US2156076A - Automatic fidelity control - Google Patents

Description

April 25, 1939. J,- E. BEGGS 2,156,076

AUTOMATIC FIDELITY CONTROL File d Oct. 17, 1955 I 2 Sheets-Sheet 1 Inventor: James E. Beggs,

s Attorngg.

Patented Apr. 25, 1939 UNITED STATES PATENT OFFICE AUTOMATIC FIDELITY CONTROL.

New York Application October 17, 1935, Serial No. 45,388

10 Claims.

My invention relates to resonant networks in general and. more particularly to those which provide a means for. widening the transmission Pass band of such networks under. certain con-.- ditions.

It is now well known that the fidelity of reproduction in a radio receiving system may be greatly improved .whenlocal or strong signals are being received by wideningthepass band of the 1 systems resonant network. During the early stages of development of the radio art it was felt that the pass band must bev relatively narrow in order to provide proper selectivity for the system. Although this useofa narrow pass band did pro- 15 vide greater selectivity, it had the disadvantage that currents having certain desired frequencies were strongly attenuated in their transmission through the coupled circuits of the resonant network. It has been conclusively shown, however, 20 that sufficient selectivity may be obtained when local or strong signals are being received. by a relatively wider pass band. This wider pass band permits a larger group of side band frequencies to be transmitted through the system.

One of themethods of effecting a wideningof the pass band is by symmetrically detuning two coupled oscillatory circuits of the resonant network. That is to say, the resonant frequency of one coupled circuit is lowered while-the resonant frequency of the other circuit is raised an equal and opposite amount.

It is an: object of' my'invention to provide an improved method" of" symmetrically detuning a pair of coupled oscillatory circuits'in a resonant network.

It is a further object of my invention to provide an' improved means for symmetrically detuning two coupled oscillatory circuits by means allof whose component parts are stationary with respect to each other.

The novel features which I believe to be characteristic of my invention'are set forth with particularity in the appended claims. My invention 45 itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying so drawings in which Figs. 1., 2 and'3 represent different embodimentsofmy invention. Fig. 4 is a chart illustrating the region of maximum selectivity, the expanding and contracting region and the high fidelity region of'the circuits illustrated in Figs. 1, 2 and 3; Fig. 5 is a vector diagram 11- lustrating the operating characteristics of the equivalent capacity network of Fig. 3;

Referring to Fig. 1, it will be seen that the upper portion of the figure represents a conventional super-heterodyne radio receiving system which includes an aerial or pick-up device 1, a radio frequency amplifier stage 2, a detector and oscillator stage 3, a stage of intermediate frequency amplification .4, a second detector and automatic volume control stage 5, an audio frequency amplifier stage 6 and a loud-speaker I. Only those portions of the radio receiving system to which my invention forms an integral part have been shown in detail.

The first detector stage 3 is coupled to an amis plifier 8 of the electron discharge type through the coupling transformer 9. Condensers Ill and II are connected across the primary and secondary windings l2 and I3 respectively of the coupling transformer 9 and are adjusted to tune the windings l2 and I3 to the intermediate frequency. By-pass condensers l4 and I5 connect the lower side of windings l 2 and I3 to ground in the customary manner. Amplifier 8 is illustrated as being an electron discharge device having a cathode [6 which is maintained at the desired potentialby a self-biasing resistor I1. A by-pass condenser I8 is placed across the self-biasing resistor "and is connected to ground in the well known manner. Amplifier 8 is also provided with control grid I9, a screen grid 20 and an anode 2|. The output of the electron discharge device 8 is fed to a diode detector 22 through the coupling transformer 23. The primary winding 24 of transformer 23 is provided with a condenser 25 5 which tunes this winding to the intermediate frequency. One side of the secondary winding 26 of transformer 23 is connected to the anode 21 of the diode 22 while the other side of Winding 26 is connected through a potentiometer 28 to'the 40 cathode 29 of diode 22. A by-pass condenser 30 is placed across potentiometer 28. The audio frequency amplifier stage 6 is fed from the potentiometer 28 through the movable contact 3|. Potentiometer 28 also provides automatic volume control in a manner well known in the art through conductor 32'.

In the application of my invention to the radio receiving system hereinbefore described advantage is taken of the fact that the input capacitance of a three element electron discharge amplifier may be considered as the grid to cathode capacitance plus the voltage amplification of the electron discharge device multiplied by the grid to plate capacitance. Thus by properly choosing the impedance in the output circuit of the electron discharge device, the input side of the device may reflect substantially pure undamped capacitance. As the voltage amplification of the triode is varied in accordance with its grid bias, the capacitance reflected in the grid circuit is also varied in accordance therewith. Since the necessary value of capacity required to detune a pair of coupled oscillatory circuits is small, the triode may be used of this puropse by connecting the grid and cathode in the oscillatory circuit.

In the receiving system of Fig. 1, the control grid 5| of the electron discharge device 52 is connected to the upper side of winding l2 of transformer 9 through the condenser 53. The cathode 54 of electron discharge device 52 is connected to ground through the self-biasing resistor 55 and a by-pass condenser 56. The anode 51 of discharge device 52 is connected through a-resistor 58 and an inductance 59 to the high potential source (indicated as B+). The magnitude of the capacitive reactance reflected in the input circuit of the electron discharge device 52 is a function of the resistance 58 and the inductance 59 in the anode circuit. The grid 62 of a similar electron discharge device 60 having a cathode 6| and an anode 63 is connected to the upper side of the winding l3 of coupling transformer 9 through condenser 64. The biasing resistor 55 and by-pass condenser 56, which provide a suitable potential on the cathode 54 of electron discharge device 52 also provide a suitable potential on cathode 6| of electron discharge device 55. It will be understood that with electron discharge devices 52 and 60 connected as thus described in the circuit of the intermediate frequency stage 4, condensers I0 and H must be so adjusted that the coils i2 and I3 are tuned to the intermediate frequency.

The oscillatory circuits associated with wind ings l2 and I3 may now be symmetrically detuned by decreasing the negative bias on the control grid 5| of electron discharge device 52 and by increasing the negative bias on the control grid 62 of electron discharge device 60. This causes an increase in the voltage amplification of device 52 and a decrease in the voltage amplification of device 60 which in turn causes the reflected input capacity of the device 52 to increase and the reflected input capacity of device 60 to decrease.

The symmetrical detuning operation may be made completely automatic by providing a separate intermediate frequency channel. This separate channel which will hereinafter be termed the fidelity control channel is coupled to the main intermediate frequency channel 4 through condenser 33. The electron discharge device as, having a cathode 35, a control grid 36, a screen grid 31 and an anode 38, operates in a manner similar to the electron discharge device 8. It has, however, one important difference from the electron discharge device 8 in that the cathode 35 is provided with an adjustable self-biasing resistor 39 in the place of the ordinary fixed self-biasing resistor. The active portion of the resistor 39 is provided with a by-pass condenser 45. The output of the electron discharge device 34 is fed to diode 4| through a coupling transformer 42 and is preferably adjusted so that its primary windings 43 and 44 are criticallycoupled. Condensers 45 and 46 tune the primary winding 43 and the secondary winding 44 respectively to the intermediate frequency. By critically coupling windings 43 and 44 the fidelity channel becomes extremely selective and passes only a narrow band of frequencies closely adjacent to the intermediate frequency. This has the advantage of forcing the operator to tune the receiving system to the center of the signal. One side of the secondary winding 44 is connected to the anode 41 of the diode ii while the other side is connected to the cathode Q8 of the diode 4| through the potentiometer 49. A by-pass condenser 5|! is shunted across potentiometer 45.

The control grid 5i of the electron discharge device 52 is connected to the cathode 48 of diode 4| through a resistor st and a conductor 68. The control grid 52 of the electron discharge device 56 is connected to a movable contact 56 on potentiometer 29 through resistor 61 and conductor 69. By connecting a resistor 75 between conductors 68 and 59 and grounding the mid-point, the grid bias of one triode increases as the grid bias of the other triode decreases. By-pass condensers H and 12 are connected across resistor I5 in the manner shown.

In order to overcome the negative bias on control grid 35 of the electron discharge device 34 so that current will flow in the output circuit when a signal is being received having greater than a predetermined intensity, control grid 36 is also connected through a resistor l3 to a diode 'M. The diode M is so connected that a positive unidirectional voltage is applied to control grid 36 when a signal is being received. Thus anode 15 of diode 14 is grounded and a resistor 16 is connected between anode 15 and cathode 11 of diode M. A condenser l8 connects cathode Tl to the anode circuit of electron discharge device 8.

Since the cathode of diode '64 becomes positive by an amount which is a function of signal level, it will be understood that the grid bias on electron discharge device 34'varies as a function of carrier intensity. Now since the potential level of the cathode 35 with respect to the control grid 36 may be varied by shifting the movable contact 19 on resistor 39, it is'obvious that movable contact 19 is in effect a sensitivity control for it determines the signal intensity level at which the fidelity control channel becomes operative. Since the unidirectional voltage swing on the grids 5| and 62 is limited by the position of the contact 55 on potentiometer 49, contact 55 may properly be termed a tone control for its position determines the maximum width of band expansion.

The operation of the receiving system is as follows: An incoming high frequency signal is received on the pick-up device l, is amplified in the radio frequency stage 2 and is changed to an intermediate frequency in the first detector and oscillator stage 3. The signal is transmitted from stage 3 through the coupling transformer 9 to the intermediate frequency amplifier 8. After being amplified by device 8, the signal is transmitted through the coupl ng transformer 23 to the second detector 22, where it is demodulated to reproduce the transmitted audio wave. The audio frequency is amplified in the audio frequency stage 5 and the electrical signal is finally emitted as a sound Wave through the loud-speaker 1.

If the intensity of the incoming signal is above a predetermined value as determined by the setting of movable contact 19, an intermediate frequency wave is amplified by electron discharge device 34 and is supplied to the diode 4| of the fidelity control channel. As the current begins to fiow in the diode 4|, the unidirectional voltage applied to the control grid 5| becomes less negative and the unidirectional control voltage applied to control grid 62 becomes morenegative. This increase in the amplification of discharge device 52 causes capacitance to be added to the oscillatory circuit associated with winding 12 of coupling transformer 9. The decrease in the amplification of discharge device 68 causes capacitance to be removed from the oscillatory circuit associated with winding I3 of coupling transformer 9. This causes one of thetwo coupled circuits to be detuned from the intermediate frequency in one direction while the other circuit is detuned an equal amount in the opposite direction.

The extent of detuning is determined by the position of movable contact 66. The particular signal intensity value, however, at which the de tuning operation begins is determined by the po sition of contact 79. The reason for this is that as soon as the bias on control grid 36 is reduced to substantially zero the grid 36 begins to draw current and any further increase in potential across resistor it due to a further increase in carrier intensity produces a voltage drop across resistor 13 due to grid current flowing therein. This voltage drop is, of course, of opposite polarity to that on resistor 16 and thus opposes further change in the grid voltage in a positive direction. l'ience, the voltage on grid 36 remains substantially constant for values of received signal intensity greater than that at which the voltage on resistor 16 equals that on the portion of resistor 38 to the left of contact 19.

if the incoming signal should for any reason fade so that its intensity level is below the predetermined value for which the position of contact "5B is adjusted, the discharge device 34 ceases to supply intermediate frequency to the diode i! and the triodes 52 and '68 resume their normal value of amplification and, as a consequence thereof, the oscillatory circuits are again tuned to the intermediate frequency.

The automatic fidelity control means operation may be more readily understood by reference to Fig. 4 in which the principal curve F shows the relative output intensity from the fidelity control channel plotted as ordinates against received signal intensity plotted as abscissa. From. points A to B, the signal strength is not sufficient to overcome the bias on discharge device 34 which is determined by the movable contact 19 and the fidelity control channel is, therefore, in an inoperative condition. For this reason, the range of signal strength from A to B may be termed the region oi maximum selectivity. From B to C the signal strength is just sufilcient to cause intermediate frequency current to flow in the fidelity control channel but the signal strength is not sufficient to cause full expansion of the pass band. From C on, no further increase in signal intensity will cause a further widening of the pass band and this region may be termed the high fidelity region with band fully expanded. Curve M indicates the shape of the transmission resonant curve for the region of maximum selectivity. Curve N indicates the characteristic doubie peak transmission curve when the band is expanded.

A different means for effecting symmetrical detuning without the use of moving parts is shown in Fig. 2. In this modification symmetrical detuning is brought about through the use of an electron beam tube whose electron stream path may be varied as a function of signal intensity. Those portions of the radio receiving system which are similar to the system described in Fig.

l have been given the same reference numerals andonly those portions which differ from Fig. 1 will be described in detail. In order to simplify the drawings, the second detector stage which is shown in detail in Fig. l is now represented generally as 5. It is, of course, obvious that an additionalamplifying device may be used in the intermediate frequency stage and such a device is indicated generally as 4'.

To the ordinary radio receiving circuit are added two small condensers of equal value 8!] and Bi. One side of condenser is connected to the upper side of'winding I2 of coupling transformer 9 while one side of condenser 8| is connected to the upper side of winding l3 of coupling transformer 9. The lower sides of condensers 88 and 81 are connected to collector plates 82 and 83 respectively of an electron beam discharge device 84. Electron discharge device 84 is alsoprovided with an electron emitting element 85 and two:

deflector plates 86 and 8'1. Plates 82 and 83 are provided with a suitable positive bias by being connected through resistors and 9'! respectively to the high voltage source (indicated as B+). The cathode or electron emitting element 85 is connected through a by-pass condenser 88' and a self-biasing resistor 81 to ground. It will be understood that so long as the stream of electrons emitted from cathode 85 are striking collector plate 82 and not collector plate 83, con-1 denser 85 will be connected across the oscillatory circuit associated with winding l2 while condenser 8| will be merely floating across the circuit since no current path is provided from col lector plate 83 to cathode 85. With condenser 86 in the circuit and condenser 8! out of the circuit, condensers I8 and H are adjusted to tune the windings l2 and I3 to the intermediate frequency. If nowthe path of the electron stream in the electron beam discharge device 84 be,

changed so that substantially all of the electrons leaving cathode 85 strike collector plate 83 capacity will have been removed from across winding l2 but an equal value of capacity will have been added across winding I3. This causes a symmetrical detuning of the oscillatory circuits.

The path of the electron stream is determined by regulating the negative bias on deflector plates 86. and 81. Assuming for the moment that no bias is placed on deflector plates 85, deflector plate 81, due to the fact that its potential is negative with respect to the cathode, causes most of the electrons emitted from cathode 35 to strike collector plate 82. If a negative bias substantially greater than the bias on deflector plate 81 be placed on deflector plate 86, the stream of electrons will be deflected so that substantially all of them strike collector-plate 83. It is obvious that as the stream of electrons is swung from one plate to the other both condenser 80 and condenser 8! effect their =1" associated oscillatory circuits to some extent.

As is indicatedin Fig. 2, a second pair of coupled oscillatory circuits may be symmetrically detuned at the same time by a second electron beam discharge device 88.

Discharge device 88 is provided with two collector plates 89 and 99, two deflector plates 9! and 82, and an electron emitting element or cathode 93. Two condensers 94 and 95 similar to condensers 80 and 8! are connected to the upper side of windings 2d and 25 respectively of coupling transformer 23. The opposite sides of condensers 8d and 95 are connected to collector plates 89 and 900i. discharge device 88. Cathode 93 connected to ground through self-biasing resistor 8? and by-pass condenser 86 which are common to both cathodes 85 and 93. Collector plates 89 and 99 are given a suitable positive bias by being connected through resistors 94 and 95 to the high voltage source (indicated as B+) Since the detuning operation depends upon the magnitude of the negative bias placed on deflector plates 86 and 9|, it will be seen that the operation may be made completely automatic by applying a unidirectional negative voltage thereto which is a function of the signal level. This may be done in a manner similar to that described in connection with Fig. 1. In this case, however, the intermediate frequency amplifier 34 is coupled to the main intermediate frequency channel through a coil 98, coupled, for example to coil 26. Coil 98 is tuned to the intermediate frequency by condenser 99. The cathode 35 of amplifier 34 is provided with a fixed self-biasing resistor I99 in the place of the variable resistor 39 of Fig. 1. Since it is not necessary to provide a simultaneously increasing and decreasing bias. no resistor with its midpoint grounded is used in conjunction with potentiometer 49. A single condenser I93 now takes the place of the two condensers 1| and 12. The fidelity control conductor I9! is connected to pontentiometer 49 through resistor I92 and movable contact 96. Conductor I9I is connected to deflector plates 86 and 9! through resistors I94 and I95 respectively. Cathodes 85 and 93 are connected by conductor IN to cathode 49 of diode 4|. Since the signal intensity level at which the detuning operation begins depends upon the potential level of the cathodes 85 and 93 above ground, it is obvious that the point at which detuning begins will depend upon the position of the movable contact I99 on the selfbiasing resistor 81'. The end of resistor 81' opposite to the grounded end is connected to the high voltage source (indicated as B+).

The operation of the radio receiving system of Fig. 2 is similar to the radio receiving system of Fig. 1 except for the portion of the circuit which causes the symmetrical detuning. In the system of Fig. 2 condensers 89 and 94 are actively in the oscillatory circuits associated with windings I2 and 24 respectively when the signal level is below the predetermined point as governed by the sensitivity control I96. Condensers 8i and 95 under these conditions are merely floating on their associated circuits. If now the signal intensity level rises above the predetermined value determined by contact I99, the negative bias on deflector plates 86 and 9| deflect the electron stream so that substantially all of the electrons emitted from the cathodes 85 and 93 strike collector plates 83 and 99 respectively. This removes condensers 89 and 94 from their associated oscillatory circuits and adds condensers 8| and 95 to their associated oscillatory circuits. The pass band is then in its fully expanded condition.

A third means for effecting symmetrical detuning without the use of moving parts is shown in Fig. 3. In this modification symmetrical detuning is brought about through the use of a pair of phase shifting networks, each network including an electron discharge device. For the purpose of simplicity of illustration only the intermediate frequency stage of the radio receiving system is shown. Those portions of the intermediate frequency stage which are similar to the system described in Fig. 1 have been given the same reference numerals.

Shunted across winding I2 of the intermediate frequency coupling transformer 9 is the plate circuit of electron discharge device I91. The anode I99 of the electron discharge device I91 is connected through a by-pass condenser I99 to the upper side of winding I2. The cathode H9 of electron discharge device I91 is connected through by-pass condensers III and H2 to the lower side of winding I2. Cathode H9 is provided with a self-biasing resistor H3 in the customary manner. If the current through the plate circuit of the electron discharge device I91 is caused to lead With respect to the applied voltage by approximately 90, the discharge device I 91 will be equivalent to a condenser.

The plate current in discharge device I91 may be caused to lead with respect to the applied voltage by approximately 90 by connecting a condenser H4 and a resistor H5 in series between the anode I98 and the cathode H9 and by connecting the control grid H6 of the discharge device I91 between condenser H4 and resistor I I5. The reasons why this circuit operates as an equivalent capacity network may be more readily understood by referring to Fig. 5. The applied voltage may be represented by a vector E. The voltage across condenser H4 due to the infiuence of resistor I I5 slightly lags voltage vector E as is indicated by E0. The current through condenser I I4 leads its voltage E0 by 90 and may be represented by a vector Io. The voltage across resistor H5 is in phase with current vector Io and may be represented by a vector ER,- ER also represents the voltage EG applied to the control grid H6. Since the plate current flowing in an electron discharge device is always in phase with the voltage on the control grid, the current in the plate circuit of discharge device I91 may be represented by a vector IP. As may be seen, IP is approximately 90 ahead of the applied voltage E and therefore discharge device I9! is equivalent to a condenser whose current flowing through it is IP. Since the amount of capacitance which is shunted across winding H2 depends upon the lengths of the current vector IP, it is obvious that the equivalent value of capacitance represented by discharge device I91 may be varied by regulating the value of unidirectional voltage applied to control grid H6.

The anode circuit of a second electron discharge device I I1 is shunted across winding I3 of coupling transformer 9 in a manner similar to that of discharge device I91. Discharge device H1 is provided with an anode H8, a cathode H9 and a control grid I29. A condenser I2I and a resistor I22 are connected across the anode H8 and cathode H9. The control grid I29 is connected between condenser IZI and resistor I22 in order that the current flowing from anode H8 to cathode H9 will lead with respect to the applied voltage by approximately 99". A selfbiasing resistor I23 and a by-pass condenser I24 are connected to cathode H9 in the customary manner.

In order symmetrically to detune the oscillatory circuits associated with windings I2 and I3, it is only necessary that the negative bias on control grid I I9 be decreased while the negative bias on control grid I29 be increased an equal and opposite amount. This may be accomplished automatically, by providing a separate fidelity chan nel as was done in the radio receiving systems described in connection with Figs. 1 and 2. In this case the separate fidelity channel is coupled to the main intermediate frequency channel through coil I25 which may be coupled, for

example, to coil I3. Coil I25 is tuned to the intermediate frequency by condenser I26. Although to anode I3 oi diode I2-I while-the lower side of winding I29 is connected to cathode I34 Of diode I2'I through a by-pass condenser I35 and a resistor I36'as is shownin the drawings. Cathode I34-is grounded and the unidirectional control voltage for control grid I20 is taken from the ungrounded side of resistor I36; The upper side of winding I30 is connected to anode I3 'I of diode I28 while the lower side of winding I30 is grounded. Cathode I38 of diode I28 is provided with a self-biasing resistor I39 and a by-pass'condenser I68 in the customary manner. The unidirectional control voltage for control grid H6 is taken from the cathode I38.

The operation of the symmetrical detuning circuit shown by Fig. 3 is asfollows: Assuming that the intensity of the incoming, signal is below the level. at which it is desired to widen the pass band, condensers II and II are adjusted so that windings l2 and I3 are tuned to the intermediate frequency. The receiving system is now in its most selective condition as is indicated by curve M of Fig. 4. If the intensity level of the incoming signal now rises above the predetermined value corresponding to the value for which the system is adjusted for expansion of the transmission band, diode I28 causes the unidirectional negative voltage on control grid H5 to decrease and diode I2? causes the unidirectional negative voltage on control grid I20 to increase. This action causes the plate current flowing in discharge device IIE'I to increase which is equivalent to decreasing the capacitive reactance across winding 52. The plate current fiowing in discharge device l i? is simultaneously decreased which is equivalent to adding capacitive reactance across winding I3. As a consequence thereof, the oscillatory circuits associated with windings I2 and I3 are symmetrically detuned.

If it is desired to detune the oscillatory circuits by changing the inductance rather than capacitance, an equivalent inductance network rather than an equivalent capacitance network may be used by simply reversing the position of condenser I24 and resistor H5 and by reversing condenser I2I and resistor I22 in the circuit shown in Fig. 3. This causes the current flowing through discharge devices IIlI and II! to lag approximately 99 with respect to the applied voltage. In other respects, however, the operation is exactly similar to that described for the circuit of Fig. 3.

Although in each of the radio receiving systems shown by Figs. 1, 2 and 3 a separate fidelity control channel is shown, it is obvious that automatic control may be obtained for the various detuning circuits by connecting the control elements thereof through suitable connections to the conventional automatic volume control bus, which is indicated, for example, at 32 in Fig. 1.

While I have shown particular embodiments of my invention it will of course be understood that I do not wish to be limited thereto since many modifications may be made both in the circuit arrangement and in the instrumentalities employed, and that I therefore contemplate by the appended claims to cover all such modifications as fall within the true-spirit and scope of my invention. What I claim asnew and desire to-secure. by

Letters-Patent of the United Statesis:

1. In combination, apair of coupled oscillatorycircuits, a plurality of electron discharge devices having their input circuits connected. to said oscillatory circuits-and their. output circuits-connected through a resistor to a source of high voltage, means,.including means for varying the input capacitanceof saidelectrondischarge devices, for symmetrically. detuningsaid. oscillatory circuits.

2. In. combination, apair of coupled oscillatory. circuits, an electron discharge. devicehaving its grid-cathode circuit connected across one of said oscillatory circuits and its anode connectedv througha-n impedance element to a source of high potential, a second electron discharge device having its grid cathode circuit connected across the other of said oscillatory circuitsv and having its anode connected. througha second impedance element toa source. of high. potential andmeans for increasing the: amplification of. one of said dischargev devices and simultaneously. decreasing the amplificationvofi the other of saiddischa-rge devices. in response to. the-intensity level of the signal in said oscillatory circuits.

3. In a superheterodyne radio receiving system, the combination of a plurality of cascaded amplifiers including an amplifier operating at the intermediate frequency of said system, a pair of coupled oscillatory circuits tuned to said intermediate frequency, means for symmetrically detuning said circuits about said intermediate frequency, automatic means for actuating said detuning means when a signal is being received above a predetermined intensity, means for varying the intensity level at which said second mentioned means actuates said first mentioned means and means for limiting the effectiveness of said second mentioned means.

4. In a superheterodyne radio receiving system, the combination of a plurality of cascaded amplifiers including an amplifier operating at the intermediate frequency of said system, a pair of coupled oscillatory circuits, and means including an electron beam discharge device for symmetrically detuning said circuits about said intermediate frequency.

5. In a radio receiving system, the combination of a plurality of cascaded amplifiers and couplings between adjacent amplifiers, the couplings between one pair of adjacent amplifiers comprising a pair of oscillatory circuits tuned to the desired frequency, a capacitance element connected across one of said circuits, a second capacitance element connected across the other of said circuits, an electron beam discharge device having one collector plate and its cathode connected in the circuit of said first capacitance element and a second collector plate and its cathode connected in the circuit of said second capacitance element, and means for shifting the electron stream in said discharge device from one collector plate to the other.

6. In combination a pair of coupled oscillatory circuits, means for applying a signal voltage to one of said circuits, a plurality of electron discharge devices each having its output circuit connected' across one of said circuits and having its input circuit controlled by said signal voltage, means for shifting the phase of the currents flowing through said discharge devices approximately with respect to the applied voltage,

and means for varying the conductance of said discharge devices in response to the intensity of the signal in said coupled circuits.

7. In combination, a pair of coupled oscillatory circuits, means for applying a signal voltage to one of said circuits, a plurality of electron discharge devices each having its anode-cathode circuit connected across one of said circuits and having its grid cathode circuit connected to receive said signal, means for shifting the phase of the currents flowing through said discharge devices approximately with respect to the applied voltage, and means for simultaneously increasing the conductance of one of said discharge devices while decreasing the conductance of the other of said discharge devices, thereby symmetrically to detune said circuits.

8. In a radio receiving system a pair of electron discharge devices and a coupling between adjacent discharge devices, said coupling comprising a pair of oscillatory circuits tuned to a desired frequency, electron discharge means for increasing the capacity of one oscillatory circuit and decreasing the capacity of the other oscillatory circuit in response to an increase in intensity of currents in said oscillatory circuits.

9. In a radio receiving system of the superheterodyne type, the combination of a plurality of cascaded amplifiers including an amplifier operating at the intermediate frequency of said system, means for applying a signal voltage thereto, a pair of coupled oscillatory circuits tuned to said intermediate frequency, one of said circuits being connected with the output circuit of said intermediate frequency amplifier, a plurality of electron discharge devices each having its anodecathode circuit connected across one of said oscillatory circuits and having its grid cathode circuit connected to receive said signal, means for shifting the phase of the anode current through said devices substantially 90 with respect to the applied voltage and means for increasing the mutual conductance of one of said discharge devices and decreasing the conductance of the other of said discharge devices.

10. In a radio receiving system a pair of electron discharge devices, a coupling therebetween comprising a pair of oscillatory circuits tuned to a desired frequency, and means all of whose component parts are fixed with respect to each other for increasing the capacity of one oscillatory circuit and simultaneously decreasing the capacity of the other oscillatory circuit in response to an increase in intensity of the currents in said oscillatory circuits.

JAMES E. BEGGS.

DISCLAIMER 2,156,076.James E. Beggs. Schenectady, N. Y. AUTOMATIC FIDELITY CONTROL. Patent dated April 25, 1939. Disclaimer filed September 17, 1940, by the assignee, General Electric Company. Hereby enters this disclaimer to claim'l of said Letters Patent.

[Ofiicial Gazette October 22, 1.940.]

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