479,599

PATENT SPECIFICATION

Application Date: July, 8, 1936. No. 18892/36.

Complete Specification Left: June, 9 1937.

Complete Specification Accepted: Feb. 8, 1938.

-------------------------

PROVISIONAL SPECIFICATION

Improvements in or relating to Modulated Carrier Wave Receivers

We, ALAN DOWER BLUMLEIN, of 32 Audley Road, Ealing, London, W.5, and WILLIAM HORACE CONNELL, of 4, Dorset Waye, Hillingdon, Middlesex, both British Subjects, do hereby declare the nature of this invention to be as follows:-

This invention relates to modulated carrier wave receivers and has particular, but not exclusive, reference to receivers of the superheterodyne type.

Various attempts have been made with a view to providing means for varying, either automatically or manually, the selectivity of receivers, and in cases where selectivity is controlled automatically, it has been suggested to employ a valve shunted across the circuit the selectivity of which is to be controlled, the valve being fed with rectified currents for varying its shunting effect whereby the response curve of the circuit is likewise varied. Such an arrangement is however, expensive owing to the provision of the additional valve shunting the circuit.

It is the chief object of the present invention to provide an improved circuit whereby variation of the selectivity can be obtained either automatically, manually, or by combination of automatic and manual means.

According to the invention, variable selectivity is obtained in a modulated carrier wave receiver by providing means for producing a negative feed-back, the feed-back being frequency selective for the purpose of reducing the remote side bands relatively to the carrier frequency, the effect of the feed-back being adapted to be varied for affording the required variation in selectivity. The frequency selective feed-back functions as will herein after be more fully explained by producing at certain frequencies a variation in the effective slope of the associated valve and the effect of the feed-back is varied as aforesaid or by providing means for additionally varying the slope of the valve. The variation in effect of the feed-back may be obtained automatically by additionally varying the slope of the valve to which the negative feed-back is applied, such variation in slope being obtained for example, by the application to the valve of automatic volume or gain control bias potentials derived in known manner. Preferably, the negative feed-back is obtained by the provision in the anode-cathode path and the grid-cathode path of the valve, of an impedance which is arranged to be of low value for the carrier frequency and of a high value for frequencies remote from the carrier frequency, that is to say, side-band frequencies. Where the invention is applied to a superheterodyne receiver, the negative feed-back may be employed in conjunction with the variable amplification intermediate frequency stage thereof. Whilst the invention is particularly advantageous when the selectivity is automatically controlled, as by varying the effect of the negative feed-back by varying the slope of the valve by the use of automatic volume or gain control potentials, the invention can, nevertheless, be employed where the effect of the negative feed-back is adjusted manually. The provision of a manual adjustment in conjunction with an automatic control is also desirable as will appear hereinafter.

The invention will now be described by way of example, with reference to the accompanying drawings in which:-

Figs. 1 and 2 illustrate circuits embodying the basic principles of the invention.

Figs. 3A, 3B, 3C and 3D illustrate modifications of the cathode circuits of the arrangements shown in Figs. 1 and 2, and

Figs. 4 to 9 illustrate further embodiments of the invention.

Referring now more particularly to Fig. 1 of the accompanying drawings, the reference numerals 1 and 2 represent respectively the input and output circuits of an intermediate frequency amplifying valve 6, the invention, as will be appreciated, being described for use in a superheterodyne receiver. The invention is also described in this figure in conjunction with automatic volume or gin control the purpose of which will be hereinafter more fully explained.

The input circuit 1 is coupled to the control grid of the valve 6 and is decoupled by the condenser 3, automatic volume or gain control potentials derived in any suitable manner being applied to the control grid of valve 6 through resistance 4 from a line 5. Negative feed-back for the valve 6 is obtained by the provision of an inductance 7a and series condenser 7b in the cathode lead of the valve 6, the feed-back circuit being generally designated by the dotted line rectangle 7. For the purpose of by-passing anode current a choke 8 is provided in parallel with the circuit 7, the choke 8 being roughly tuned if desired. The circuit 7 is arranged to present a low impedance at its resonant frequency as by tuning the circuit to the same frequency as the circuits 1 and 2 which, in the case of a superheterodyne receiver, corresponds to the intermediate frequency of the receiver. At this frequency small negative feed-back is produced by the residual resistance of the circuit 7, but for side-bands the impedance of the circuit 7 increases, thus reducing the gain for such side-bands. If the slope of the valve is high the feed-back will attenuate or cut off the side-bands, but at low slopes the effect of the feed-back will be negligible. In the example of Fig. 1, the slope of the valve measured in terms of cathode current will be the combined slope to screen and anode, but in the example shewn on Fig. 2 which illustrates a modification of that shewn in Fig. 1. and in which like reference numerals indicate similar parts, the slope of the valve is only the slope to the anode. In Fig. 2 it will be observed that the circuit 7 is coupled to the output circuit 2 through an anode circuit decoupling condenser 9. Assuming the slope of the valves of Figs. 1 and 2 to be a, and z, the impedance of the circuit 7 with the choke 8 in parallel therewith, then the effective slope of the valve in the presence of the negative feedback will be

EQUAT. HERE

 

or the effective ratio of the slopes with and without negative feed-back will be

EQUAT. HERE

 

As stated above, z is small at carrier frequencies and larger at side-band frequencies, so that, providing z is sufficiently great, there will be an attenuation or cut-off of side-band frequencies and a corresponding increase of selectivity. This increase in selectivity will be most noticeable when g is large, that is to say, when there is little automatic volume control negative bias corresponding to distance stations. On a local station the automatic volume control voltage will reduce the bias thus reducing the slope and so making the negative feed-back, and hence the extra selectivity, negligible. It will be appreciated that instead of employing the automatic volume control to vary the slope of the valves 6 for selectivity control, the bias on the valves may be adjusted manually, or alternatively, a manual control may be employed in addition to the automatic volume potentials so that the slope of the valves may be maintained high whereby extra selectivity is available for a less distant station which is subjected to unusually severe interference.

The circuit 7 of Figs. 1 and 2 may be replaced by the circuits shewn in Figs. 3A, 3B, 3C and 3D. In Fig. 3A the inductance 7c is bridged across the condenser 7b so as to by-ass the anode current thus obviating the use of the inductances 8 of Figs. 1 and 2. The inductance 7c may be made sufficiently high to avoid a parallel resonance with the associated condenser 7b within the working range the circuit shewn in Fig. 3A being tune by the condenser 7b so as to present a low impedance at the carrier frequency.

In Fig. 3B the circuit 7 is replaced by a condenser 7d shunted by an inductance 7c in series with another condenser 7f shunted by an inductance 7g. The circuit of Fig. 3B can be adjusted to present a low impedance at the carrier frequency and a high impedance at two frequencies on either side of the carrier frequency so that a comparatively sharp cut can be effected at the undesired side-band frequencies without necessitating the high coil efficiency required in the circuits of Figs. 1, 2 and 3A.

Fig. 3C illustrate a modification of the circuit of Fig. 3B, in which an inductance coil 7h in series with a condenser 7i is shunted by a condenser 7i and by a parallel inductance 7k. The circuit of Fig. 3C is less difficult to adjust than the circuit of Fig. 3C, the inductance 7h and condenser 7i are disconnected and the condenser 7i and inductance 7k are tuned to the carrier frequency to afford maximum negative feed-back, that is, a reduction of gain at the carrier frequency. The inductance 7h and condenser 7i are then connected and tuned to pass the carrier frequency, the circuit then affording an approximately symmetrical cut-off of side-bands on each side of the carrier frequency.

The circuit shewn in Fig. 3D is similar to that shewn in Fig. 3C with the difference that the coil 7h is shunted by a condenser 7i which permits the use of a smaller inductance for the coil 7k. Typical valves for the components of the circuit 3D are for a carrier frequency of

EQUAT. HERE

 

Inductance 7h = 3.125 microhenries

Condensers 7i & 71 = 250 micro-micro- farads

Inductance 7k = 31.25 microhenries

Condenser 7j = 50000 micro-microfarads

With components having the values stated, high impedance resonances occur at approximately 3200 cycles per second on each side of the carrier frequency.

With coils in which EQUAT. HERE = 200, the impedance of the circuit is about 50 ohms at the carrier frequency and about 2500 ohms at 3200 cycles per second off the carrier frequency. A valve having a slope of 4 milliamps. per volt will therefore lose 1.6 decibels gain at the carrier frequency and 20.8 decibels gain at 3200 cycles per second off the carrier frequency. A reduction of slope to 0.4 milliamps per volt will make the carrier loss negligible and at 3200 cycles per second off the carrier frequency the loss will be about 6 decibels. At 0.04 milliamps. per volt, both losses will be negligible.

With the circuits shewn, especially with that of Fig. 1, there may be a tendency for oscillation, since with a capacitive cathode load the input impedance of the grid (due to grid-cathode capacity) has a negative resistance component. In the construction shewn in Fig. 4 (in which only the grid and cathode circuits are illustrated) neutrodyning condenser 11 is provided to prevent such oscillation. The choke 8, which corresponds to the choke 8 of Fig. 1, is extended by a further portion giving a potential opposite to that of the cathode. The condenser 11 approximates to the grid-cathode capacity, assuming equal effective ratio for the two portions of the choke 8. Similarly, the inductance 7k of Fig. 3D may be extended downwards and condenser 7j correspondingly decreased so as to provide a neutrodyne potential. In practice, the inductance 7k may with advantage consist of a small part of a much larger coil, the condenser 7k being then bridged across the whole coil, thus requiring a much smaller value for condenser 7j; the neutrodyne voltage can then be obtained from a further tapping point along this coil.

Fig. 5 shews a further alternative which enables a less efficient circuit 7 to be used than that required by the arrangement of Fig. 1. The tuned resistance of circuit 7 in Fig. 1 causes feed-back at the carrier frequency, thus reducing the gain and reducing the difference of feed-back for carrier and remote side-bands. In Fig. 5 a tightly coupled and roughly tune choke is provided having tappings 13, 14, 15 and 16 connected as shown. If the ratio between 13, 14 and 14, 15 is unity, and if resistance 12 equals the tuned resistance of circuit 7, no feed-back will be produced at resonance, assuming negligible leakage inductance between 13, 14 and 14, 15, since the bridge will be balanced. At the side band frequencies, however, negative feed-back will be produced. The grid circuit decoupling condenser 3 may be tapped at 15, but is preferably taken to a more remote tapping point 16. In effect, a step-up of feed-back is obtained between the tapping points 14, 15 and 14, 16. This arrangement does not require such a high impedance for circuit 7, which might otherwise require an inconveniently high inductance/capacity ratio. To prevent oscillation, the neutrodyne condenser 11 may be provided connected to an extension 17 of the roughly tuned coil. If desired, the resistance 12 may be replaced by a parallel resonant circuit affording a resistance equal to resistance 12 at its resonant frequency and so serving to earth the tapping point 15 at frequencies remote from the carrier frequency. Many other alterations of this bridge circuit are possible. For example, the circuit 7 may even be replaced by a comparatively high resistance and the resistance 12 replaced by a parallel tune circuit, thus obtaining a similar effect to that which results when the resistance 12 is replaced by a parallel resonant circuit as aforesaid. Similarly, the bridge circuit may be placed in the anode decoupling lead as in Fig. 2. This would correspond to earthing the cathode and tapping 14, and returning the anode decoupling condenser to the junction of the circuit 7 and resistance 12. With the screen of the valve earthed, tapping 17 and the neutrodyne condenser would not be required. If the screen decoupling were also returned to the junction of circuit 7 and resistance 12 (to increase the effective slope), then the tapping point 17 would require to lie on the other end of the choke, (beyond the point 13) and condenser 11 would neutrodyne the screen to grid capacity.

With the bridge circuits shown, small positive feed-back may be introduced at the carrier frequency, as for example, by making the resistance 12 slightly greater than the tune resistance of 7. This, however, will not alter the method of action, since for frequencies remote from carrier frequency the required selectivity will be obtained by introducing negative feed-back as against the positive feed-back for the carrier frequency.

The invention may also be applied to receivers other than the superheterodyne type, in which case the circuit 7 would be arranged to be tunable with the other circuits. Similarly the invention may be applied to more than one amplifier stage.

Fig. 6 shows selective intermediate frequency feed-back applied to the mixing valve of a superheterodyne circuit, the bias on the signal grid of the mixing valve being varied by automatic volume control. In this figure the tune input circuit is indicated by the reference numeral 1, automatic volume control potentials being applied through the resistance 4 from the line 5, the signal grid being decoupled by condenser 3. The intermediate frequency anode circuit is indicated at 2. The screening grids are fed through a high impedance choke 8 and decoupled to cathode. Local oscillations are fed between the cathode and the first grid of the mixing valve. The intermediate frequency anode currents flow through circuit 7 and cause negative feed-back of intermediate frequency voltage between the signal grid and cathode. The circuit 7 is of the type shown in Fig. 3c so as to by-ass the radio frequency currents though the shunt condenser.

If desired, a switch may be arranged to remove the automatic volume control bias from a valve having selective feed-back, to enable the selectivity to remain for large input voltages if required. However, the removal of the automatic volume control negative bias may cause too much output from the receiver, unless there is an efficient automatic volume control, operative also on earlier valves. The same switch may, therefore, be arranged to make the bias of the earlier valves more negative and so counteract this effect.

Fig. 7 whose a schematic arrangement of the direct current cathode potential arrangements in accordance with a modification of this invention. This bias on the automatic volume control line starts at a positive potential to earth for maximum gain. The two amplifier valves 16 and 6 are given a positive potential by means of resistances 22 and 21 in their cathode leads. The resistance 22, however, is smaller than the resistance 21, but is connected to the high tension supply through a resistance 23 which supplied extra steady current to maintain the cathode positive. An increase of signal strength causes the automatic volume control line to become less positive, or more negative, causing a drop in anode currents. A drop of the current in valve 6 effectively lessens the positive potential on the cathode and so counteracts the change of bias. A similar effect takes place in valve 16, but as resistance 22 is less than resistance 21, the effect is less marked, and sol valve 16 is biased more negatively than valve 6. This prevents the selectivity decreasing too rapidly with increase of signal strength. An interchange of resistance 22 and resistance 23 and resistance 21 would produce the reverse effect. Valve 16 my of course be a high frequency mixing valve or an intermediate frequency valve. 7 represents the negative feed-back impedance in the cathode of valve 6.

Fig. 8 of the drawings shows a combined automatic and manual selectivity control. The resistance 23 feed currents from the high tension supply to cathodes of the two valves shown through the potentiometer 24. By adjusting the point of connection of the resistance 23 and potentiometer 24 to the right, the negative bias is reduced on the first valve and increased on the second thus increasing the selectivity while keeping the gain approximately constant.

It is also found with variable selectivity that it is advantageous to cut the very low modulation frequencies at the same time as cutting the very high modulation frequencies, in order to preserve a better balance of the sound output. This invention can be applied to perform this bass cut automatically at the same time as the higher modulation frequencies are cut. Fig. 9 whose such a circuit in which automatic volume control is also employed in conjunction with one of the low frequency amplifying stages to maintain the sound output constant. In this circuit 23a represents the last intermediate frequency valve, and 21a and 22a are coupled circuits arranged as shown. The anode of 29a of the double diode 24a provide automatic volume control voltage to the automatic volume control line 5, which voltage is developed across resistances 25, 26, 27. The resistance 27 is comparatively small and is used with condenser 34 to decouple any possible modulation frequency arriving from resistances 27a and 28. A "delay" for the automatic volume control is provided both by the voltage drop across the resistance 27a, and also by the potentiometer 29, 30 connected to the high tension supply, the cathode of valve 24a being decoupled by condenser 38. This latter potentiometer draws comparatively little current in comparison with the current of the low frequency amplifier valve 35. This valve obtains its bias through potentiometer 33, and resistance 31 from a tapping on the resistances 25, 26 and 27 across which the automatic volume control potentials are developed. The grid bias is decoupled to earth by a condenser 37.

Suppose that a "delay" of 7 volts is provided on the cathode of valve 24a, and suppose also that approximately 21 volts bias, not allowing for the change in potential of the cathode of valve 35, is required to reduce the preceding valves to their minimum sensitivity then there will be a change of signal strength over the whole automatic volume control range of 4:1. The tapping providing the grid bias of valve 35 is so chose as to change the slope of this valve by about 4:1 in this range, so that the final output for the same modulation depths is maintained approximately constant. This involves a rather large current in valves 35, which may necessitate a transformer coupling to the output valve in place of the resistance capacity coupling shown, if overloading is to be prevented at the position of minimum slope. A rectified modulation input for valve 35 is obtained from the anode 29b of diode 24a across the load circuit 32 and potentiometer 33. Further smoothing may be introduced at this point if required in order to remove intermediate frequency input to the valve 35. The cathode load of valve 35 is composed of resistance 27, 28 shunted by condenser 36. This combined resistance is preferably made several times the inverse of the slope of the valve. If condenser 36 is not made sufficiently large, a loss of low frequencies will occur when the inverse of the slope of valve 35 in parallel with the cathode resistance is less that the impedance of the condenser at the low frequencies. This bass loss, will be much less marred when valve 35 has a low slope, since the impedance of condenser 35 will no longer be high compared with the slope. Thus an automatic cut-off of the bass frequencies will be obtained for distance stations in the same manner as the automatic cut-off of the high frequencies prior to the detector. If preferred, high frequency cut-off may also occur after the detector, as for example, by inserting an inductance in series with condenser 36, so as to introduce the negative feed-back again for the higher modulation frequencies. As the valve 35 does not change its slope (in the example given) as much as do the earlier variable mu valves, this high frequency cut-off cannot be made as sharp nor will it be found a satisfactory as the high frequency cut-off applied in the manner described in the preceding examples.

Dated this 7th day of July, 1936.

F. W. CACKETT,

Chartered Patent Agent.

COMPLETE SPECIFICATION

Improvements in or relating to Modulated Carrier Wave Receivers

We, ALAN DOWER BLUMLEIN, of 32 Audley Road, Ealing, London, W.5, and WILLIAM HORACE CONNELL, of 4, Dorset Waye, Hillingdon, Middlesex, both British Subjects, do hereby declares 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 tuned modulated carrier wave receivers. The invention has particular, but not exclusive, application to tune modulated carrier wave receivers of the superheterodyne type.

Various attempts have been made with a view to providing means for varying the selectivity of receivers, either manually, or automatically with the strength of the received signals, and in cases where selectivity is controlled automatically, it has been proposed to employ a valve shunted across the circuit the selectivity of which is to be controlled, the valve being fed with rectified currents, corresponding to the received signal strength for varying its shunting effect whereby the response curve of the circuit is likewise varied. Such an arrangement is, however, expensive owing to the provision of the additional valve shunting the circuit.

It is the chief object of the present invention to provide and improved circuit whereby variation of the selectivity can be obtained s the signal strength varies.

According to the invention, a tuned modulated carrier wave receiver is provided comprising a thermionic valve having a negative feed-back path associated therewith and an arrangement for applying a volume or gain control bias potential in accordance with the strength of the received signals to a control electrode of said valve, the feed-back path being so designed and arranged that for signals at frequencies adjacent to the frequency of signals to which the receiver is tuned and on both sides thereof, a relative reduction of amplification with respect to the amplification of the signals to which the receiver is tuned is produced in such a way that upon increasing or decreasing the gain of said amplifier valve the relative reduction of amplification of signals having adjacent frequencies is such that the selectivity of the amplifier is increased or decreased respectively. If desired a gain control bias depending on the strength of the received signal may also be applied to the control electrode of a thermionic amplifying valve not having a negative feed-back path associated with it, but associated with the valve to which feed-back is applied, the two valves being so connected that the effect of the gain control bias potentials applied to the two valves is to vary the gain of each valve differentially in such manner that the variation of selectivity with gain of the receiver will be modified. An arrangement including two valves combined in this way may operate in such a way that as the gain control potential varies, the amplification of the valve to which feed-back is applied, varies less than that of the other valve whereby the selectivity of the receiver may be prevented from decreasing too rapidly with increase in signal strength.

An arrangement according to the invention comprising two associated thermionic valves, one of which as a negative feed-back path associated with it, may include means whereby the distribution of volume control bias potentials to each of the two linked stages may be adjusted manually, the arrangement being such that notwithstanding the change in volume or gain control bias applied to each valve and the consequent change in selectivity of the receiver, the gain of the receiver in respect of signals to which it is tuned remains unchanged.

A receiver in accordance with the invention may also include a stage of low frequency amplification including a thermionic amplifier valve having a feed-back arrangement similar to that used with the amplifier valve in the selectivity control stage or stages, this arrangement being used to operate on the low or base frequencies of the reproduced sound in a manner corresponding to the way in which the high frequencies thereof are effected by the variation of the selectivity.

The negative feed-back path for variable selectivity may comprise an impedance which is common to the anode-cathode path and to the control grid-cathode path of the valve, said impedance being arranged to be of low value for signals of the frequency of the carrier to which the receiver is tuned and of high values for adjacent frequencies on either side of the carrier frequency. Alternatively in an arrangement according to the invention the control grid-cathode path and the anode cathode path of the valve to which feed-back is applied may be associated through a bridge circuit, one arm of which includes a tune impedance and another arm of which includes a further impedance which at the carrier frequency presents a resistance of value at least approximately equal to the impedance presented by the tune impedance, the bridge circuit being so arranged that no or a slightly positive feed-back is produced in respect of currents of the carrier frequency, the feed-back voltage becoming increasingly more negative in respect of currents of frequencies on either side of the carrier frequency.

In a carrier wave receiver of the superheterodyne type in which the invention is applied for producing automatic variation of selectivity with the varying signal strength, the invention will usually be applied to a stage of intermediate frequency of the receiver. In a superheterodyne receiver a selectivity control according to the invention may be applied to the frequency changer valve of the receiver. In other carrier wave receivers the invention will usually be applied in connection with a stage of carrier frequency amplification.

In order that the said invention may be clearly understood and readily carried into effect, the same will now be described with reference to the drawings accompanying our Provisional Specification in which:-

Figs. 1 and 2 illustrate circuits embodying the basic principles of the invention,

Figs. 3A, 3B, 3C and 3D illustrate modifications of the cathode circuits of the arrangements shown in Figs. 1 and 2 and Figs. 4 to 9 illustrate further embodiments of the invention. In the drawings similar components are indicated by the same reference numerals.

Referring now more particularly to Fig. 1 of the drawings, the reference numerals 1 and 2 represent respectively the input and output circuits of an intermediate frequency amplifying valve 6, shown as a screened grid valve of the variable mu type, the invention, as will be appreciated being described for use in a superheterodyne receiver. The invention is also described in this figure in conjunction with automatic volume or gain control the purpose of which will be hereinafter more fully explained.

The input circuit 1 is coupled to the control grid of the valve 6 and is decoupled to earth by a condenser 3, automatic volume or gain control potential derived in any suitable manner being applied to the control grid of valve 6 through resistance 4 from a line 5. The anode circuit may be completed to earth through a suitable source of high potential connected to the lead A. Negative feed-back for the valve 6 is obtained by the provision of an inductance 7a and series condenser 7b in the cathode lead of the valve 6, the feed-back circuit being generally designated by the dotted line rectangle 7. For the purpose of by-passing anode current a choke 8 is provided in parallel with the circuit 7, the choke 8 being flatly tuned if desired with the parallel condenser shown. The circuit 7 is arranged to present a low impedance at its resonant frequency as by tuning the circuit to the same frequency as the circuits 1 and 2 which, in the case of a superheterodyne receiver, corresponds to the intermediate frequency of the small receiver. At this frequency a small negative feed-back is provided by the residual resistance of the circuit 7, and for side-band frequencies the impedance of the circuit 7 increases, thus reducing the gain for such side-band frequencies. If the slope or mutual conductance of the valve is high, the feed-back will attenuate or cut of the side-bands, but at low slopes the effect of the feed-back will be negligible. In the example of Fig. 1 the slope of the valve measured in terms of cathode current will be the combined slope to screen and anode, but in the example shown in Fig. 2 which illustrates a modification of that shown in Fig. 1, and in which like reference numerals indicate similar parts, the slope of the valve is only the slope to the anode. In Fig. 2 it will be observed that the circuit 7 is coupled to the output circuit 2 though an anode circuit decoupling condenser 9. Assuming the slope of the valves of Figs. 1 and 2 to be g, and z the impedance of the circuit 7 with the choke 8 in parallel therewith, then the effective slope of the valve in the presence of the negative feed-back will be

EQUAT. HERE

 

or the effective ratio of the slopes with and without negative feed-back will be

EQUAT. HERE

 

As stated above, z is small at carrier frequencies and larger at side-band frequencies so that, providing z is sufficiently great, there will be an attenuation or cut-off of side-band frequencies and a corresponding increase of selectivity. This increase in selectivity will be most noticeable when g is large, that is to say, when there is little automatic volume control negative bias as when distant stations are being received. On a local station the automatic volume control voltage will reduce the bias thus reducing the slope of the valve and so making the negative feed back, and hence the extra selectivity, negligible. Thus it will be appreciated that with the circuits shown in Figs. 1 and 2, automatic variable selectivity occurs, the selectivity decreasing when local stations are being received and increasing with more distant stations. It will be understood that instead of employing the automatic volume control to vary the slope of the valve 6 for selectivity control, the bias on the valves may be adjusted manually, or alternatively, a manual control may be employed in addition to the automatic volume control potentials so that the slope of the valves may be maintained high whereby extra selectivity is available for a less distant station which is subjected to unusually severe interference.

The circuit 7 of Figs. 1 and 2 may be replaced by the circuits shown in Figs. 3A, 3B, 3C and 3D. In Fig. 3A an inductance 7c is bridged across the condenser 7b so as to by-ass the anode current thus obviating the use of the inductances 8 of Figs. 1 and 2. The inductance 7c may be made sufficiently high to avoid a parallel resonance with the associated condenser 7b within the working range, the circuit shown in Fig. 3A being tune by the condenser 7b so as to present a low impedance at the carrier or intermediate frequency for the reasons referred to above.

In Fig. 3B the circuit 7 is replaced by a condenser 7d shunted by an inductance 7e in series with another condenser 7f shunted by an inductance 7g. The circuit of Fig. 3B can be adjusted to present a low impedance at the carrier or intermediate frequency and a high impedance at two frequencies on either side of the carrier or intermediate frequency so that a comparatively sharp cut can be effected at the undesired side-band frequencies without necessitating the high coil efficiency required in the circuits of Figs. 1, 2 and 3A.

Fig. 3C illustrates a modification of the circuit of Fig. 3B, in which an inductance coil 7h in series with a condenser 7i is shunted by a condenser 7j and by a parallel inductance 7k. The circuit of Fig. 3C is less difficult to adjust than the circuit of Fig. 3B. For adjusting the circuit of Fig. 3C, the inductance 7h and condenser 7i are disconnected and the condenser 7j and the inductance 7k are tuned to the carrier or intermediate frequency to afford maximum negative feed-back, that is a reduction of gain at the carrier or intermediate frequency. The inductance 7h and condenser 7i are then connected and tuned to pass the carrier frequency, the circuit then affording an approximately symmetrical cut-off of side-bands on each side of the carrier or intermediate frequency.

The circuit shown in Fig. 3D is similar to that shown in Fig. 3C with the difference that the coil 7h is shunted by a condenser 71 which permits the use of a smaller inductance for the coil 7h. Typical values for the components of the circuit 3D are for a carrier frequency of

EQUAT. HERE

 

Inductance 7h = 3125 microhenries

Condensers 7i and 71 = 250 micron-micro-

farads

Inductance 7k = 31.25 microhenries

Condenser 7j = 50000 micro-microfarads

With components having the values stated, high impedance resonances occur at approximately 3200 cycles per second on each side of the carrier frequency.

EQUAT. HERE

 

With coils in which EQUAT. HERE = 200, where 2p w is the frequency in cycles per second, L the inductance, and R the resistance, the impedance of the circuit is about 50 ohms. at the carrier frequency and about 2500 ohms. at 3200 cycles per second off the carrier frequency. A valve having a slop of 4 milliamps. per volt, will therefore lose 1.6 decibels gain at the carrier frequency and 20.8 decibels gain at a frequency 3200 cycles per second off the carrier frequency. A reduction of slope to 0.4 milliamps. per volt will make the carrier loss negligible and at a frequency of 3200 cycles per second off the carrier frequency the loss will be about 6 decibels. At 0.04 milliamps per volt, both losses will be negligible.

With the circuits shown, especially with the of Fig. 1, there may be a tendency for oscillation, since with a capacitive cathode load the input impedance of the grid (due to grid-cathode capacity) has a negative resistance component. In the construction shown in Fig. 4 (in which only the grid and cathode circuits are illustrated) a neutrodyning condenser 11 is provided to prevent such oscillation. The choke 8, which corresponds to the choke 8 of Fig. 1, is extended by a further portion giving a potential opposite to that of the cathode. The condenser 11 approximates to the grid-cathode capacity, assuming equal effective ratio for the two portions of the choke 8. Similarly, the inductance 7k of Fig. 3C or 3D may be extended downwards and condenser 7j correspondingly decreases so as to provide a neutrodyne potential. In practice, the inductance 7k may with advantage consist of a small part of a must larger coil, the condenser 7j being then bridged across the whole coil, thus requiring a much smaller value for condenser 7j; the neutrodyne voltage can then be obtained from a further tapping point along this coil.

Fig. 5 shows a further alternative which enables a less efficient circuit 7 to be used than that required by the arrangement of Fig. 1. The resistance of circuit 7 in Fig. 1 at resonance causes feed back at the carrier frequency, thus reducing the gain and reducing the difference of feed-back for carrier and remote side-band frequencies. In Fig. 5 a tightly coupled and flatly tuned choke is provided having tappings 13, 14, 15 and 16 connected as shown. If the ratio between 13, 14 and 14, 15 is unity, and if resistance 12 equals the tune resistance of circuit 7, no feed-back will be produced at resonance, assuming negligible leakage inductance between 13, 14 and 14, 15, since the bridge will be balanced. At the side-band frequencies, however, negative feed back will be produced. The grid circuit decoupling condenser 3 may be connected at 15, but is preferably taken to a more remote tapping point 16 as shown. In effect, a step-up of feed back is obtained between the tapping points 14, 15 and 14, 16. This arrangement does not require such a high impedance for circuit 7, which might otherwise require an inconveniently high inductance/capacity ratio. To prevent oscillation, the neutrodyne condenser 11 may be provided connected to an extension 17 of the flatly tune coil. If desired, the resistance 12 may be replaced by a parallel resonant circuit affording a resistance equal to resistance 12 at its resonant frequency and so serving to earth the tapping point 15 at frequencies remote from the carrier frequency. Many other alterations of this bridge circuit are possible. For example, the circuit 7 may even be replaced by a comparatively high resistance and the resistance 12 replaced by a parallel tuned circuit, thus obtaining a similar effect to that which results when the resistance 12 is replaced by a parallel resonant circuit as aforesaid.

In the arrangement of Fig. 5 the anode-cathode circuit would be completed to earth from the anode through a suitable high potential source and thence over the bridge circuit to the cathode, a direct current path being completed over resistance 12 and the inductance between tappings 15 and 14. If desired, the bridge circuit may be placed in the anode decoupling lead as shown in Fig. 2. This would correspond to earthing the cathode and tapping 14, and returning the anode decoupling condenser to the junction of the circuit 7 and resistance 12. With the screen of the valve 6 earthed, tapping 17 and the neutrodyne condenser would not be required. If the decoupling condenser (not shown) usually associated with the screen of valve 6 were also returned to the junction of circuit 7 and resistance 12 (to increase the effective slope), then the tapping point 17 would require to lie on the other end of the choke, (beyond the point 13) and condenser 11 would neutrodyne the screen to grid capacity of the valve.

With the bridge circuits shown, small positive feed-back may be introduced at the carrier frequency, as for example, by making the resistance 12 slightly greater than the resistance of the circuit 7 at its resonance. This, however, will not alter the operation, since for frequencies remote from carrier frequency the required selectivity will be obtained by introducing negative feed-back, as against the positive feed-back for the carrier frequency.

The invention may also be applied to receivers other than of the superheterodyne type, in which case the circuit 7 would be arranged to be tunable with the other circuits. Similarly the invention may be applied to more than one amplifier stage.

Figs. 6 shows the invention as applied to the mixing valve (a hexode valve being shown) of a superheterodyne circuit, the bias on the signal grid (the third grid from the cathode) of the mixing valve being varied by the application thereto of bias potentials derived from an automatic volume control circuit. In this figure the tuned input circuit is indicated by the reference numeral 1, automatic volume control potentials being applied through the resistance 4 from the line 5, the signal grid being decoupled to earth by condenser 3. The intermediate frequency anode circuit is indicated at 2. The screening grids are fed through a high impedance choke 8 a decoupling condenser being provided between the screening grids and cathode as shown. Local oscillations are fed between the cathode and the first grid of the mixing valve, the source of local oscillations being conventionally represented in the Figure. The intermediate frequency anode currents flow through circuit 7 and cause negative feed-back of intermediate frequency voltage between the signal grid and cathode. The circuit 7 is of the type shown in Fig. 3C and presents a low impedance to frequencies other than the side-bands of the intermediate frequency.

If desired, a switch may be arranged to remove the automatic volume control bias form a valve having selective feed-back, to enable the selectivity to remain for large input voltages if required. However, the removal of the automatic volume control negative bias may cause too much output from the receiver, unless there is an efficient automatic volume control, operative also on other valves. The same switch may, therefore, be arranged to make the bias of the other valves more negative and so counteract this effect.

Fig. 7 shows a modification of the invention. In this figure the valve 6 having the negative feed-back circuit 7 in its cathode lead is associated with a further amplifier valve 16. Automatic volume control bits potentials are applied to the grids of the two valves, the bias potential of the automatic volume control line in this case is arranged to be at a positive potential to earth for maximum gain. The cathodes of the two valves 16 and 6 are given a positive potential by means of resistances 22 and 21 in their cathode leads. The resistance 2, however, is smaller than the resistance 21, but is connected to the high tension supply through a resistance 23 which supplies extra steady current to maintain the cathode of valve 16 positive. An increase of signal strength causes the automatic volume control line to become less positive or more negative, causing a drop in anode currents. A drop of the current in valve 6 effectively lessens the positive potential on its cathode and so counteracts the change of bias. A similar effect takes place in valve 16, but as resistance 22 is less than resistance 21, the effect is less marked, and so valve 16 is biassed more negatively than valve 6. This prevents the selectivity decreasing too rapidly with increase of signal strength. An interchange of resistances 22 and 23 with resistance 21 would produce the reverse effect. Valve 16 may of course be a high frequency, mixing, or intermediate frequency valve.

Fig. 8 of the drawings shows a combined automatic and manual selectivity control. The arrangement of the two valves 6 and 16 is similar to that shown in Fig. 7. the biasing resistances 21 and 22 in this case, however, being equal. The resistance 23 feeds current from the high tension supply to cathodes of the two valves shown through the potentiometer 24. By adjusting the point of connection of the resistance 23 and potentiometer 24 to the left, the positive cathode potential is increased on the valve 6 and decreased on the valve 16 so increasing the feed of the second valve and thus increasing the selectivity while keeping the gain approximately constant.

It is also found with variable selectivity that it is advantageous to cut the very low modulation frequencies at the same time as cutting the very high modulation frequencies, in order to preserve a better balance of the sound output. This bass cut may be effected automatically at the same time as the higher modulation frequencies are cut. Fig. 9 shows such a circuit in which automatic volume control is also applied to one of the low frequency amplifying stages to maintain the sound output substantially constant. In this circuit, 23a represents the last intermediate frequency valve of a superheterodyne circuit, and 21a and 22a are coupled circuits arranged as shown, the circuit 21a being connected to a source of anode current not shown. The anode 29a of a double diode valve 24a supplied automatic volume control voltage to the automatic volume control line 5, which voltage is developed across resistances 25, 26, 27 and 28 connected as shown. The automatic volume control bias potentials are applied by line 5 to one or more of the preceding stages, in which the circuit 7, for affording a negative feed back which varies with frequency, is associated with one or more of said stages as described above. The resistance 27 is comparatively small and is used with condenser 34 to decouple any possible modulation frequency arriving from resistances 27a and 28 arranged between the cathode and earth of a low frequency amplifier valve 35 shown as a pentode. A "delay" for the automatic volume control is provided both by the voltage drop across the resistance 27a, and also by the potentiometer 29, 30 connected to the high tension supply, the cathode of valve 24a being decoupled to earth by condenser 38. This latter potentiometer draws comparatively little current in comparison with the current of the low frequency amplifier valve 35. This valve obtains its bias through potentiometer 33, and resistance 31 from a tapping on the resistances 25, 26, 27 and 28 across which the automatic volume control potentials are developed. The grid bias is decoupled to earth by a condenser 37. The "delay" is not intended to imply any time delay but has the significance attributed to the word when employed in the term "Delayed Automatic Volume Control".

Suppose that a "delay" of 7 volts is provided on the cathode of valve 24a and suppose also that approximately 21 volts bias, not allowing for the change in potential of the cathode of valve 35, is required to reduce the preceding valves to their minimum sensitivity then there will be a change of signal strength over the whole automatic volume control range of 4:1. The tapping providing the grid bias of valve 35 is so chose as to change the slope of this valve by about 4:1 in this range, so that the final output for the same modulation depth is maintained approximately constant. This involves a rather large current in valve 35, which may necessitate a transformer coupling to the output valve in place of the resistance capacity coupling shown, if overloading is to be prevented at the position of minimum slope. A rectified modulation input for valve 35 is obtained from the anode 29b of diode 24a across the load circuit 32 and potentiometer 33. Further smoothing may be introduced at this point if required in order to remove any intermediate frequency input to the valve 35. The cathode load of valve 35 is composed of resistances 27a, 28 shunted by condenser 36. This combined resistances is preferably made several times the inverse of the mutual conductance or slope of the valve. If condenser 36 is not made sufficiently large, a loss of low frequencies will occur when the inverse of the mutual conductance of valve 35, in parallel with the cathode resistance is less than the impedance of the condenser at the low frequencies. This bass loss will be much less marked when valve 35 has a low slope, since the impedance of condenser 35 will no longer be high compared with the slope. Thus an automatic cut-off of the bass frequencies will be obtained for distant stations in the same manner as the automatic cut-off of the high frequencies prior to the detector. If preferred, high frequency cut-off may also occur after the detector, as for example, by insuring an inductance in series with condenser 36, so as to introduce the negative feed back again for the higher modulation frequencies. This may be employed independently or in conjunction with the attenuation of the bass response. As the valve 35 does not change its slope (in the example given) as much a do the earlier variable mu valves, this high frequency cut-off cannot be made as sharp nor will it be found as satisfactory as the high frequency cut-off applied in the manner described in the preceding examples.

The circuit shown in Fig. 9 is intended to operate so as to provide a potential on the automatic or gain control lead 5, which for weak signals, is positive with respect to earth. The circuit indicated at 5 will of course have the usual smoothing means in it. The controlled valves must have cathode resistances which cause the cathodes to be initially yet more positive than this potential, so that the valves are not operating with the grids positive with reference to the cathodes. These cathode resistances may be shunted by condensers so as to remove any negative feed-back, or may be left unshunted so as to reduce overload effect in the valve by providing negative feed-back, or finally, the resistances may be constituted by a resistance of the circuit "7" used to provide automatic variable selectivity, the resistance being an resistances such as the choke 8 in Fig. 1, coils 7K in Figs. 3c, 3d, or the resistance 12 in Fig. 5.

In designed the circuit shown in Fig. 9, the value of combined resistance 27a and 28 is made sufficiently high to produce at maximum sensitivity the required drop in low frequency response for valve 35. Part of this resistance 28 is used to produce the standing positive potential on the lead 5 via the resistance 27 (decoupled by 34 and the resistance 26 and 25). The whole positive potential available on the cathode of 35 is used via resistance 29 to provide positive "delay" volts for the diode 24a as regards its anode 29a. in most cases it may be found that there is insufficient positive potential on the cathode of the valve 35, having regard to the positive already applied from resistance 28 to the anode 29a, in which event it is necessary to provide additional positive potential by means of the resistance 30 connected to H.T. positive supply. An alternative arrangement is to connect the right hand end of resistance 29 to earth and either increase resistance 29 or reduce resistance 30 to obtain the desired voltage.

In some cases, particularly with the arrangement shown in Fig. 5, it is possible to operate the circuits satisfactorily with considerably positive feed back at the carrier frequency, the feed-back still being positive despite variation in the slope of the valve. For remote side-band frequencies, however, the feed-back is negative.

In the following claims the term "carrier frequency" is intended to include not only the actual carrier frequency but also the carrier frequency modified to the intermediate frequency of a superheterodyne receiver or to a further intermediate frequency in a multiple superheterodyne receiver.

We are aware that tuned modulated carrier wave receiving arrangements are well know, in which signals or noises of unwanted frequencies are suppressed by providing a tuned amplifier stage, having a negative feed-back arrangement which is effective at the unwanted frequency. For example in British Patent Specification No. 403,709 arrangements are described in each of which is provided a thermionic valve circuit comprising a resonant circuit tunable to a wanted frequency and a thermionic valve having means comprising a second resonant circuit common to its anode and control grid feed back circuits to produce the desired anti regenerative feed-back at the unwanted frequency.

We are also aware that in British Patent Specification No. 290,202 a receiving or detecting arrangement is described comprising a thermionic valve having a plurality of differently tuned circuits effectively connected between its input and output circuits, the arrangement being such that the reaction voltage feed-back into the valve is substantially zero only over the band of frequencies spanned by the tuned circuits whereby constant amplification is obtained over this band but signals of frequencies outside this band are suppress.

We are further aware that in British Patent Specification No. 440,337 arrangements are described each comprising a thermionic valve having a negative feed-back coupling between its anode and control grid circuits, which valve serves as a load impedance for a thermionic amplifier in which the effective impedance of the said valve is varied in accordance with the mean amplitude of the signals in the amplifier channel in order that the amplification effected by the thermionic amplifier is automatically adjusted in accordance with said signals. In a telephone signal amplifier the feed-back circuit of the control valve may be such that the amount of feed-back at middle frequencies is greater than the feed-back at the extreme frequencies of the audio frequency range so that automatic tone control is provided by the automatic variation of impedance of the control valve.

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. A tuned modulated carrier wave receiver comprising a thermionic amplifier valve having a negative feed-back path associated therewith, and an arrangement for applying a volume or gain control bias potential in accordance with the strength of the received signals to a control electrode of said valve the feed-back path being so designed and arranged that for signals at frequencies adjacent to the frequency of signals to which the receiver is tuned and on both sides thereof a relative reduction of amplification with respect to the amplification of the signals to which the receiver is tuned is produced in such a way that upon increasing or decreasing the gain of said amplifier valve the relative reduction of amplification of signals having adjacent frequencies is such that the selectivity of the amplifier is increased or decreased respectively.
  2. A tune modulated carrier wave receiver according to claim 1 wherein a gain control bias potential depending on the strength of the received signal is also applied to the control electrode of a thermionic amplifying valve not having an associated negative feed-back path but associated with the valve to which feed-back is applied, the two valves being so connected that the effect of the gain control bias potentials applied to the two valves is to vary the gain of each valve differentially in such manner that the variation of selectivity with gain of the receiver may be modified.
  3. A tuned modulated carrier wave receiver according to claim 2 wherein as the volume or gain control bias potentials applied to the two valves vary, the amplification of the valve having the associated negative feed-back path varies less than that of the other valve to which the bias potential is applied whereby the selectivity of the circuit is prevented from decreasing too rapidly with increase of signal strength.
  4. A tune modulated carrier wave receiver according to claim 2 in which the amplifier valve having a negative feed-back path associated with it is associated with another stage of amplification including another thermionic valve so linked with the first mentioned valve that when the gain of one valve is increased or decreased the gain of the other valve is decreased or increased as the case may be to such an extent that the amplification of the two valves taken together remains substantially constant whereby the selectivity of the receiver may be varied by varying the gain of the first mentioned valve but without varying the gain of the receiver.
  5. A tune modulated carrier wave receiver according to any of the preceding claims and of the superheterodyne type in which the thermionic valve having a negative feed-back path associated therewith is the mixer or frequency changer valve of the receiver.
  6. A tune modulated carrier wave receiver according to any of the preceding claims adapted to reproduce sound signals, comprising an audio frequency amplifier valve of which the gain is simultaneously varied with the gain of said carrier amplifier the audio frequency amplifier valve having a negative feed-back path associated therewith and designed to modify the frequency response of the amplifier in such manner that as the response of the receiver to signals of high audio frequency is effected by variation in the selectivity of the receiver, the response of the receiver in respect of signals of low audio frequencies is correspondingly varied.
  7. A tuned modulated carrier wave receiver according to any of the preceding claims in which the negative feed-back path associated with said amplifier valve comprises an impedance which is common to the anode-cathode path and to the control grid-cathode path of the valve, said impedance being composed of elements tuned to the frequency of the carrier to which the receiver is tuned and arranged to be of low value for signals of the frequency of the carrier frequency and of higher values for adjacent frequencies on either side of the carrier frequency.
  8. A tuned modulated carrier wave receiver according to any of the preceding claims in which the control grid-cathode path and the anode-cathode path of the valve to which feed-back is applied are associated through a bridge circuit, one arm of which includes a tuned impedance and another arm of which includes a further impedance which at the carrier frequency presents a resistance of value at least approximately equal to the impedance presented by the tune impedance, the bridge circuit being so arranged that no or a slightly positive feed-back is produced in respect of currents of the carrier frequency, the feed-back voltage becoming increasingly more negative in respect of currents of frequencies on either side of the carrier frequency.
  9. A tuned modulated carrier wave receiver according to claim 8 in which the other two arms of the bridge circuit are constituted by two equal parallely connected parts of a tightly coupled flatly tuned inductance coil, the difference of the electro-motive forces set up in dependence on the fluctuation of the respective currents in each part determining the voltages fed-back over the feed-back path.
  10. A tuned modulated carrier wave receiver according to claim 9 in which said inductance coil is extended at one end and connected though its extension to the control grid whereby a step-up of the voltages fed-back on the grid is obtained.
  11. A tuned modulated carrier wave receiver according to any of preceding claims 7 to 10 inclusive wherein there is included between the cathode and earth a portion of an inductance from a remote point on which a voltage is applied through a condenser to the input grid of the valve, whereby the effect of the capacity between the cathode and grid is reduced or eliminated.
  12. A tuned modulated carrier wave receiver according to claim 9 or 10 and 11 in which the portion of inductance from a remote point on which a voltage is applied to the input grid of the valve is constituted by an extension of the said tightly coupled inductance.
  13. A tuned modulated carrier wave receiver having means for varying the selectivity thereof substantially as described with reference to any of the Figures 1 to 8 of the drawings accompanying the Provisional Specification.

Dated this 4th day of June, 1937.

F. W. CACKETT,

Chartered Patent Agent.

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Leamington Spa: Printed for His Majesty’s Stationery Office, by the Courier Press. – 1938.