US2395413A - Demodulator device for frequency and amplitude modulation - Google Patents

Description

Feb. 26, 1946 I LANGE 2,395,413

DEMODULATOR DEVICE FOR FREQUENCY AND AMPLITUDE MODULATION FiledApril s, 1944 INVENTQR Patented Feb. 26, 1946 BEMODULATOR DEVICE FOR FREQUENCY AND AMPLITUDE MODULATION Edward H. Lange, Baltimore, Md.

Application April 3, 1944, Serial No. 529,359

18 Claims. (01. 250--20) This invention pertains to receiving systems for receiving either frequency-modulated carrier waves or amplitude-modulated carrier waves, and more particularly to demodulator devices for such receiving systems, capable of demodulating either frequency-modulation or amplitude-modulation.

The principal object of this invention is to provide such demodulator'devices with a maximum of economy of necessary electronic equipment and compartment circuit parts, with improved facilities for compensating amplitude-modulation, and. improved facilities for tuning and. for control of unmodulated signal amplitudes.

A first object of this invention is to provide a frequency-demodulator requiring but a single conventional type of electronic tube, and capable of operating upon a variable norm of signal strength of frequency-modulated signals, compensating audio-frequency variations of signal amplitude and permitting lower frequency, i. e. subaudio frequency variations to pass uncompensated, and requiring no readjustment of circuit elements to attain compensation of amplitude variations when the average value of signal amplitude changesfrom one norm to another. Asecond object of this invention is to provide a simple form of demodulator circuit with a'con- 'trol-impedance carrying the sum total of thermionic currents of both diode rectifying means employed for frequency-demodulation, to attain unidirectional control-voltages independent of frequency-modulation and directly proportional to the amplitude of carrier currents, for the essential controls of demodulation.

A third object of this invention is to utilize the control-impedance in a novel compensating system for compensating undesired variations of amplitude of the carrier currents during the r ception of frequency-modulated carrier currents. In systems heretofore disclosed, such compensation is generally effected in relation to a definite norm of carrier current amplitudes, such that the norm or average value of-the carrier current amplitudes is an essential part of a balance established, compensation being with reference to amplitude variations from a specific norm, and such that when the norm is changed, a new balance must be set up to attain compensation with ref erence to the new norm, thus requiring periodic adjustment for various magnitudes of the norm. In the present invention, the necessity of this periodic adjustment is eliminated, slow changes in the norm of insufficient rapidity to modify the I acoustic resultant from the frequency-demodulatcd' output-voltages are permitted; filter means.

are employed to transmit voltages from the control-impedance which are a consequence of changes of carrier current amplitudes in the audio-frequency range, and employed to substantially annul the changes in the carrier current amplitudes of the carrier currentscoupled with the resonance network of the demodulator, when such changes are sufiiciently rapldto fall within the audio-frequency range.

A fourth object of this invention is to provide a control means for the average amplitude of carrier currents employed upon the demodulators, for regulating the maximum values of the demodulated output-voltages automatically in relation to the strength of received signals, applicable selectively for use during reception of freequency-modulated signals, or for use during reception of amplitude-modulated signals, or for both, and employing the control-impedance of this invention.

A fifth object of this invention is to provide a simple tuning means for tuning the resonance network of the demodulators to a predetermined centre frequency, employing the control-impedance of this invention and an electron-ray tube.

A sixth object of this invention is to provide automatic correction of tuning to coincidence with a predetermined centre frequency of the resonance networks of the demodulators, when demodulating frequency-modulation as Well as when demodulating amplitude-modulation, and in a superheterodyne receiver having a mixer-device for reducing the carrier frequency, including a thermionic alternating current generator having a bias-voltage control means for controlling the frequency of the generator.

These objects, and others, are hereinafter pointed out in further detail, and will he better understood by reference to the drawing, and to the appended claims.

In the drawing,

Fig. 1 illustrates a demodulator for both frequency-modulation and amplitude-modulation, with means for compensating amplitude-modulation, means for regulating an average amplitude of carrier currents, selector means for selecting the above means, including means for selectively connecting an audio-frequency push-pull amplifler for amplifying either frequency-demodulated output-voltages or amplitude-demodulated out put-voltages, and means for tuning the resonance network to a centre frequency coincident with the centre frequency of the carrier currents.

Fig. 2 illustrates a modification of Fig. 1, in which the grid-controlled thermionic conductbias-voltage control circuit of the mixer, steady bias voltages, independent of amplitude variations of the frequency-demodulated voltages, and proportional to the deviation of the centre frequency of the carrier currents from the centre frequency of the resonance network, for automatically modifying the generated frequency of the mixer-generator to annul the undesired deviation between said centre frequencies.

Fig. 4 illustrates certain quantitative relations with reference to the resonance networks, in relation to a centre frequency for the networks, and for the control-impedance of this invention, and tuning of the demodulators.

Fig. 5 illustrates a conventional type of mutualconductance characteristic of super-control or variable mu types of thermionic tube, employable in connection with the dynamic-type of compensation for amplitude-modulation of thisinvention, in which an equilibrium of carrier current amplitudes is determined by the erasure of the original corrective stimulus, as distinguished from a static-type of compensation, wherein a fixed magnitude of change of carrier current amplitude causes a proportionately fixed magnitude of corrective stimulus, to maintain the frequency-voltage conversion sensitivity of the demodulator constant, while permitting changes in amplitude of the. carrier currents.

Referring to Fig. 1, at 4 is an inductance coil, shunted by the variable condenser l9, the elements 4-!!! forming a resonance circuit, which is coupled by mutual inductance with the primary coil 5; also at 3 is an inductance coil forming a resonance circuit with the variable condensers i8 and 11, and at 6 is a primary coil coupled by mutual inductance with 3. At l is a common terminal upon the resonance circuits 4-19 and 3-H. At 42 is indicated a mechanical inter-connection between the rotors of the variable condensers l9, l1, and N3, the condenser i8 being a spacer-condenser, for simultaneously spacing the resonant frequency of the resonance circuit 3-H a predetermined frequency-interval from the resonant frequency of the resonance circuit 4-I9, and for regulating the amount of the frequency-interval in relation to the magnitude of the resonant frequency. At 31 is a source of unidirectional voltage, having its negative terminal connected to the ground 36. At Ill is a thermionic tube having the diode-anode l2, diodeanode I3, anode ll, common cathode l4, controlgrid 1, screen-grid 8, and suppressor-grid 9. The cathode M has the cathode terminal I6, and the common terminal 1 upon the resonance circuits 3-H and 4-H is connected through the control-impedance 2-2l to the terminal 16; controlimpedance 2-21 has the resistance 2 shunted by the condenser2l. The terminal a of inductance coil 3 is connected through impedance 22-20 and conductor d to the diode-anode l2, likewise the terminal b of inductance coil 4 is connected through impedance 23-24 and conductor to diode-anode [3; the impedances 22-20 and 23- 24 are made up of the respective equal resistances 22 and 23, each shunted by an equal capacitance, the resistance 22 being shunted by the condenser 26, and the resistance 23 being shunted by the condenser 24. The resistances 22, 23 are large in relation to the respective shunt capacitive reactances of 20 or 24, at the frequency of the carrier currents, to provide a large time-constant in relation to the half-cycle time of the carrier currents, sufiicient to smooth out the'rectified currents in resistances 22 and 23. The control-impedance 2-2! carries the total thermionic currents of both the diode-anodes I2 and I3, and likewise has a large time-constant sufiicient to smooth out the voltages developed across the resistance 2, the control-impedance being discussed hereafter in further detail, and in reference to Fig. 4. Connected between the diodeanodes l2, I3, is the output-resistance for frequency-demodulated voltages, 52-52, consisting of the resistance 52 connected in series with the resistance 52', and having the mid-point y between these resistances connected to ground, at 35. Resistance 52 is connected to diodeanode !2, through condenser 5| and choke-coil 29, and resistance 52' is connected to diode-anode l3, through condenser 32 and choke-coil 30. Chokecoils 29 and 36 each have a high reactance at the carrier frequency in relation to respective inductance' coils 3 and 4, and thecondensers 5| and 32 each have negligible reactance ataudiofrequencies in relation to respective resistances 52 and 52', and serve as stopping condensers. Resistances 52 and 52' are equal. Condenser h is shunted across resistance 52, and condenser 1' is shunted across resistance 52'; these condensers serve to by-pass the small carrier frequency currents which traverse the choke-coils, aroundthe resistances 52 and 52 respectively, that is, to substantially eliminate voltages upon 52 and 52' of carrier frequency. At S1 and S2 are switchblades, capable of connecting respectively with terminals F1 and F2, for reception of frequency demodulated voltages, or with A1 and A2, forIre-. ception of amplitude-demodulated voltages. At is a bias-resistance connected from the pathode M to ground 3'6, for carrying the unidirectional current which flows from the anode H. to the cathode 14 in tube Ill, by reason of connec- .tion of anode H with the positive terminal of source 3'1. Resistance 35 is shunted by the bypass condenser 34, for by-passing alternating current components around resistance 35. At 25-26 is a transfer-filter for transferring voltage variations from the control-impedance 2-2! to grid control means, the reactance of condenser 26 at audio-frequencies being negligible incom parisonwith the resistance of 25. Condenser 26 and resistance 25, are. connected in series, one

terminal of resistance 25 being connected to ground v3t, and one terminal of condenser 26 being connected to the common terminal I, At 2'l-28-42, is a second transfer-filter, for transferring steady voltages from the control-impedance 2-2l to grid control means, the reactance of condenser 28 at audio-frequencies being negligible in relation to the resistance 21, and re- In Fig. 1, the anode II is energized by connection through the primary coils B and in series, to the positive terminal of the source 31. Suppressor-grid 9 is connected to cathode terminal I6, and screen-grid 8 is connected to a positive terminal upon the source 31', by the variable connector 38. Connected between the terminals 44 and 46, is the input impedance 43, the terminal 46 being connected to the control-grid 1. The junction-terminal a, between the resistance 25 and condenser 26, is connected to terminal 44 through a switch-blade S3; also the terminal 12 is directly connected with the terminal A1. At 43 is a variable contactor upon the resistance 42, the contactor 43' being selectively connectable to the terminal 45 by means of the switch-blade S the terminal 45 is understood to be connected togrid control elements of one or more thermionic tubes of a thermionic amplifier having its out put-impedance coupled with the input-impedmice 43 of tube I0, and so that conductive connection of the grid control elements of said amplifier to the respective cathodes of said amplifier'is completed through the terminal 45 in a manner well understood, for example by a conductive connection of terminal 45 to the ground 36, and by conductive connection of the cathodes of said amplifier through individual voltagebias resistors to ground 36. Uses of the switchblades S3, S4, and other selector means. are hereafter illustrated. The switch-blades Sr and S2, are understood to be respectively connected to control-grids of a push-pull audio-frequency amplifier, through connections indicated by G1 and G2. The terminal A2 is understood to be for phase-inversion, that is, to be connected through connection P2 to the output-impedance of the audio-frequency amplifier in a manner well un- I derstood, and so as to apply between ground 35 and terminal A2 equal voltages of opposite phase to those applied between ground 35 and terminal A1, by the amplitude-demodulated voltages upon resistance 25.

Referring to Fig. 2, at 5a and 5a are terminals for applying the carrier currents to be demodulated. Primary coils 5 and 6 in series, are connected between the positive terminal of source 31 and the terminal 6a; the terminal 5a is connected to ground 36, and to the positive terminal of source 3'! by the by-pass condenser M of negligible impedance-for carrier frequencies, for carrying the alternating current component of currents through primary coils 5 and 6. The terminals 5a and 6a, are understood to be for connection to the output side of a thermionic amplifier for carrier currents, for example, such as illustrated -at terminals65 and 66, of amplifier 68, Fig. 3. In tube m, suppressor-grid 9 is connected to cathode terminal I6, and screen-grid 8 is connected by variableconnector 38 to a positive terminal upon source 31; by-pass condenser 40 is connected from screen-grid 8 to the oathode terminal IG. At F is an audio-frequency transformer, having primary 25a connected across a part of resistance 2, and secondary 25b with one terminal grounded to 36, and the other terminal connected to control-grid I. The terminal A1 is connected to terminal n. Terminal 45 is connected to switch-blade S4, which is connectable either with resistance 42, or with ground 36. The anode II is energized by connection from the positive terminal of source 31 through switchblade Se, and choke-coil 78, to anode I I; also anode II is connected to terminal Ba through stopping condenser 11 The thermionic alternating current'conductance between anode II and cathode I 4 is thus shunted across primary coils 5 and- 6 in series, the reactarice of condenser 11 being negligible in relation to said conductance, as is also the reactance of condensers 4| and 34, for carrier frequencies. An alternative connection for anode II can be employed, in which condenser I1 is replaced by switch-blade St, and the connection to the positive terminal of source 31 through choke-coil I8 omitted, the anode I I then being directly energized from terminal 6a when the switch S6 is closed.

Referring to Fig. 3, at I5 is a thermionic douhie-diode with diode-anode I2, diode-anode I3, and respective diode-cathodes Ida and I Ib. Connected between the cathodes I ia and MI) are the two resistances 22' and 23' in series, the terminal I6 being a junction-terminal between seriesconnected resistances 22 and 23; the terminal of resistance 23' opposite terminal I5, is grounded at 35. Shunted across resistance 22' is condenser 20, and shunted across resistance 23' is condenser 24', each condenser with respective shunted resistance providing a large time-constantin relation to the half-cycle time of the carrier currents. Connectedto the terminal a of the resonance circuit 3I'I is the diode-anode I2, connected through conductor 01, and connected to the terminal b of the resonance circuit 4-49 through conductor is the diodeanode I 3. Connected betweenthe common terminal I and the junction-terminal I 6, is the control-impedance 2I--2, carrying the sum. total of the thermionic currents to diode-anode I2 and diode-anode I3. At I [la is a thermionic tube having the anode II, cathode I4, control-grid I, screen-grid 8, and the suppressor-grid 9. The suppressor-grid 9 is connected to the cathode l4, and screen-grid 8 is connected to a positive terminal upon the source 37 by the variable connector 38. Connected betweenthe control-grid 1 and the terminal 44' is the input-impedance 43. Cathode I4 is connected to ground through the bias-resistance 35. The anode II is energized by connection through the primary coils 6 and 5 in series, to the positive terminal upon the source 37. At 58 is a thermionic amplifier for carrier frequency currents of reduced carrier frequency, and illustrated in block-diagram form, the inputterminals of the amplifier being indicated by 55 and 55, and the output-terminals of the amplifier being indicated by 55 and 55. At 54 is an output impedance for the amplifier 68 coupled with the input-impedance 43 of the demodulator, by mutual inductance between 54 and 43. At 58 is a terminal conductively connectedto a controlgrid of one stage of the amplifier 58, and at 55 is another terminal conductively connected to a control-grid of another stage of the amplifier 68. Terminals 58 and 59 are connected together, and tothe terminal 45, it being understood that the cathodes of the amplifier 68 are connected to ground 36 through individual bias-resistances in a manner Well understood, and for example such as resistance 35 connected to cathode I4; also that the anodes of the tubes of amplifier 55 which have thermionic currents controlled by the con trol-grids connected to terminals 58 and 59, are supplied from the source 31, for example by con nection 53, and that screen-grids of the tubes of amplifier 58 may be supplied by the variable connector 35. At 48 is a mixer-device, having a thermionic alternating'current generator with a biasvoltage control means for controlling the frequency of generated currents, and illustrated in block-diagram form. At 41 is a receiving antenna connected to the mixer-device, and at 62 and 6 3 are output-terminals of the mixer-device, terminal 62 being connected to terminal 56, and terminal 63 being connected to terminal 55 of ampiifier 68. The mixer-device is understood to supply a reduced carrier frequency to the terminals 62, 63, obtained from a combination of the generator frequency with the received frequency, received upon antenna 41; also the generator is understood to have its generated frequency controllable by the bias-voltage supplied through conductor t in a manner well understood, for example by reactance-modulator means having the conductor t and connected terminal 6| connected to a control-grid of a thermionic tube, controlling a quadrature component of current of the generator.

At F is anaudio-frequency transformer having the primary winding 25a, and the secondary winding 25b. The primary winding 25a is connected across a part of the resistance2 of the control-impedance 2-2 I, for transferring voltage variations from the control-impedance to the secondary 2511. It will be understood that transformer F has the usual characteristics of audiofrequency transformers, and passes a band of frequencies approximately between 100 cycles per second and 10,000 cycles per second, that the phase-shift is small over a substantial part of this range, and that the part of the resistance 2 included across the primary 25a is small in relation to the primary impedance. One terminal of the secondary 25b is connected to the ground 36, the other terminal of the secondary 25b is connected to the terminal M of a switch controlled by the switch-blade S34. The switch-blade S34 is connected to terminal 44' of the input-impedance 43, and terminal 44' is connected to terminal 45 of the control-grids of amplifier 58 by means of the conductor 39'.

At -16-14, is a second transfer-filter for transferring steady voltages, independent of amplitude-modulation, and proportional to an average amplitude of the carrier currents. The condenser 15 has one terminal connected to ground 36, and the other terminal connected through resistance 15 to the junction l6 between resistances 22' and 23. Resistance 14 is shunted across condenser 16. The terminal V of the switch controlled by switch-blade S34, is connected to resistance 14 by a variable contactor upon resistance 14. The reactance of condenser 16 is negligible at audio frequencies, in relation to the resistance of 15, and'the resistance of 14. Shunted across resistance 23' is the condenser 12 connected in series with the resistance 13, one terminal of condenser 12 being connected to the junction It. The reactance of condenser 12 is negligible at audio frequencies in relation to the resistance of 13. At S1 is a switch-blade for connecting the control-grid of an audio-frequency amplifier by means of connection G1, with the terminal F1 for frequency-demodulated voltages, or with the terminal A1 for amplitude-demodulated voltages. Terminal A1 is connected to the terminal of resistance 13 opposite the ground connection 35, and between the condenser 12 and resistance 13. v

At 49-50 is a third transfer-filter for transferring steady bias-voltages proportional to an average frequency-deviation of the carrier currents from a centre-frequency of the resonance circuits 3-i1-l8, and 4-19, for automatic correction of tuning, independent of the audio-frequency variations of the frequency deviations with reference to a centre-frequency of the frequency-modulated carrier currents. condenser connected in series with the resistance 58, one terminal of the condenser 49 being connected to the ground 35, and one terminal of the resistance 50 being connected to diode-cathode Ma, so that the series-connected resistance 50 and condenser 49 are shunted across the diodecathodes I ia-Mb. between the condenser 49 and resistance, 50 is connected through the conductor t to terminal 6!, upon the mixer-device, for transmitting biasvoltage for regulatingthe generated frequency of the thermionic generator of the mixer-device.

The reactance at audio-frequencies of condenser purpose of equalizing any unbalance of the diode circuits introduced by employment of the impedances 15-15-14 and 12-13 shunted across the resistance 23'. It will be understood that various equivalent structures can be connected across the resistance 22; for example, an obvious equivalent being ance 22' of impedances identical with those of 15-16-14, and 12-13, the resistances 225 and 23' being equal.

In Fig. 1, tuning means is shown for indicating the appropriate tuning for a centre-frequency of the resonance circuits, off-resonance from the resonant frequency of either resonance circuit by a predetermined amount, employing the con trol-impedance and an electron-ray tube 86. The tube 85 has cathodes 19, a control-grid 82,-a luminous-indicating anode control electrode 85 for controlling electronbeams from cathode 19 to anode 83. Theanode 83'is energized by conductor 8!! connected to a positive terminal upon source 31, and anode 85 is energized through the resistance 8| connected to anode 83. Control-grid 82 is connected to resistance 2 by a variable contactor upon 2, and cathodes 19 are connected to the cathode terminal N5 of cathode l4.

. Certain further details with reference to the operation of the control-impedance, and principles of compensation for amplitude-modulation by a dynamic balance are hereafter given, in relation to Fig. 4, and Fig. 5.

Before considering Figs. 4 and 5, some of the important features of operation of the structures will be pointed out, for Figs. 1, 2, and 3.- Referring to Fig. 1, voltages are developed across the control-resistance 2, directly proportional to the amplitudes of the carrier currents in the primaries 5, 6, and independent of the frequencydeviations from a centre-frequency of the carrier currents, over the entire range in which frequency-modulation is employed. Variation voltages are developed upon resistance 25 substantially equal to the variation voltages across resistance2, caused by amplitude variations of the carrier current through primaries 5. 6, at audiofrequencies. However, very slow changes of At 49 is a- The junction-terminal 50 the connection across the resist- 83, and an anode amplitude of the carrier currents, produce no compensating voltages upon resistance 25. When the audio-frequency push-pull amplifier is connected with terminals F1 and F2, and the carrier currents in the primaries 5, B, are frequencymodulated and have a constant amplitude, no compensating voltage are produced across resistance 25, and the difierential unidirectional voltages across 23 l-22 resulting from voltage rectification and frequency-modulation, are transmitted to resistances 52, and 52'. Also, if the amplitude of the carrier currents ch'angesat a slow rate, e. g. if the carrier current amplitude increases to double its original value in the course of thirty seconds, at a substantially uniform rate, no compensating voltage is provided upon resistance 25. When the switch-blade S3 connects with. terminal 44,as it does during reception of frequency-modulated carrier currents, compensating voltages are transmitted to the controlgrid 7. if the carrier current amplitude varies at a specified rate, e. g. at a rate sufficient to cause an undesired acoustic resultant from the cross-modulation caused by variation of the carrier amplitude. It will be noted that the cathode terminal I6, of cathode I4, is positive relative to the ground 35; also that the direction of fiow of rectified current through resistance 2 is from 2 to junction I, the rectified voltage upon resistance 2' being positive at the terminal I 6 with reference to terminal I, and that an increased negative voltage is introduced upon control-grid 1 through the path |34 25-1 when the voltage across resistance 2 is increased by reason of an increase in the amplitude of the carrier currents through primaries 5, and 6. This increased negative bias upon control-grid is capable of introducing a large decrease of mutual-conductance in the tube In, sufficient to substantially erase the original increase in carrier current; likewisea decrease in the carrier current amplitude at an audic frequency rate, causes a reversed flow through resistance 25, and a decreased negativebias voltage upon control-grid l, capable of in troducing a large increase of mutual-conductance sufficient to erase the decrease of the original carrier amplitude. The input-impedance 43 is understood to be coupled with the output-impedance of an amplifier for amplifying carrier currents, or carrier currents of reduced frequency,

such as the amplifier 68, Fig. 3, and the compensating voltage upon resistance 25 may also be employed to change the. mutual conductance of the tubes of this amplifier, to attain a very high rate of change of mutual-conductance in relation. to incremental voltage upon control-resistance 2, or'resistance 25. For example, the control-grids of amplifier 68 connected to terminal 45, can be connected to terminal 44 by the connector 39. The switch 85-394: is omitted when the connection 39 is employed. When it is desired to receive amplitude-modulation, switch-blades S1, S2, S3, and S4, are deflected to a downward position so as to connect respectively with the ter minals A1 and A2, so as to open the circuit from terminal 11 to terminals 44 and 45 through switchblade 53, and so as to connect the switch-blade Sawith the terminals 44 and 45. With these connections amplitude modulation produced across resistance 2 is transmitted to terminal A1 from resistance 25; also the steady bias-voltages fromresistance 42 are introduced upon controlrld'l, and upon the control-grids of the amplifier, 68 in relation to the amplitude of the carrier currents- ;It will be noted that impedance 21-28 is in parallel with resistance 2- for the flow of currents from terminal IE to the terminal I and that the voltage across condenser 28, for example by traversing the path from terminal it through resistance 2 and resistance 21, is composed of a steady voltage, and equal and opposite variation voltages, the latter voltages annulling each other; resistance 42, and contactor E3 being provided to proportion the magnitude of steady voltage employed between terminal l5 and variable con-. tactcr 43. With this particular connection, the bias-voltage from resistance is eliminated from the control-grid I, only the negative bias-voltage from terminal I through resistance 42 to ccntactor 53 being upon control-grid 1, and increasing inrelation to the amplitude of theunmodulated carrier currents, is increasing in relation to the average amplitude of the amplitude-modulated carrier currents; the bias-voltage however upon the control-grids ofthe amplifier Bills relative to the ground 36, and the bias voltage from terminal 45 to terminal Hi, is opposed to the bias voltage from terminal I6 to ground 36. Thus, resultant negative bias voltage upon the control-grids of the amplifier 68 is not operative until the amplitude of the carrier currents exceeds a predetermined. minimum value, after which increased carrier amplitudes operate to reduce the mutual-conductance, and control the demodulated output-voltageupon A1. Other selective means for employing the transfer-filterawill be evident. For example, the conductor 39 may be omitted, and a switch S539a employed, in. which switch-blade S5 is connected to the ground 36, and terminal Btu connected'to terminal t4, the switch being open for frequency demodulation and closed for amplitude-demodulation. With this connection, no amplitudemodulation compensating voltages areemployed upon the amplifier 68, and when receiving a'mpli tude-modulation intelligence bias-voltage of resistance 35 is operative upon control-grid 1. Control voltages for controlling maximum strength of the amplitude-demodulated output voltages are then only applied to the control-grids of the amplifier 68, the switch-blade S4 being permanently closed upon the switch-terminal connected with terminal 45. Maximum strength of output voltages. is then regulated both for freduencydemodulated voltages and amplitude-demodulated voltages. Still other combinations'will be'evident to anyone skilled in the art,ifor employing the transfer-filters selectively with the controlgrid 7, and the control grids of amplifier 68. Tuning of the resonance circuits of Figs. land 2,

' will be apparent from the consideration hereafter, of Fig. 4. p 1

In the operation of Fig. 2, anode l l is energized through switch-blade Seonly when recep-. tion of frequency-modulation is desired; when amplitude-modulation is selected the switchblade Se is disconnected from the positive'terminal of source 31- When frequency-modulae tion is selected for reception, amplitude-.modu lation compensating voltages are impressed upon control-grid 7 from the secondaryi'afib, The equivalent impedance of the primary circuit) 5 and 5, is substantially pure resistance for a range of frequencies including the operating range of frequency-modulation, as will be evident from consideration of Fig. 4, and the alternating current conductance of tube It] between cathode l4 and anode H, is shunted across coils 5 and thy way of the circuit 6a77-l l l4--34-36--M,

,the reactances of. condensers 11, 34, and ifybeing negligible in relation to the thermionic resistance H--I4. In this circuit the fixed negative biasvoltage determining the normal mutual-conductance of tube 10 is supplied from resistance 35, and the order of connection of the secondary 25b is such that when the currents through resistance 2 are increased by an increase in carrier current amplitudes a positive bias-voltage is introduced upon the control-grid '1, increasing the con ductance between cathode l4 and anode II, and reducing the carrier current through the primaries 5 and 6. For the reception of amplitudemodulation the switch-blade Si is deflected to the downward position, connecting the terminal 45 with the resistance 42, and providing control of maximum demodulated output voltages. In the upward position of switch-blade'si, the terminal 45 is connected to the ground 33.

In the operation of Fig. 3, for reception of frequencyemodulation terminal F1 connects with switch-blade S1, and terminal M connects with switch-blade S34. Compensation of amplitudemodulation is provided by the bias-voltages introduced upon the control-grid I, and upon the control-grids of amplifier 68 by the secondary 23b of audio-frequency transformer F. The order of secondary connection is such as to increase the amount of negative bias-voltage upon the control-grid I when the current through controlresistance 2 increases through increase of amplitude of carrier currents in the primaries 5 and 6. For reception of amplitude-modulation, the switch-blade S1 is connected with terminal A1; and the audio-frequency voltages superposed upon the rectified voltages on resistance 23 are taken ofi: from resistance I3, one terminal of which is connected to ground 36. Switch-blade S34 is connected with terminal V, and steady negative bias-voltages are taken off from resistance 14 for control of maximum amp1itude-demodu lated output voltages, in relation to the amplitudes of applied carrier currents.

During reception of frequency-modulation, the difierential rectified voltages between cathode Ma and cathode Mb are taken off from the resistance 52, one terminal of which is grounded at 36.

It will be noted that only when the frequencyv of the carrier currents coincides with the centrefrequency of the resonance circuits, that is, with a frequency mid-way between the individual resonant frequencies of the resonance circuits, will the voltage between the cathodes Ma and Mb be zero for the balanced diode circuits. When the carrier current frequency does not coincide with the centre-frequency of the resonance circuits, then a constant diiference of potential exists between the cathodes Ma, and 13b. If this carrier current is frequency-modulated at audiofrequencies, then in addition to the constant difference of potential which is an index of the deviation between the carrier frequency and the centre-frequency of the resonance circuits, there is present an audio-frequency component of voltage. When such a demodulator is intended only for amplitude-demodulated voltages, the frequency-deviation between the carrier frequency and the centre-frequency of the resonance circuits, due tomistuning, is not subject to rapid changes, and the resultant substantially constant difference of potential between the cathodes 14a and Nb can be directly applied to the conductor 15 and terminal 6 I, to automatically correct the mistuning. If directly applied however, when frequency-demodulated voltages from a frequencymodulated carrier are present between cathodes means Ma and Mb, undesired frequenoy-modul tlofi at audio-frequencies would be introduced into the receiver. I

An important feature of Fig. 3, isthe means for providing steady bias-voltages for automatic correction of small amounts of mistuning, in a receiver for frequency-modulated carrier "currents. Condenser E9 in series with resistance 60 is shunted across the cathodes 14a and l4b'; 'v'ari-' ation voltages between the junction 60 of can denser 49 and resistance 50, and the ground 36, are therefore of opposite phase, in the circuit 3i50-22-23--36, so that only the stea y 1' sultant voltage is applied to conductor "t to modify the generated frequency tobring' the mrier current centre-frequency into "coincidence with the centre-frequency of the resonance 'cir cuits. It will be evident that provision of a capacity differential between the tuning condensers l1 and 19, for example by a spacer-condenser It, can be made either upon resonance circuit 3-l'! as illustrated, or upon resonance circuit 4-19, thus determining the polarity of the oathode Ma relative to ground 36, and relativewto positive or negative deviations of carrier frequency from the centre-frequency of the resonance circuits, for purposes of compensating these frequency-deviations.

Referring to Fig. 4, graphs are illustrated, the horizontal axis or abscissas indicating frequencies. At I1 is shown a resonance curve for rectified currents in the diode circuit through the resonance circuit 3-H, through resistance 22 and resistance 2; at I2 is shown a'resonance curve for rectified currents in diode circuit through the-resonance circuit 4-49, through resistance '23 and resistance 2. The above applies also to'Fig. 3, resistances 22' and 23 of Fig. 3 replacing 22 a'nd 23, respectively. At i1 is indicated'the' resonant frequency for resonance circuit 3 -11, and at f: is indicated the resonant frequency for the resonance circuit 4-!9. Midway between {i and f: is the centre-frequency .fc of the resonance circuits. In Fig. 4 the irequency deviations are indicated by Afo and Afo respectively, for the dif ferences f'c-f2, and fc-fi, sufficient tode-ph'ase the alternating currents in circuits 4-H and 3-48 by 45 degrees. At p is indicated the intersection of the resonance curves I1 and I2. TAt

I1 and I2 are indicated respectively tangent lines to the curves I1 and I2, through the point p these lines provide a substantial coincidencewith the respective curves I1 and I2, and for a range oi frequency-deviations A) and -AI fromthe centre frequency somewhat less than one half of Aft. It will be noted that Within this range, an increase of current I1 is accompanied by an equal decrease of current I2 as the frequency is varied, and that the sum of the currents I1+Iz is constant, and independent of the frequency modulation In order to aid in a better understanding of the principles of operation of the control-impedance 2-2l with reference to tuning, andas an index of amplitudes of carrier currents, and the principles of operation of the dynamic balance means for annulling. undesired amplitude-modulation during reception of frequency-modulation,

the following quantitative relations are pointed out in a simplified form, it being understood that these indicated relationships are not to be construed as any manner of limitatiton upon-the structures disclosed, or otherwise than to facilitate a better understanding'oi the devices. f

The curves I1 and I2 may also be taken to indi cate the respective alternating'voltages acrom the resonance circuits 3-41 and 4-l9. If these respective voltages are indicated by Ea and Eb, and the amplitude of the carrier currents in the primary coils and S is indicated by Ip, then the voltages EB. and Eb within the above-stated range of frequency-deviations are:

spend-KAI) 1) Eb=BI (l+KAJ) (2) from well known circuit relations for the coupled resonance circuits, the constants B and K are readily shown to be related to the circuit factors as follows:

B=\/21r'fcMQ (3) and wherein Q is the reactance-resistance ratio of the coil 3 or coil 4 of the resonance circuits, and M is the mutual inductance between the primary 5 and coil 4, or the primary 6 and coil 3. In terms of the rectified currents, the foregoing voltages can be indicated with reference to the corresponding resistances of the diode circuits. Designating by Re the resistance of the control-resistance 2, and by R the resistance of 22 or 23 Fig. 1 or 2, and of 22 or 23, Fig. 3, then:

Ea=RoIo+RI1 (5) and Eb=RoIo+RI2 ('6) Thus wherein the total current through control-resistance 2, is Io=I1+I2. The total rectified current through resistance 2, in terms of the primary current, orcarrier current Ip, is therefore indicated by: i a

The control voltage is Role, and the dependence of this control voltage upon'carrier current amplitude, and independence of frequency-modulation is indicated.

The frequency-demodulated voltages available are indicated by:

' Eb'- EG=2 KI,,A; and from (5), (6),

EbEa= R(I2-I1) (9) In Fig. 4, the maximum rectified currents are indicated by Ilm and I2m respectively, for the currents I1 and I2. The points q and s, are corresponding points upon the curves for is and I1 re- Ea= EO PElZ and Eb=Ec-{-E2' and the voltage difference Eb-Ea,=E2'E1' Referring to Fig. 1, utilization of the negative bias voltageestablished betweenthe cathode terminal l6 and a point upon the controlresistance 2-, for tuning the carrier frequency to coincidence with the centre-frequency of the resonance circuits, is shown, with reference to Fig. 4. This negative bias-voltage is a maximum for the coincidence of the carrier frequency, or centre-frequency of frequency-modulated carrier currents, with the centre-frequency of the resonance ci cuits, as indicated by fc. As the system is tuned by varying the mechanically connected condensers, the intermediate position between the two positions for which a falling off of negative biasvoltage is evident, is the position corresponding to exact tuning. It will be understood that the various selector switches described herein, can be readily connectedmechanically, so that but a single switching operation is needed to switch from a position for reception of amplitude-modulation, to a position for reception of frequencymodulation. Since this type of receiver must provide switching means for selecting either amplitude-modulation or frequency-modulation, it will be evident that an intermediate position of the switching means may be employed for a tuning position, wherein th audio-frequency amplifier connections G1, G2 are open, and whereby in-,

termediate contacts are provided to properly ground to 33 the control-grid terminal 44 and control-grid terminal 45 of amplifier 68, eliminating for example, the connection of these controlgrids with resistance 42, and the bias controlvoltage upon resistance 42.

Referring to Fig. 5, a mutual-conductance characteristic is indicated by gm, with reference to negative bias-voltages along the axis of'abscissas, for a super-control type of tube such as may be employed for-the tube ID, or Illa. At ml) is indicated a normal value of the mutualconductance, established for example by a. biasvoltage Ec, provided by flow of current through resistance 35'. At Agm is indicated a positive increment of the mutual-conductance established for example by a decrease of current through control-resistance 2, and similarly at Agm is a negative increment established by an increase of current through resistance 2. The principles of ,the dynamic balance employed for compensation of undesired amplitude-modulation,i. e. for substantially annulling amplitude-modulation upon the demodulators, are hereafter stated in simplified form.

The impedance of tube ID, or Illa, is usually large in comparison with the load impedance of primaries 5 and 6, so that the amplitude of the carrier currents Ip through the primaries is normally indicated by the relation: I1z=gm0Eg in which Eg is the amplitude of the carrier voltage impressed upon a control-grid, for example control-grid 1. Without any compensation for amplitude-modulation, the mutual-conductance is constant and gm=gm0. Designating the uncompensated carrier current through primaries 5 and 6, as I o, and the uncompensated carrier voltage upon the control-grid as EgU, then:

Howeve'n with. compensation by mutual-cone From'(13 and (14) the reduction in amplitudemodulation index as a consequence the structures heretofore disclosed, is derived. Thus: The resultant compensated carrier current increment is F WD IaaeAas Equation 15 gives the change of carrier amplitude remaining after compensation, in relation to'the original uncompensated change of carrie amplitude; defining the compensated modulationindex as r then from Equation 15 Equation 16 illustrates the effectiveness, in reducing the undesired amplitude-modulation, of the structures herein disclosed. It will be evident, that when the mutual-conductance or" several initial'stages of amplification is also controlled, the

resultant eifective change of mutual-conductance with change 03f unidirectional current through the control-impedance 2 is greatly increased, that is, a large value of D is provided. As an illustration of the'operation with reference to Equation 16, ii suificient negative bias-voltage is produced upon the control-resistance 2 with a positive increment of carrier current corresponding to percent amplitude-modulation, to make a change of effective' mutual-conductance substantially equal to ymo, then and o 1+1O(1|m under which conditions an amplitude modulation of 70 percent would be reduced by the compensation to about 4 percent. For a negative increment of carrier current, values of the coefficient B may be three or four times larger than the above illustrative value. It will be noted that the change of carrier current amplitude, indicated by AIp, only provides a modification of mutual-conductance when the rate of change of Al is suiliciently rapid to come within the audio-frequency range, that is, when the changes in the envelope of the carrier frequency currents are at an audio-frequency.

Reference for example to Equation 8, for the sensitivity of the frequency-demodulator for providing demodulated voltages in relation to the.

amount of frequency deviations, shows that the primary current Ip is a factor. Thus changes of Ip cross-modulate the frequency-demodulated voltages, however, it is evident that only such changes as occur at 'a sufiiciently rapid rate would be objectionable. Gradual change of Ip, merely have the effect of increasing or decreasing the strength of the demodulated output voltages, and the acoustic output. This is an important feature of this invention, eliminating the necessity of continual readjustment of a balance, as required in compensating systems fo amplitude-modulation wherein a static balance is determined by the absolute magnitude of the carrier currentshot permitting any change without readjustment.

With reference to Figs. 1 and 2, it will be evident that an alternative transformer coupling can be used from the demodulator to the audio-frequency amplifier; with this connection the connections of F1, F2, and ground 36 to resistances 52, 52', are omitted, the primary of the coupling transformer being shunted across the resistances 52 and 52 in series, one terminal of the secondary being connected to F1, the other terminal of the secondary being connected to terminal F2, and a centre-tap terminal of the secondary being connected to the ground 36.

Having described several illustrative embodiments of my invention, it will be evident that changes can be made in the form and arrangement of parts, and by substitution in part of other well known structures, or otherwise, without departing from the spiritof my invention, as set forth in the appended claims, and I do not there fore limit the scope of the invention to such particular embodiments, o otherwise than by the terms of the appended claims.

What is claimed is: r V

1. In a frequency demodulator for demodulating frequency-modulated carrier currents, reson-ant circuit means for determining alternating voltages responsive to frequencies of said carrier currents, and responsive to amplitudes of said carrier currents, a first voltage-rectifying means including a first thermionic diode means serially connected with a first resistance, a second volt-.- age-rectiiying means including a second thermionic diode means serially connected with a second resistance equal to said first resistance, a connection from a first terminal upon said resonant circuit means to said first rectifying means, a

connection from a second terminal upon said resonant circuit means to said second rectifyin means, a common connection between said first and second rectifying means, a connection from a third terminal upon said resonant circuit means to said common connection including a controlimpedance carrying the sum total thermionic currents of both diode means; an output-impedance for frequency-demodulated voltages connected to said first and second resistances, thermionic amplifier means including control-grid means energizing said resonant circuit means with amplified carrier currents, and transfer-filter means coupling said control-impedance with said controlgrid means, transferring compensating voltages to said control-grid means, substantially annulling amplitude-modulation of said carrier 'currents at audio-frequencies, and permitting carrier current amplitude variations at lowersubaudio-frequencies. I i 3 7 I 2. In combination with the structure of claim 1, a second transfer-filter means including a filter-condenser and filter-resistance, selectively connectable with said control-grid means, selec.. tively applying negative bias-voltage to said control-grid means proportional to an average amplitude of said carrier currents, independent of said audio-frequencies, for regulating amplitudes of demodulated voltages upon said demodulator.

3; In combination with the structure of claim 1, filter means including a serially connected filter-condenser and filter-resistance connected to terminals of said first and second resistances, and conductive circuit connections through said filter-resistance and said first and second resistances, providing steady bias-voltage independent of audio-frequency variations of said frequencydemodulated voltages, and proportional to frequency deviation of the centre-frequency of said carrier currents from the centre-frequency of said resonant circuit means, for modifying the centrefrequency of said carrier currents to coincide with said centre-frequency of said resonant circuit means.

4. In a frequency-demodulator device for demodulating frequency-modulated carrier currents, resonant circuit means determining alternating voltages responsive to frequencies of said currents, and responsive to amplitudes of said currents, voltage-rectifying means connected with said resonant circuit means, including a thermionic double-diode with a first anode, a second anode, and a common cathode, and a conductive impedance connected between said resonant circuit means and said common cathode, carrying the sum total thermionic current of both said anodes, said conductive impedance including a resistance shunted by a condenser; an output-impedance for frequency-demodulated voltages connected to said anodes, thermionic amplifier means energizing said resonant circuit means with amplified carrier currents, including said common cathode and a control-grid means, and a transfer-filter means coupling said resistance with said control-grid means, transferring compensating voltages to said control-grid means, substantiall annulling amplitude-modulation of said carrier currents at audio frequencies, and permitting amplitude variations of said carrier currents at lower sub-audio frequencies. 5.. In combination with the structure of claim 4, a second filter means coupled with said voltage-rectifying means, providing steady negative bias-voltage proportional to average amplitude of said carrier currents, independent of said audio frequency amplitude-modulation, and selector means for selectively transferring to said controlgrid means said compensating voltages, or said steady negative bias-voltage.

6. In a receiver for frequency-modulated and amplitude-modulated carrier waves, a frequencydemodulator device for demodulating frequencymodulated carrier currents, having a first resonance circuit, a second resonance circuit resonant at a frequency interval from the resonant frequency of said first resonance circuit, a first primary coil coupled with said first resonance circuit, a second primary coil coupled with said second resonance circuit, a first voltage-rectifying means including a first thermionic diode means serially connected with a first resistance, a second voltage-rectifying means including a second thermionic diode means serially connected with a second resistance equal to said first resistance, a common connection between said first and second rectifying means, a connection from said first resonance circuit to said first rectifying means, a connection from said second resonance circuit to said second rectifying means, a common terminal upon said first and second resonance circuits, and a control-impedance connected from said common terminal to said common connection, carrying the sum total of thermionic currents of both said diode means; an output-impedance for frequency-demodulated voltages connected to said first and second resistance, thermionic amplifier means coupled with said first and second primary coils, including grid control means, controlling the amplitude of carrier currents through said primary coils, and transfer-filter means coupling said grid control means with said controlimpedance, transferring compensating voltages to said grid control means, substantially annulling amplitude-modulation of said carrier currents at audio frequencies, and permitting carrier current variations at lower sub-audio frequencies.

'7. In combination with the structure of claim 6, a second transfer-filter means including a filter-condenser and a filter-resistance, for transferring to said grid control means a steady nega tive bias-voltage, proportional to an average amplitude of said carrier currents, independent of audio frequency modulation of said carrier currents, and selector means for selecting frequencydemodulated or amplitude-demodulated voltages from said demodulator, including a. switching means for interrupting said coupling of said transfer-filter means with said grid control means.

8. In combination with the structure of claim 6, said receiver having a mixer-device coupled with said grid control means, for reducing the carrier frequency of said carrier waves, including a thermionic alternating current generator with bias-voltage control means for controlling.

the frequency of said generator, said demodulator device having a second transfer-filter means comprising a filter-resistance and a filtercondenser connected in series and connected between a terminal of said first resistance and a terminal of said second resistance, providing a conductive path comprising said filter-resistance,

said first resistance, and said second resistance in series, and the circuit connections of said conductive path in series with said bias-voltage control means, whereby a steady bias-voltage is provided in said conductive path for modifying said reduced carrier frequency to coincidence with a frequency midway within said frequency interval, independent of frequency-modulation of said carrier currents at audio frequencies.

9. In a frequency-demodulator for demodulating frequency-modulated carrier currents, a first, resonance circuit, a second resonance circuit resonant at a frequency-interval from the resonant frequency of said first resonance circuit, coupling means coupled with said first and second resonance circuits for coupling said circuits with said carrier currents, a first voltage-rectifying means including a first thermionicv diode means serially connected with a first resistance, a second voltage-rectifying means including a second thermionic diode means serially connected with a second resistance equal to said first resistmice, a first common terminal upon said first resonance circuit and said second resonance circuit, a connection from said first resonance circult to said first rectifying means, a connection from said second resonance circuit to said second rectifying means, a second common terminal upon said first and-second rectifying means, and

a control-impedance connected between saidfirst and second common terminals, carrying the sum total of thermionic currents of both said diode means, including a control-resistance shunted by a control-capacitance, providing a large time-constant in relation to the half -cycle time of said carrier currents, whereby unidirectional control-voltages are provided upon said control-resistance, proportional to amplitudes of saidcarrier currents, and independent of frequency-deviations over a range of positive and negative frequency-deviations from a carrier ire.- que'ncy. midway within said frequency-interval.

l; Incombination with the structure of claim 9', the tuning means for simultaneously tuning said first and second resonance circuits for res.- onant-irequencies differing respectively from the unmodulated carrier frequency of saidcarrier currents by equal predetermined frequency-intervals, respectively above and below said unmodulated carrier frequency, said tuning: means including the connection; of av volta-gerindicating device to said control-impedance, and the me-' chanical.interconnection of the tuning elements of each of said. resonance circuits.

1:11. In combination with the structure of claim 9, the rmionic amplifier means including grid controllimearisand said coupling means, and a second coupling j means coupling said control-impedance wit saidgrid control means, transferring coming voltages to said grid control means, substantially annulling amplitude-modulation. of said carrier currents.

12, In combinationwith the structure of claim 9,1thermionic amplifier means including grid contrIol means and said coupling means, and a'transfen-filter means couplingsaid control-impedance with saidgrid. control means, transferring steady negativ'Ieb'iasevoltage-to said grid control means, independent of amplitude-modulationofsaid er currents, and proportional to an average 1 V 3 current amplitude, for regulating the magnitude of, demodulated voltages uponsaid demodulator.

Theco'rnpensating device for compensating unde d amplitude-modulation of carrier cur- "frequency-demodulator for converting: frequent: -mcdulated carrier currents into amplitude-modulated output-voltages, said demodulatorhavingf resonant circuit means responsive to frequency modulatio'n and amplitude-modulation of iI D'ressed' carrier currents, voltage-rectifying means; including a thermionic diode and connected resistance, providing unidirectional voltages'upoii' saidiresistance directly proportional to amplitudes of said' carrier currents and independent of frequency-modulation of said carrier currentsand thermionic amplifier means coupled Witll $flid resonant circuit means, including a thennionic'tube with gridimeans for controlling carrier; currents impressed upon said resonant circuitmeans sai'dco'mpensatin device having a transfer-filter coupled with said resistance and with said? grid means, transferring compensating-voltagestosaid gridjmeans substantially annullingamplitude-modulation of said carrier currents only' when said amplitude-modulation is within an audio frequency range, and permitting variationsof amplitude of said carrier currents whensaid variations are within a sub-audio frequen'cy' rangei '14: In combination with the structure of claim 13", a sec'ond'transfer-filter coupled with said .rectify'irig'means, and'selectcr means for selectively,

coupling said transfer-filters; with said} grid means, said: second transfer-filter including afilter-condenser and a filter-resistance, deters mining steady negative bias-voltagesa'0I'bSS:S-2Lld 1 'ing amountsof irequency-deviationfrom a-vcentrex frequency of said reduced carrier'waveinto dk rectly proportional amounts of resultant output? voltages, ofoppositepolarity for positivevf-res" quency-deviations in reference to; polarity for negative frequency deviations, including? an: out:-" put-resistance shunted by" capacitance: for said? resultant output-voltages andresonant circuit means tunable in relation to said centreyfree' quency, a transfer-filtermeanscomprisin'ga filter-resistance and arfilter-condenser connected in series and shunted across said output res-ists ance, providing aconductive paththrough: said filter-resistance and, said: output-resistance? in? series, and. the-circuit connections ofsaid? con; ductive path in series with-said?bias-voltagemom trol means, whereby stead-y;v compensating? voltage is provided upon said bias-voltage control; means, independent of audio-frequency; ampli=.-=-

' tude variations of said outputevoltage;g to estabs lish a coincidence of tuning ofsaid:resona-ntcin-'- cuitmeans'w-ith said= centre frequency:

16L The combinationwitm a 'frequency-demod ulator: for demodulating frequency modulateds carrier waves, of anamplitude-envelope:stabiliz ing device-for,substantiallyremoving amplitudes envelope modifications with rates or change? inz: an; audio-frequency range; ,saidv demodulator: hav? inga firstoutputsimpedance' for? frequencysdes modulatedoutput-voltages rfrom constantamplie tude= frequency modulated-carrier waves; anth means--for energizing said-first dutputeimpedance: from;s-aidcarrierwavesincluding; athermionid amplifier having,- gridcontrolsmea'risciorrcontrols-' ling, saidcarrier Waves; saidw'device:includingrai secon d-i output-impedance for amplit-ud'esdemodu lated.output voltagesw energized: from: said thermionicamplifier; and transfer-filter means? cons nected tosaid. second output-impedance? and to?- saidgrid control, means;j including. impedance; means capable of transferring;- voltages sufficient: for modifying; the;transconductancewofsaid am."- plifier controlled-bysaidgrid control means;. onlyz when said rates of changei oftheicarrier amplis tude-envelope areabovea sub-audio threshold range, to suppress the; amplitudeemodulatiom stimulus applied to: said second I outputeimpedi ance. I

17. a The; combination withv a demodulator for demodulating frequency-modulated. and am'plrtude-modulated carrier waves; of an amplitude; modulation.- reducing device for; substantiallyianinulling amplitude-modulation; upon iawfrequencyi-f modulated carrier wave; said demodulatorshavingz a first output-impedance-for frequenoy+demodue;

device having a transfer-filter connected with said second output-impedance and connected to said grid control means, including impedance means for transferring to said grid control means amplitude-modulation annulling voltages only when the frequency of said amplitude-modulation voltages is above a sub-audio threshold range of frequency.

18. A frequency-demodulator having a thermionic tube with a cathode, an anode, grid control means for controlling thermionic currents between said anode and cathode, a first diode-anode with said cathode and a second diode-anode with said cathode, an input-impedance connected between said grid control means and said cathode, a three-branch network having a common junction, a first branch including a first resonance circuit connected in series with a first impedance between one of said diode-anodes and said junction, a second branch including a second resonance circuit detuned relative to said first resonance circuit connected in series with a second impedance between the other of said diode-anodes and said junction, and a third branch including a control-impedance connected between said cathode and said junction; coupling means coupling said anode with said resonance circuits and with a source of unidirectional anode-supply voltage,

and circuit connections for transferring frequency-demodulated voltages from said network including a terminal upon each of said impedances of said first and second branches.

EDWARD H. LANGE.

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