit consisting mainly of resistance R 717 shunted by inductance, the transformer T 601 having a suitable air gap in its core, into a series of abrupt changes in potential, alternately in opposite directions, followed by exponential decays to zero. The time constant of the decay is considerably less than the interval between pulses so that the output is substantially zero for a large part of the cycle. The output from the secondary winding of the transformer T 601 is fed from terminal t17 to terminal t18 (Fig. 5) to the primary winding of a transformer T3 (Fig. 5), the two secondary windings of which are respectively connected in series with the X plates of the cathode ray tube VI.
The application of this output results in lateral extension of the indicating spot, giving the appearance of wings, the length of which is, as will later be explained, inversely proportional to the distance of the target from the observer. Owing to the character of the wave form referred to, namely, that the potential is zero for a large part of the cycle, the intensity of the spot is kept almost as great as the centre of the "wings" as it is in the absence of "wings", thus giving a definite point always for estimating bearings, and also appearing like the body of an aircraft. Further the sudden excursions of potential followed by the exponential decay have actually slightly rounded tips, due to a suitable amount of capacity, mostly stray, shunting the output circuit, and this causes the tips of the "wings" to be brighter than the main portion thus making the width of the "wings" more clearly visible.
The amplitude of the "wings" depends on the mean current passing through the valve V 620, since the pulses on the grid are always of sufficient amplitude to modulate this current fully. The mean current depends on the mean grid potential which is supplied from the range voltage applied from terminal t2 to terminal t3 and on the cathode circuit resistance of the valve V 620 which is sufficiently high to linearise the relation between mean current and mean bias potential. As the range decreases after finding a target echo, and proceeding to overtake it, the range voltage rises. Until the range decreases to about 10,000 feet, the valve V 620 is cut off having an open circuit voltage of about 100 volts positive on its cathode, but for shorter ranges and higher range voltages, reaching finally 125 volts positive for minimum range, the mean current through the valve V 620 increases from zero to a certain limit and the "wings" correspondingly expand. This limit occurs when the range voltage reaches 125 volts positive and the strobe pulses cannot be allowed to move nearer to the transmitter pulse owing to its finite width. As the target continues to come nearer therefore, its echo gradually merges into the transmitter pulse and is no longer actually aligned with the strobe pulse so that the effective signal strength decreases. This causes the automatic gain control circuit, to be described later, to increase the gain of the receiver in an effort to keep the output constant. Such increase of gain is used to give a final extra increase of "wings" amplitude in the following manner.
The eventual limit to increase of gain is set by the noise limiting valve V 610, the diode anode 5 of which supplies rectified noise voltages taken from the anode of the receiver output valve V 611 and the triode portion of which amplifies the resultant negative voltage, the output taken from the cathode of the valve V 610 being also negative. By means of the diode-anode 4 and the automatic gain control bias connection from terminal t23, a limit is eventually set to the gain of the receiver, since increasing gain increases the noise level. Before this point is reached, however, the lowering of the cathode potential owing to a connection from terminal t19 to terminal t20 through a resistance R 708 causes anode current to flow in valve V 619 (Fig. 3), and therefore the anode current of valve V 620 to increase, giving an increased "wings" amplitude. This action will also occur during fading of an echo at any range and to prevent this, any cathode current in the valve V 619 is normally passed to the diode anodes 4 and 5 of the valve V 619 instead of to the triode-anode of that valve, unless the range is less than about 800 feed when the fraction of the range voltage applied from terminal t3 and the potentiometer including the resistance R 711, which applied to the grid of the valve V 619, is sufficient to lift the cathode potential above that of the diode anodes and transfer the current, if any, dependent upon the valve V 610, to the anode.
So long as searching is proceeding, the steady negative rate of change of potential of the cathode of the valve V 602 (Fig. 2) is differentiated by the condenser C 667 and the resistance R 663 and keeps a steady negative potential on the control grid of a valve V 612, whilst during the rapid return stroke of the saw tooth voltage controlling the drift-cycle the diode anode 4 of the valve V 617 serves to prevent the same grid from becoming positive, so that no output is passed from the output terminal t21 of the valve 612 to the indicating cathode ray tube. Once the strobe pulse D has found and become locked to an echo signal, however, and after a short delay imposed by a condenser C 668 to allow for the establishment of automatic gain control, the cessation of the steady drift current through the resistance R 663 and condenser C 667 allows the grid of the valve V 612 to rise to zero voltage during the periods of coincidence of the strobe pulse and echo signal, and to apply a brightening pulse from terminal t21 to terminal t22 (Fig. 5) connected to the cathode of the indicator tube, which is normally biassed to suppress the electron beam. The brightening pulse allows the signal to appear as a spot on the screen of the tube.
The pair of valves V 610 and V 613 (Fig. 4) serve to limit the noise-level, and to apply automatic gain-control to the intermediate frequency amplifiers in the receiver. The function of the valve V 610 has already been described in connection with the production of the range indicating "wings". When locked to a sufficiently strong echo, the valve V 613 takes over the gain control by lowering the potential of the diode-anode 4 of the valve V 610 and with it the gain control bias available from the terminal t23 until the diode-anode 4 no longer takes current. Owing to a connection between terminal t24 (Fig. 4) and t25 (Fig. 5), the valve V 613 is caused to pass current when the average signal potential on the anode of the valve V 614 rises sufficiently to neutralise the large negative bias applied through the resistance R 668, and then lowers the gain by the connection from its anode through resistances R 671 and R 648 to the gain control voltage terminal t.23 which is connected to the grid bias input for the intermediate frequency amplifying valves until the said neutralisation is only just maintained. This ensures that the mean signal level of the anode of valve V 614 is held at a desired value.
The connections of the suppressor valve V 601 and the delay network DN 600, are shown in detail also in Fig. 4. The synchronising pulses from the modulator M (Fig. 1) are fed in to the delay network DN 600 at terminal t and the controlling pulses for the valve V 602 (Fig. 2) are taken from terminal t28 connected to a tapping p5.
A pulse derived from a second tapping is applied to the control grid of the valve V 601, the resulting pulse on the screening grid being taken from terminal t26 and applied, as previously stated to inhibit the local oscillator of the frequency changer, during the transmission of the signal, the connection for this purpose corresponding with lead G in Fig. 1. Suppression pulses produced in the anode circuit of the valve V 601 and taken from terminal t27 are simultaneously applied to the screening grids of the radio and intermediate frequency amplifiers of the receiver, this connection corresponding with lead H in Fig. 1.
Although the particular application of the invention has been described in connection with visual indicating means, it will be understood that aural indicating means may be employed, the local signals co-operating with selecting signals to cause audible signals to e supplied to headphones for example. Again, the indicating means referred to may include any means for taking cognisance of the coincidence of a local signal and a selected signal.
Dated this 13th day of October, 1943.
C. STRATTON CROSS,
Chartered Patent Agent.
Improvements in or relating to Pulse Signal Selecting and Indicating Systems
We, FREDERIC CALLAND WILLIAMS, of 1, The Lynches, Albert Road South, Great Malvern, Worcestershire, ERIC LAWRENCE CASLING WHITE, of Watchfield, 167, Sutton Court Road, Chiswick, London, W.4., and DOREEN BLUMLEIN, of Lanherne, Lescudjack, Penzance, Cornwall, legal representative of ALAN DOWER BLUMLEIN, deceased, late of 37, The Ridings, Ealing, London, W.5., all British Subjects, do hereby declare the nature of this invention and in what manner the same is to be performed, to be particularly described and ascertained in and by the following statement:-
This invention relates to circuit arrangements for detecting or rendering effective electrical signals and is more particularly concerned with electrical signals which are regularly recurrent. Such electrical signals are frequently indicated visually be means of a cathode ray tube.
It is one of the objects of the invention to render such signals effective by performing a searching operation and according to the invention in a circuit arrangement for rendering effective at a receiving point a regularly recurrent electrical signal by performing a searching operation, the searching operation is effected automatically over a period of time between two limiting values by means of a signal generated at the receiving point and is repeated continuously until coincidence in timing I obtained between the recurrent electrical signal and the signal generated at the receiving point whereupon the searching operation is terminated and the recurrent electrical signal is rendered effective.
According to a further feature of the invention, in a circuit arrangement for selecting, at a receiving point, one of a number of recurrent electrical signals in which the timing of a locally generated recurrent signal is adjusted to coincide with the timing of the desired recurrent electrical signal, means are provided for controlling the subsequent timing of the locally generated signal in accordance with the timing of the recurrent electrical signal in order that the condition of coincidence is maintained.
It will be understood that the recurrent electrical signal may be rendered effective in a number of different ways. For instance the coincidence between the recurrent and local signals may cause an indication of the recurrent signal to be given, for example, on a cathode ray tube. Alternatively the coincidence between the two signals may give rise to a voltage which serves to control the operation of one or more further circuits.
The invention has particular application to radio location or "radar" systems of the type in which a carrier wave modulated by signal pulses of short duration is radiated usually in a directional manner and echoes of the radiated signals reflected from any objects within the zone and range of radiation are demodulated and applied to a cathode ray tube. In the absence of means to suppress echoes which it is not desired to observe, such echoes will be rendered visible on the screen of the cathode ray tube without discrimination. The echoes may, for example, be due to direct reflection from the ground, the sea, tress or buildings and they may be rendered visible as deflections in a continually visible trace of any desired form produced on the luminescent screen of the cathode ray tube, such trace being circular or linear and bearing any desired relationship to an indicating scale of measurement applied to the screen. Again, the trace may normally be invisible and only rendered visible on reception of echo signals. Unless suppressing or discriminating means are provided, however, a confusing number of echoes are likely to appear and render observation of a particular echo or class of echoes extremely difficult.
In the case of radio location equipment installed in an aircraft for example, and intended to facilitate the detection and interception of other aircraft, it is important that an indication of echoes received from a particular reflecting aircraft should appear clear of possibly misleading echoes in order that the position of the aircraft under observation may readily be ascertained.
A further object of the invention is therefore to provide apparatus which will enable a substantial degree of automatic discrimination to be effected between signals applied to visual indicating equipment, particularly of the kind employed for radio location purposes, in order that substantially only those signals which it is desired to observe shall be rendered visible.
According therefore to a feature of the invention, in a circuit arrangement for providing an indication of recurrent input signals, local signals, whose timing relation is automatically varied progressively, are generated and compared with the input signals, the generation of a local signal confident in timing with an input signal serving to produce an indication of said input signal.
When the input signal coincides in timing with one of a series of locally generated pulses, according to a further feature of the invention, the subsequent timing of the local signals is arranged to be coincident with the timing of the input signals to provide a recurrent indication of said input signal.
According to a further feature of the invention, in a circuit arrangement for providing an indication of recurrent input signals, the coincidence of an input signal with one of a series of locally generated signals, whose timing relation is adapted to be varied, serves to produce an indication of said input signal and the subsequent timing of said local signals is automatically controlled so that it remains coincident with the timing of said input signal to provide a recurrent indication of said input signal.
According therefore to a further feature of the invention, in circuit arrangements for discriminating between wanted and unwanted recurrent input signals, the input signals are compared with recurrent locally generated signals of predetermined duration and an indication of an unwanted input signal is prevented when the duration of the input signal approaches the duration of a locally generated signal.
According to a further feature of the invention, in circuit arrangements for providing an indication of recurrent input signals and including means for discriminating between wanted and unwanted input signals, when an input signal coincides in timing with one of a series of locally generated pulses, the timing relation between which is automatically varied progressively, an indication of said input signal is produced provided that the duration of the input signal is not greater than the duration of one of a second series of locally generated signals the timing between which is automatically varied progressively in a similar manner to that of the first series of signals.
According to another feature of the invention, a plurality of series of local signals are generated, the timing of individual signals of each series being automatically varied progressively in a similar manner, one series of local signals controlling the production of an indication when an input signal coincides in timing with one of said series of local signals whereupon a second series of local signals causes the timing of all the series of local signals to be coincident with the timing of said input signals to provide a recurrent indication of said input signals while discrimination against unwanted input signals is effected by a further series of local signals which prevent the second series of local signals being rendered effective if the duration of the input signal exceeds a predetermined value.
As applied to radio-location systems provided with indicating means for displaying echoes of a pulse-modulated carrier wave, according to a further feature of the invention, arrangements are provided for automatically effecting a progressive searching operation over ranges within predetermined limits until an echo signal is encountered, when the searching operation is stopped and a recurrent indication of the echo signal is given by said indicating means.
Where the indicating means comprise at least one cathode ray tube for displaying echoes of a pulse-modulated carrier wave reflected from distant bodies, echo signals cause an indication of the direction of a distant body to be given by the position of the spot on the screen of one or more of the cathode ray tubes and arrangements are provided for automatically preventing the simultaneous presentation of echo signals from more than one distant body.
Preferably the simultaneous presentation of more than one echo signal is prevented by automatically effecting a progressive searching operation until an echo signal is encountered when the searching operation is tapped and a recurrent indication is given on the said echo signal. Further the progressive searching operation is effected by generating local signals, the timing relation between which is automatically varied progressively, the generation of a local signal coincident in timing with said echo signal serving to terminate the searching operation and to provide an indication of said echo signal.
It will be understood that where the radio location equipment is employed in an aircraft, for instance, for the detection and interception of other aircraft, echo signals will be returned from the ground at a range substantially equivalent to the height of the aircraft. Assuming that no aircraft echo has been encountered, then the automatic searching operation will cease and he locally generated signals will become "locked" to the ground echoes. In order to prevent this, according to a further feature of the invention, the duration of the echo signals are timed by comparison with a locally generated signal and if the duration is such as to indicate that the echo signal is due to refection from the ground, the automatic searching operation is resumed and no indication is given of the echo signal.
Normally the locally generated signals will "lock" to the echo signal at the shortest range but arrangements are provided according to a further feature of the invention whereby an echo signal may be rejected for another echo signal at a greater range, the subsequent indications being representative of the second echo signal.
The local signals generated and used in the various ways outlined above will be referred to subsequently as "strobing" or "strobe" pulses.
These and other features of the invention will be better understood from the following detailed description of the application of the invention to an airborne radio-location system employed for the detection and interception of enemy aircraft. The description should be read in conjunction with the drawings accompanying the Provisional Specification and with the accompanying drawings which for convenience are numbered from 6 onwards. In the drawings:-
Fig. 1 is a block diagram showing the main connections between the component parts of an airborne radio-location transmitter and receiver equipment,
Fig. 2 is a circuit arrangement including the strobe pulse generating, timing and associated control valve,
Fig. 3 is a circuit arrangement for ground echo rejection and range indication,
Fig. 4 is a circuit arrangement including a receiver output valve, a noise limiting valve, a gain control valve and a receiver suppression valve,
Fig. 5 is a circuit arrangement including a signal measuring valve, a signal amplifier and the connections of an output switch to a cathode ray indicating valve,
Fig. 6 shows the waveforms obtained from the strobe pulse generator,
Fig. 7 shows the timing and duration of three strobe pulses relative to a synchronising pulse,
Fig. 8 shows certain waveforms which are generated during the automatic searching operation,
Fig. 10 is a simplified circuit diagram of arrangements for applying blackout potentials to the cathodes of two cathode ray tubes.
Fig. 11 shows waveforms generated for suppressing the operation of the receiver during the operation of the transmitter,
Fig. 12 shows a noise waveform such as may be applied to the noise limiting valve and
Figs. 13 and 14 show waveforms developed in circuits for modifying the spot indication in accordance with the range.
Referring to Fig. 1 of the drawings a transmitting dipole aerial TA is mounted at the forward end of the fuselage of an aircraft carrying the equipment and is designed to radiate exploring signals over a wide forward angle of search. A pulse modulated carrier wave is fed to the aerial from a transmitter T comprising an oscillator which generates a carrier frequency of, say, 200 megacycles per second. The oscillator, the details of which are not shown, comprises a pair of valves having their anodes coupled by a Lecher tuned circuit including a capacity-coupled sliding bridge of known form which is ganged for simultaneous tuning with the cathode circuits of the valves which are also Lecher tuned. The aerial feeder is variably coupled with the anode conductors by a sliding capacity bridge similar to the tuning bridges. The control grids of the oscillator valves are connected directly together and to a reflecting delay network which simulates s 2000 ohms leak resistance and, due to reflection effects, serves rapidly and effectively to damp the generated oscillations at the termination of each modulating pulse.
The modulator circuit represented by M, may be of any suitable type and generates modulating pulses of two micro-seconds duration separated by intervals of approximately 1,500 micro-seconds, which pulses are fed through choke coils to the oscillator anode circuit. The modulator also supplies two synchronising pulses (pulses A and B) which serve to control the whole of the receiving and display equipment as described in detail later. The power supply unit for the modulator includes a relay which is energised to open the supply circuit to the primary winding of the rectifier transformer if the transmitter valves cease to take the modulator load.
The receiving aerial RA comprises two pairs of dipoles. Those comprising one pair are mounted spaced apart along the wings of the aircraft at equal distances from the fore and aft axis and discriminate between reflecting object spaced in azimuth. The dipoles comprising the other pair are mounted one above and one below a wing of the aircraft and the echo signals picked up by these aerials provide an indication of the elevation of the reflecting object with respect to the aircraft.
The signals picked up by the aerials RA are fed through a rotary switch AS in sequence to a radio frequency amplifier RFA from which they are passed to a frequency changer FC, thence to an intermediate frequency amplifier and second detector IFD followed by a low frequency amplifying stage LF and the final receiver output stage V 611.
When the aircraft carries an observer and a pilot, two indicators are employed. The observers indictor S includes two cathode ray tubes, one of which is employed for range determination only while the other is employed for the determination of direction. The pilots indicator VI consists of one cathode ray tube which provides an indication of both range and direction. The observers range tube is provided with a horizontal time base and return signals or echoes applied thereto via the lead R are displayed as vertical deflections of the time base. In both direction tubes, the echoes from the four dipole aerials are applied in turn to the deflecting plates so that he position of the spot with reference to the centre of the tube screen gives the direction of the aircraft to be intercepted. In addition as regards the pilots indicator, range is determined by modification of the spot, the arrangement being such that the spot is expanded in a line or "grows wings" to an extent dependent on the range. A more detailed description of this type of range determination is given in co-pending application No. 1251/44 (Serial No. 581,375).
Consideration will now be given to the synchronising pulses A and B derived from the modulator M. The A pulse is fed direct to the observers indicator S where it control the generation of the horizontal time base for the observers range tube. The B pulse is fed via the pilots control PS to a delay network DN600. The delay network has two outputs, one of which is fed to a suppression unit V601 while the other is fed to the strobe timing circuit V602. The suppression unit V601 provides two negative-going pulses, one of which is fed via lead G to the control grid of the local oscillator in the frequency changer FC and prevents the operation of the oscillator for the duration of the transmitter pulse to avoid overloading of the receiver. The other output is fed via lead H to the screening grids of the radio and intermediate frequency amplifying valves to relieve the automatic gain control circuit at close ranges as described subsequently.
The strobe timing circuit V602, in conjunction with the drift circuit V615/6/7, controls the timing, relative to the transmitted pulse, of the strobing pulses generated by the strobe pulse generator V604/5. This generator includes a delay network into which generated pulses are fed and from which three strobing pulses having selected duration and timing separation between then are taken. The three strobing pulses will be referred to as pulses E, F and D. Pulse E is fed to the observers indicator unit S and the signal measuring rectifier V606 via lead E; pulse F is fed to the ground echo rejection rectifier V608/9 via lead F and pulse D is fed to the drift stop rectifier V607 via lead D. The signal output from the receiver output stage V611 is also fed to the signal measuring rectifier V606, the ground echo rejection rectifier V608/9 and the drift stop rectifier V607 via leads I, J and K respectively.
The signal measuring rectifier V606 is arranged to pass a rectified voltage when the strobe pulse E coincides with and covers an echo. This rectified voltage is representative of the signal received by the directional receiving aerials and is fed through an amplifier V614 and an output switch S601, which rotates synchronously with the aerial input switch AS, to the deflecting plates of the parallel-connected direction tubes in the observers and pilots indicators. The output from the amplifier V614 is also fed to the automatic gain control circuit V613, the DC output of which is mixed with the DC output from a noise limiter V610, fed from the receiver output stage V611, to reduce the gain of the intermediate frequency amplifier IFD to prevent overloading in the event of high signal strength and/or high noise level.
The drift stop rectifier V607, when the echo signal overlaps the strobe pulse D, develops a voltage which is applied through lead L to the drift circuit V615/6/7 which interrupts the automatic searching operation and causes the strobe pulse D to follow the echo signal i.e. the strobe pulse D becomes "locked" to the echo signal. The drift circuit also feeds the range indicator circuit V619/20/1 via lead N to provide the wing-growing feature previously mentioned while a voltage taken via lead O from noise limiter valve V610 controls the wing-growing at closer ranges than can be determined by the drift circuit alone as described in detail subsequently. The output from the drift circuit is also fed via the lead P to a black-out valve V612 which biases off the direction tubes in the two indicators as long as automatic searching is in operation. When a strobing pulse locks to an echo, this bias is removed.
As previously mentioned the strobe pulse E is fed directly to the observers indicator unit. This together with a signal output from the receiver output stage V611, serves to provide the display for the range tube, the presence of the strobing pulse being necessary in order to enable automatic searching to be interrupted and manual searching to be effected, if desired, as will be described in greater detail subsequently.
The ground echo rejection rectifier V608/9 is provided to prevent the automatic searching operation from being interrupted by ground echoes. A signal output from V611 is fed to V608/9 together with the pulse F. This pulse has a longer duration than the pulse E and the recurrence of signals having a duration considerably greater than that of a signal returned by an aircraft, as determined by comparisons with the pulse F, causes a potential to be built up which after a predetermined number of pulses is sufficient to trip the search return circuit and the searching cycle is recommenced.
Certain of the units shown in Fig. 1 will now be described in greater detail and reference will first be made to the receiver output stage V611 shown in Fig. 4 in which the valve V611 corresponds with the similarly lettered unit shown in Fig. 1. It will be understood that in order to assist in an understanding of the invention, specific values will be mentioned for various components and voltages while certain waveforms developed in different parts of the circuits will be illustrated. These values are given purely by way of example and the invention should not be considered as limited thereto. Further the waveforms shown are theoretical only and do not necessarily represent the waveforms which would be exhibited by suitable test equipment.
The chief function of the receiver output stage V611 is to act as a phase splitter for the receiver output, delivering a positive-going signal across the cathode load and a negative-going signal across the anode load. The cathode load is composite and consists of the inductance L632 in series with the resistance R658; a resistance/condenser combination (C661, C662, C663, R653, R654, R655), the values of the components being such that the load presents a substantially constant resistance at all frequencies; and finally a circuit consisting of a condenser C664 and resistance R657.
The function of the resistance/condenser combination is to provide a circuit having a time constant of approximately 3.4 micro-seconds. The pulse width of the transmitted signal is approximately 2 micro-seconds so that any echo signals will be passed through the combination substantially unchanged except for a reduction in amplitude. Interfering signals having a pulse width considerably in excess of 3.4 micro-seconds, for instances interrupted continuous wave interference, will be reduced to sharp pulses at the instants of switching on. This question of interference will be referred to subsequently.
The output from the anode of V611 is fed to a continuous wave interference and noise limiting valve V610, the operation of which will be described subsequently. The cathode load provides three outputs which, assuming the input to V611 consists of echo signals with a pulse width not greater than 2 micro-seconds, are as follows:-
(a) one tenth of the differentiated output from the resistance/condenser combination is taken from the junction of R655 and C663 and fed via terminal t4 to a signal amplifier (not shown) for the range cathode ray tube in the observers indicator S.
(b) the full amplitude of the differentiated output from the resistance/condenser combination is taken from the junction of R653 and C662 and fed via terminals t12 and t15 (Fig. 5) to the signal measuring rectifier V606 and via terminals t12 and t11 (Fig. 2) to the drift stop rectifier V607. The operation of these circuits will be described in detail subsequently.
(c) the third output is fed from the junction of the condenser C664 and resistance R657 via terminals t8 and t7 (Fig. 3) to the ground echo rejection rectifier V608. The condenser C664 and resistance R653 have a time constant of 20 micro-seconds and the function of the circuit is to determine whether the signal lasts as long as 5.5 micro-seconds, indicating a ground echo, and if so to trip the search return circuit. The impedance of the condenser and resistance is sufficiently high not to load the valve V611 appreciably.
The operation of the strobe generating and timing circuits will now be described with reference to Fig. 2. The strobe pulse generator consists of a multivibrator type of circuit comprising the valves V604 and V605, the timing of the circuit being fixed by the timing valve V602 to which the synchronising pulse B from the modulator M is applied. Considering the multi-vibrator circuit, it will be noted that the cathodes of V604 and V605 are directly connected together and that the anode of V604 is coupled to the grid of V605 through a condenser and a resistance, the time constant of this circuit being long compared with the interval between the synchronising pulses. Such an arrangement is inherently stable and request to be triggered by an applied impulse. In fact there are two critical values for the potential of the control grid of V604. Thus assume that the control grid potential of V604 is above the greater of the two critical values, then V604 will be conducting and V605 will be non-conducting since its cathode potential is held by the cathode of V604 considerably above the potential of its control grid, which is low owing to the coupling from the anode of V604. As the grid potential of V604 is lowered, the anode voltage and hence the control grid voltage of V605 rises while the common cathode voltage falls. When the grid potential of V604 reaches the lower critical potential which in this particular equipment may be, say, +126 volts, the circuit triggers to the condition where V604 is non-conducting and V605 is conducting. In this condition the cathode of V604 is held at a sufficiently high potential with respect to the grid to maintain the anode current zero. Further since the time constant of the coupling circuit is long, this condition will exist until the grid potential of V604 rises to the upper critical value. As this grid potential rises, anode current beings to flow through V604 so that the grid potential of V605 is lowered with a consequent reduction of common cathode potential. When the grid potential of V604 reaches the upper critical value which may be, say, +132 volts, the circuit triggers back to the original condition.
It is this second triggering action, in which the anode current of V605 is reduced sharply to zero, which is employed to form the strobe pulses. Its timing, which is determined by the time taken for the grid potential of V604 to move from the lower critical value downwards and then upwards to the higher critical value, is fixed by the timing valve V602. The valve V602 has the synchronising pulse (pulse B) applied to its grid from a tapping on the delay network DN600 (Fig. 4) via terminals t28 and t1 (Fig. 2) and has a self-biasing arrangement consisting of the condenser C624 connected between the cathode and earth. The valve of the condenser (say, 0.1 uF) is such that the potential across it can only vary by a negligible amount in a single pulse period, but its mean potential may be anything from say, +25 volts to +125 volts and this mean potential determines the strobe pulse timing. During the long interval between the termination of a strobe pulse and the arrival of the next synchronising pulse, the valve V602 is conducting due to the connection between the cathode and control grid. Due to the flow of current in the anode circuit and also through R619 and the diode 4 of V618, the anode potential remains at say, +138 volts for this period. This potential is above the upper critical value for the grid of V604 and V604 remains conducting for this period as shown at ab on curve J (Fig. 6).
When a synchronising pulse is applied through terminal t1 to the control grid of V602, the anode voltage falls to substantially that of the cathode owing to the high anode load, the waveforms for values of cathode potential of +125 volts, +100 volts and +25 volts being shown at J1, J2 and J3 in Fig. 6. The minimum voltage in each case is less than the lower critical value for the grid of V604 so that the multivibrator circuit is triggered as previously explained. The anode potential remains at this low potential until the end of the synchronising pulse. It will be noted that a resistance/condenser combination, consisting of R609, R610, R612, C622, C623 and the stray capacity of the screen grid across resistances R610 and R612, is connected to the anode and screen grid circuits. When the synchronising pulse ends, the effect of this combination on the anode voltage is to allow an initial fairly rapid rise in anode voltage followed by a rise which takes place at a slower rate. For high vales of cathode bias, the initial rise will carry the anode voltage to 138, above which it cannot go owing to the diode portion of V618. For low values of cathode bias, however, the rapid rise is insufficient to raise the anode voltage to 138 and the rise to this value takes place at a slower rate as shown in Fig. 6. The upper critical voltage for the grid of V604 is 132 i.e. just below 138, and hence the timing of the second triggering of the multivibrator circuit depends upon the value of the cathode bias of V602. Curves K1, K2 and K3 of Fig. 6 show the grid potentials of V605 corresponding to the curves J1, J2 and J3 respectively. The shortest delay after the end of the synchronising pulse is arranged to be approximately 0.35 micro-seconds and the longest is approximately 67 micro-seconds.
The strobe pulses are generated by connecting the anode of V605 to a delay network DN601 having a short-circuited end 6_ sections away from the input, the other end of the network extending for 6 sections from the input and being terminated in the characteristic impedance R627 of the network. The component values of the network may be such that each section introduces a delay of 0.42 micro-seconds. The three pulses E, D and F are obtained respectively from tapping points P2, P3 and P4 on the network. The pulse E is obtained from a tapping three sections away from the input and hence begins at 3 x 0.42 = 1.3 micro-seconds after the input is applied to the point P1/ The pulse lasts until it is reflected by the short-circuited end and travels back in phase opposition along the line to the point P2. This period is 2 x 3_ x 0.42 = 2.9 micro-seconds. The pulse D will start at the same time as the pulse E but will have a duration given by 2 x 6_ x 0.42 = 5.5 micro-seconds. Finally the pulse F will start 5 x 0.42 = 2.1 micro-seconds after the input is applied to the point P1 and will also last for 5.5 micro-seconds. The pulse E is fed via terminals t13 and t15 (Fig. 5) to the signal measuring rectifier V606; the pulse D is fed to the drift stop rectifier V607 and the pulse F is fed via terminals t5 and t6 (Fig. 3) to the ground echo rejection rectifier V608. The operation of these circuits in response to the pulses will be described subsequently. The timing and duration of the three strobe pulses relative to the pulse B are shown in Fig. 7, the negative-going pulse being the grid potential of V605.
The strobe pulses are produced at instants determined by the prevailing potential of the cathode of V602 and, in applying the present invention, provision is made for varying the timing of the pulses over what may be termed a searching period during which the potential of the cathode of the valve V602 is automatically and cyclically varied between two predetermined limits. Such variation has the effect of gradually increasing the time interval between the signal radiated by the transmitter and the production of the strobing pulses until one of the strobing pulses coincides with an echo signal, as is inevitable if an object producing echoes is within range of the equipment during any cycle of drift of the cathode potential from one extreme to the other. It will be seen from the subsequent description that when an echo signal coincides with a strobing pulse, the normal progress of the drift cycle is arrested, the echo signal and the strobing pulse become "locked" together and an indicator appears on the two direction tubes, one in the pilot' indicator VI and the other in the observer' indicator S.
The voltage applied to the cathode of V602 is varied slowly from +125 volts to +25 volts followed by a quick return to +125 volts under the control of the valves V615, V616 and V617. The variation is effected by varying the current flow through V615, the anode of which is connected through a small resistance R683 to the cathode of V602 and also through R606 and R605 to the positive H.T. line. It is to be noted that the cathode voltage of V602 cannot rise above +125 volts owing to its connection to the diode anode 5 of V618, the cathode of which is connected to +125 volts derived from the potentiometer P7. Assume that the cathode of V602 and hence the anode of V615 is at the maximum value of +125 volts. The control grid of V615 is connected to +10 volts with respect to earth and hence with respect to the cathode and in normal operation this would result in heavy anode current and a rapid fall in anode potential. However, the condenser C683 is connected between the anode and grid with the result that any fall in anode voltage, due to the grid voltage tending to rise, is fed back to the grid so as to oppose the rise in grid voltage. As long as the control grid is connected to a fixed positive voltage source, this action will continue, the grid voltage endeavoring to rise while the anode voltage falls due to the feedback action which takes place to prevent the grid voltage rising. Now the change in control grid potential necessary to cause a change of 100 volts at the anode will be small owing to the magnification of the valve. In a particular type of pentode employed, the magnification is of the order of 250 so that the necessary change in control grid potential is 0.4 volts and the actual change is from, say, -2.1 to -1.7 volts approximately. The resistance R681 is high, say 5 megohms, so that there is thus a substantially constant potential of 12 volts across R681 passing a current of 2.4 micro-amps through it. It is assumed that the valve is operating within its grid base so that this current flows into the condenser C683 thus changing the voltage across it at the rate of EQUAT. HERE = 48 volts per second. Since the control grid may be regarded as at a substantially constant potential, the anode voltage must fall at a rate of 48 volts per second and takes 2.1 seconds to fall from +125 volts to +25 volts. In Figure 8 curves L and M show respectively the cathode potential of V602 and the anode potential of V615 while curve N shows the grid potential of V615 on an enlarged voltage scale. It will of course be understood that the slow rate of fall of anode voltage could have been obtained by inserting a condenser between the control grid of V615 and earth but in this case the value of the condenser would have to be multiplied by the magnification of the valve in order to obtain the same rate of fall.
The anode potential of V615 continues to fall and if left to itself would eventually reach about +5 volts and remain at this voltage which is on the knee of the anode volts/anode current characteristic of the type of pentode employed. It will, however, be seen from Figs. 2 and 3, that the resistances R605 R606 (Fig. 2), R711, R710, R737 and R728 (Fig. 3) form a potentiometer between the high tension positive (300 volts) and a source of -100 volts, terminals t2 (Fig. 2) and t3 (Fig. 3) being connected together. The values of these resistances are such that when the anode of V615 falls to +24 volts, the junction of R710 and R711, which is connected to the diode cathode 4 of V609, falls to earth potential. Any further fall in the anode potential of V615 causes the potential of the diode cathode to become negative with respect to the anode and the diode conducts. This causes a negative potential to be applied via terminals t9 (Fig. 3) and t10 (Fig. 2) to the grid of V617. The valve V617 is normally conducting as its grid and cathode are both connected to earth and the negative voltage applied to the grid thus causes a rise in anode voltage which, through R693 and R689, causes a rise in the control grid potential of V616. Normally the control grid potential of V616 is well beyond cut off and the effect of the rise in anode voltage of V617 is to rise the control grid voltage of V616 to earth whereupon V616 conducts. It will be noted that the suppressor grid of V615 is connected via R685 and R686 to a source of -100 volts and via R684 to the anode of V616. Normally the suppressor grid is at a small positive potential but the fall of anode voltage of V616 causes a negative potential to be applied to the suppressor grid of V615 sufficient to cut off the anode current. The anode potential of V615 thus rises exponentially as determined by C624 and R605, R606 (curve M, Fig. 8). The condensers C683, C685 and C667 are also charged during the rise in anode potential and the voltage developed across R689 by the charging of C685 maintains a positive potential on the grid of V616 and hence a negative potential on the suppressor grid of V615 (curve P, Fig. 8) thus allowing the return stroke to be completed even though V617 becomes conducting as soon as the return stroke starts. The return stroke is terminated by the action of the diode 5 of V618 which prevents the cathode voltage of V602 rising above +125. As the rate of rise of anode potential of V615 falls to zero, the voltage on the control grid of V616 begins to return to its original value beyond cut off with a time constant which is determined by C685 and a resistance network consisting of R689 in parallel with R693, the latte being in series with the parallel-connected impedance of V617 and resistance R694. When V616 is cut off, the anode voltage rises followed by the voltage of the suppressor grid of V615. The latter conducts and the cycle begins again.
The above description of the operation of V615, V616 and V617 which are known collectively as the "drift circuit," has been based on the assumption that there are no return signals. If a return signal or echo is encountered during the searching operation, the searching operation is stopped. This action is caused by the drift-stop rectifier V607.
It will be noted that the inner control grid G1 of this valve is connected to a source of -30 volts while the potential of the outer control grid G2 will tend towards earth in view of its connection to the diode 5 of the V616. Further a source of -15 volts is connected to the cathode. The grid base of the valve is greater than 15 volts and hence there will at this stage, be some valve current which will be divided between the outer control grid and the screen grid which is connected to a source of +100 volts. The majority of this current will flow to the screen grid but that flowing to the outer control grid will reduce the potential thereof to that of the cathode. No anode current will flow at this time since the anode, being connected to the control grid of V615 is at the potential of that grid i.e. approximately -2 volts as previously explained. The valve current at this time gives rise to a potential drop across the cathode load of +5 volts so that the cathode potential is -10 volts and there is a potential difference between anode and cathode of +8 volts.
The D pulse, having a duration of 5.5 micro-seconds is applied to the inner control grid and, being positive-going, increase the valve current. This will cause a rapid fall in the screen-grid voltage with the result that anode current will now flow as long as the potential of the outer control grid does not fall below cathode potential. The negative-going wave form on the screen travels along the delay network DN602, the end of which is connected via C653 to the outer control grid. The delay network is arranged to introduce a delay of, say, 0.9 micro-seconds and at the end of this period the voltage of the outer control grid is reduced well below the cathode potential and the anode current is cut off. This operation is clearly seen from Fig. 9 where Q shows the variation in potential at the inner control grid due to pulse D, curve R shows the potential variation at the outer control grid and the lower curve shows the resulting variation in anode current. This current is determined almost entirely by the value of the series resistance R731, since the actual anode/cathode potential drops to about 1 volt even for very small currents. The value of R731 is about 3500 ohms so that the current flow for the 0.9 micro-seconds duration of the pulse will be EQUAT. HERE = 2.0 milliamps. If a pulse is generated every 1500 micro-seconds, the mean anode current is 1.2 micro-amps. This current flow is in opposition to the current flowing into the condenser C683 through the resistance R681 and hence the current flow to the condenser is reduced from 2.4 microamps to 1.2 microamps with a consequent increase in the time of fall of the anode voltage of V615 or the "drift rate" from 2.1 to 4.2 seconds.
Now consider the operation of V607 when a signal coincides with the strobe pulse. As previously explained this signal pulse is fed via the cathode load of V611, terminals t12 (Fig. 4) and t11 (Fig. 2) and condenser C648 to the anode of V607. Now the current flowing in the anode circuit of V607 varies linearly with the signal voltage existing at the instant of the strobe pulse, since, as pointed out previously, this current is substantially completely determined by R731. This is true of positive signals and negative signals down to -8 volts. Owing to the A.C. coupling to the anode of V607, the average of all random signals is zero and hence no increased rectification will occur. Thus moderate interference or noise has no appreciably slowing up effect on the drift rate, but an echo of a transmitted pulse is always positive at the instant when it is coincident with the strobe pulse, so that an increase takes place in the rectified current. As the amount of overlap of the strobe pulse and the echo increases, the extra rectified current becomes sufficient to counterbalance the 1.2 microamps flowing in to the condenser C683 through R681. Feedback through C683 thus ceases and the potential of the control grid of V615 is maintained constant and hence the timing of the strobe pulses also remains constant i.e. the drift cycle has been stopped. It will be understood that, once the drift cycle has been stopped, any movement of the echo in one direction of the other will cause variation of the overlap between the echo and the strobe pulse and the cathode potential and hence the timing of the strobe pulse will steadily follow the variation.
It will be remembered that the cathode load or V611, which provides the signal voltage for V607, has a time constant of 3.4 micro-seconds and the purpose of the circuit is to reduce the effect of I.C.W. interference. In such circumstances it will now be appreciated that in the absence of the differentiating circuit rectification by V607 will take place for the whole of each drift cycle as long as the mark period of the I.C.W. exists and the strobe pulses would be driven backwards towards zero range during the marking periods of the I.C.W. and would be unable to hold on an echo. Even when the differentiating circuit is employed, the echoes may be lost during the marking period, due to overloading in the receiver stages if the interference is strong, but if the ratio of marking to spacing periods is not greater than, say 1:1 the equipment will still be capable of holding on an echo and giving a bearing indication which will be correct qualitatively.
The searching operation may also be effected manually if desired in which case no echoes are visible on the two direction tubes i.e. these tubes are completely blocked out. All the echoes will be visible on the observers range tube. This feature is useful in that it enables the observer to select any echo and align the strobe pulse and echo whereupon by switching over to automatic working the strobe will lock to this echo and an indication will be given on the direction tubes. In order to change over from automatic to manual working, a switch in the observers indicator is operated, the effect of which is to connect earth to terminal t30 (Fig. 2) so that the anode of V616 is earthed. This causes the potential of the suppressor grid of V615 is cut off completely. This prevents the automatic variation in the cathode potential of V602 and hence also the automatic strobing operation. By the operation of the switch, however, terminal t2 is connected to a source of potential through a potentiometer in the observers indicator, the source of potential having such a value that variation of the potentiometer setting between two limits is sufficient to vary the cathode potential of V602 between +25 and +125 volts.
The pilot can, if desired, reject an echo to which the strobe pulse is locked whereupon the automatic search is continued for another echo. This rejection is effected by the operation of a push-button switch on the pilots control unit PS, the effect of which is to apply earth via terminal t16 (Fig. 2) to the junction of R692 and C687. This point is normally held at +300 volts while the diode anode 5 of V616 is at earth potential. The connection of earth to t16 thus causes -300 volts to be added to the existing -10 volts on the outer control grid of V607. This prevent signal rectification by V607 and the automatic operation of V615 is therefore resumed. The negative potential on the outer control grid however leaks back to -10 volts after a time which is determined mainly by the time constant of C687 and R690. Hence even if the push button switch is maintained operated, ability to hold is restored after a time sufficient to slip past one echo, and the earth has to be removed for a short time, sufficient to enable C687 to charge up again through R692 and the diode 5 of V616, before an echo can again be rejected or "slipped".
As previously explained, the two direction cathode ray tubes in the pilots and observers indicators are rendered effective only when an echo coincides with the strobe pulse. At all other times, no beam current flows and the tubes are "blacked out". This is effected by applying a positive potential to the cathodes of the tubes by means of two potentiometers. A simplified circuit diagram as shown in Fig. 10 in which the tube CRT300 forms part of the pilots indicator and the tube CRT410 forms part of the observers indicator. Normally approximately +60 volts is applied to the cathodes of the two tubes by means of the potentiometers R305, R306, R307 and R480, R481, R482. With this positive potential on the cathodes no beam current flows in either tube. The black-out may be lifted automatically be means of the valve V612 or manually be operation of either of the switches S402 or S802. The connection of earth to the junction of R308 and R484 by the operation of one of the switches lowers the voltage on the cathodes to approximately +25 volts when beam current will flow. In the case of automatic control by V612, the blackout line is not earthed but when V612 becomes conducting, the voltage on the line falls from 195 volts to 2 volts and the effect is substantially the same as a direct earth connection. The switch S402 is a three position switch of which two positions only are shown in Fig. 10. In position 1 both tubes are required to be permanently blacked out even through V612 is conducting and this is effected by limiting the available anode current of V612 to 2 milliamps at any anode potential by maintained the screen potential at +60 volts and connecting the anode to +300 volts through R485. By this means the voltage on the blackout line is maintained at a sufficiently high value to ensure that the tubes are blacked out. The pilot can however still override this control by the operation of the switch S802.
Referring again to Fig. 2, the operation of the valve V612 is as follows. During the automatic search period, the cathode voltage of V602 is decreasing linearly at a rate of 48 volts per second. The cathode voltage of V602 is fed to the grid of V162 via C667 and R663, the time constants of which is small compared with the time fall of the cathode of V612 from +125 volts to +25 volts. The condenser and resistance thus form a differentiating circuit with the result that a steady potential of approximately -6 volts is applied to the grid of V612 and the valve is cut off. When the strobe sticks on an echo, the grid potential rises towards earth and the value will become conducting in about 1 second and the tubes will be brightened. This however is too quick, as the automatic gain control circuits take about 3 seconds to set the average signal level correctly and bearing indications are inaccurate before this is done. To delay the conduction of V612 the anode is coupled to the grid by C668 which is effectively multiplied by the gain of the valve, which is about 250, giving, with R663 and R664 a time constant of about 3 seconds. It will be understood that the rapid rise in voltage of the cathode of V602 during the return stroke would normally cause the valve V612 to conduct, but the grid of V612 is prevented from rising above earth potential due to the connection to the diode 4 of V617 and the time constant of R664 and C669 is sufficient to prevent the grid voltage of V612 from rising above cut off during the return stroke.
It has previously been mentioned that the signal output from the receiver output stage V611 (Fig. 4) is fed to the signal measuring rectifier V606 via terminals t12 and t15 (Fig. 5). The signal output is applied to the anode of V606 via the condenser C645 while the strobe pulse E, having a duration of 2.9 micro-seconds is applied to the control grid of V606 via terminal t14, C698 and R630. Normally the control grid of V606 is at -30 volts and the valve is cut off. When the E pulse is applied to the control grid, valve current flows but, assuming that there is no signal output from V611, this current flows to the screen grid. When the E pulse and a signal coincide, V606 operates substantially as a peak rectifier and the rectified output of V606 is fed through a smoothing circuit, comprising resistances R631, R634 and condensers C647 and C673 to an amplifier V614. The output from the valve V614, representing the peak values of the incoming signals from the various aerials in turn is fed through a condenser C676 and resistance R679 to the movable contacts S1, S2 of the rotary output switch S601, which is driven synchronously with the serial input switch A1 so that each signal is fed in proper sequence to the four deflector plates of the two direction cathode ray tubes of which one only, is shown in Fig. 5 for simplicity. Voltages representative of the signals received by the two elevation aerials are fed through contacts C1 and C2 of the rotary output switch to the Y plates and voltages representative of signals from the two azimuth aerials are fed through contacts C3, C4 to the X plates. The valve V606 also supplied from its cathode load the E pulse which is fed to the observers range cathode ray tube via t31 so that an indication is given of the position of the strobe pulse by means of a brightening of the time base trace.
An auxiliary voltage is simultaneously applied to the reflecting plates of the pilots direction tube so that, in addition to showing the orientation of the target, the spot also indicates its relative distance from the observer. The indication is given by causing a horizontal line to grow from the spot i.e. the spot grows "wings", the length of the line being inversely proportional to the distance. For this purpose a valve V621 (Fig. 3) having its anode and control grid coupled through a transformer T602, generates a rectangular waveform having frequency of approximately 7 kc/s. The total amplitude of the control grid wing is about 100 volts so that over the majority of the positive half-cycle the cathode voltage follows the grid, owing to the series resistance R725, and exceed that of the diode anodes 4 and 5. No current, therefore flows to the diode anodes and the potential of the anodes is at approximately 125 volts. During the negative half cycle, however, the control grid potential falls followed by that of the cathode until the latter voltage is less than that of the diode anode i.e. 125 volts. Diode anode current then flows and the diode anode voltage falls by about 5 volts to 120 volts (see Fig. 13), the cathode voltage also remaining at this potential so that the triode anode is cut off completely (see Fig. 14 which shows the waveform son the cathode and grid of V621). The waveform at the diode anodes is passed through C693 to the control grid of V620, the negative-going portion of the waveform being sufficient to cut off the valve entirely. The anode waveform will thus be a substantially symmetrical waveform which is distorted by the anode load consisting mainly of the resistance R717 and the shunt inductance of the transformer T601 having a suitable air gap in its core. The distortion of the square wave results in a series of abrupt changes in potential, alternately in opposite directions. Followed by exponential decays to zero. The time constant of the decay is considerably less than the interval between pulses so that the output is substantially zero for a large part of the cycle. The output from the secondary winding of the transformer T601 is fed from terminal t17 to terminal t18 (Fig. 5) and thence to the primary winding of a transformer T3, the two secondary windings of which are respectively connected in series with the X plates of the pilots direction tubes.
The application of this output results in the lateral extensions of the indicating spot, giving the appearance of wings, the length of which is, as will later be explained, inversely proportional to the distance of the target from the observer. Owing to the character of the waveform applied to the X plates i.e. that the potential is zero for a large part of the cycle, the intensity of the spot is kept almost as great at the centre of the wings as it is in the absence of wings, thus always giving a definite point for estimating bearings. Further the sudden excursions of potential followed by the exponential decay have actually slightly rounded tips, due to a suitable amount of capacity, mostly stray, shunting the output circuit, and this causes the tips of the wings to be brighter than the main portion, thus making the width of the wings more clearly visible.
The amplitude of the wings depends on the mean current passing through the valve V620 since the negative-going pulses applied to the control grid through C693 always have a sufficient amplitude to cut off the valve. The mean current depends upon the mean grid potential which is controlled by the range voltage developed at the cathode of V602, by means of the connection between t2 (Fig. 2 and t3 (Fig. 3) and upon the cathode circuit resistance R716 of V620 which is sufficiently high to linearise the relation between mean current and mean bias potential. When a target echo has been found and the following aircraft proceeds to overtake the target, the range voltage rises. Until the range decreases to about 10,000 feet, the valve V620 is cut off since the cathode potential is about +100 volts due to the potentiometer R716 , R720 and the applied grid voltage is not sufficient to overcome this voltage and cause the valve to conduct. As the range decreases below 10,000 feet, the range voltage and hence the mean grid bias, increase to such a value that the mean current through the valve V620 increases from zero to a certain maximum value and the wings correspondingly expand. The maximum value occurs when the range voltage reaches +125 volts which represents a range of approximately 800 feet. Below this value the amplitude of the echo will of course continue to increase but its width will be decreased due to the fact that the receiver is suppressed for the duration of the transmitter pulse. To overcome this difficult it is arranged that the wings amplitude is increased at close ranges by the operation of a local circuit. This operation will be described after consideration has been given to the automatic gain control circuits.
The gain of the receiver is varied by varying the grid bias potential applied to the first and third intermediate frequency valves either automatically, by a combination of signal strength and the prevailing noise or continuous wave interference level, or manually from the observers indicator. The input to the automatic gain control valve V613 (Fig. 4) is derived from the anode of the signal amplifier V614 (Fig. 5), a connection existing between terminal t24 (Fig. 4) and t25 (Fig. 5). Let it be assumed that the average amplitude of echoes from all the four aerials shall be 25 volts at the output of the receiver i.e. at the cathode of V611 and that this value would make the average anode voltage of V614 approximately 170 volts in which case the potential divider consisting of R675, R666 and R668 serves to apply -3 volts to the control grid of V613, at which point the valve is just passing anode current. A variation in grid bias from -3.5 to -2.5 volts varies the anode current of V613 from, say, 0 to 0.8 milliamps and hence varies the potential of the junction of R671 and R672 from say, - 2.2 volts to -9 volts. This potential is fed via R648 and t23 to the receiver where it controls the gain of the two intermediate frequency valves. The gain control of the receiver is thus varied over its whole range from zero to maximum with a variation of average signal level of one volt at the grid of V613 or a variation of 1.75 volts at the anode of V614 in an average of 102 volts. The variation in average output signal level thus being less than 2 per cent, over the whole range of usable input levels. It is to be understood that "average" in this connection refers to the average on all four receiving aerials, i.e. in a switch period.
The smoothing of the gain control voltage must be sufficient to allow no appreciable change of gain during a switch period (1/10 second) since otherwise the position of the slot with respect to the centre of the tube will not be a true indication of the position of the followed aircraft. At the same time the time constant must not be too great or the gain control will now follow rapid fading and the strobe will slip off the selected echoes and search for the next echo. Double smoothing is therefore employed, the condenser C671 being provided to remove most of the switch frequency ripple while R667 together with C670 provides the main smoothing. The values of the components are such that the angle between the true direction and the direction given by the spot is less than 2 degrees, which is scarcely perceptible, and at the same time the gain can cover its whole range in 1.5 seconds.
Manual gain control is effected by operating a switch in the observers indicator which applies earth to terminal t29. This applies a bias of -20 volts to the grid of V613 which is thus biassed beyond cut off. The gain is then controlled by a potentiometer in the observers indicator, the potentiometer being connected to terminal t30.
The gain is also subject to a maximum limit in dependence upon the level of the noise output. If the noise output of the receiver tends to exceed a total swing of approximately 40 volts the limitation of the gain should become effective, as with this value of noise level the strobe tends to stick at the beginning of the search. The method employed makes use of the D.C. component of the noise and of the gaps cut in the noise by the suppression pulses. Or this purpose the negative-going output from the anode of V611 is applied to the diode anode 5 of the valve V610, the waveform on the diode anode being shown in Fig. 12. The positive-going pulses produced at the anode of V611 by operation of the suppression pulses cannot exceed the cathode potential so that the whole waveform is depressed with respect to the cathode as shown in the diagram.
The diode 5 of V610 rectifies the signal output from V611 and the resulting D.C. component, shown by the dotted line in Fig. 12 is smoothed by R651 and C699 and applied to the control grid of V610. The load resistance of V610 is R649 in the cathode lead so that the valve acts as an amplifier of the rectified output of the diode without reversal of phase. Hence the greater the amplitude of the noise, the more negative becomes the potential of the cathode. In the absence of noise the cathode potential is positive with respect to the diode anode 4 but as the noise output of the receiver rises to 40 volts, the cathode potential falls until when the noise exceeds 40 volts, the diode passes current, the gain control voltage is made more negative, and the gain is reduced.
A D.C. component of the signal output due to anything else besides noise, for example continuous wave or interrupted continuous wave interference will also lower the gain the same way sot that it is possible for the equipment to work through moderate interference which might otherwise saturate the receiver and prevent pulse signals getting through and operating the automatic gain control even though the pulse signals may be several times the amplitude of the interference.
It will be noted that the gain of the receiver will increase at very short ranges since, although the amplitude of the received signal increase, its width is reduced as an increasing portion of it is lost due to overlapping with the suppression pulse.
As previously mentioned the wings amplitude increases with decreasing range under control of the range voltage down to a range of approximately 800 feet. It is also desirable that the wings amplitude should increase below this range value and for this purpose a local circuit is employed. Such a local circuit must be acted upon by a voltage which varies either directly or inversely with the range. In the embodiment shown the noise voltage as represented by the cathode voltage of V610 (Fig. 4) is employed. As the range decreases, the noise voltage becomes less positive and by taking a connection from terminal t19 (Fig. 4 to terminal t20 (Fig. 3), a control is exerted on the current flow through V619 which controls the amplitude of the wings voltage generated by V620. At ranges above 800 ft. the grid of V619 is negative with respect to the fixed voltage applied to the diode anodes, due to the fraction of the range voltage applied to the grid from terminal t3 and the potentiometer R711, R710, R737 and R728. Under such circumstances, therefore, the valve current flows to the diode anodes. Below a range of 800 ft. however, the grid potential of V619 is above that of the diode anodes and the valve current passes to the triode anode.
It has previously been mentioned that the equipment is arranged so that the strobe pulses are prevented from locking to the ground, echoes and the valve V608 (Fig. 3) provides this facility. The strobe pulse F having a duration of 5.5 micro-seconds is fed to the inner grid of V608 via terminal st5 (Fig. 2) and t6 (Fig. 3) while the echo signals are fed from the cathode circuit of the receiver output stage V611 (Fig. 4) via C664, terminals t8 and t7 (Fig. 3) to the outer grid of V608, this feed circuit having a time constant of approximately 20 micro-seconds. The pulse F starts at approximately the same time as the echo when the apparatus is holding on an echo and anode current will flow in the anode circuit for the duration of the echo. Now the anode voltage is 300 when the valve is cut off and is reduced to, say, +10 volts when the echo and strobe pulse coincide, and remains at this value for the duration of the echo if this is less than 5.5 micro-seconds or for 5.5 micro-seconds if the echo has a duration greater than this. This change of potential is fed through the long time constant circuit R644, C657 to the diode cathode of V609. The normal potential of this cathode is +40 volts obtained through r647 and the diode is thus non-conducting. If the echo lasts for 3.8 micro-seconds the diode cathode potential is reduced to zero while an echo lasting for longer than this period causes the diode to conduct and lowers the potential of the condenser C688. One echo lasting for the whole of the 5.5 micro-second pulse reduces this potential by 0.6 volts and a number of successive similar coincidences of long echoes with strobe pulses reduces the potential of C688 sufficiently to trigger the drift return circuit as previously described and the automatic searching action is resumed. The condenser C688 discharges through R696 during the return stroke.
As previously explained means are provided for preventing the receiver being violently overloaded by direct pick-up of the transmitter pulse. The valve V601 (Fig. 4) is provided for this purpose, the B pulse from the modulator being applied to the grid of V601 via a delay network in the pilots indicator and thence via terminal t (Fig. 4) to the delay network DN600. The delay network DN600 also provides the synchronising pulse which is applied via terminals t28 and t1 (Fig. 2) to the grid of V602. The pulse applied to V602 is delayed by approximately 3 micro-seconds by the delay network in the pilots indicator and by DN600 while the pulse applied to the grid of V601 is delayed by a further 2.5 micro-seconds. The pulse applied to the grid of V601 is positive-going and gives rise to a negative-going pulse on the screen which is coupled to the grid of the receiver local oscillator though C618 and terminal t26. The waveform fed to the grid of the oscillator is shown at G in Fig. 12 in its relation to the timing pulse B applied to the grid of V602 and to the transmitter pulse C. during the pulse B, the oscillator is biased off by the portion bc of curve G and on the bias being removed at c it does not being oscillating at full amplitude owing to the time taken to build up energy in the oscillatory circuit. This is indicated by the fact that from c the bias drops to nearly zero for about 0.7 micro-seconds, but when the oscillator is nearing its normal amplitude, at d it starts to generate the normal self-bias and is in full normal operation at e.
The valve V601 is also used to reduce the gain of three stages of the receiver to a very low value before and during the transmitter pulse and then to allow this gain to rise to full value with a time constant of 14 micro-seconds. This is effected by coupling the screen grids of the radio frequency and intermediate frequency valves to the anode of V601. When anode current flows, the fall of anode voltage is applied as a negative-going wave to the screen grids. The reduction of the screen potential not only reduces the gain to a value more suitable for close range echoes, thus relieving the automatic gain control, but it increases the damping of the grid circuits of the radio frequency valves in the receiver, and thus materially improves receiver the suppression and hence the ability to receiver near echoes.
Having now particularly described and ascertained the nature of our said invention and the manner in which the same is to be preformed we declare that what we claim is:-
Dates this 10th day of January, 1945.
C. STRATTON CROSS,
Chartered Patent Agent
Leamington Spa: Printed for His Majestys Stationery Office, by the Courier Press. 1946. Published at The Patent Office, 25, Southampton Buildings, London, W.C.2, from which copies, price 1s. 0d. each (inland) 1s. 1d (abroad) may be obtained.