US2413593A - Radio receiver - Google Patents

Description

G. C. SZIKLAl RADIO RECEIVER Dec. 31, 1946.

Filed Aug. 5, 1944 m R. mm r W m a M W S. M I c T E Mun vfi i QWDQQQET w E bmiw g 6 HE Q95 Y Y llV QR B w! Q SQ 365 H ER QQTE QQEQ E Patented Dec. 31, 1946 RADIO RECEIVER George C. Sziklai, Princeton, N. .L, assignor to Radio Corporation of America, a corporation of Delaware Application August 5, 1944, Serial No. 548,162

9 Claims. (Cl. 250-27) Y My present invention generally relates to receivers of radio signals whether angle modulated or amplitude modulated, and more particularly to an improved detector of frequency modulated (FM) carrier waves.

In my U. S. Patent No. 2,251,382, granted August 5, 1941, there are shown various arrangements for reducing the effect of carrier amplitude variations during detection of angle modulated carrier waves, as for example frequency modulated carrier waves. In the patented circuits a special amplitude limiter prior to the discriminator section of the detector was dispensed with. Variations in carrier amplitude were substantially reduced by varying the mutual conductance of the balanced detector tubes, or of the audio amplifier tube, inversely with the carrier amplitude thereby to overcome the effect of the amplitude variations.

One of the important objects of my present invention is' to improve the method of compeneating for carrier amplitude variation of my aforesaid patent by changing the internal resistances of the balanced rectifiers proportionally to the carrier amplitude whereby there may be employed gridcontrolled diode rectifiers in the detector circuit-gin place of the multi-grid tubes of my aforesaid patent.

Another important object of my invention is to utilize the rectifier device furnishing the compensation voltage during frequency modulation detection, for supplying modulation voltage in response to reception of amplitude modulated (AM) carrier waves.

Another important object of my invention is to impart a double function to arectifier device embodied in a frequency modulation-amplitude modulation receiver; the rectifier furnishing modulation signals during amplitude modulation reception, and providing compensation voltage to reduce the efiect of carrier amplitude variation during detection of frequency modulated carrier waves. Another object of my present invention is to provide an FM-AM receiver which does not employ a special amplitude limiter stage for FM reception while being relatively non-responsive to carrier amplitude changes, existing tubes of the FM receiver being employed for AM reception.

Still other objects of my ,invention are to improve generally the operation of FM receivers. and to provide a compact, efficient and stable FM-AM receiver. V v

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims; the inven- 1 tion itself, however, as to both its organization and method of operation will best be understood by reference to the following description, taken in connection with the drawing, in which I have indicated diagrammatically two circuits whereby my invention may be carried into effect.

''-In the drawing:

Fig.1 shows an FM-AM receiving system embodying the invention; and

Fig. 2 is a modification of the demodulator circuit of Fig. 1.

Referring now to the accompanying drawing, there is shown in Fig. 1 so much of an FM-AM receiver as is essential to a proper understanding of my invention. The various objects mentioned above are all achieved in the system shown. Let it be assumed that tube I is the final intermediate frequency (I. F.) amplifier of a superheterodyne receiver, adapted to be operated in one or more frequency ranges of the radio communication spectrum.

The superheterodyne form of receiver is widely used for FM and ,AM reception. While my present invention is not restricted to any specific frequency ranges so far as FM or AM reception is concerned, by way of specific example it may he assumed that FM reception is had in the asvalue of 4.3 mc..is found most desirable.

signed FM broadcast band of 42-50 megacycle (mc.), while AM .reception is had in the 550-1700 kilocycle (kc.) AM broadcast band. Those skilled in the-art or radio communication are fully acquainted with multi-range superheterodyne receiver construction-wherein a plurality of frequency ranges are selectively operated for signal reception. Multi-range superheterodyne receivers are provided in the signal selector circuits and inthe local oscillator circuit with suitable band switching devices so as to produce a suitable I. F. signal depending upon the frequency range of reception. For example, in receiving FM signals in the 42-50 mc. band, anoperating I. F. On the other hand when receiving signals in the AM broadcast band of 550-1700 kc., a usual operating I. Fjvalue is 455 he. Hence, there are usually provided in the plate circuit of the con-. verter tube independent resonant circuits which are respectively tuned to the FM or AM operating I. F. values. The respective I. F. signal energies are developed in these independent I. F. circuits without interference with each other. The independent I. F. circuits are utilized in the input in the plate circuit of a prior I. F. amplifier tube. Furthermore, let it be assumed that circuit 2 is tuned to the operating 1. F. value of 4.3 me. which is employed when receiving FM signals, while circuit 3 is tuned to the LF. value of 455 kc. which is employed when receiving AM signals. The

+B voltage supply line is bypassed to ground for I. F. currentsby condenser 4 in order to provide return path to the cathode circuit of the prior I. F. amplifier tube. Circuit 2 is magnetically coupled to resonant circuit 5, whereas circuit 3 is magnetically coupled to resonant. circuit 6. Cir-- cuits 5 and 6 are arranged in series in the grid circuit of the I. F. amplifier tube i. Circuits 5 and 6 under the conditions set forth above are tuned respectively to 4.3 mo. and 455 kc. The lower end of circuit 6 is preferably connected to a point. of negative direct current potential so as to bias the grid 1 of the triode section of amplifier tube I., the cathode 8 of the amplifier tube being grounded. Condenser 9 provides an intermediate frequency return path to ground-for the grid circuitofthe tube 1.

It is to be understood that circuits 2 and 5 are magnetically coupled to provide a passband width sufiicient to transmit the maximum frequency variations of the signal waves. In the 42-50 mo. band the maximum frequency swing is.150 kc., and it is preferred to" have the passbands of .the. coupled selector circuits of the FM channel of the receiver each 200 kc. wide. As those skilled in the art of radio communication l:now,an FM wave has itsfrequency varied in accordance with the. amplitude of the-modulating signal which is'applied at the transmitter transmit the. sideband' components: of. the AM.

waves... In the AM broadcast bandthe permissible channel of transmission is 10 kc;, but it is preferred to have the circuits 3 and 6. coupled to transmit a band which is. 20. kc. Wide. Similarly in the plate circuit of the triode section of tube I there are arranged in series the resonant circuits it; and H. Circuit iii is tuned to 4.3 mo... while circuit H is. tunedto4'55 kc. The low potential side of circuit H is returned to ground by condenser 12' for I. F. currents. A selective. means is provided with each of the. series-connected resonant circuits. for selectively short-circuiting' circuits 3; 6 and H' respectively. These circuits are shcrt-circuited. Whenit isdesired to receive FM signals. Thus, adjustable .switch arms. I3, I14" and I5 are connected to the, low potential sides Qfc'ircuits. 3, 6 audit respectively. Eachof the switch. arms 13., Mend 15 has. associated therewithlan. AM contact. The FM contact for each. of. the switches. is. connected to the junction of eachrespective pair of. series-connectedresonant circuits. Accordingly, the. receiver. is adjustcd. for FM- reception when. each of, switch arms is... it and I5. isadiustedto its. respective FM contact thereby short-circuiting the. re spective. circuits; 3,. E. and H. However, when eachof these-switch arms is adjusted. to the free.

AM contacts then the receiver is conditionedto receive AM signals, since each of circuits 2, 5 and ID has no effect on the associated circuits 3, E and I i at the lower I. F. value of 455 kc. This follows from the fact that the capacity elements of respective circuits 2, 5 and II] have very low impedance at the lower LF. value of 455 kc. It is to be clearly understood that the various selector circuits preceding circuits 2 and 3, and not shown herein to preserve simplicity of description, will be constructed in the manner describedherein.

The tube 1 is provided with an auxiliary diode anode Eli which is located adjacent cathode 8,, but out of the main electron stream to plate 2 l Tube i may-be of the 63K? type. Cathode 8 and anode 20. provide a diode rectifier which performs a double function in the receiving system. These dual functions will be described at a later point. While I have shown the tube i' as embodying a diode section, those skilled in the art will realize that a separate diode tube could be used instead. The anode is connected by lead 22 to the midpoint of coil 23. The latter coil is actually the secondary winding of the transformer M whose primary circuit in is tunedto the operating I. F; value of 4.3 me. The coil 23 is shunted by condenser 24 and tunes the former to the 4.3 mc. frequency'value. The coupling between primary circuit Ill and secondary circuit 23. 24 is such that a band pass response having a 200 kc. width is provided.

The plate side of circuit H3 is connected to the phase of the I. F. currents-passed therethrough.

The condenser functions essentiallyas a direct current blocking condenser and acts asa direct connection for FM currents from the high alternating potential side of circuit Hi to the midpoint of coil 23. There is thus provided a method of translating applied FM voltage into a pair of AM voltages of the same frequency but Whose relative magnitudes are a function of the sense and extent of frequencydeviation ofthe FM signal waves. The frequency discriminator shown is that described and claimed by S. W. Seeley in his U. S. Patent No. 2,121,103, granted June- 21, 1938. The present invention is not restricted to that type of frequency discriminator, as there may be employed in place of circuit 23, 24 a pair of resonant circuits tuned to frequencies located on opposite sides of the frequency of input circuit it; That form of discriminator is shown by G. L. Usselman in U. S. Patent No. 1,794,932, grantedMarch 3, 1931. In general, then, my invention is independent of the particular construction of the frequency discriminator con nected to the pair of detector tubes 26 and Z'L Each-of detector tubes 25 and 21 is shown as a triode; they are used specifically as gridcontrolled diode rectifiers. The tubes 26 and 21 may each be of the 6SL7 type, but the invention is in no way restricted to any specific types of tubes. It will be seen that except for the internal resistance control of tubes 26-and 21, the latter function as if. they were diodes arranged in opposed relation. This is readily observed from th fact that anodes 28 and 29 of respective tubes Ziiand 21 are. connected to opposite ends of coil 23,,while cathodesfiil and 3| are respectively-connected to opposite ends of output load resistors 32 and 33. The. midpointof coil 23.is connected by I..F.. choke coil 34 tothe junction. of load resistors.32.and 33.

Each of load resistors 32 and 33 is shunted by Y a respective I. F. bypass condenser and 36. The upper end of load resistor 32 is connected through the modulation signal coupling condenser 3'! to the output lead 38 terminating in the contact FM. The internal resistance between the anode and cathode of each of tubes 26 and 21 is regulated automatically in dependence on the AM effects which may exist on the FM carrier wave energy at output circuit Hi. In other words, the internal resistances of the opposed rectifiers are changed proportionally to the FM carrier amplitude variations. The control is secured by providing control grids and 4| in the respective space current paths of tubes 26 and 21. The grids 43 and 4| are conductively connected to slider 42 arranged to be adjusted along resistor 43 shunted by condenser 44.

The resistor 43 is arranged in the space current path of diode section 8, 20 by connecting the upper or ungrounded end of resistor 43 to anode 20 through leads and 22. Since cathode 8 of tube I is grounded, it will be seen that resistor 43 is traversed by the rectified signal cur rents. The condenser 25 applies the amplified I. F. signal currents to anode 20. The condenser 44 is chosen to bypass I. F. currents, but not modulation components. In other words, the constants of condenser 44 and resistor 43 should be so chosen that network 43, 44 has a relatively short, i. e., fast, time constant. The time constant should be capable of permitting the highest modulating signal frequency (say about 15,000 cycles) expected to be encountered to develop a voltage across resistor 43.

Hence, the voltage applied to grids 43 and 4! is proportional to AM components thatmay exist on the FM carrier wave. Such AM components are created as a result of diverse effects. Even though an FM wave is transmitted from the transmitter as a constant-amplitude wave, yet by the time it is applied to the frequency discriminator it may well have acquired AM components. Special amplitude limiters have been employed prior to the discriminator. These, while effective, require a high signal gain at the limiter tube input electrode. In the case of portable or compact radio sets, for example those of the battery-operated type where space is at a premium, it is virtually impossible to attain the relatively high gain needed for driving a limiter tube.

Hence, in my present system there is no special limiter tube employed. Instead the control. grids 40 and 4! are provided in the usual FM detector diodes, and the grids have applied thereto control voltage derived from a rectifier embodied in an existing I. F. amplifier tube. No extra tube has been added to the FM receiver, and yet any AM components existing on the FM signals at circuit H) are instantly rectified and caused to bias grids 40 and 4! in a sense to increase the internal resistance of each detector tube.

Furthermore, rectifier 8, 23 is caused to per form an additional function. Upon adjustment of the receiver for AM carrier wave reception (arms l3, l4 and i5 adjusted to AM contacts), amplified AM waves developed at circuit ll will be applied through condenser 25 to the rectifier 8, 261. The rectified voltage appearing across resistor 43 in that case corresponds to the ampli tude modulation components existing on the desired AM carrier wave. Such modulation is utilized by connecting the AM contact of switch arm to the upper end of resistor 43 through coupling condenser 5|. Switch arm 50, of course,

iii

may be connected to any desired audio frequency amplifier network (not shown).

Considering, now, the operation of the FM-AM system in detail, and assuming first that all reception switches are adjusted to FM position, there will be developed across circuit I 0 signals whose center frequency is 4.3 mc. The various circuits 3, 6, l l have been short-circuited, and switch arm 50 is on contact FM. Due to the magnetic coupling between the primary and secondary circuits l0 and 23, 24 there will be induced in each half of coil 23 a signal voltage in phase quadrature with the primary signal voltage. However, the induced voltages are of opposite polarity due to the efiective grounding of the midpoint of coil 23. The midpoint is, also, varied in potential in the same manner as the plate side of primary circuit Hi. This directly injected voltage appears across each half of coil 23 in like polarity. Hence, at the anodes 28 and 29 appear the respective resultant voltages across each half of coil 23, and these resultants are the vector resultants of the quadrature-related induced and directly-injected voltages across each half of coil 23. When the I. F. signals are at the center or reference frequency of 4.3 mc., the vector resultant voltages at anodes 28 and 29 are equal. However, as the frequency of the signal energy across circuit l0 deviates from 4.3 mc., the resultant voltages become unequal in the direction of frequency deviation due to the fact that the aforesaid quadrature relation exists only at the instant when the signal energy frequency is equal to the reference frequency of 4.3 mc.

As is explained in the aforesaid Seeley patent, the variable-amplitude signal voltages at the anodes 28 and 29 are rectified by the respective diode rectifiers 28, 30 and 29, 3|. The rectified voltage output of rectifier 28, 30 appears across load resistor 32, while the rectified voltage out put of the other rectifier appears across load resistor 33. Since the cathode end of resistor 33 is grounded, there will be taken off from the cathode end of resistor 32 the differential resultant voltage of the voltages across resistors 32 and 33. At the instant when the I. F. signals are 4.3 mc., the voltages across resistors 32 and 33 will be equal and the modulation output will be zero. The rapid frequency variations or deviations, corresponding to the modulating signals applied to the carrier at the transmitter, will. appear as a modulation voltage output at the cathode end of resistor 32 as described in Seeley Patent 2,121,103 and as is well known in the art.

Should there suddenly appear an undesired AM component on the carrier, such undesired amplitude modulation of the carrier will appear as a variable voltage across resistor 43. It is to be understood that during the normal FM reception there will be a rectified carrier voltage across resistor 43. Of course, as long as the carrier amplitude is uniform or constant the grids 4! and 43 will have substantially constant potentials, which will establish the normal internal resistance existing between the cathode and anode of each of tubes 26 and 21. As a matter of fact the slider 42 may be adjusted to a point on resistor 43 so as to provide a minimum value of internal resistance for each of the detector tubes. Having set slider 42 to a minimum internal resistance point, an increase in carrier amplitude will resultin a variation of the potential of each of grids 40 and 4|.

It will be noted that the internal resistance of ondary circuit 23,:24' asshown in Fig. 1.

grids 10 and ii will become proportionally more negative. Consequently the internal resistance of each of the detector tubes will be increased, and this increase will be proportional to the increase of the FM carrier wave amplitude. Since the internal resistance of each of the detector tubes has been increased, any increased voltage impressed upon diodes 23, 30 and 29,- 3! will not be reflected across load resistors 32 and 33, but will be compensated for by virtue of the fact that the internal resistance of each of tubes 26 and 21 has been increased. In this way amplitude modulation of the FM carrier wave will be very largely counteracted at the detector tubes 25 and 21 so as to prevent any undesired increase in rectified voltage across each of resistors 32 and 3.3.

If, now, it is desired to receive AM carrier waves it is only necessary to switch the various band switching devices of the receiver, and more particularly switch arms i3, i4, i5 and 50 are adjusted to the AM contacts. When the receiver is thus conditioned, the circuits 3, 6 and H will have developed across each of them the I. F. signal energy at 455 kc., while the circuits 2, 5, l0 and 23, 2'4 will be ineffective at the lower I. F. value. The tube i will function as an I. F. amplifier, whereas tubes 26 and 2'! will be ineffective. The diode rectifier 8, 29 will have applied thereto the I. F. signal energy of 455 kc. by virtue of the condenser 25. It is to be understood that the resonant circuit II will be given a response curve such as to pass eiiiciently the entire I. F. signal energy. There will be produced across resistor 13 the modulation signal voltage representative of the AM components of the AM carrier wave energy. These modulation signal components are taken ofi through coupling condenser 51 and switch 50, and utilized in the common audio fre quency amplifier network.

It will, therefore, be seen that I have provided a compact and highly economical circuit arrangement for receiving either FM or AM car rier waves, and in the FM- reception condition use of AM limiting is made unnecessary by the provision of means for varying the internal resistances of the rectifier tubes in a manner to compensate for the existence of any AM components on the received FM waves. It will be observed that there is no need for a special limiter tube prior to the frequency discriminator network, and that the device which provides the compensating voltage for the detector tubes is included within the last I. F. amplifier tube. It is to be clearly understood, however, that the diode 8, may be located outside the amplifier tube i. Furthermore, my invention is not restricted to the utilization of independent transmission channels for FM and AM signal waves, since a common transmission channel may be employed in the receiver where the FM and AM signal waves lie in a common range of frequencies.

In Fig. 2 I have shown a modification of the system wherein the I. F. amplifier i has an output circuit ii! magnetically. coupled to. the seca matter of fact the system shown in Fig. 2 is substantially similar to that shown for the FM transmission channel of Fig. 1, and for this reason identical circuit elements are indicated by the same reference numerals, except that in the case of Fig. 2 a prime designation is employed. Furthermore, in Fig. 2 the detector electrodes are shown as embodied in a common tube envelope 26'; It will be understood that in Fig. 2 there may be developed across resonant circuit iii FM or AM signal waves having a carrier frequency which will be equal to the operating I. F. value of 4.3 mc. Hence, the secondary circuit 23', 24' will be tuned to the same I.'F. value.

For FM reception it is only necessary to switch arm 5!! to the FM contact which connects through condenser 31' to the cathode end of resistor 32'. In this position of the switch arm 5!! the frequency discriminator input network produces the hereinbefore described resultant vector voltages at anodes 28 and 29', and the differential rectified voltage is produced across load resistors 3?." and 33 in series. The diode 26'', whose cathode 8' is connected to the output terminal of condenser 25 and whose anode 20' is connected to the ungrounded end of resistor 53, provides the compensating voltage. It will be noted that in this case the diode 29" has its cathode 8' returned to ground through a path comprising lead 45, choke coil 34. and resistor 33'. Undesired amplitude modulationinthe FM carrier wave is compensated for by applying rectified voltage across resistor 43'to grids M and 2-21. The functioning of the compensating circuit in Fig. 2 is precisely the same as inthe case of Fig. 1. To receive AM signal waves, switch arm 59 is adjusted to the AM contact. In this case the AM signal waves, also at an I. F. value of 4.3 me, are applied to the rectifier 28" through the coupling condenser 25'. The modulation signal voltage across resistor. 43' is utilized by the following audio frequency amplifier. Of course, the circuits i0 and 23, 25' may be tuned to any common desired I. F.'value. Furthermore, in the modification of Fig. 2 the diode 2G" may have its electrodes included as a part of the I. F. amplifier tube i in the manner shown in Fig. 1, if desired.

While I have indicated and described several systems for carrying my invention into efiect, it will be apparent to one skilled in the artzthat my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention.

What I claim is:

1. In combination in a frequency modulated wave receiver of the type comprising a demodulator having an input circuit upon which ,is impressed frequency modulated waves,.said .demodulator consisting of a pair of trio-des connected as diode rectifiers in a push-pull circuit constructed and arranged to convert the waves into modulation voltage; the improvement which comprises means coupled to said input circuit for producing a uni-directional voltage in response to amplitude variations in the impressed waves, and means responsive to said uni-olirectional voltage for controlling the internal resistance between the cathode and anode of each diode rectifier in like senses and to'an extent to compensate for said amplitude variations.

. 2. In combination, a frequency-modulator detector system of the type utilizing a pair of opposed diode rectifiers, a signal amplifier including a diode rectifier section, said amplifier being provided with an output circuit coupled to said rectifiers, means for applying frequency modulated carrier wave energy developed in said amplifier output circuit to said rectifier section, the improvement comprising means. responsive to rectified voltage developed from rectified currents of said diode section, and said last means for varying the internal resistance of each of said opposed diode rectifiers in like senses.

3. In a combined frequency modulation and amplitude modulation receiver, a signal amplifier tube provided with an amplifier section and a diode rectifier section, separate signal input channels connected to said amplified section inut electrodes tuned to diiferent operating frequencies for respective frequency modulation and amplitude modulation reception, a similar pair of transmission circuits connected to the output electrodes of said amplifier section, a pair of independent grid-controlled diode rectifiers connected in a push-pull circuit and having an input circuit coupled to said amplifier output circuit which is tuned to the frequency modulation signals, means for applying frequency modulated signal waves developed in the frequency modulation output circuit of the amplifier section to said diode section, means for deriving a control voltage from said diode section, and additional means for automatically varying the grid potential of each of said opposed rectifiers thereby to vary the internal resistanoe of the latter in a sense to compensate for any amplitude variations which exist on reoperatively associated with each rectifier, and

prcvement which comprises means coupled to Y input circuit for producing a uni-directional voltage in response to amplitude variations in the impressed Waves, and means responsive to said uni-directional voltage for controlling the internal resistance between the cathode and anode of each diode rectifier in like senses.

5. In combination with a frequency modulator detector system of the type utilizing a pair of triodes connected as opposed diode rectifiers, a signal amplifier including a diode rectifier section, means coupling the amplifier output circuit to said opposed rectifiers, means for applying frequency modulated carrier wave energy developed in said amplifier output circuit to said diode rectifier section, means responsive to rectified voltage developed from rectified currents of said diode section, and said last means varying the internal resistance of each of said opposed diode rectifiers in like senses.

6. In an angle modulated wave receiver of the type comprising a demodulator having an input circuit upon which are impressed angle modulated Waves, said demodulator consisting of a pair of diode rectifiers connected in a pushpull circuit constructed and arranged to convert the waves into modulation voltage; the improvement which comprises rectifier means coupled to said input circuit for producing a unidirectional voltage in response to amplitude variations in the impressed waves, a control grid means responsive to said uni-directional voltage and connected to each control grid for control- -ling the internal resistance between the cathode and anode of each diode rectifier in like senses and to an extent to compensate for said amplitude variations.

'7. In combination in a frequency modulated wave receiver of the type comprising a demodulator having an input circuit upon which is impressed frequency modulated waves, said demodulator consisting of a pair of triodes connected as diode rectifiers in a push-pull circuit constructed and arranged to convert the waves into modulation voltage, means to utilize said modulation voltage; the improvement Which comprises rectifier means coupled to said input circuit for producing a uni-directional voltage in response to amplitude variations in the impressed. waves, means responsive to said unidirectional voltage for controlling the respective control grids'of said triodes in like polarity senses thereby to vary the internal resistance cetween the cathode and anode of each diode rectifier in like senses and to an extent to compensate for said amplitude variations, and means for selectively switching the connection of the utilizing means from said demodulator to said rectifier means for reception of amplitude modulated waves.

8. In a combined frequency modulation and amplitude modulation receiver, a signal amplifier tube provided with input and output electrodes, a rectifier, separate signal input circuits tuned to different operating frequencies for respective frequency modulation and amplitude modulation reception, means connecting the input circuits between said amplifier input electrodes,.a similarly tuned pair of amplifier output c cults connected between the output electrodes of said amplifier, a pair of independent grid-controlled diode rectifiers connected in a push-pull circuit and having an input circuit coupled to the amplifier output circuit of said pair which is tuned to the frequency of frequency modulation signals, means for applying frequency modulated signal Waves developed in said last-named amplifier output circuit to said rectifier, means for deriving a control voltage from said rectifier, additional means for automatically varying the grid potential of each of said opposed rectifiers thereby to vary the in ternal resistance of the latter in a sense to compensate for any amplitude variations which exist on received frequency modulation signals.

9. In a frequency modulated Wave receiver of the type comprising a demodulator having an input circuit upon which are impressed frequency modulated Waves, said demodulator consisting of a pair of diode rectifiers, connected in a push-pull circuit constructed and arranged to convert the waves into modulation voltage; the improvement which comprises a control rectifier coupled to said input circuit for producing a uni-directional voltage in response to amplitude variations in the impressed waves, means responsive to said uni-directional voltage for con-trolling the internal resistance between the cathode and anode of each diode rectifier in like senses, and means for selectively deriving modulation signals from said control rectifier in response to amplitude modulated carrier waves being applied to said input circuit.

GEORGE C. SZIKLAI.

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