400,976

PATENT SPECIFICATION

Application Date: April 4, 1932. No. 9651/32.

Complete Left: May 2, 1933.

Complete Accepted: Nov. 6, 1933.

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PROVISIONAL SPECIFICATION

Improvements relating to Oscillatory Electric Circuits, such as may be used, for Example, in connection with Cathode Ray Devices

We, ELECTRIC & MUSICAL INDUSTRIES LIMITED, a British Company of Hayes, Middlesex, and ALAN DOWER BLUMLEIN, a British Subject, of 32, Woodville Road, Ealing, London, W.5, do hereby declare the nature of this invention to be as follows:-

The present invention relates to oscillatory electric circuits such as may be used for example as time bases in connection with cathode ray devices, for the purpose of producing periodic deflection of the ray.

The invention is mainly concerned with the generation of non-sinusoidal oscillations adapted to produce deflection of the cathode ray of the kind in which each cycle of deflection comprises a relatively slow motion in one direction followed by a relatively rapid motion in the opposite direction. Arrangements of this kind are, for example, used in television receivers in which reconstitution of a line of the transmitted image occurs during the slow motion of the ray and the rapid motion constitutes the return stroke which is necessary before the next line of the image can be reconstituted. The requirements of such arrangements are, amongst other thins, that the slow motion shall be a uniform one and that the return stroke shall be rapid.

According to the present invention there is provided an oscillation generator adapted to generate periodic electrical oscillations o other than sinusoidal wave form in which the wave form of one portion of each cycle of the generated oscillations is determined by a circuit comprising a reactive element or a plurality of reactive elements in which the wave form of another portion of the cycle is determined by a circuit having different reactive properties from the drift circuit switching means being provided for transferring the control of wave form periodically from one of the circuits to the other. Where the natural frequency of the generated oscillations is high, the switching means may conveniently be in the form of a thermionic device and impulses serving to control the frequency of the generated oscillations may be applied to the grid circuit of the device. The wave form of the generated oscillations can thus be made independent of the wave form of the impulses within the wide limits. The thermionic device can be made to function as a switch by arranging that its anode-cathode impedance is very small compared with the impedances with which it is associated during the "closed" periods and very large compared with these impedances during the "open" periods. Where lower frequencies are to be generated, the switching means may be mechanical.

The present invention further provides apparatus for generating periodic electrical oscillations of other than sinusoidal wave form, comprising means whereby the wave form of the generated oscillations is adapted, during the greater part of each cycle, to be controlled substantially by an inductance and, during the remainder of each cycle, by the resonant frequency of a tuned circuit. The apparatus may be such that the generated current, during the greater part of each cycle, is substantially that due to the voltage of a source applied across an inductance of it may be such that the generated voltage, during the greater part of each cycle, is substantially that build up across a condenser due to the flow of current thereto from a source through an inductance. In both cases, however, the current or voltage during the remainder of each cycle may be that due to a half cycle of oscillation of a tuned circuit.

According to a feature of the present invention as applied to cathode ray devices, the rapid motion of the ray is effected under the control of one half cycle of oscillation of a resonant circuit. Preferably, the rate of the slow motion is determined by the value of an inductance and the rate of the rapid motion is determined by the natural frequency of a resonant circuit.

According to a further feature of the present invention there is provided a source of electrical energy and means for applying from this source to the control coil or electrodes, which serve to deflect a cathode ray, a periodic voltage of pre-determined wave form having a maximum voltage greatly exceeding that of the source.

The invention will be described, by way of example, in two embodiments but it will be realised that there are many other embodiments within the scope of the invention.

The first embodiment to be described comprises a cathode ray tube in which the ray is deflected magnetically by means of an electric current passed through a control coil. The periodicity of the deflections is determined by electrical impulses, for example synchronising impulses transmitted from a television transmitter.

The synchronising impulses are applied to the grid circuit of a thermionic triode having a thermionic diode shunted across its anode-cathode circuit, the cathode of the diode being connected to the anode of the triode. The anode of the triode is connected through a parallel resonant circuit and a suitable impedance, such as a resistance, to the positive terminal of a source of high tension, the negative terminal of the source being connected to the cathode of the triode. The tuned circuit comprises, as inductance, the control coil of the cathode ray device and this is shunted by a condenser in series with an auxiliary parallel resonant circuit. The purpose of this auxiliary circuit will be described later and for the purpose of the preliminary description it will be assumed to be short-circuited. In any case its impedance at the resonant frequency of the main tuned circuit is low. A large condenser is connected from the cathode of the triode to point between the tuned circuit and the resistance for the purpose of maintaining the voltage of the high tension source at a steady value. Suitable biassing means are provided to maintain the grid of the triode slightly positive in relation to the cathode in the absence of a received impulse.

The cycle of deflection will be assumed to being with the ray in its mid position, that is to say with the scanning spot in the middle of the reproducing screen. Under these conditions the current through the control coil is zero but increasing in a direction which will for convenience of description be referred to as positive. The ratio of the resistance of this coil to its inductance is made low and the anode-cathode impedance of the triode is also low due to the positive bias on its grid and therefore the rate of increase of current through the control coil and triode due to the voltage of the source is determined substantially entirely by the inductance of the coil. The current through the coil thus rises at a uniform rate until a scanning impulse is received. This impulse serves to make the grid of the triode negative relative to the cathode and the anode-cathode impedance high. The flow of current through the control coil cannot be stopped suddenly because of the inductance thereof and, since the path through the triode has become of high impedance, the current is deflected into the condenser of the tuned circuit which is thereby charged. The voltage across the anode-cathode of the triode will now rise to a high value and the grid thereof must therefore by this time have been raised to a sufficiently high negative value, but the received impulse, to prevent appreciable current flow through the triode. After a time interval, (measured from the moment when the synchronising impulse was received) equal to that of one quarter of a cycle of oscillation of the tuned circuit at its resonant frequency, the current through the control coil has fallen to zero and the condenser commences to discharge thus causing current to flow through the coil in a negative sense. By the time that the condenser has become fully discharged a further quarter cycle of oscillation at the resonant frequency of the tuned circuit has been executed. Due to the negative current now flowing in the coil, the condenser commences to change in the reverse direction but this reverse charging ceases almost immediately due to flow of current through the diode. The high tension voltage of the source opposes the flow of current in a negative direction through the coil and this current is reduced at a steady rate to zero, thus completing the cycle.

It will be noted that during the grater portion of each cycle (constituting the scanning stroke) the wave form of the current through the control coil is determined substantially by the inductance of this coil and the voltage of the source and during this portion of the cycle the rate of change of current in the coil is, therefore, substantially uniform. The cathode ray is thus swept at a uniform rate across the reproducing screen. During the remainder of the cycle (constituting the return stroke) the wave form of the current in the coil is determined by the natural frequency of the tuned circuit which during this period executes one half cycle of oscillation at its resonant frequency. Further, the voltage developed across the control coil reaches, during each cycle, a peak value greatly exceeding the voltage of the source.

The wave form of the generate wave will be substantially independent of the wave form of the received impulse provided that this impulse makes the grid sufficiently negative and that it does not maintain the negative grid polarisation for longer than one half the scanning stroke.

The circuit described depends upon a half cycle of oscillation of a tuned circuit to produce the back stroke and is liable to give alternate scanning stokes of different mean positions due to persistence of oscillations at one half the scanning frequency. The auxiliary tuned circuit which is arranged in series with the condenser of the min tuned circuit is provided to prevent this. The auxiliary tuned circuit is arranged to resonant at half the scanning frequency whilst the condenser and inductance constituting it are of low impedance and good efficiency. Further the value of the inductance is so chose that series resonance with the condenser of the main tuned circuit occurs at a frequency not lower than one third of the scanning frequency. The auxiliary tuned circuit may, if desired, be connected in the high tension supply between the large shunt condenser and the main tuned circuit.

In determining the capacity of the main tuned circuit this should be taken as including, with the condenser thereof, the self-capacity of the control coil and the associated valves.

The diode should be selected to have a reasonably low impedance and the triode should also have a low impedance with a suitable value of positive grid bias. Both valves should be capable of withstanding high voltages and should have high impedance, in the case of the triode with suitable negative potential on the grid and in the case of the diode for negative voltages applied to its anode. The impedance of the valves at low currents may be decreased by arranging, in series with the diode, a source of E.M.F. adapted to produce a small normal circulating current between the tubes.

The negative pulse required to raise the impedance of the triode is usually relatively high and it must be supplied against the maximum capacity of the valve due to the "Miller" effect.

In order to decrease the amplitude of the negative pulse required there may be provided a coil connected to the grid circuit of the triode and coupled, not very tightly, to the control coil in such a sense that the change in current in the control coil due to the synchronising impulse serves to increase the negative potential on the grid. Further the leakage inductance of this additional coil together with the capacity of the triode serves to delay the negative pulse.

In a second embodiment of the invention a triode of the thyratron type is used. This type of triode has the property that its anode-cathode impedance remains high so long as its grid is maintained at a suitable negative potential relative to the cathode. On suitably reducing the negative potential, however, the impedance falls to a low value and remains at this low value, irrespective of any potentials applied to the grid, until the anode voltage has been reduced to a certain value.

Between the anode and cathode of a triode of this kind there is connected an inductance in series with a condenser constituting a tuned circuit. The condenser consists of the capacity between the electrostatic control electrodes of a cathode ray tube increased by parallel condenser to a suitable value, for example 0.002 microfarads. The point of connection between the inductance and condenser is connected through an inductance and a resistance (which will be termed the feed impedance) to the positive terminal of a source of high tension, the negative terminal of the source being connected to the cathode of the triode.

Once more assuming the cycle to commence with the scanning spot in the centre of the reproducing screen, the charge upon the condenser is zero but increasing due to current from the source flowing through the feed impedance. The grid of the triode is held at a suitable negative value and the current through the inductance of the tuned circuit is thus negligible. The voltage across the condenser therefore rises until a positive synchronising impulse is applied to the grid of the triode. The impedance of the triode is now very small and the condenser discharges through the inductance of the tuned circuit at a rate dependent upon the natural frequency thereof. the feed impedance is made sufficiently high to prevent appreciable flow of current through it during the short period of the condenser discharge. When the tuned circuit has executed one quarter of a cycle of oscillation the condenser has discharged and, due to the current now flowing in the inductance, charges up in the reverse direction, the tuned circuit thus executing a second quarter cycle of oscillation. The current through the inductance is now zero and the voltage across the anode circuit of the triode is therefore zero also and its impedance has accordingly returned to its original high value. The condenser cannot draw current through the inductance of the tuned circuit and accordingly current commences to flow through the feed impedance until the voltage across the condenser is once more zero and the cycle has been completed.

In this arrangement the wave form of the voltage across the condenser is controlled ruing the greater part of each cycle, that is during the scanning stroke, by the flow of current thereinto through the inductance of the feed impedance, and during the remainder of the cycle, that is during the return stroke, by the natural frequency of the tuned circuit which executes one half cycle of oscillation at its resonant frequency.

The positive impulse applied to the grid need only last for long enough to trigger the thyratron but it may if desired persist until the middle of the scanning stroke, after which the grid must be negative.

In this example also, the voltage developed across the control electrodes can exceed greatly the voltage of the source since the voltage is determined by the flow of current through the inductance of the feed impedance.

In this case also it is desirable to provide an auxiliary circuit to prevent persistence of oscillations at one half the scanning frequency and this may take the form of an inductance and a condenser connected in series across the condenser of the main tuned circuit. The auxiliary circuit is arranged to resonant at the undesired frequency and thus makes the voltages across the control electrodes at this frequency negligible. It should also be arranged that the inductance and condenser of the auxiliary circuit have individually a high impedance and that the inductance resonates with the condenser of the main tuned circuit at a frequency not higher than say two thirds the scanning frequency.

In determining the capacity of the main tuned circuit it should be take as including the self-capacity of the feed inductance.

Both the circuits described are such that there is theoretically, no power dissipation. The actual power dissipation is therefore only that due to unavoidable losses and can be made relatively small.

Clearly, forms of switch other than thermionic valves may be used. At relatively low frequencies mechanical switches can be used and they can be made to operate substantially sparklessly.

Although the invention has been described as applied to the control of cathode ray tubes it is clearly not limited to such uses but may be employed in many other fields where periodic oscillations of predetermined but other than sinusoidal wave form are required.

Dated this 4th day of April, 1932.

REDDIE & GROSE,

Agents for the Applicants,

6, Breamís Buildings, London, E.C.4.

COMPLETE SPECIFICATION

Improvements relating to Oscillatory Electric Circuits, such as may be used, for Example, in connection with Cathode Ray Devices

We, ELECTRIC & MUSICAL INDUSTRIES LIMITED, a British Company, of Hayes, Middlesex, and ALAN DOWER BLUMLEIN, a British Subject, of 32, Woodville Road, Ealing, London, W.5, 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 any be the following statement:

The present invention relates to oscillatory electric circuits such as may be used for example as time bases in connection with cathode ray devices, for the purpose of producing periodic deflection of the ray.

The invention is concerned with the generation of oscillations of the kind in which the rate of change of current or voltage is relatively small or even zero over the greater part of each cycle and is relatively high over another part of each cycle. For convenience these two parts of the cycle will be referred to as the quick and slow strokes, respectively. Oscillations of this kind are, for example, used in television receivers in which reconstitution of a line of the transmitted image occurs during the slow motion of the ray produced by a slow change of current or voltage and the rapid motion, produced by a rapid change of current or voltage, constitutes the return stroke which is necessary before the next line of the image can be reconstituted. The requirements of such arrangements are, amongst other things, that the slow motion shall be a uniform one and that the return stroke shall be rapid.

In connection with multiplex telegraphy it has been proposed to generate oscillations having a slow forward stroke and a quick return stroke by means of a circuit comprising a rectifier having connected in series therewith a source of potential, a resistance and a choke. Across the choke there is placed a condenser. The condenser is slowly charged from the source through the resistance until the potential across the rectifier is sufficient to break down the rectifier. There is then a sudden discharge through the rectifier which continues by virtue of the choke past the zero potential point of the condenser.

According to the present invention there is provided an oscillation generator adapted to generate electrical oscillations of the kind specified under the control of signals applied thereto from an external source, the generator comprising two reactive circuits having different reactive properties and means responsive to said signals for transferring the control of the generated wave form from one of said circuits to the other.

Where the natural frequency of the generated oscillations is high, the transferring or switching means may conveniently be in the form of a thermionic device and applied signal impulses serving to control the frequency of the generated oscillations may be applied to the grid circuit of the device. The wave form of the generated oscillations can thus be made independent of the wave form of the impulses within wide limits. The thermionic device can be made to function as a switch by arranging that is anode-cathode impedance is very small compared with the impedances with which it is associated during the "closed" periods and very large compared with these impedances during the "open" periods. Where lower frequencies are to be generated, the switching means may be mechanical.

The present invention also provides a circuit for generating electrical oscillations of the kind specified, under the control of signals applied thereto from an external source, the wave form of the slow stroke being determined by the rate of decay and built up either of current in an inductance or of voltage across a condenser, the moment of termination of the slow stroke being determined by the applied signal and the voltage maintained across the inductance or, as the case may be, the current fed to the condenser being maintained substantially constant during the slow stroke.

Thus the arrangement of the present invention employing a condenser to control the slow stroke is clearly distinguished from known arrangements for generating oscillations of similar wave form under the control of receiving signals in which the wave form of the slow stroke is determined by the rate of build up of voltage across a condenser since according to the present invention, during the slow stroke, the voltage across the controlling condenser first falls to zero and then builds up in the opposite sense.

Other features of the invention will be apparent from the following description and the appended claims.

The invention will be described, by way of example, with reference to the accompanying drawings, in three embodiments but it will be realised that there are many other embodiments within the scope of the invention.

In the drawings, Figs. 1 and 4 are circuit diagrams showing alternative arrangements according to this invention, Fig. 3 shows a modification of the circuit of Fig. 1, and Figs. 2 and 5 are diagrams illustrating the operation of the arrangements of Figs. 1 and 4 respectively.

The first embodiment to be described with reference to Fig. 1 is suitable for use with a cathode ray tube in which the ray is deflected magnetically by means of an electric current passed through a control coil. The periodicity of the deflections is determined by electrical impulses, for example synchronising impulses transmitted from a television transmitter.

The synchronising impulses are applied through a transformer 1 to the grid circuit of a thermionic triode 2 having a thermionic diode 3 shunted across its anode-cathode circuit, the cathode of the diode being connected to the anode of the triode. The anode of the triode is connected through a parallel resonant circuit 4 and a suitable impedance 5, such as a resistance, to the positive terminal of a source of high tension 6, the negative terminal of the source being connected to the cathode of the triode 2. A large condenser 10 is connected from the cathode of the triode to a point between the tuned circuit 4 and the resistance 5 for the purpose of maintaining the voltage applied to the tuned circuit 4 at a steady value. The value of condenser 10 is so great that this condenser has no effect upon the circuits other than that of maintaining the voltage steady. The resistance 5 is included to limit the current which can flow to the tuned circuit 4 so as to prevent damage if this circuit ceases to oscillate.

The tuned circuit 4 comprises, as inductance, the control coil 7 of the cathode ray device (not shown) and this is shunted by a condenser 8 in series with an auxiliary parallel resonant circuit 9. The purpose of this auxiliary circuit 9 will be described later and for the purpose of the preliminary description it will be assumed to be short-circuited. In any case its impedance at the resonant frequency of the main tuned circuit is low. Suitable biassing means 11 are provided to maintain the grid f the triode 2 slightly positive in relation to the cathode in the absence of a received impulse.

The operation of the circuit will be described with reference to Fig. 2 in which time is the abscissa and the ordinates are: in curve (a) the current IL in inductance 7; in (b) the voltage Vg impulses applied to the grid of triode 2; and (c) the voltage VC developed across the tuned circuit 4.

The cycle of deflection will be assumed to begin with the ray in its mid position, that is to say with the scanning spot in the middle of the reproducing screen. Under these conditions the current through the control coil is zero but increasing in a direction which will for convenience of description be referred to as positive and indicated by the arrow IL in Fig. 1. The ratio of the resistance of the coil 7 to its inductance is made low and the anode-cathode impedance of the triode 2 is also low due to the positive bias on its grid and therefore the rate of increase of current through the control coil and triode due to the voltage of the source 6 is determined substantially entirely by the inductance of the coil 7. The current IL through the coil thus rises at a uniform rate from d to e (Fig. 2 (a)) until a scanning impulse is received through transformer 1 (Fig. 2 (b)). This impulse serves to make the grid of the triode negative relative to the cathode and the anode-cathode impedance high. The flow of current through the control coil 7 cannot be stopped suddenly because of the inductance thereof and, since the path through the triode has become of high impedance, the current is deflected into the condenser 8 of the tuned circuit 4 which is thereby charged. The voltage across the anode-cathode of the triode will now rise to a high value and the grid thereof must therefore by this time have been raised to a sufficiently high negative value, by the received impulse, to prevent appreciably current flow through the triode. After a time interval, (measured from the moment when the synchronising impulse was received) equal to that of one quarter of a cycle of oscillation of the tuned circuit at its resonant-frequency and represented by the distance along the abscissa in Fig. 2 (a) between points e and f, the current through the control coil 7 has fallen to zero and the condenser commences to discharge thus causing current to flow through the coil in a negative sense. By the time that the condenser 8 has become fully discharged a further quarter cycle of oscillation at the resonant frequency of the tuned circuit 4 has been executed and the current is at point g Fig. 2 (a). Due to the negative current now flowing in the coil, the condenser commences to charge in the reverse direction but this reverse charging ceases almost immediately due to the cathode of the diode 3 becoming negative with respect to earth with consequent flow of current through the diode 3. Thus the diode 3 operates to pass current whenever the rate of change of current flowing in a negative direction through the inductance 7 exceeds a predetermined value represented by the slope of the line g e1 in Fig. 2. The high tension voltage of the source 6 opposes the flow of current in a negative direction through the coil 7 and this current is reduced at a steady rate to zero as represented by g-d1 (Fig. 2 (a)), thus completing the cycle.

It will be noted that during the greater portion of each cycle (constituting the slow forward or scanning stroke), namely g to e1 Fig. 2 (a), the wave form of the current through the control coil 7 is determined substantially by the rate of decay and build up of current through the inductance of this coil, the voltage source 6 maintaining a substantially constant voltage across the inductance during this portion of the cycle. The rate of change of current in the coil is, therefore, substantially uniform. The cathode ray is thus swept at a uniform rate across the reproducing screen. During the remainder of the cycle (constituting the return stroke), and represented between points e and g Fig. 2 (a), the wave form of the current in the coil is determined by the natural frequency of the tuned circuit 4 which during this period executes one half cycle of oscillation at its resonant frequency. The function of the diode 3 is to prevent more than one half cycle of oscillation occurring. Further, the voltage developed across the control coil reaches, during each cycle, a peak value greatly exceeding the voltage of the source 6. This is shown clearly in Fig. 2 (c) where the distance VS represents the voltage of the source 6.

In the generator described, the control of wave form is transferred, by the valve 2 under the influence of an applied negative impulse upon its grid, from the inductance 7 along to the tuned circuit 7, 8. The transfer of the control of wave form from the tuned circuit back to the inductance is effected by the unidirectionally conductive diode 3. Thus the control of wave form is shared between two circuits having different reactive properties, namely the circuit through inductance 7 and valve 2 and the tuned circuit comprising the inductance 7 and the condenser 8.

The wave form of the generated wave will be substantially independent of the wave form of the received impulse provided that this impulse makes the grid sufficiently negative and that is does not maintain the negative grid polarisation for longer than one half the scanning stroke. The grid voltage may for example extend anywhere within the shaded areas in Fig. 2 (b).

The circuit described depends upon a half cycle of oscillation of a tuned circuit to produce the back stroke and is liable to give alternative scanning stokes of different mean positions due to persistence of oscillations at one half the scanning frequency. The auxiliary tuned circuit 9 which is arranged in series with the condenser 8 of the main tuned circuit 4 is provided to prevent this. The auxiliary tuned circuit 9 is arranged to resonate at half the scanning frequency whilst the condenser and inductance constituting it are of low impedance and good efficiency. Further the value of the inductance is so chose that series resonance with the condenser 8 of the main tuned circuit occurs at a frequency not lower than one third of the scanning frequency. The auxiliary tuned circuit, may if desired, by connected in the high tension supply between the large shunt condenser 10 and the main tuned circuit 4.

In determining the capacity of the main tuned circuit 4 this should be taken as including, with the condenser 8 thereof, the self-capacity of the control coil 7 and the associated valves 2 and 3.

The diode 3 should be selected to have a reasonably low impedance and the triode 2 should also have a low impedance with a suitable value of positive grid bias. Both valves should be capable of withstanding high voltages and should have high impedance, in the case of the diode for negative voltages applied to its anode.

The negative pulse required to raise the impedance of the triode is usually relatively high and it must be supplied against the maximum capacity of the valve due to the "Miller" effect.

In order to decrease the amplitude of the negative pulse required, coupling in a suitable sense may be provided between the coil 7 and the transformer 1. Such an arrangement embodying also certain further modifications of the circuit of Fig. 1 is shown in Fig. 3. In this Figure like parts have the same references as in Fig. 1.

Referring to Fig. 3, the grid of the triode 2 is connected through a resistance 20 and a coil 21 to one terminal of the transformer 1. The resistance 20 serves to prevent the flow of excessive grid current in the triode 2. The coil 21 is coupled to the coil 7 and is connected in such a sense that a decrease of current in coil 7 flowing in the direction of the arrow IL tends to make the grid of the triode more negative. Thus when a small negative pulse is applied to the grid through transformer 1, the triode 2 insulates and the current IL begins to decrease (point e Fig. 2 (a)). As already pointed out in connection with Fig. 1 and as shown in Fig. 2 (c), the voltage across the anode circuit of the triode soon rises to a high value and unless the grid is by then sufficiently negative, the triode will cease to insulate. The decrease in the current IL, however, and the increase thereof in the opposite sense (that is the current change taking place between points e and g in Fig. 2 (a)) induces in coil 21 an electromotive force which makes the grid more negative. A small initial negative impulse is therefore sufficient to operate the circuit efficiently, the major portion of the pulse being applied by the coil 21.

A further modification included in Fig. 3 is the source 22 which is connected to the anode of the diode 3 through a resistance 23. This resistance serves, with resistance 5, to reduce the voltage to the correct value during operation of the device and to prevent the flow of excessive current if oscillation of the circuit 7, 8 should cease. The condenser 24 serves to maintain the voltage at the anode of the diode 3 substantially constant. The voltage applied to the diode 3 by the source 22 is made such that a small current normally circulates between the diode 3 and the triode 2. This serves to reduce the impedance of the valves 2 and 3 at small currents.

The circuit of Fig. 3 is capable of producing in coil 7 a current having a wave form which approximates fairly closely to saw-tooth form even when sinusoidal oscillations are fed to the transformer 1.

A circuit such as 9 of Fig. 1 my be provided in the arrangement of Fig. 3 if desired to damp out oscillations at half the scanning frequency.

The inversion of the diode in Figs. 1 and 3, that is to say the arrangement of the anode thereof at earth potential with regard to the oscillations, may be avoided by connecting the diode between the terminals of a coil coupled to the coil 7, the cathode of the diode being earthed if desired. The connections are made such that a decrease in the current IL flowing in the direction of the arrow tends to make the cathode of the diode positive in relation to the anode thereof. the diode is so biassed that the negative potential induced on the cathode relatively to the anode during the scanning stroke, which is proportional to the slope of the line g e1 in Fig. 2 (a), is just insufficient to cause current to flow. When one half cycle of oscillation of the tune circuit 7, 8 has taken place, however, and point g, Fig. 2 (a), has been reached, any tendency for the oscillation to continue would result in the curve of Fig. 2 (a) becoming much steeper than the line g e1 and would, for reasons already explained, cause the potential of the cathode of the diode to become strongly negative relatively to the anode thereof. the current which would then flow through the diode, and therefore in the coil coupled to coil 7, would immediately damp out the persisting oscillation. The effect is therefore, as before, that the diode arrests the oscillation after one half cycle.

The whole or a part of the coil connected to the diode as above described may be used to provide part of the negative pulse applied to the grid of the triode.

If desired, instead of the inductance 7 of the tuned circuit itself constitute the scanning coil, the scanning coil may be fed from the coil 7 through a condenser. In other words the scanning coil is coupled to the tuned circuit by what in loudspeaker coupling would be known as choke-capacity coupling, the inductance 7 constituting the choke. In this way, flow of direct current through the scanning coil is prevented.

The voltage pulses applied to the grid of the valve 2 may, if desired, be modified in such a way as to give a measure of correction for deformation of the wave from of the generated oscillations which may occur during the scanning stork due to the resistance of the circuits not being negligibly small.

In practice it my be found that the self-capacity of the scanning windings and the stray capacities, such as that of the leads, are sufficient to supply the capacity to resonate with inductance 7 without the provision of any additional condenser 8.

In a further embodiment of the invention illustrated in Fig. 4, triode 12 of the thyratron type is used. This type of triode has the property that its anode-cathode impedance remains high so long as its grid is maintained at a suitable negative potential relative to the cathode. On suitably reducing the negative potential, however, the impedance falls to a low value and remains at this low value, irrespective of any potentials applied to the grid, until the anode voltage has been reduced to a certain value.

Between the anode and cathode of a triode of this kind there is connected an inductance 13 in series with a condenser 14 constituting a tuned circuit. The condenser represented by 14 consists of the capacity between the electrostatic control electrodes of a cathode ray tube (not shown) increased by a parallel condenser to a suitable value, for example 0.002 microfarads. The point of connected between the inductance 13 and condenser 14 is connected through an inductance 15 and a resistance 16 (which will be termed the feed impedance) to the positive terminal of a source 17 of high tension, the negative terminal of the source being connected to the cathode of the triode. The inductance 16 has such a high value that its only effect is to maintain a steady current from the source 17. The resistance 16 serves, in the same manner as resistance 5 of Fig. 1, to prevent damage if oscillation should cease.

The curves of Fig. 5 illustrate the behavior of this circuit. In each curve the abscissa is time and the ordinates are, in (a) the voltage V across condenser 14; in (b) the voltage impulses Vg applied to the grid of the triode 12; and in (c) the current I flowing in inductance 13.

Once more assuming the cycle to commence with the scanning spot in the centre of the reproducing screen, as represented by point k Fig. 5 (a), the charge upon the condenser 14 is zero but increasing due to current from the source 17 flowing through the feed impedance 15, 16. The grid of the triode 12 is held at a suitable negative value by biassing means 18 and the current through the inductance 13 of the tuned circuit is thus negligible. The voltage across the condenser 14 therefore rises until a positive synchronising impulse is applied to the grid of the triode as represented by point l. The impedance of the triode 12 is now very small and the condenser discharges through the inductance 13 of the tuned circuit at a rate dependent upon the natural frequency thereof. The feed impedance 15, 16 is sufficiently high to prevent appreciable flow of current through it during the short period of the condenser discharge. When the tuned circuit has executed one quarter of a cycle of oscillation (points l to m Fig. 5 (a)) the condenser 14 has discharged and, due to the current now flowing in the inductance 13, charges up in the reverse direction, the tuned circuit thus executing a second quarter cycle of oscillation (points m to n Fig. 5 (a)). The current through the inductance 13 is now zero and the voltage across the anode circuit of the triode 12 is therefore zero also and its impedance has accordingly returned to its original high value. The condenser cannot draw current through the inductance 13 of the tuned circuit and accordingly current commences to flow into the condenser through the feed impedance 15, 16 until the voltage across the condenser is once more zero (point k1) and the cycle has been completed.

In this arrangement the wave form of the voltage generated is controlled during the greater part of each cycle, that is from n to l1, during the scanning stroke by the decay and build upon of voltage across the condenser 14 due to the flow of a substantially constant current thereto from the source 17 and during the remainder of the cycle, that is during the return stroke, l to n or l1 to n1 by the natural frequency of the tuned circuit 13, 14 which executes one half cycle of oscillation at its resonant frequency. Oscillation of the tuned circuit over more than one half cycle is in this case prevented by the unidirectional conducting property to the triode 12 instead of by the unidirectional property of the diode as in Fig. 1.

The positive impulse applied to the grid need only last for long enough to trigger the thyratron 12 but it may if desired persist until the middle of the scanning stroke, after which the grid must be negative. The impulses may extend anywhere within the shaded areas of Fig. 5 (b), for example.

In this example also, the voltage developed across the control electrodes can exceed greatly the voltage of the source since the voltage is determined by the flow of current through the inductance of the feed impedance.

Moreover, as in the arrangement of Fig. 1, two reactive circuits are provided to control the wave form of the generated oscillations namely, in this case, the circuit from the source 15, 16, 17 through condenser 14 and the resonant circuit including condenser 14 and inductance 13. The valve 12 acts as a switch to transfer the control from one of the reactive circuits to the other.

In this case also it is desirable to provide an auxiliary circuit to prevent persistence of oscillations at one half the scanning frequency and this may take the form of an inductance 19 and a condenser 20 connected in series across the condenser 14 of the main tuned circuit. The auxiliary circuit is arranged to resonate at the undesired frequency and thus makes the voltages across the control electrodes at this frequency negligible. It should also be arranged that the inductance 19 had condenser 20 of the auxiliary circuit have individually a high impedance and that the inductance 19 resonates with the condenser 14 of the main tuned circuit at a frequency not higher than say two thirds the scanning frequency.

In determining the capacity of the main tuned circuit it should be taken as including the self-capacity of the feed inductance 15.

The wave forms shown in Figs. 2 and 5 are typical examples of the kind of wave form with which this invention is concerned. Thus the slow stroke is in curves (a) and (c) of Fig. 2 contained between vertical lines through points g and e1 and in the curves (a) and (c) of Fig. 5 between verticals through points n and l1 whilst the quick strokes are contained between verticals through e and g in Fig. 2 and l and n in Fig. 5. The slow stroke in Figs. 2 (a) and 5 (a) is constituted by a relatively small and constant rte of change of current and voltage respectively and in Figs. 2 (c) and 5 (c) by constant voltage and current respectively. It may be noted that the wave forms of curves (a) and (c) in both Figs. 2 and 5 can readily be derived from on another because the curves (c) are the differentials of curves (a).

Both the circuits described are such that there is, theoretically, no power dissipation. The actual power dissipation is therefore only that due to unavoidable losses and can be made relatively small.

Clearly, forms of switch other than thermionic valves such as 2 in Fig. 2 or 12 in Fig. 3 may be used. At relatively low frequencies mechanical switches can be used and they can be made to operate substantially sparklessly.

Although the invention has been described as applied to the control of cathode ray tubes it is clearly not limited to such uses but may be employed in many other fields where periodic oscillations of the kind specified are required.

Having now particularly described and ascertained the nature of our said invention, and in what manner the same is to be performed, we declare that what we claim is:-

  1. An oscillation generator adapted to generate electrical oscillations of the kind specified under the control of signals applied thereto from an external source, the generator comprising two reactive circuits having different reactive properties and means responsive to said signals for transferring the control of the generated wave form from one of said circuits to the other.
  2. An oscillation generator according to claim 1, wherein said transferring means comprise a thermionic valve the anode circuit of which is conductive during a part of each cycle of the generated oscillation and non-conductive during another part of each cycle.
  3. An oscillation generator according to claims 1 or 2 wherein the transferring means are adapted to be actuated by received electrical impulses.
  4. A circuit for generating electrical oscillations of the kind specified, under the control of signals applied thereto from an external source, the circuit comprising an inductance, means for maintaining a substantially constant voltage between the terminals of said inductance during the slow stroke, means whereby the wave form of the slow stroke is determined by the rate of decay and build up of current in said inductance and means whereby the moment of termination of the slow stroke is determined by said signal.
  5. A circuit for generating electrical oscillations of the kind specified, under the control of signals applied thereto from an external source, the circuit comprising a condenser, means for maintaining current supplied to said condenser substantially constant during the slow stroke, means whereby the wave form of the slow stroke is determined by the rate of decay and build up of voltage across said condenser and means whereby the moment of termination of the slow stroke is determined by said signal.
  6. An oscillation generator for generating electrical oscillations of the kind specified, the frequency of said oscillations being determined by the frequency of applied electrical impulses, said generator comprising a tuned circuit, means for causing said tuned circuit to commence oscillation in response to one of said impulses and means for automatically stopping said oscillation after one half cycle thereof.
  7. An oscillation generator adapted to generate electrical oscillations of the kind specified, the frequency of said oscillations being determined by the frequency of applied electrical impulses said generator comprising an inductance having a condenser connected in parallel thereto, the inductance being connected in series with a switching device across a circuit adapted to maintain a substantially constant voltage at its terminal, and a unidirectionally conducting device associated with the inductance in such a manner that it is adapted to pass current when the rte of change of current in the inductance in one direction exceed a predetermined value.
  8. A generator according to claim 7, wherein said switching device is a thermionic valve the anode circuit of which is conductive during the greater part of each cycle of the generated oscillation and is adapted to be rendered substantially non-conductive in response to a received electrical impulse.
  9. A generator according to claim 8, wherein there is provided between said inductance and the control grid circuit of said thermionic valve a coupling adapted to feed back to the grid circuit a voltage which assists the received impulse in maintaining the anode circuit non-conductive.
  10. A generator according to any of claims 7 to 9, wherein means are provided for maintaining a current through said unidirectionally conductive device during the greater part of each cycle of the generated oscillation.
  11. An oscillation generator adapted to generate electrical oscillations of the kind specified under the control of signals applied thereto from an external source said generator comprising a condenser, means for maintaining a substantially constant flow of current to the condenser and, connected in parallel with the condenser, an inductance and a switching device arranged in series, means being provided for preventing flow of current in other than one direction through said inductance and said switching device being adapted to initiate the flow of current in said inductance in response to said signals.
  12. A generator according to claim 11, wherein said switching device is adapted to prevent flow of current through said inductance during the greater part of each cycle of the generated oscillation.
  13. A generator according to claims 11 or 12, wherein said switching device is a thermionic valve of the thyratron type and serves also to prevent flow of current in other than one direction through said inductance.
  14. An oscillation generator according to any of claims 7 to 13, wherein there is provided a circuit for preventing the persistence of oscillations of one half the frequency of the generated oscillations.
  15. An oscillation generator substantially as hereinbefore described or as shown in either of Figs. 1, 3 or 4 of the accompanying drawings.

Dated this 2nd day of May, 1933.

REDDIE & GROSE,

Agents for the Applicants

6, Breamís Buildings, London, E.C.4

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Redhill: Printed for His Majestyís Stationery Office, by Love & Malcomson, Ltd. Ė 1933.