Jan. 7, 1947. W. R. KOCH 2,413,977 ANGLE-MODULATION WAVE RECEIVER F1198 Nov, 18, 1944 Am 2 Sheets-Sheet 1 INVENTOR ATTORNEY Jan. 7, 1947. 'w_ R, KQCH- \ 2,413,977 ANGLE-MODULATION WAVE RECEIVER ~ Filed Nov. 18, 1944 , 2 Sheets-Sheet 2 ' Tlclju. ?v'pa/vwt > Kano/vi: 4r @ya 2'2 I 712,4. BY ATTO'RN EY Patented Jan. 7, 1947 attain UNITED STATES-PATENT OFFICE ANGLE-MODULATION WAVE RECEIVER l/Vin?eld H. Koch, Haddon?eld, N. J., assignor to Radio Corporation of America, a- corporation of‘ Delaware Application November 18, 1944, Serial No. 564,047 12 Claims. (Cl. -250—-27) 1 2 My present invention relates to receivers of frequency modulated (FM) or amplitude modu lated (AM) carrier waves, and more particularly to radio receivers utilizing novel FM detector cir cuits. 50 me. band ‘is presented by way of illustration, since it is the FM broadcast band presently as signed to such transmission. The 550 to 1700 kc. band is the present AM‘ broadcast band assigned to transmission of AM signals. ' An important object of my present invention is It will further be understood that in the present to provide a novel detector of angle modulated description and ‘following claims the generic ex carrier waves wherein each of a pair of recti?er pression “angle modulated” is intended to include tubes concurrently functions as a load impedance frequency modulation or phase modulation. for its companion tube. 10 Froma very general viewpoint my invention re Another important object of my invention is lates to a demodulator network having separate to provide an FM-AM detection network wherein input circuits for carrier waves of different fre no actual detector output load resistors are em quencies and of different ‘modulation character ployed, but instead the detector tubes per se are istics. employed for that purpose. The numerals I and 2 in Fig. 1 denote respec Another object of my invention is to provide tively different sources of modulated‘ carrier a switchless FM-AM detector network wherein waves. Source I may be the usual signal ‘col triode detection is utilized, and each triode serves l'ector, such as a dipole, employed‘for collecting as the output load element for another triode. ‘FM Waves. The FM carrier waves are transmit Still other objects of my present invention are 20 ted from FM transmitters at a center, or carrier, to improve generally the ef?ciency of FM-AM re frequency assigned to the particular transmitter. ceivers, and more especially to provide economical In the assumed FM band of 40*to 50'mc. the radi detector circuits for FM receivers. ated. carrier wave frequency would be in that Still other objects of my invention will best be range, and would be ‘a wave of variable frequency understood by reference to the following descrip tion, taken in connection with the drawings, in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect. In the drawings: Fig. 1 shows, in partial schematic form, an FM~ AM receiver employing one embodiment of my in vention; Fig. 2 shows a modi?cation ‘of the detector net work of Fig. 1; Fig. 3 illustrates the FMv detection characteris tic of the detector circuit of Fig. 2; and and substantially uniform amplitude. As is Well known, the frequency modulation of the carrier wave would be in accordance with the modulation signals at the transmitter. The extent of fre quency‘deviation of the carrier frequency is a function of the modulation signal amplitude, while the ‘rate of frequency deviation is dependent upon the modulation signal frequencies per se. The permissible extreme frequency deviation in the FM band of 40 to 50 Inc. is '75 kc. to either as.CT side of the carrier frequency. These frequency values are purely illustrative. Source 2 may be a suitable grounded antenna Fig. 4 shows a further modi?cation of the FM circuit employed in AM broadcast reception. detector circuit. The allotted channels ‘are 10 kc. wide in this band. Referring now to the accompanying drawings, an In AM transmission the ‘carrier wave is modulated wherein, like reference characters in the different in amplitude in accordance with the modulation ?gures designate similar circuit elements, Fig. 1 signals. The carrier frequency is maintained shows an illustrative receiving system embodying constant in value at the transmitter. The nu a demodulator network adapted to provide audio meral 3 designates a tunable radio frequency am frequency voltage in response to FM or AM signal plifier having ‘suitable signal selector circuits for reception. The receiver circuits prior to the de FM and AM reception. Switching devices 4 and modulator are schematically represented. Those it are provided for separate connection of the skilled in the art of radio reception are well ac sources I and 2 to respective selector circuits of quainted with the nature of the circuits cus ampli?er 3. It will be understood that when tomarily employed in multi-band receivers. switch 4 is in closed position, collected FM signal While my ‘invention is readily adapted for FM energy will be applied to selector circuits of am and AM reception on respective bands of 40 to 50 pli?er 3 capable of selectively amplifying the FM megacycles (me) and 550 to 1700 kilocycles (kc), signals over a‘ band at least 150 k'c.'wide. Upon it is to be clearly understood that the invention is closing of switch '5,_and opening switch 4, the not limited to such frequency bands.v The 40 to 155 same ampli?er 3 will have the FM selector cir 2,413,977 3 4 cuits thereof operatively replaced by AM selector nized that circuits 8, I2 and I5 provide the well known frequency discriminator network of U. S. Patent No. 2,057,640 to Conrad. The action of circuits. These latter circuits will select the col lected AM signals, and permit ampli?er 3 to am— plify the same over a 10 kc. band. Multi-band selector circuits and switching devices for suit able change-over are well known to those skilled in the art of radio communication. this form of discriminator circuit is well known It functions to trans to those skilled in the art. late FM high frequency signals into correspond ing AM high frequency signals. At the 4.3 mc. frequency used for FM recep Assuming the system is of the superheterodyne tion the impedance of circuit I5, I8 is negligible. type, as is the usual practice at present, the con verter 6 and intermediate frequency (I. F.) am 10 Therefore, the low potential side of circuit I5 is pli?er ‘I will, also, be provided with suitable FM and AM signal selector circuits. At the converter 6 the FM signals will have the center frequency effectively at the cathode potential of detector triode II. The detector triodes are shown ar ' ranged in series relation so far as the space cur rent paths thereof are concerned. The plate 20 thereof reduced to a value which may be chosen from a range of 1 to 20 mc., as for example 4.3 15 of triode I0 is connected to the +13 terminal (say, for example, +200 volts) of a suitable direct cur mc. The AM signals are reduced to an I. F. of rent supply source. The cathode 2I is connected 455 kc., as an illustrative frequency value, the directly to the plate 22' of triode I I. The low latter being a commonly employed frequency in potential side of input circuit I2 is connected to AM broadcast receivers of the superheterodyne type. The I. F. ampli?er ‘I, which may consist of 20 cathode ‘2L The control grid 22 of trio-dc I0 is connected by direct current blocking condenser one or more separate stages of ampli?cation, will 23 to the high alternating potential side of input have an ultimate output circuit from which may circuit I2. Resistor 24 provides a direct current be derived, at separate points thereof, the ampli return path for grid 22. Similarly in the case ?ed FM signals or AM signals. The selective circuits 8 and 9 are to be under 25 of detector triode II, the resistor 25 returns grid I3 to the grounded cathode I9. stood as being arranged in series in the plate cir It will be seen that a unique feature of my cuit of the last I. F. ampli?er tube. Each of cir present circuit is the complete absence of output cuits 8 and 9 is resonated to its respective op or load resistors external of tubes I0 and I I. The erating I. F. value for PM or AM reception. Thus, circuit 8 is tuned to 4.3 mc., while circuit 30 internal cathode to plate impedance of each of triodes I0 and II is utilized as the load imped 9 is tuned to 455 kc. There will be developed ance for the companion triode. The internal re across tuned circuit 8 the FM signals at the 4.3 sistive impedance of triodes I0 and II are indi mc. mean frequency when switch 4 is closed. and cated by numerals 26 and 21 respectively. The all FM selector circuits of ampli?er 3, converter 6 and I, F. ampli?er ‘I are in operative electrical ' output resistive impedances 26 and 21 are indi cated by dotted lines to show that they are the connection. Conversely, when switch 5 is closed, internal resistive impedances of the triodes I0 and switch 4 is open, and all AM selector circuits and I I. The junction of resistive impedances 26 are in operative electrical connection, there will and 21 is connected to the audio frequency cou be developed across circuit 9 the AM signals at the I. F. value of 455 kc. The impedance of cir 40 pling condenser 28, the carrier bypass condenser 29 connecting the junction point to ground. cuit 9 is negligible at 4.3 mc. Hence, the inser Leads 30 function to feed the modulation signals tion of circuit 9 in series with circuit 8 will not to a subsequent suitable audio output circuit. affect the’ development‘ of FM signal voltage The detectors may function as grid leak de across circuit 8. Similarly, the impedance of cir cuit 8 is negligible at 455 kc., and circuit 8 will 45 tectors, or grid current-biased plate detectors. Thus, if the condensers 23 and III were each as not affect development of AM signal voltage signed a value of 50 micromicrofarads (mmf.) across circuit 9. ’ and each of resistors 24 and 25 a value of 200,000 The present demodulator comprises two elec ohms, the detectors would perform as grid recti tron discharge devices of the triode type. The triode I0 is provided with a resonant input cir 50 ?cation devices. By assigning a value of 0.005 mmf. to each of condensers 23 and I4 and a cuit I2 which is inductively coupled to the cir value of 10 megohms to each of resistors 24 and cuit 8. vTriode II has its cathode I9 established 25, the detectors perform as grid current-biased, at ground potential, while its grid I 3 is connected plate recti?cation. devices. ' through condenser I4 to the high alternating po During AM signal reception the I. F, signal en tential side of its resonant input circuit I5. Cir 55 ergy produced in the circuit 9 will be transferred cuit I5 is, also, inductively coupled to the circuit to input circuit I6, I8. Each of circuits 9 and 8. The low potential side of circuit I5 is con I6, I8 is tuned to the operating I. F. value of 455 nected to the grounded cathode I9 through the kc. The circuit I5 and the grid to cathode path coil I6. Coil I6 is magnetically coupled to cir cuit 9, and condenser I8 shunts coil I6 to pro 60 of triode I I are in a series circuit with tuned cir cuit I6, I8. The circuit I5, resonant close to vide a resonant circuit IB-I8 tuned to 455 kc. 4.3 mc., has no appreciable effect on the series The input circuits I2 and I5 of respective tri circuit, since it acts as an extremely low imped odes I0 and I I are oppositely and equally mis ance connection at the 455 kc. frequency value. tuned with respect to the operating I. F. value for FM reception. In other words, if the circuit 8 65 The modulation (AM) signal voltage component of the recti?ed I. F. energy developed across load has an operating frequency FL: of 4.3 mc., then resistive impedance 26 during grid leak detection 'circuits I2 and I5 will be detuned in opposite at tube II is applied through condenser 28 and senses by equal predetermined frequency values leads 30 to the common modulation signal out relative to Fe. Thus, circuit I2 is indicated as tuned to a frequency in excess of Fe, and circuit 70 put circuit. Hence, it will be seen that during AM signal reception the tube II functions as the I5 is represented as being tuned to a frequency detector tube for the AM signals at input circuit less than Fe. For ‘example, circuit I2 could be I6, I8. The output resistor in that case is the tuned to 100 kc., more than 4.3 mc. (4.4 mc.) internal cathode to plate resistance of tube I0. whilecircuit I5 could be tuned to 4.2 mc. These are only illustrative values. It will be recog 76 In other-words, the audio frequency voltage de 2413,07? 5 6 veloped across resistiveimpedancez?? is utilized through'the output leads 30. reception, andv upon grid‘ current ?owithrough resistor 25, AVG biasis produced. To‘ detect FM‘ signals the tuned circuits 8’ For FM‘signal reception, on the other hand,'in ternal ‘resistances 26' and 21' are alternately used and [2' are employed. ‘Resonant circuit 8' is connected ; in series with circuit 9 in the I. F. as the frequencyolf the I.‘ F; energy deviates relative to its mean or» center frequency value. amplifier output circuit, and is tuned approxi Assume, ?rst, thatthe FM signal‘is at4.3 me. mately to Fe (4.3 mc.). The resonant circuit I2’ In such case both circuits l2 and i5 apply equal is tuned somewhat above Fe. The control grid signal energy to the respective detector tubes. 22 of ‘triode I9 is coupled by condenser 23 to Assuming that tubes Ill‘ and H are of like con l0 the upper end of coiLSil. The latter is magneti struction, the'resistance values of'impedances 26 cally coupled to circuit 8’, and has its lower end and 2'! will be equal. Therefore, there will- be connected to the cathode 2| of triode Ill. The equal voltage drops across impedances '26‘ and resistor '24 connects grid 22 to cathode 2 l . Hence, 21, and theiipotential‘at junction‘ point a will during FM reception triode II] has applied there have a predetermined normal or mean: value. . to the signal energy across circuit 8’, while triode Suppose; now, that’ the applied FM'signal H has applied‘ to it the signal energy existing energy at: circuit ii? deviates ini‘frequency‘ to in the circuit including both circuits 8’ and [2’. wards the frequency of" circuit l2l (4.4 incl). Of course, circuit 9 has no effect at FM reception,’ “ The current flow through the tube It} would since its impedance is negligible at 4.3 mc. decrease, assuming the condenser 23 and resist; 2%) The aforesaid Reid patent fully discusses‘ the tor Hare chosen to provide grid leak‘detection. operation of an FM discriminator of the type There would occur a relatively larger voltage shown in Fig. 2. By virtue of the discriminat drop across resistiveimpedance 26. This fol— ing action of circuits 8' and [2' the triodes l0 lows from these considerations. When a‘ signal and l l will have amplitude modulated waves ap is applied to the grid 22, rectification makes the plied thereto which were derived from the re average grid potential more negative ‘relative to ceived FM signal waves. Resistive impedance cathode 2|. This tends to stop electrons from 2S acts as the load for triode H, and develops flowing to plate 28; Theelectrons start piling recti?ed voltage thereacross. Resistive imped~ up on the cathode 2! thereby ‘making it more neg ance 2i’ acts as the load for triode It}, and de ative relative to the‘ plate. Therefore, the'voltage 1‘ 1 velops recti?ed voltage thereacross. across the internal impedance 2B ‘of the tube is The FM signal voltage across circuit 8’ is ap greater. This would mean that‘a smaller voltage plied to grid 22 through coil 38. Triode grid l3 drop existed across resistive impedance 21. has applied to it the signal voltage across its Hence, the point a would‘ be relatively. less‘posi own input circuit. At the frequency where cir tive relative to ground. If; now, the FM‘ signal 3*‘ cuit l 2‘ is anti-resonant the voltage across cir cuit l2’ will be high, while that at grid l3 will frequency deviates towards the resonance fre be very low. At a somewhat lower frequency, quency of circuit Hi, the reverse‘ takes place. circuit IE’ will have an inductive reactance which That is, the point a becomes more positive .rel will series resonate with the capacity between ative to ground. The sense anddegree of poten tial change of point a will be dependent upon the 40 grid l3‘ and cathode l9 thereby making the volt age applied to the triode l i very high. It is thus direction. and amount respectively of'ifrequency apparent that the radio frequency voltage ap deviation of the FM. signaLenergy. Itwill be plied to triode l I changes very rapidly with fre noted that the audio output circuit is. single quencies between th'ese points. At the ?rst fre ended. Condenser. 29 will be chosen to bypass quency the voltage across circuit 8’ will be high all high frequency components, whether at .455; thereby causing the voltage applied to triode It) kc. orat 4.3‘ me. > . to be high. At the second frequency the’ series The detection networlsof Fig.1 mayv assume. resonant circuit formed by circuit l2’ and the in di?erent forms. Figs. 2 and 4, show. different put capacity of triode ll will load‘ down circuit modi?cations of the invention. InFig. 2 I_ have 8’ thereby causing the voltage applied to triode I!) to be low. Hence, the recti?ed voltage output shown a modi?cation of the invention wherein for FM signal reception thereis used a. discrimi» nator which is of the type. disclosed ‘and-claimed by John D. Reidin U. S. Patent. No. 2,341,240 of triode. H changesin a direction opposite to that of triode l0 as the instantaneous frequency granted February 8,. 1944. In this embodiment 55 of the applied FM signals varies. Correct cir cuit design produces the desired characteristic. of the invention the AM- circuit 9- providesv the In Fig. 3 there is illustrated the frequency re» input for. triode-detector H; The condenser. Ill sponse characteristics at grid l3 (solid line curve) and resistor 25 function to provide grid‘; leak and grid 22 (dotted. line curve) respectively. The detection for AM‘ reception. Here, again,_ the resistive impedance 26 acts as the: plate ‘load 60 cross-over frequency is at 4.3 me. In Fig. 4 there ‘is shown a further modi?cation for the grid‘ leak detector tube l I. It’ will be of my invention, wherein the AM input circuit noted that the high alternatingrpotential side of 9 is omitted and the‘I. F. ampli?er includes the input circuit’ 9, is'icoupled’ to grid; l3'by‘a; series F. i. circuit 8 tuned to 4.3 Inc. The latter is cou~ path consisting of .tuned‘f circuit'i8', condenser l4 pied magnetically to resonant circuit 3i tuned to and resonant circuit 12'. Since circuits 8! and 65 4.3 me. The plate side of circuit 3 is connected i2’ are tuned: in the:vicinity' 0f‘4;3. mo.‘ theyahave by lead 432 tothe midpoint ofcoil 33 of secondary negligible impedance at 455-kc; Hence;v during AMjreception the grid l3 is-in- eflfectdirectlyv ‘ ‘ connected to the high potential side‘ of circuit‘ 9. An automatic volume control '(AVC) connec-i tion 40 isr'nade‘ to the grid‘end of'resistor' 25v to provide AVC' bias for‘ prior-gain controlled tubes. Filter; networkrlll isemploycd'to suppress any. pulsating: components: of‘ thenAVC' voltage; " circuit 3?. Control grid22 is coupled byv direct current blocking, condenser to one side of cir~ » cuit‘iii, and control grid‘lli is coupled by direct 70 current blocking ‘condenser l 4 to the opposite side of circuit 3!‘. The frequency]v discriminator oi - Fig. 4 is of the typedisclosed and ‘claimed by S. ' W. Seeley in-U. S. Patent No. 2,121,103 granted As- the. carrier;- amplitude:- increases1 duringv AM. 75 June 21, 1938. , The discriminator: circuit of Fig. 4 functions 2,413,977 8 7 to provide at each of grids 22 and l 3 a signal volt age whose magnitude is the resultant of two sig nal voltages. The magnetic coupling between pri mary circuit ii and secondary circuit ill results in the application to grids 22 and I3 ofsignal voltages in polarity opposition and in phase quad connecting the plate to cathode impedances there of in series relation, a frequency discriminator input circuit, separate connections from said fre quency discriminator circuit to a respective con trol grid of each of said triodes, a modulation signal output circuit connected across one of said plate to cathode impedances, a Signal input cir cuit tuned. to a frequency sufficiently different age. The direct connection of circuit 8 to the from the resonant frequency of said discriminator midpoint of coil 33 causes signal voltages to be applied to grids 22 and i3 in like polarity and 10 circuit to have negligible impedance effect on the latter, and connections between the input elec in parallel. Hence, at signal frequency Fc each trodes of one of said triodes and said signal input grid has applied thereto the resultant vector volt circuit. age of a phase-shifted signal voltage and a non 4. In a frequency modulation receiver, a pair shifted voltage in normal phase quadrature. The of detector tubes each including at least a cath resultant vector voltages at grids 22 and i3 are ode, control grid and anode, means for establish equal at Fe. However, for signal frequencies ing the cathode of one of the tubes at ground po different from F0 (the operating I. F. value) the tential, means connecting the anode of said one resultant vectors become unequal, because the tube directly to the cathode of the second tube, shift due to the magnetic coupling between rature relative to the primary circuit signal volt circuits 8 and 3| changes. The respective re~ sultant vector voltages at grids 22 and f3 wili vary in sense and magnitude in response respec tively to the direction and extent of signal fre quency deviation. These resultant signal voltages will cause cor means for applying a positive potential to the anode of the second tube whereby the space cur rent paths of the two tubes are in series relation, modulation signal output connections connected‘ to the anode and cathode of one of said tubes, and a frequency discriminator network having sepa responding voltage drops across the internal re sistive impedances 26 and 21. The operation is quite similar to that described in connection with Fig. i. If desired, an AM input circuit could be added to cooperate with one of the detector tubes it or H. Of course, the triodes could be located rate signal input connections to the control grids of said pair of tubes. in a single tube envelope, in the manner of a source to said detector tube input and cathode electrodes, a second tube provided with an input, 5. In combination with a source of amplitude modulated carrier waves, a detector tube having input, cathode and plate electrodes, an input cir cuit tuned to the carrier frequency coupling said twin triode, if compact construction were desired. cathode and plate electrodes and having its inter Grid leak detection can be made to give only a small variation in audio frequency output volt- "P nal plate to cathode impedance in series relation with the plate to cathode impedance of said de age for a considerable range of input signal am tector tube, means connecting the plate of the plitudes. A reduction in noise and distortion second tube to a source of positive voltage, and is accomplished in the circuits described through this type of detection. Low mu tubes are recom modulation. signal output connections connected mended where there is available sufficient signal amplitude to provide AVC, AVC action could be secured in either of the systems of Fig, l or Fig. across the plate to cathode impedance of said de tector tube. 4, in which case low mu tubes are preferred. For rier waves, a pair of triodes Whose respective in ternal plate to cathode impedances are connect~ ed in series between a point of relatively high positive voltage and ground, means for varying 6. In a demodulator circuit for modulated car maximum sensitivity it is preferred to use high mu tubes, and will be suitable for signals too weal: to furnish AVC bias. While I have indicated and described several the voltages of the control grids of said triodes systems for carrying my invention into e?ect, it in accordance with frequency deviation of a fre will be apparent to one skilled in the art that quency-variable wave to be demodulated, and my invention is by no means limited to the par means responsive to the voltage variations of the ticular organization shown and described. but junction of said plate to cathode impedances for that many modi?cations may be made without producing a modulation signal representative of departing from the scope of my invention. said frequency deviations. What- I claim is: 7. In a detection system for frequency modu 1. In a detection system for angle modulated lated carrier waves, a pair of electron discharge carrier waves, a pair of electron discharge de tubes having the space current paths thereof con vices having the space current paths thereof con nected in series relation, discriminator means for nected in series relation, means for deriving from deriving from said Waves a pair of carrier voltages said waves a pair of carrier voltages each variable each variable in amplitude, control grid means in amplitude, means separately controlling the 60 separately controlling the'space current flow of space current ?ow of each device in response to each tube in response to the relative amplitudes the relative amplitudes of said pair of voltages, of said pair of voltages, and modulation signal and modulation signal output connections con output connections connected across the internal nected across the space current path of one of impedance of one of said tubes. said devices. 8. In combination with a pair of grid leak de 2. In combination with a pair of triodes, means tector triodes, means connecting the internal im connecting the internal plate to cathode imped pedances thereof in series relation, a frequency ances thereof in series relation, a frequency dis discriminator input circuit, separate connections criminator input circuit, separate connections from said frequency discriminator circuit to a re from said frequency discriminator circuit to a spective control grid of each of said triodes, and respective control grid of each. of said triodes, and a modulation signal output circuit connected a modulation signal output circuit connected across one of said internal impedances. across onec‘f said internal plate to cathode im _ _ 9. In combination with a pair of grid detection triodes, means connecting the internal plate to _ 3. In combination with a pair oftriodes, means 75 cathode impedances thereof in series relation, a Dcdances.‘ ‘ ' ' ' ‘ " " ’ » 7 2,413,977 frequency discriminator input circuit, separate connections from ‘said frequency discriminator circuit to a respective control grid of each of said triodes, an audio signal output circuit connected across one of said plate to cathode impedances, an amplitude modulation signal input circuit tuned to a frequency su?icien-tly different from the res onant frequency of said discriminator circuit to have negligible impedance e?’ect on the latter, and connections between the input electrodes of solely i one of said grid detection triodes and said sig nal input circuit. 10. In a frequency modulation receiver, a pair of grid leak detector tubes each including at least a cathode, control grid and anode, means for es [5 tablishing the cathode of one of the tubes at ground potential, means connecting the anode of said one tube directly to the cathode of the sec ond tube, means for applying a positive potential to the anode of the second tube whereby the space current paths of the two tubes are in series rela tion, audio frequency signal output connections connected to the anode and cathode of one of said tubes, and a frequency discriminator network 10 having respective signal connections to the con trol grids of said pair of tubes. 11. In a receiver circuit for frequency modu lated carrier waves, a pair of grid leak detector triodes whose respective internal plate to cathode impedances are connected“ in series between a point of relatively high positive direct current voltage and ground, means for varying the volt ages of the respective control grids of said tri odes in accordance with frequency deviation of a frequency modulated wave to be demodulated, and means responsive to the voltage variations of the junction of said plate to cathode impedances for producing a modulation signal representative of said frequency deviations. ' 12. In combination, a pair of grid leak detector ‘ tube circuits, means connecting the internal im~ pedances of said detector tubes in series relation, means for varying the voltages of the control grids of the tubes in accordance with received fre quency modulated waves, and a modulation signal output circuit connected across the internal im' pedance of one of said detector tubes. WINFIELD R. KOCH.