US2472301A - Frequency modulated-amplitude modulated receiver - Google Patents

Description

June 7,1949, w R, KOCH 2,472,301

FREQUENCY MODULATED-AMPLITUDE MODULATED REGEIVERS w. R. KocH I 2,472,301

FREQUENCY MODULATED-AMPLITUDE MODULA'IED RECEIVERS June 7, 1949.

2 Sheens-SheerI 2 Original Filed Feb. 5, 1945 SLOW ,4.1%6.

ELAV D/ODE R Em Am 0 rm a ai H mm F v n c M 3 .C nu L S r f 6 AAAAAAAAAA .rc s AV FA INVENToR. M//NF/ELD Q KOCH Patented June 7, 1949 UNITED STATES PATENT OFFICE FREQUENCY MODULATED-AMPLITUDE MODULATED RECEIVER Winfield R. Koch, Haddonleld, N. J., assignor to Radio Corporation of America, .a corporation of Delaware (Cl. Z50- 20) 4 Claims.

My present invention relates to receivers of frequency modulated (FM) or amplitude modulated (AM) carrier waves, and more particularly to novel and improved receivers of FM or AM carrier waves, This application is a division of my application Serial No. 521,193, led February 5 1944, now Patent No. 2,429,726.

In his U. S.. Patent No. 2,296,092, granted September 15, 1942, M. G. Crosby has disclosed a differential detector circuit adapted to receive FM and/or AM carrier waves. In his detector circuit Crosby provides automatic volume control (AVC) voltageat the detector output in response to either FM or AM reception. The AVC voltage is derived from the separate rectified voltages of the balanced rectiiiers added in aiding phase, while the modulation signal corresponding to the -frequency modulation of a carrier wave is derived separate and independent FM and AM signal' channels to the detector and yet lbe able to provide the same output voltages as in the Crosby system. When receiving FM signals with a receiver having a substantially flat-topped selec.- tively characteristic and no amplitude limiter used, it is desirable to provide AVC voltage from the opposed diodes of the discriminator-detector circuit. Again, where such a receiver is provided with a separate signal channel for AM broadcast reception, it is desirable to use one of the opposed diodes for AM detection and AVC rectication without switching. It is an important object of my invention to provide such an .FM-AM receiver with -minimum circuit components and maximum detected voltage output.

In the customary limiter, or fast-acting AVC, used in FM receivers s low variations in carrier amplitude are controlled along with rapid varia.- tions. It is another object of my present invention to improve the receiver performance by providing separate slow-acting AVC and fast-acting Other objects of my present invention are to improve generally the efficiency of IFM-AM receivers, and more especially to provide economical detector circuits for such receivers.

Still other features will best be understood by reference to the following description, 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. l shows, in partial schematic form, an FM-AM receiver employing one embodiment of my invention;

Fig. la shows an ideal selectivity characteristic of the FM signal channel;

Fig. 1b shows an ideal selectivity characteristic of the AM signal channel;

Fig. 1c illustrates the FM detection characteristic;

Fig. 1d shows the Frequency vs. AVC voltage characteristic for FM reception;

Fig. 2 is the circuit diagram of an FM signal channel using separate slow-acting and fastacting AVC and Fig. 3 shows a modication of the circuit of Fig. 2 applied to an FM and AM receiver.

Referring now to the accompanying drawings, wherein lile reference characters in the dilerent figures designate similar circuit elements, Fig. 1 shows an illustrative receiving system embodying a demodulator network adapted to provide audio voltage and AVC voltage in response to FM or AM signal reception. The receiver circuits prior to the demodulator are schematically represented. Those skilled in the art of radio reception are well acquainted with the nature of the circuits customarily employed in multi-band receivers. While my invention is readily adapted for FM and AM reception on respective bands of 42 to 50 megacycles (rnc.) and 550 to 1700 kilocycles (kc), it is to be clearly understood that the invention is not limited to such frequency bands. The 42 to 50 mc. band Lisl presented by way of illustration, since it is the FM broadcast band presently assigned to such transmission. The 550 to 1700 kc. band is the present AM broadcast band assigned to transmission of AM signals.

It will further be understood that in the following description and claims the generic expression angle modulated is intended to include frequency modulation, phase modulation or hybrid modulations possessing characteristics common t0 either form of modulation. From a very general viewpoint my invention relates to a demodumunication.

lator network having separate input circuits for carrier waves of different frequencies and of different modulation characteristics.

The numerals I and 2 in Fig. 1 denote respectively diferent sources of modulated carrier waves. Source I may be the usual signal collector, such as a dipole, employed for collecting FM waves. The FM waves are transmitted from FM transmitters at a mean, center or carrier frequency assigned to the particular transmitter. In the assumed FM band of 42 to 50 mc. the radiated carrier wave frequency would be in that range, and would be a wave of variable frequency 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 frequency 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 42 to 50 mc. is '75 kc. to either side of the carrier frequency; the allotted FM channels are 200 kc. wide. These values are purely illustrative.

Source 2 may be the customary grounded antenna circuit employed in AM broadcast reception. The allotted channels are Iii kc. wide in this band. In AM transmission the carrier Wave is modulated in amplitude in accordance with the modulation signals. The carrier frequently is maintained constant in value at the transmitter. The numeral 3 designates a tunable radio frequency amplifier having suitable signal selector circuits for FM or AM reception. Switching devices 4 and 5 respectively provide separate connection of the sources I and 2 to respective selector circuits of amplifier 3. It will be understood that when switch 4 is in closed position, collected FM signal energy will be applied to selector circuits of amplifier 3 capable of selectively amplifying the FM signals over a band at least 150 kc. wide. Upon closing of switch 5 and opening switch 4 the Same amplifier 3 will have the FM selector circuits thereof operatively replaced by AM selector circuits. These latter circuits will select the collected AM signals and permit amplier 3 to amplify the same over a 10 kc. band. Multi-band selector circuits and switching devices for suitable change-over are well known to those skilled in the art of radio com- Switching devices 4 and 5 affect the demodulator circuit only in so far as they determine the character of the modulated wave to be delivered to the demodulator.

Assuming the system is of the superheterodyne type, as is the usual practice at present, the converter E and intermediate frequency (I. F.) amplifier 'I will also be provided with suitable FM and AM signal selector circuits. At the converter 6 the FM signals will have the mean or center frequency thereof reduced to a value which may be chosen from a range of 1 to 2O rnc., as for example 4.3 mc. The AM signals are reduced to an I. F. of 455 kc., as an illustrative frequency value, the latter being a commonly employed frequency in AM broadcast receivers of the superheterodyne type. The I. F. amplifier l, which may consist of one or more separate stages oi amplification, will have an ultimate output circuit from which may be derived, at separate points thereof, the amplied FM signals or AM signals.

The selective circuits 8 and 9 are to be understood as being arranged in series in the plate circuit of the last I. F. amplifier tube. Each of circuits 8 and 9 is resonated to its respective operating I. F. value for FM or AM reception. Thus, circuit 8 is tuned to 4.3 mc., while circuit 9 is tuned to 455 kc. There will be developed across tuned circuit 8 the FM signals at the 4.3 mc. mean frequency when switch 4 is closed, and all FM selector circuits of amplifier 3, converter 6 and I. F. amplifier 'I are in operative electrical connection. Conversely, when switch 5 is closed, and switch 4 is open, and all AM selector circuits are in operative electrical connection, there will be developed across circuit 9 AM signals at the I. F. value of 455 kc. The impedance of circuit 9 is negligible at 4.3 mc.; therefore, the insertion of circuit 9 in series with circuit will not affect the development of FM signal voltage across circuit 8. Similarly, the impedance of circuit 8 is negligible at 455 kc., and circuit 8 will not affect development of AM signal voltage across circuit 9.

The demodulator comprises but two electron discharge devices, shown as diodes by way of illustration. The electrodes of the pair of diodes may be housed within a common tube envelope, or they may be in separate envelopes. By way of specific illustration the diodes I Il and II are shown as being separate tubes. The diode I0 is provided with a resonant input circuit I`2 which is inductively coupled to the circuit 8. The anode of diode I0 is connected to one side of the input circuit I2, while the cathode of diode Ill is connected to the opposite side of input circuit I2 through the load resistor I3. Resistor I3 is bypassed by condenser I4 for high frequency currents.

Diode I I has its cathode established at ground potential, while its anode is connected to the high alternating potential side of its resonant input circuit I5. Circuit I5 is also inductively coupled to the circuit 8. The low potential side of circuit I5 is connected to ground through the coil I6 and load resistor I"I. Coil I6 is magnetically coupled to circuit 9, and condenser I8 shunts coil I6 to provide a resonant circuit IE5- I3 tuned to 455 kc. Condenser I 9 shunts resistor Il to bypass high frequency currents.

The input circuits I2 and I5 of diodes Ill and II respectively are oppositely and equally mistuned with respect to the operating I. F. value for FM reception. In other words, if the FM signals developed across circuit 8 have a center frequency of Fc (4.3 mc.), then circuits I2 and I5 will be detuned in opposite senses by equal predetermined frequency values relative to Fc. It will be recognized that circuits 8, I2 and I5 provide the well known discriminator network of Conrad U. S. Patent No. 2,057,640. The action of this form of discriminator circuit is well known to those skilled in the art. It functions to translate FM high frequency signals into corresponding AM high frequency signals.

At the 4.3 mc. frequency used for FM reception the impedance of circuit I6-I8 is negligible, and hence the load resistor I1 is effectively in series with input circuit I5 and diode II. lThe upper end of resistor I1 is coupled to the lower end of resistor I3 through the condenser 20. Condenser 2n has a low impedance for the modulation frequencies developed during detection of the FM signals, but has a high impedance to high frequency currents. In other words, condenser 20 is a modulation frequency coupling condenser.

Assuming that the modulation signals on the received FM waves are of audio frequency, then the audio frequency amplifier of the receiver wil-l have its input lead connected to the cathode end of load; resistor I3. As already known, a deemphasis network 2l' may be employed in the audio frequency output connection in order to compensate for pre-emphasis of higher audio frequencies at the transmitter. In accordance with well-understoodprinciples of FM signal detection. the alternating current components in the rectiiied signal voltage across each of resistors Il and i3v will be combined in phase opposition due to the connection of the anode end ofA resistor Il to the anode end of resistor I3 by coupling. condenser 2D..

The diferential voltage resulting from the phasefopposed voltages corresponds to the audio modulation signal voltage originally applied to the. FM carrier wave at the FM transmitter.. At the same time there is. provideda conductive connection between the negative or anode end ofv resister II and the cathode or positive end of resistor I. This conductive connection includes resistor 22; Considered relative to ground the direct current voltage components of the rectied voltages appearing across resistors. I3 and I'I` are added in phase-aiding sense. In other words, the direct current voltage components of rectiers t and I Ir are combined in additive manner duringy FM signal reception, while the alternating current. (audio) outputs. ci rectiiiers Ill and. III are combined in phase-opposed relation.

An AVC connection 2t is provided between the gain control electrodes, as for example. the signal grids, of the various tubes in networks t, 6 and 1 and the negtive endof resistor I3. The AVC connection 23 includes a lter resistor 24, whose lower end isv bypassed to ground by an audio frequency condenser 2.5, so as to prevent alternating cur.- rent components. from being transmitted over the. connection 23. Network24-2 5. therefore acts as. a` time constant network to. produce slow AVC action.

The functionr of the AVC connection is well known to those skilledv in the art. Should there be.. any carrier amplitude variation at the input terminals of each of rectifiers lil and II, such amplitude variation will be translated into a corresponding change in direct current voltage across the corresponding load resistors. I3 and Il. The AVC voltage applied over connection 23- to the controlled tubes will reduce the gain of the tubes to counteract undesired carrier amplitude increase.

During AM signal reception` the I. F. signal energy produced in the circuit 9 will be trans.- ferredto. input. circuit lli-I8. Each of circuits 9 and l-I 8 is tunedto the operating I. F. value of` 455 kc. The circuit I5 and diode II are both included in a series circuit with tuned circuit Iii-I8 and load resistor I'I. The circuit I5, resonant close to 4.3 mc., has no appreciable efrect on the series circuit, since it acts as an eX- tremely low impedance connection at the 455 kc. value. The modulation voltage component of the rectiedl. F. energy developed across bypassed load resistor Il is applied through condenser and resistor I3 to the common-modulation signal output circuit. The direct current voltage component across resistor Il is applied over AVC path 23 to the prior tubes. The AVC line 23 connects to the ungrounded end of. resistor Il, through a series path consisting of resistor 24, resistor I3 and resistor 22. Here, again, the network 24--25 acts to introduce timedelay into. the AVC action.

In Fig.. lai I' have shown the form of selectivity characteristic. which is preferred for use during FMsignal reception. The curve is idealized, and represents a flat-topped characteristic at least kc. wide. The characteristic represents the idea-l pass band of the receiver circuits up to the opposed rectiers: Ill- Ii during FM reception. The dat-topped selectivity characteristic, if the FM carrier is correctly centered on it, insures against production of' amplitude modulation on the-FM wave as the latter passesv through the cas;- caded resonant circuits to the FM detector cire cuit, and lessons the importance of the use of an amplitude limiter stage in FM reception. The AVC circuit acts effectively to reduce the gain of the receiver tubes in response to increases in amplitude of the FM carrier.

By Way of contrast to Fig. la I have shown the'AM selectivity characteristic in Fig. 1b. This curve is idealized, and represents the flat-topped l0. kc. pass band of the receiver circuits up to the rectier during AM signal reception. This enables faithful AM reception and permits the AVC action to function in the well understood manner. Fig. 1cv shows the FM detection characteristic of opposed rectifiers I0 and II and their associated input circuits t2 and I5. It is desirable to have the spaced peaks of the ideal curve sepa,- rated by a frequency value in excess of the 150 kc. band width. Further, the curve should be as linear as possible between the peaks thereof.

With a detection characteristic as shown in Fig. 1c, the AVC voltage vs. Frequency characteristic durinng FM reception will be substantially of the form represented in Fig. 1d. It will be noted that the AVC (negative in polarity) voltage, with changes in carrier frequency but notin amplitude, becomes a maximum at spaced peaks of the curve with a decrease towards the center frequencyv Fc. In other words, the AVC bias Will' be a maximum on each side of Fc thereby providing an audible aid in differentiation between exact tuning of the receiver and off-center tuning thereof.

In Fig. 2 Iv have shown a receiving system of the type schematically represented in Fig. l, but arranged; to receive FM signals only. For this reason the circuits shown in Fig. 2 omit the AM signal receiving circuits. The FM detector circuit is substantially the same as that shown in Fig. 1 for receiving FM- signals. It will be seen that the AVC line 231s connected through lter resistor 2li to they anode end of resistor I3, and therethrough to the anode end of resistor I1 through resistor 22. The time constant circuit 24;-25 provides slow-acting AVC action. In order to secure a. measure of delay there is employed a well known delay device consisting of a diode 50 whose cathode is grounded, but whose anode isV connected to the lead 23. A permanent positive bias is applied to the anode of diode 50 thereby effectively establishing the lower end of resistor 24 at ground. potential until the diode 50 is rendered non-conductive. The diode 5G becomes non-conductive upon the negative potential atthe anode end of resistor I3 assuming a sufficient negative value relative to ground to overcomeY the positive bias on the anode of diode 5I). From that point on the AVC bias will be suppliedV over line 23 to the control grids of the various controlled tubes. This form of delayed AVC action is well known.

Only the I; F. ampliers preceding the discriminatorl networkare shown, since it is tobe understoodithatvthe.. slow-acting AVCv circuit 23 may be connected to'one or more of the transmission tubes preceding the I. F. amplifier tubes I and 52. 'I'he tuned I. F. transformers 53 and 54 will, of course, be constructed so as to pass the required frequency swing of the FM signals. It will be seen that the I. F. amplifier stages shown in Fig. 2 are of well known form, and hence they need no further explanation. The input electrode of amplifier 52 is magnetically coupled to the resonant secondary circuit of I. F. transformer 54, and, therefore, derives its signal energy therefrom.

A diode rectifier 55 has its electrodes coupled to the opposite sides of the secondary circuit of I. F. transformer 54. The resistor 56. bypassed by condenser 5l for I. F. currents, is connected between the grounded cathode of diode 55 and the low potential side of the secondary circuit of transformer Eli. Alternating current components of the rectified voltage developed across resistor 56 are applied to the control grids of amplifiers 5I and 52 by connecting the grid circuits of these tubes to any desired point on resistor 56 by means of an audio coupling condenser 58. The feedback to the amplifier control grids from across resistor 56 is performed degeneratively so as to compensate for undesired relatively fast amplitude variations of the FM signals. This action will be referred to hereinafter as control of fast variations or fast AVC. It will be noted that the slow-acting AVC lead 23 is also connected to the grids of tubes 5I and 52 through a filter resistor 59.

No amplitude limiter stage per se is employed prior to the discriminator network. Furthermore, it will be observed that there is utlized separate control by diodes 55 and Ill-II respectively of fast variations and slow variations in carrier amplitude. In the familiar and customary form of limiter, or fast-acting AVC, slow variations in carrier amplitude are controlled along with the rapid variations. The customary limiter circuit of necessity must reduce the -average amplification of the receiver in order to be able to increase and decrease the overall gain to compensate for rapid variations of amplitude. By using a separate slow-acting delayed AVC circuit, together with fast AVC, the average amplification of the receiver will not be cut for weak signals. However, variations in the carrier amplitude, for example as the carrier is swung across a round-top selectivity curve, will be compensated for by the amplitude correction device. Hence, weak signals will be received with less distortion.

Since the gain of a special limiter stage is usually low, because it is usually operated with low screen and plate voltages, there is lanother advantage secured by the circuit of Fig. 2. By using the separate controls high gain amplification is obtained. It will be noted that the slow-acting AVC voltage is derived from the discriminatordetector output circuit as explained in connection with Fig. l. This AVC voltage is considerably larger than can be secured in the case where a limite-r stage precedes the discriminator thereby giving better regulation and having less likelihood of overloading preceding stages.

The fast-acting AVC rectifier applies its rectified voltage to points prior to, and following, the point from which it derives its signal energy. This insures a substantially fiat gain control action. The slow-acting AVC voltage is applied to many tubes in addition to the tubes to which the fastacting AVC bias is applied. By virtue of the combined AVC actions of Fig. 2 the response char- 8 acteristics of the coupling transformers between stages need not be ideally flat, since the AVC circuit will tend to compensate for any curvature in the response characteristics.

In Fig. 3 I have shown a further modification wherein the fast-acting AVC voltage may be secured from the discriminator network. This has the advantage that the control voltage obtainable is much greater, and is, therefore, more effective. It will be recognized that in Fig. 3 the FM-AM networks feeding the diodes I and I I are very similar to those shown in Fig. 1. Assuming first that FM signals are being received, the load resistors I3 and I'I develop thereacross audio voltages which are combined in phase opposition and the resultant voltage is transmitted through resistor 2| to the subsequent audio amplifier. The direct current voltage components across resistors I3 and I'I are combined in additive phase through resistor 22. The slow-acting AVC connection 23 goes through resistor 24 to the negative end of resistor I3. The AVC action is delayed by connecting the lead 23 to a source of positive bias. Instead of using the diode 50 of Fig. 2 for the delaying action, however, the resistor 59 of Fig. 2 may be omitted and the control grids of the various tubes to be controlled may be permitted to draw grid current thereby rendering the AVC circuit ineffective until the negative voltage at the anode end of resistor I3 becomes sufficiently negative to overcome the positive bias. This is, also, a well-known means for securing delayed AVC action.

The fast-acting AVC action is secured by means of an `auxiliary diode rectifier E0 whose cathode is connected to the same side of circuit I2 as the anode of diode I0. The anode of diode 6U is connected to the opposite side of circuit I2 through a resistor SI bypassed by condenser G2. The resistor 6I functions as the load resistor for diode 60, and the condenser 62 bypasses only the I. F. currents. The fast-acting AVC connection 63 utilizes the alternating current voltage components of the rectified voltages across resistor 5I and resistor I'I. Since the positive end of resistor SI is connected through the audio coupling condenser 20 to the negative end of resistor I1, it will be seen that the audio voltages across resistors Il and BI are combined in additive phase for transmission through the audio coupling condenser 'ID to connection 63 when switch 'II is closed. Switch II is provided to selectively connect or disconnect connection 63 from the condenser 10. Condenser 'I0 and resistor l2, the latter having its lower end grounded, remove the direct current components from the voltage and provide the fast time constant network for connection 63.

It is contemplated that during FM signal reception the AVC connect-ion 63 will be made to at least the prior I. F. amplifier l, and will act degeneratively as described in reference to Fig. 2. The slow-acting AVC connection 23 will also be made to one or more of the transmission tubes preceding stage l. For AM reception the switch 'II may be opened thereby removing the fastacting AVC circuit. In this case the AVC lead 23 is connected through resistor 2li, resistor I3 and resistor 22 to the negative end of resistor II. In other words the direct current voltage component across resistor I l is still used for the slow AVC action. In AM reception the audio voltage component across resistor I'I is applied to the audio output connection through the path cornprising condenser 29 and resistor I3. It will, therefore, be appreciated that in Fig. 3 I have provided a circuit wherein both slow-acting AVC and fast-acting AVC derive their voltages from the combination of rectified voltages in additive phase and in response to FM signal reception. For AM reception only one of the three rectifiers need be employed with its associated load resistor. In this Way distortionless AM reception can be secured with only the slow-acting AVC circuit operating.

While I have indicated and described several systems for carrying my invention into effect, it will he apparent to one skilled in the art that 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 a frequency modulation receiver, a frequency discriminator including a pair of load resistors across which direct current voltages appear as a result of detection, conductive means for connecting said resistors in series-aiding relation with respect to said voltages, a slow-acting circuit for applying the resultant of the voltages to a portion of the receiver as a first gain control voltage thereof, means for deriving from the received frequency modulated signal a second control voltage which is representative of relatively fast amplitude variations of the signal, means coupled to said resistors for deriving a detected modulation voltage representative of the frequency modulation of the received signal, and a fast-acting circuit for applying said second control voltage to a portion of the receiver as a second gain control voltage.

2. In a yfrequency modulation receiver, a frequency discriminator including a pair of load resistors across which direct current voltages appear as a result of detection, a conductive impedance element for connecting said resistors in series-aiding relation with respect to said voltages, a slow-acting circuit for applying the resultant of the voltages to a portion of the receiver as a gain control voltage thereof, means I:for deriving from the received frequency modulated signal a modulation voltage which is representative of relatively fastamplitude variations of the signal, and a fast-acting circuit for applying said modulation voltage to a portion of the receiver as a gain control voltage, means for connecting said resistors in series-opposing relation with respect to the modulation voltages thereacross representative of desired frequency modulation thereof, and means for utilizing the resultant of the series-opposed modulation voltages.

3. In a frequency modulation receiver provided with an amplifier, a frequency discriminator coupled to the amplier providing a pair of signal voltages whose relative magnitudes are dependent on the frequency Variation of received waves, a pair of rectiflers each having an input electrode separately coupled to said discriminator to have a respective one of the pair of signal voltages applied thereto, a separate load impedance in circuit with each rectifier, a circuit connected across said load impedances deriving a modulation voltage in response to the added rectied voltages developed across said load impedances, a conductive impedance element connecting opposite polarity ends of said load impedances in series relation, a iirst gain control connection to similar polarity ends of said impedances for deriving from said impedances in additive polarity sense slow-acting rectified voltages developed thereacross, means to apply the additive voltage to said amplifier, an auxiliary rectifier having an input electrode coupled to said discriminator and including a third load impedance in circuit therewith, means of low impedance to modulation currents connecting opposite polarity ends of one of said first two impedances and said third impedance, a second gain control connection between said amplifler and one end of the third impedance for deriving lfrom said third impedance and said one impedance in additive polarity sense the fast-acting rectified modulation voltages thereacross and for impressing them on said amplifier.

4. In a frequency modulation receiver, a frequency discriminator including a pair of load resistors across which direct current voltages appear as a result of frequency discrimination, a conductive impedance element for connecting said resistors in series-aiding relation with respect to said voltages, a slow-acting circuit for applying the resultant of the voltages to a portion of the receiver 'as a first gain control voltage thereof, means for deriving from the received frequency modulated signal a second control voltage which is representative of relatively f-ast amplitude variations of the signal, a circuit connected to said -resistors for deriving a detected modulation voltage representative of desired frequency modulation of the received signal, said means for deriving comprising a rectifier in circuit with said discriminator, a third load resistor in circuit with the rectifier, means connecting one of the two load resistors and one terminal of the third resistor in series-aiding relation with respect to said second control voltage, and a fastacting circuit connected to the other terminal of the third resistor.

WINFIELD R. KOCH.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS- Number Name Date 2,057,640 Conrad Oct. 13, 1936 2,264,724 Schonfeld Dec. 2, 1941 2,330,902 McCoy Oct. 5, 1943

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