US2361625A - Frequency and phase modulation receiver - Google Patents

Description

oct. 31, 1944. C AW, HANSELL 2,361,625

FREQUENCY AND PHASE MODULATION RECEIVER Filed Dec. 22, 1941 3 Sheets-Sheet l d INVENTOR Za/Jene@ .IYamx/elly ATTRNEY Oct. 31, 1944. Q W HANSELL 2,361,625

FREQUENCY AND PHASE MODULATION RECEIVER Filed Dec. 22, 1941 3 Sheets-Sheet 3 Patented Oct. 31,1944 2,361,625

UNITED STATES PATENT oFFicE FREQUENCY AND PHASE MOD'UIATION RECEIVER Clarence W. Hansell. Port Jefferson, N. Y., assignor to Radio Corporation oi America, a corporation of Delaware Application December 22, 1941, Serial No. 423,881

19 Claims.

My present invention relates generally to receivers of frequency modulated carrier waves (FM) and phase modulated carrier waves (PM), and has particular reference to improved and simplified circuit arrangements for rendering such receivers substantially unresponsive to am plitude modulation (AM) of the carrier currents.

In receivers for signals transmitted by means of frequency and phase modulated radio waves, of which reception of frequency modulated (FM) radio broad-cast programs is a common example, there are undesired amplitude modulations of carrier currents in the receivers which tend to add unwanted noise to the output of the receivers. These amplitude modulations usually are present because of hum modulations in the transmitter; because of interfering radiations produced in the space circuit due to natural causes, or due to operation of electrical equipment of many kinds; because of hum and microphonic noises in the receivers; because of thermal agitation and shot eil'ect noises in early circuits in the receivers; and because of insuiiicient uniformity, or fiatness of response, of selective circuits within the frequency band occupied by the frequency and phase modu lated waves and currents. In the prior art it has been customary to employ an amplitude limiter for removing amplitude modulations from the carrier current before the carrier current is applied to a frequency, or phase, modulation demodulator. By removing amplitude modulations with a limiter it has been found possible to make a very substantial improvement in signal to noise ratio in the output of frequency and phase moduiation receivers.

Limiters of the prior art have usually been some form of amplifier so designed, adjusted and operated that changes in input power gave little, if any, change in output power. These liimters require some input power level to be exceeded before a satisfactory limitng condition is reached. and. often. they show a reduction in output power if the input power becomes relatively high. Within a range of input power levels the output power from the limiters may be nearly constant.

In designing limiters it has been found that the amplifier-limiter system preceding the demodulator must be capable of great overall amplification, or increase of power, when weak sig nais are to be received. To provide for this great amplification and for adequate limting at the same time, it has been necessary so to construct (Cl. Z50-27) frequency and phasf` modulation receivers that K modulation has not increased as rapidly as might be desired, and some receiver manufacturers, to avoid the cost, have sold frequency and phase modulation receivers to the public which omitted amplitude limiters even though such receivers do not adequately suppressV amplitude modulation noise.

In this application, wherever used, the generic terms angular velocity modulated carrier waves and timing modulated carrier waves, are to be understood as covering either frequency or phase modulated carrier waves, or combinations thereof. In this connection it is pointed out that frequency modulations of a carrier current in which the higher modulation frequency currents are emphasized or creased with respect to lower frequencies res ts in a type of modulation which is more accurately described as phase modulation than as frequency modulation. Thus, the above terms are intended to include any form of frequency modulation, or any form of phase modulation, with any kind of modulation frequency emphasis or deemphasis including the kinds described in my U. S. Patent No, 2,179,182, issued November '7, 1939.

One of the objects of my present invention is to provide frequency and phase modulation receivers without amplitude limiters, but having overall characteristics similar to those obtainable through the use of limiters so that the cost of good receivers may be reduced. In accomplishing this purpose I provide a demodulator for frequency and phase modulation which, when provided with a correct level of input power, is inherently unresponsive to the effects of amplitude modulation so that no limiter is required.

The new type of demodulator, in its preferred form, comprises a pair of amplitude modulation detectors of the grid rectification type with their two demodulated outputs combined differentially, in combination with frequency selective dis- `criminator circuits for converting frequency modulation into differential amplitude modulation of the inputs to the two detectors. This demodulator is so designed and operated that, over a range of input power levels, the demodulated output is substantially free from the eiects of amplitude modulation of the input power.

A further object of the invention is to provide a frequency modulation demodulator system having low response to amplitude modulation over a range of amplitudes, and which also provides for deemphasis of the highermodulation frequency currents in frequency modulation receivers so that no separate circuit elements are required to accomplish the deemphasis. Thus, the system is adapted to reception oi frequency modulated transmitters inV which the higher frequency modulations have been increased in relative strength to reduce the effects of noise.

Another object is to provide a demodulator substantially correctly responsive to phase modulation, but unresponsive to amplitude modula tion.

Another object is to combine with the new demodulating system, in frequency and phase modulation receivers, manually operated and automatic volume control circuits for increasing the percentage range of receiver input power levels over which the receiver as a whole is substantially, unresponsive to amplitude modulations. This automatic volume control is not to be confused with devices to accomplish limiting, which are instantaneous in their action, whereas automatic volume control is relatively slow acting, being too slow to affect amplitude modulations substantially within the useful range of modulation frequencies.

Other objects of the invention are to improve generally the simplicity and efliciency of frequency and phase modulation receivers, but, more especially, to eliminate any need for an amplitude limiter.

In the drawings:

Fig. 1 shows one embodiment of the invention,

Figs. 2 and 3 show some experimental response characteristics obtained with the arrangementy of Fig. 1.

Figs. 4 and 5 show modifications of the invention,

Fig. 6 shows a block diagram of a complete frequency and phase modulation receiver including the demodulator of Fig. 4.

Referring now to the accompanying drawings, wherein like reference characters in the different figures designate similar circuit elements, there is shown in Fig. 1 a demodulator network constructed according to my present invention. In general, the demodulator circuit comprises a pair of tubes l and 2. While shown as of the triode type, lt is to be understood that the tubes may be of any other desirable type. For example` each of the tubes could be of the 6SJ7 type, used in some experiments made in connection with the present invention, wherein the screen grid, suppressor grid and anode are tied together. The cathodes of the detector tubes are connected together to provide a common cathode circuit, and the common cathode connection is established at ground potential. The control grid of tube l is connected to its cathode through a path comprising the grid leak resistor 3, shunted by carrier bypass condenser 4, and the resonant circuit l. The control grid of tube 2 is connected to its cathode through a series path comprising grid leak resistor and the parallel resonant input circuit 8, the grid leak resistor 5 being shunted by carrier bypass condenser 6.

The anodes of the detector tubes are connected to the positive terminal of'a common direct current energizing source. The anode of tube I is connected to the anode of tube 2 through the series anode load resistor I3, and the approximate mid-tap of this resistor is connected to the positive terminal of the direct current energizing source. The carrier bypass condenser Il is connected between the anode of tube I and the grounded cathode connection, and carrier by-` pass condenser l2 similarly is connected between the anode of tube 2 and the grounded cathode (l ll connection. The output connections I4 take off the modulation voltage developed across the anode load resistor I3, and transmit the modulation voltage to any desired utilization network.

Where the modulation voltage is of audio fre quency, the modulation utilization network would usually embody one or more audio amplifier stages followed bya reproducen The parallel resonant input circuits 'l and 8 are yoppositely mistuned by substantially equal frequency amounts with respect to the center frequency Fc of the applied FM waves. While it is not believed necessary to give specic frequency values, those skilled in the art know that for receiving standardized frequency modulation transmitters in the United States frequency F1 ci' the input circuit l may be removed some 75 kilocycles (kc.), or more, from Fs, whereas the frequency F2 of input circuit 8 may be correspondingly located 75 kc. or more on the opposite side of Fc. However, by tuning the circuits apart by a greater amount, and utilizing sufficiently broadly tuned circuits, less distortion in the demodulated output from the demodulator may be obtained. Thus, by broadening the cir cuit tunings and setting the tuning peaks further apart in frequency in a manner to reduce the percentage differential response of the circuit to a given change in frequency, the distortion of the detector systems output may be reduced. For additional information on this subject reference is made to my application Serial No. 297,777, filed October 4, 1939, which is a division of my U. S. Patent No. 2,179,182, issued November 7, 1939.

Where the receiver is of the superheterodyne type, which ls the type universally employed in present day FM broadcast reception, the input circuits of the detector tubes will be operating at a. lower or intermediater frequency (I. FJ. Such I. F. value would generally be chosen from a range of between 2 and 8 megacycles (mc). The assigned FM broadcast band in the United States at the present time is 42 to 50 megacycles, and the permissible channel width is 200 kc. As is well known, the carrier is deviateda maximum of substantially 75 kc. to either side, and, hence,

in the best receivers, the lntertube networks preceding the demodulator of Fig. 1 are designed to transmit nearly equally a band Width of approximately kc. plus twice the band width of the modulation frequencies.

It will be understood that the primary circuit 9, which is magnetically coupled with input circuits l and 8 respectively, will be tuned to Fc but will be a broad band circuit capable of responding substantially equally to the Whole signal band. The primary circuit 9 may be arranged in the anode circuit of the last I. F. amplifier stage, and each of networks 9 and 1; 9 and 8 may be arranged to have a pass band of the required kc. width. It is not believed necessary to describe the networks between the signal collector and the circuit 9 since they may follow standard FM broadcast receiver practice, and those skilled in the art are fully aware of the construction thereof. It is to be clearly understood that in the present case no amplitude limiter is utilized in the system preceding the input to the demodulator at coil 9. Since no limiter is employed, it will be clear that there will be imv pressed upon the input circuits 1 and 8 the FM waves `whose carrier has imposed thereon AM components. These AM components, as stated before, are due to noise and unwanted responses in the cascaded frequency selective circuits prior to the demodulator. The demodulator system shown inherently functions to suppress effects of these AM components in the output from the receiver. Thus, at no point in a receiver employing the present invention, is there any production or utilization of carrier current power which has been subjected to amplitude limiting. This is entirely the opposite practice of what has been cause equal potential drops across the two portions of resistor I3. As the input to the demodulator is increased, 'the anode current flowing in each tube will decrease. At the mid-frequency Fc, where the two inputs are equal, the anode currents will decrease equally and the voltage drop across resistor I3 will be substantially zero' for all values of input. When the applied frequency is shifted to one side so that circuits l and t deliver unequal inputs to the two tubes I and 2, the grid bias of one tube will increase faster than the other as the input voltage is increased. Hence, the anode current of one tube will, at first, decrease faster than the other, but atsome very large value of input both tubes will be practically eut off. The result is that the output potential appearing across Iii will rise from zero, go through a maximum, and return to low values as the input is increased.

At the point where the output stops increasing and begins to decrease again, for increasing input. the rate of change of output with respect to input, which is a measure of amplitude modulation response, will be zero. All that is necessary. then, to obtain an output due to the frequency modulation components without the AM components is to make the maximum portion of the input vs. output voltage characteristic flat, or constant, over a large range of inputs sumclent to contain the percentage of amplitude modula tion which may be encountered in practice. This is automatically accomplished to a considerable degree in the arrangement of Fig. 1 by utilizing the grid leak and condenser type of detector circuit, and by designing and operating the receiver to provide an input level to the demodulator which will obtain substantially minimum response to amplitude modulation.

In practice it has been found that a greater percentage range of inputs might be applied without substantial change in output if relatively large values of grid leak resistances 3 and 5 are used. In a particular case, grid leak resistances of 0.5 megohm gave about a 2 to 1 range of input amplitude within which the output potential was nearly constant. When the resistances were each increased to 2 megohrns a Very substantial increase in the percentage range of input which would not produce a substantial change in output was noted. A further increase to 10 megohms gave a range of over ten to one in input amplitude within which no substantial amplitude modulation response was obtained.

Eig. 2 comprises a setof experimentally obtained response characteristics of a demodulator of the type illustrated in Fig. 1. In taking these characteristics the frequency of the input current was set ofi.' balance with respect to the resonant frequencies of circuits I and 8, but within the range of operating frequencies which would be utilized in practice. Then the amplitude of input potential and current was varied, and the resulting output potential at Il was measured. As may be seen from the curves of Fig. 2, the higher values of grid leak resistance gave the greatest range of input within which the rate of change of output with respect to input was small. Curves I to 4 were secured with grid leak resistors of 0.5; 2; 5 and 10 megohms respectively. Furthermore, as will be observed from the curves, the higher values of range likely to be occupied by the frequency modulated carrier current. (In the figure the indicated frequencies are not accurately determined so that they cannot be used to construct curves of frequency modulation response.)

In using large grid leak resistances the time constant of the grid leak and condenser circuit tends to become too great to permit full response of the grid bias potential to changes in input at the higher modulation frequencies. Therefore, the relative differential response of the anode currents of tubes I and 2 to frequency modulation of the input current tends to decrease with increasing modulation frequency. This brings about a deemphasis of higher modulation frequencies which can be made substantially to balance preemphasis of higher modulation frequencies required by the Federal Communicaamplitude modulation over a large range of input amplitudes, but which also accomplishes deemphasis of the higher modulation frequency output currents to correct for preemphasis introduced at the transmitter.

In those cases where no preemphasis is used at a frequency modulation transmitter I may still use a large time constant in the grid leak biasing circuits, Ain order to obtain a large range of inputs within which amplitude modulation responses are small if, in the circuits coupled to output I4, I introduce equalization of the modulation frequency currents. The art of equalizing frequency responses in communications circuits is so well known as not to require exposition here. See, for example, my Patent No- 2,179,182, and my application No. 297,777, led October 4, 1939. which describe a system of emphasis and deemphasis which ymay frequently be used in combination with the present invention.

The demodulator of Fig. 1, when provided with sulciently long time constants in the grid leak biasing circuits, is automatically adapted to reception of phase modulated carrier current signals, since phase modulation is indistinguishable from frequency modulation in which the degree of frequency modulation response of the transmitter is made inversely proportional to modulation frequency. In other words, to a first degree of approximation, the present standard system of preemphasis of higher modulation frequencies in FM broadcast transmitters is equivalent to phase modulation.

For the purposes of the present invention, as stated before, I consider that frequency and phase modulation, or any hybrid or compromise between them, are one and the same thing since, by emphasis or preemphasis of higher or lower modulation frequencies at the transmitter we may make it impossible to distinguish between them at the receiver. Either type of modulation, or any compromise between' them, such as was outlined, for example, in my Patent 2,179,182, and in my application No. 297,777, iiled October 4, 1939, may be used with my invention. It is only necessary to so equalize the overall responses of the system to currents of various modulation frequencies that a desired overall result is obtained.

In the arrangement of Fig. 1, in order to compensate for differences in characteristics of vacuum tubes available commercially, I prefer to make the values of the two portions of resistance I3, between the power lead and the tube anodes, variable for obtaining an optimum balancing of amplitude modulation response currents. In ordinary receivers this adjustment of the relative values of the resistances will be required only infrequently, or when tubes are changed, and will ordinarily not need to be available to the usual listener. Also, lin'order to provide for manual volume control of the receiver I may provide for varying the connections of output leads I4 to the output resistance I3 or, if preferred by the designer of a receiver, the manual volume control may be inserted at any later point, or stage, in the receiver. While I have shown circuits 1 and 8 variable, or adjustable, in tuning it will be understood that in practice the adjustment will usually be done in the factory where the receiver is manufactured, and may sometimes be done by trained service personnel after the receiver has been used.

The discriminator circuits 9, 'I and 8 of Fig. 1 are similar to those described in Usselman Patent 1,794,932. However, any other balanced or differential type of discriminator circuits may be used. In Fig. 4 is illustrated one of a number of other discriminator input networks to tubes I and 2 that may be used. 'I'he discriminator there shown has been described and claimed by S. W. Seeley in U. S. Patent 2,121,103, granted June 21, 1938. Since the discriminator shown in Fig. 4 is very well known at the present time, it will be suicient to describe it in general terms. The primary tuned circuit 9 is xedly resonated to Fc, which may be the operating I. F. value. The secondary tuned circuit 'I' is xedly tuned to Fc. A direct current blocking condenser I5 is connected between the high potential side of primary circuit 9 and the midpoint of the secondary coil. A large blocking condenser I6 is connected between the negative direct current supply lead and the low potential side of primary circuit 9.

The grid of tube I is connected to one side of' the secondary circuit 'I' through a carrier bypass condenser 4, while the grid condenser 6 connects the opposite end of secondary circuit 'I' to the grid of tube 2. The junction of the grid leak resistors 3 and 5 is connected to the common cathode connection of the detector tubes. As is well known to those skilled in the art, the discriminator input network functions to provide maximized signal voltage, to one tube grid or the other, on either side of Fc depending upon the frequency deviation of the I. F. carrier. In effect, the discriminator input network in Fig. 4 develops signal Voltage on the detector tube grids in the same way as signal voltage is developed in the case of Fig. 1. This is brought about by the shift in phase of carrier potential across tuned circuit 'I'with respect to the potential applied through condenser I5 as the frequency is varied. The demodulator of Fig. 4 functions in substantially the same manner as that in Fig. 1 in providing response to frequency modulation substantially without response to amplitude modulation of the input carrier current.

The discriminator input network in Fig. 5 is that`disclosed by E. H, Armstrong in his U. S. Patent 1,941,069 granted December 26, 1933. Briefly, in that circuit the source of FM waves, which may be the output circuit of the prior I. F. amplier, has its FM voltage applied to a pair of series resonant circuits each of which includes a resistor. Thus, the series resonant circuit comprising coil I 'I and condenser I8 is connected in shunt with the grid leak and condenser 3 6. The junction of coil I'I and grid condenser Il is connected by resistor I9 to the high potential side of the input circuit. The second series resonant circuit I'l-I8' is connected in shunt withy grid condenser and grid leak 6 5, while the resistor I9 connects the junction of coil Il and condenser 6 to the high potential side of the input circuit. The low potential side of the input circuit is connected to the junction'of condensers I8 and I8. As pointed out in the Armstrong patent, the series resonant circuitsA are oppositely mistuned by equal frequency amounts with respect to Fc. The plate circuits of the detector tube are connected so as to respond cumulatively for frequency variations, but differentially for amplitude modulation. The resistors I9-I9' are used to provide a substantially constant current, for any constant input potential, to the discriminator circuits l'I-I8 and I'I-I8 within the signaling band, even though the series resonant circuits change impedance with change of frequency.

It will be noted that the output of the demodulator in Fig. 5 is taken through a transformer 20 in place of the resistance couplings of Figs. 1 and 4. Thechoice of the type of output coupling is a matter to be chosen by the designer of the receiver for any particular purpose. There is, also, shown a resistance 2I in thepower supply lead to the center point of the primary winding of transformer 20. The resistance is not essential, but may sometimes be desired to modify the response characteristics, to provide a coupling for automatic volume control, or for shortcircuit protection. It will be apparent that any other discriminator circuit capable of converting frequency modulation into differential amplitude modulation may be used.

In the arrangements of Figs. l, 4 and 5 the grid to cathode paths in the tubes function as diode rectiiiers of the input signal currents. Obviously, if desired, separate diodes may be employed within either the same or different evacuated envelopes to accomplish rectification and control of grid bias potential of tubes I and 2. Again, if desired, I may insert coupling tube ampliiiers between circuit 3 and the circuits l and 8 of Fig. 1, and the corresponding circuits ln Fig. 5. There also may be used coupling tube ampliiiers between the frequency selective discriminator circuits of Figs. 1, 4 and 5 and the grid, or diode, rectiilers. I realize that by using prin. ciples exemplified by the variable mu type oi vacuum tube amplifiers I may obtain improved demodulator response characteristics through use of tubes especially designed for best results in my demodulator system. With remote cut-off tubes the demodulator will be given a larger percentage range of input within which amplitude response in the output is low.

Since in practice an FM receiver is required to function over a very large range of radio frequency power input levels, principally because of being tuned to nearby or distant transmitting sta-- tions, the percentage range of small response to amplitude modulation illustrated in Figs. 2 and 3, while large, is not likely to be sufficient to cover the entire range of possible input power levels. Therefore, it is desirable that FM receivers employing this new type of demodulator be equipped with some means of regulating the power level at the input to the demodulator. One simple means for doing this is to insert a manually controlled attenuator in the coupling between the antenna and the receiver so that the level at the demodulator may be adjusted by the person who operates the receiver. The level may be observed on an anode circuit milliameter or voltmeter, or on some type of cathode ray indicator tube. Alternatively, and preferably, the attenuator is located between stages in the receiver, sufiiciently far from the antenna input connection to assure that an adequate or optimum ratio of signal to thermal agitation and shot eiect noise is obtained down to the lowest possible input signal levels.

However, for all receivers, such as broadcast receivers, which are to be sold to the public, for all receivers requiring frequent tuning to transmitters of various received power levels and for use on radio circuits where fading may occur, I prefer to provide automatic volume control for the power input to the demodulator. This is readily accomplished by means similar to those already known in the art for use with other receiver combinations. One such automatic volume control system is illustrated in. Fig. 6. Referring, in general, to Fig. 6 I have shown a rectifier coupled to the intermediate frequency amplifier output circuit which delivers power to the demodulator. This rectifier serves to'provide a direct current and potential proportional to the mean, or average, carrier current power level at the demodu lator. This direct current and potential is then utilized to control the bias potentials and amplication of vacuum tubes preceding the rectier and the demodulator. The polarity of the control is so chosen that the rectified current, by controlling power gain of vacuum tubes preceding the rectiier, tends to hold itself constant. Thus, the receiver of Fig. 6 is capable of functioning to suppress the effects of amplitude modulation of received carrier currents having a far greater percentage range of power levels than can be accommodated by the demodulator alone.

It will be understood that the rectified current and potential used for automatic volume control (AVC) is made relatively slow acting, by means of a low pass filter or its equivalent, so that variations in gain due to automatic volume control are too slow to follow the lowest required modulation frequency to any substantial degree. Therefore. for all practical purposes, the automatic volumercontrol does not ail'ect the wave form of amplitude modulations of the carrier current, and it does not function like a limiter.

Since frequency modulation, generally, is usedA to correct, or adjust, the frequency of the heterodyne oscillator of the receiver.

Considering the system of Fig. 6 more specif-I ically, the signal pick-up device 30 may be a dipole, grounded antenna circuit or a radio frequency distribution line. The superheterodyne receiving system shown embodies a radio frequency amplifier 3| of one or more tunable stages. The first detector 32 also has a tunable input circuit; the local oscillator 33 has a tunable circuit 34. As those skilled in the art fully know, the tunable signal circuits usually are concurrently tuned to the center frequency (Fs) of the desired FM channel. The circuit 34 is concurrently tuned to a local oscillation frequency differing from Fc by the operating I. F. valve. The I. F.

amplifier 35 has its input electrodes coupled to the rst detector 32 by the I. F. transformer 36. Of course, additional I. F. stages may be used prior to amplifier 35.

The AVC rectifier 31 has its anode coupled to the anode circuit of amplifier 35, the cathode of the diode is at ground potential. -The load resistor 38, in series with an I. F. choke coil 33, is connected between the diode anode and ground. The usual AVC connection 40 is made from the control grids of the, various controlled amplifiers to a desirable point on load resistor 38. As pointed out before, the AVC line includes a low pass filter 4l to render the AVC relatively slow acting. The usual filter resistors 42 are also included in the control connections to the controlled amplifiers.

Since the demodulator shown in Fig. 6 is substantially similar to that shown in Fig. 4, it isv not necessary to describe those connections in any detail. Of course, either, of the demodulator circuits of Figs. 1 or 5 may be used in place of that shown in Fig. 6. The output circuit of the demodulator in Fig. 6 di'ers from that shown in Fig. 4. To derive the aforementioned AFC voltage from the demodulator, a split primary winding 50-5i is used in the output transformer 52. Each of the split winding sections is fed from the +B terminal of the power supply source through a resistor. Thus, resistors 53 and 54 are in the power supply connections to Winding sections 50 and 5| respectively. The anode ends of the resistors 53-54 are bypassed to ground for both I. F. and modulation frequency currents. The AFC potential is derived from the anode ends of resistors 53 and 54. The AFC leads are designated by the numeral 60. As is very well known to those skilled in the art, the AFC bias fed over connections 60 varies in polarity and magnitude in a sense to correct the tuned circuit frequency of the local oscillator 33 so as to maintain the correct value of the I. F. f

The AFC bias is usually applied to a frequency control tube, schematically represented as lil which functions to provide a simulated reactance eect across the tuned circuit 345. This is too well known at the present time to require fur2 ther description. For example, the frequency control tube arrangement shown in Seeley Patent 2,121,103 may be used for the frequency correction action. It is to be clearly understood, however, that both the AFC and AVC systems shown in Fig. 6 are purely illustrative. In the discriminator of Fig. 6, as well as of Fig. e, the midtap of the coil of circuit 'l' may, in some instances, have a resistor between it and ground to provide for some desired loading and broad ness of tuning of circuit d', or to drain off direct current leakage through other condensers. Condenser i may be omitted, if desired. It is my present intention to utilize any AVC, or AFC, system known to those skilled in the art of constructing AM or FM broadcast receivers. Further, my present invention is not limited to radio reception but may be used in connection with any system of intelligence transmission by means of frequency and phase variations of carrier waves.

While I have indicated and described several systems for carrying my invention into effect, it will be 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 or 'my invention, as set forth in the appended claims.

What is claimed is:

1. A demodulator of frequency modulated waves comprising a. frequency responsive discriminator circuit for obtaining differential amplitude modulations in response to irequency variations, a pair of grid leak detector vacuum tubes with outputs combined differentially, means to adjust the amplitude of input to the demodulator to such a high value so as to obtain low output response to amplitude modulations the grid leak and condenser circuits of the detectors having sufficiently long time constants to compensate in the output of the demodulator for preemphasis of some modulation frequencies at the transmitter.

2. A demodulator of frequency modulated waves comprising a frequency responsive discriminator circuit for obtaining differential amplitude modulations in response to frequency modulations, a pair of grid leak detectors coupled to the discriminator circuit having their outputs combined differentially, means providing said demodulator with a mean amplitude of power input adjusted to such a high value so as to result in low output response of the demodulator to amplitude modulation of the input said detectors having leaky grid condensers whose time constants are sumciently high so as to provide deemphasis of higher modulation frequencies.

3. A demodulator of frequency modulated power comprising a discriminator for producing two potentials which vary differentially as the frequency is varied; means responsive to these potentials for decreasing two currents as the po tentials increase; said last meansI being additionally constructed to provide deemphasis of higher modulation frequencies; means for deriving output power from differential variations of the two currents: and means for adjusting power input to the discriminator to a high level which will accuses give low response to amplitude modulations in the output from the demodulator.

fi. A demodulator ci frequency modulated pow er comprising a discriminator for converting ire quency modulations into differential module tions or two potentials; a pair of pushmuil leak detectors each supplied with input by 1 of the two differentially medulated patenti-ais, said detectorsk having grid leal.: resistors whose value is of the order of l0 megehms. manuaily adjustable means for adjusting the power in to the discriminator to a value which low disturbance from amplitude `niodulatio the output from the demodulator over a range of power input levels.

5. A demodulator of frequency modulated er comprising a discriminator for converting uuency modulations of a carrier current ferential amplitude modulations of two ce" currents; a pair of grid leak detectors with outputs combined differentially, each p with input of one of the differentially am modulated carrier currents, the grid l sistor of each detector having a resista. of the order of l0 megohins, and automatic for holding the input to the demodulator a range of relatively high power levels whie" suits in small disturbances from amplitude ulations of the input upon the wave form strength of demodulated output.

6. In combination, a source or" ang-ula waves, a demodulato tubes tector tubes, each oi sait. separate input circuit,

d l Sai spect to the center frequency ci applied waves, and means for combining the output currents leak detector tubes each including a grresistor whose magnitude is of the order o megohms and a shunt condenser, and the ra ance values of said grid leak resistors intensity of said waves being chosen suciently high to cause substantially complete suppression of amplitude modulation components of said waves.

7. 'in combination, a source of angular velocity-modulated carrier waves, a demodulator net work including a pair of separate grid leai; detector tubes, each of said detector tubes having a separate input circuit, said separate input circuits being oppositely and equally mistuned with respect to the center frequency of applied Waves, and means `for combining the output currents of said detector tubes in phase opposition, a grid leak resistor arranged in the grid circuit of each detector tube, a by-pass condenser in shunt with each respective grid leak resistor, and the time constant of each pair of grid leak resistors and shunt condensers being sufficiently long to pro vide deemphasis of higher modulation frequencies.

8. In combination, in a receiver of angular ve locity-modulated carrier Waves, a. demodulator nework including a pair of separate grid leak detector tubes, a. network feeding said waves to the demodulator, an automatic volume control circuit regulating said network, each of said detector tubes having a separate input circuit, said separate input circuits being oppositely and equally mistuned with respect to the center frequency opposition, said grid leak detector tubes each including a grid leak resistor and a shunt condenser, and the resistance values of said grid leak resistors being of the order of 10 megohms thereby to cause substantially complete suppression of amplitude modulation components of said waves.

9. A demodulator of frequency modulated Waves comprising a frequency responsive discriminator circuit for obtaining differential amplitude modulations in response to frequency variations, a pair of grid leak detector vacuum tubes with outputs combined differentially, said grid leak detectors each including a grid leak resistor and a shunt condenser, the resistance Value of each leak resistor being chosen from a range of 0.5 to 10 megohms thereby to cause substantially complete suppression of amplitude modulation components of said waves over a wide range of intensity thereof and the time constant of each grid leak and condenser circuit being sufficiently great to provide deemphasis of higher modulation frequencies.

10. In combination, in a receiver of angular velocity-modulated carrier waves, a demodulator network including a pair of separate grid leak detector tubes, a network feeding said waves to the demodulator, an automatic volume control circuit regulating said network, each of said detector tubes having a separate input circuit, said separate input circuits being oppositely and equally mistuned with respect to the center frequency of applied Waves. means for combining the output currents of said detector tubes in phase opposition, said grid leak detector tubes each including a grid leak resistor and a shunt condenser, the resistance values of said grid leak resistors being chosen from a range of 0.5 to 10 megohms to cause substantially complete suppression of amplitude modulation components of said waves, and the time constant of each pair of grid leak resistors and shunt condensers being such as to provide deemphasis of higher modulation frequencies.

CLARENCE W. HANSELL.

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