US2165252A - Phase modulation receiver - Google Patents

Description

July 11, 1939. H. E. GQLDSTHNE 2,355,252

PHASE MODULATION RECEIVER v Filed Feb. 10, 1957 2 Sheets-Sheet 1 AMP. +8

INVENTOR HALLAN E.GOLD5T|NE vkwp ATTORN EY y ,1939. H. E. GOLDSTINE PHASE MQDULATION RECEIVER Filed Feb. 10, 1937 2 Sheets-Sheet 2 4 4L m r m m .v N a m f 04 0 M a R C l .h F, .A 0 K I w I 0 H 2 pm .I K h mm 2 r. 7 MM M 11v C 7 2 f INVENTOR HALLAN E. QOLDSTI NE BY ATTORNEY Patented July 11, 1939 2,165,252 PHASE. MODULATION RECEIVER Hallan Eugene Goldstine, Port Jefferson, N. Y., as-

signer to Radio Corporation of America, a corperation of Delaware Application February 10, 1937, Serial No. 125,080

11 Claims.

This invention relates to the art of reception and deals especially with the art of receiving phase modulated signals. The method herewith disclosed deals with using a frequency modulation 5, receiver and by means of radio frequency selectivity, decreasing the high modulation frequencies which are predominant in a frequency modulation receiver when receiving phase modulation, so that the audio output may be essentially flat over the audio frequency range when receiving a phase modulation signal.

More in detail this disclosure concerns a phase modulated Wave receiver of the type wherein to receive phase modulation a frequency modulation receiver is used in conjunction with a correction circuit which converts the frequency modulation receiver output into a phase modulation output. Instead of the usual audio or modulation frequency correction circuit used for this purpose a high frequency or an intermediate frequency correction circuit, or both, is employed in the present system so that the phase modulation on the wave is corrected to frequency modulation before being impressed on the frequency modulation receiver portion of the system.

In the prior art, a receiver of the nature of the type herein disclosed consisted of a frequency modulation receiver with an audio frequency correction circuit inserted in the receiver output leads. Crosby UnitedStates application #618,154 filed June 20, 1932 describes such a frequency modulation receiver for reception of phase modulations. Owing to the inherent difference between frequency and phase modulation, if phase 35 modulation is received on a frequency modulation 7 receiver without the aid of a correction circuit, the receiver output is distorted in the respect that the output is directly proportional to the frequency of modulation.

It is well known that the expression for a phase modulated wave is given by e=E sin (wt-F4) sin pt) (1) where w is 21r carrier frequency, f0, is the phase deviation with modulation, and p=21r the modulation frequency" fm. wave of Equation (1) is given by its rate of change of phase or From (3) it can be seen that the effective frequency deviation of a phase modulated wave is directly proportional to and fm. Consequently,

The frequency of the the output of a frequency modulation receiver, receiving phase modulation, would be directly proportional to the modulation frequency im, in addition to the depth of modulation, Therefore, in order to remove this distortion so that the re- [5 ceiver reproduces all modulation frequencies with the same amplitude, a correction circuit must be inserted which applies acorrection inversely proportional to the modulation frequency. In the receiver of Crosby United States application #618,154 filed June 20, 1932, this correction was applied in the output audio frequency circuits.

In the receiver of the present disclosure, the correction-is applied to the modulated wave in such a manner that the side bands on both sides 5 of the carrier are attenuated an amount directly proportional to their frequency spacing from the carrier. In this manner, the depth of phase modulation on the wave is made inversely proportional to the modulation frequency. Consequently, the directly-proportional-to-frequency output, produced by the reception of phase modulation on a frequency modulation receiver is compensated for.

The phase modulated wave which has been corrected to have the characteristics of a frequency modulated wave may be impressed on any type of frequency modulation receiver, but I prefer to use a frequency modulation receiver of the back-to-back type because the filter circuits thereof may also act as the correcting'medium to attenuate the sidebands in proportion to their spacing from the carrier.

In the prior art practice amplitude modulation is normally used although phase modulation, from the transmitter standpoint, has decided advantages. However, in the prior art practice of receiving phase modulation, equipment is usually very elaborate or certain benefits that are gained in the transmission are lost by the receiver and 4, the usual practice of using a frequency modulation receiver and distorting the audio output by means of certain reactance combinations to lower the high modulation frequencies in a certain relationship so that the audio output will be essentially flat over the frequency range has disadvantages in that this system usually decreases the. low frequency output to a certain extent and also has a tendency to increase the effective noise level while in the method given herethe noise level is 550 reduced. The receiver of the present invention is also simplified because the circuits which are used in the receiver to reduce the high frequency response, i. e., attenuate the side frequencies remote from the carrier can be incorporated as a normal part of the receiver and do not require extra correction circuits. The receiver of the present invention also acts to augment the carrier, which has certain advantages such as reduction of fading etc., which have been pointed out before.

The novel features of my invention have been set forth in the claims appended hereto. The nature of my invention and the operation thereof has been described in detail in the specification which follows. In describing said invention and operation reference will be made to the attached drawings wherein,

Figures 1a, 1b and 10 show curves illustrating the operation of various portions of the circuits of my receiver. In Figure 1b I show the resonant curves of the two detectors used to demodulate the phase modulated wave and the over-all selectivity of the receiver amplifier and the back-to-back detectors. In Figure lo I show the superimposed or combined characteristic curves of the two detectors as operating in the back-to-back receiver of the present invention when the said detectors are preceded by a selective amplifier system arranged in accordance with the present invention. Figure 1a, also illustrates the corrected resonance curve denoting the selectivity of the circuits preceding the detectors. Figure 1bshows the separate selectivity characteristic curves ofthe detectors of the back-to-back detector system. Figure 1c illustrates the over-all frequency response curve of the audio output obtained from a receiver having the selectivity features of the present invention in the circuits preceding the back-to-back detectors. Figure 1 illustrates the essential elements of a phase modulation receiver arranged in accordance with my invention.

The main features of this invention consist of having the required radio or intermediate frequency selectivity. If proper selectivity is obtained in a straight radio frequency receiver it is not necessary to useintermediate frequency selectivity for obtaining the desired effect. However, in short wave work it is usually necessary to use intermediate frequency selectivity to obtain the proper characteristics to give the proper audio output response of the receiver. The receiver here disclosed uses a frequency modulation receiver of the superheterodyne type where the final detection takes place in the intermediate frequency circuit, which circuit in the present invention is known as a back-to-back receiver circuit of the type disclosed and claimed in Usselman United States Patent #1,'794,932. By havingsuitable selectivitybefore this back-to-back detector the frequency modulation receiver can be made into one suitable for receiving phase modulation. It is possible to combine several stages of intermediate frequency amplification to get the desired selectivity curve and in Figures 1a, 1b, lo I have shown by hypothetical curves radio frequency characteristics by means of which the audio frequency response may be made suitable for receiving a phase modulated wave in accordance with the method of this invention.

In Figure 1, I have shown a receiver which includes the essential features of the present invention. This receiver comprises an oscillator d of the regenerative coupled tube type having coupled circuits 8 and lb which cooperate to produce oscillations of a frequency determined by the tune of 8. The oscillator 4 is coupled by an in ductance I3 to the input inductance [6 of a first detector l2 which is also coupled to an antenna system H and by a parallel circuit i8 to an intermediate frequency amplifier tube M. If sufficient selectivity can be obtained in the detector and oscillator and the back-to-back receiver which follows a single intermediate amplifier may be used. If not, an additional stage l4 may be added in the usual manner as shown. This stage may be coupled as shown by low loss circuits to the preceding stage and to the second detector. The input and output circuits l6 and 18 of the first detector [2, and the input and output circuits 28 and 22 of the intermediate frequency amplifier M and the input circuit 28 of the intermediate frequency amplifier US are of low power factor or low loss to further increase the selectivity of the system. Phase modulated signals may be impressed on the antenna system H and from said system to input circuit 16 through an electrostatic shield or screen S. The signals of intermediate frequency are sup.- plied from a low loss circuit I8 to the low loss input circuit 26 of the intermediate frequency amplifier M and from the low loss circuit 22 to the low loss circuit 2% and from low loss circuits 2b to the input of M which has in its output circuit two resonant circuits 23 and 24 which are also preferably low loss circuits. Although I have shown two intermediate frequency stages M and 14', one or several similar stages may be used as long as the high Q circuit or circuits give the desired selectivity. In the proper frequency range piezo-electric crystals or resonant lines may be used. It is also possible, if a high Q circuit is used, to obtain both the necessary frequency discrimination and also the necessary detection characteristic in one circuit if the circuit has an over-all characteristic as shown on the drawings.

By using the intermediate frequency amplifiers to obtain high selectivity and also as limiters, the amplitude modulation may be limited out. Moreover, another possibility of the receiver is that by either having two intermediate frequency amplifiers, one with high selectivity and one with a band pass, or by having two different selectivity adjustments of the intermediate frequency the receiver can be used either for frequency or phase modulation and also if the intermediate frequency tube voltage is changed so that they do not limit and are linear amplifiers, then combining the outputs of the detectors in push-push instead of push-pull, amplitude modulation could also be received. This could be easily accomplished in one switching operation. a

The term intermediate frequency is relative, as used herein, in that it designates only a frequency equal to thesum or difference between the signal'frequency and the oscillator frequency, which difference or sum frequency may be a high radio frequency.

The circuits 23 and 2 3, which may be in parallel rather than in series as shown, are coupled to two circuits 26 and 28 connected between the input electrodes of two detectors 30 and 32 connected as shown in back-to-back relation. The anode of detector 36 is connected by a radio frequency filter circuit 34 and if desired, an ammeter 36, to a primary winding on a transformer 4B. The anode electrode of tube 32 is connected by way of a radio frequency filter circuit 35 and, if desired, an ammeter 31, to a primary winding on the transformer 40. The transformer 40 has its secondary windings connected as shown in push-pull relation to the input electrodes of a pair of amplifier tubes 42 and 44 having their anodes connected in pushpull relation and coupled by transformer 46 to any utilization circuit by way of jack 48. The filters 34, 35 and 23 and 26, 24 and 28 are the split circuits for the back-to-back detectors. The plate tuning elements of circuits 23 and 24 are tuned so that equal voltages are placed on the grids of the detectors when the secondary circuits 26 and 28 are tuned to the same frequency. The grid circuits 26 and 28 are then detuned,

one to a higher frequency and the other to a lower frequency than the mid frequency, which was the original frequency adjustment. Due to the low coupling coeflicient between the primary and secondary circuits of the transformers the detuning of the secondary (grid circuit) has very slight effect on the primary tuning. Different combinations and arrangement of series and parallel circuits, or the sloping sides of a high and low pass filter circuit may be used without changing the scope of the invention.

The selectivity of the radio frequency circuits arranged in accordance with the present invention is indicated by the resonant curve in Figure la. The circuits 26 and 28 are tuned to opposite sides of the carrier wave of the intermediate frequency output of tube I4 and these circuits are so connected to the inputs of tubes 30 and 32 that said tubes operate on the straight portions of their characteristic curves and the points of intersection of these characteristic curves is at the carrier frequency as indicated in Figure 1a.

The over-all selectivity of the radio frequency amplifiers and the detectors, considered separately, are shown in Figure 1b.

The audio frequency responses in the output of tubes 42 and 44 is indicated in Figure 1c.

The essential operation of this receiver consists in heterodyning a phase modulated signal so that it is reduced to an intermediate frequency. The intermediate frequency is then amplified in one or more stages of radio frequency amplification or it may be passed through selector circuits to give a selectivity to the intermediate frequency so that the sidebands away from the carrier are decreased in a certain relationship to produce in the detector output a signal whose audio response is essentially flat over the audio spectrum received. After the signal has been subjected to the intermediate frequency amplification where the remote sidebands carrying the higher audio frequencies are further decreased, it is then detected in a frequency modulation type of detector, which consists of the tubes 30 and 32 connected back-toback. This detector normally augments the higher frequency but since these have been previously decreased the over-all result is to give a frequency response which is essentially a reproduction of the audio input to the phase modulation transmitter. After detection the normal audio frequency amplifier comprising tubes 42 and 44 may be used to bring the detected audio signal up to the desired level.

Of course this type of correction for a frequency modulation receiver to adapt it to receive phase modulation may be applied to any type of frequency modulation receiver other than the back-to-back type. Automatic volume control and automatic frequency control may be applied to the receiver. This feature need not be explained here because it has already been taken up in previous disclosures. It is also feasible and may be sometimes desirable to have some correction in the audio circuit to compensate for variations in the audio response of the receiver to that which is desired. This may consist of a circuit for raising the low frequency response or raising the high frequency response or altering the audio frequency response in a manner which is somewhat similar to that which is disclosed in Crosby United States application #618,154 filed June 20, 1932. Where more than one intermediate frequency is used, as is sometimes desirable, image suppression may be obtained and the intermediate frequency correction or attenuating of higher modulation frequencies may be applied to either intermediate frequency channel, although it would be more likely to be applied to the lower intermediate frequency.

I claim:

1. The method of demodulating wave energy modulated in phase which includes the steps of attenuating wave frequencies remote from the carrier frequency to thereby distort the phase modulation so that it has a characteristic of frequency modulation, and demodulating said resulting distorted energy.

2. The method of demodulating wave energy modulated in phase which includes the steps of, attenuating frequencies remote from the carrier frequency to thereby distort the phase modulation so that it has a characteristic of frequency modulation, amplifying the resulting potentials and additionally amplifying the resultant potentials by amounts which are proportional to their frequency to convert said modulation having a characteristic of frequency modulationinto amplitude modulation.

3. The method of demodulating wave energy modulated in phase which includes the steps of attenuating frequencies remote from the carrier frequency to thereby distort the phase modulation so that it has a characteristic of frequency modulation, amplifying the resulting wave energy substantially in proportion to an increase or decrease of frequency thereof to convert said frequency modulation into amplitude modulation, and demodulating the amplitude modulation.

4. The method of demodulating phase modulations on wave energy which includes the steps of, attenuating sideband frequencies of the phase modulated wave energy directly in accordance with their frequency spacing from the carrier and demodulating the resultant energy.

5. The method of demodulating phase modulations on wave energy which includes the steps directly as the frequency of the resultant attenuated energy.

'7. The method of converting phase modulations on wave energy into characteristic modulations which includes the step of attenuating sideband frequencies of the phase modulated A wave energy directly in accordance with their frequency spacing from the carrier.

8. In a phase modulated wave energy demodulating system, phase modulated wave receiving means, a. sharply tuned resonant circuit having an input coupled to said Wave receiving means, said resonant circuit having an output and being tuned to th mean frequency of the wave energy so that it attenuates the side frequencies in accordance with their spacing from said mean frequency, a pair of demodulator tubes each having input and output electrodes, circuits of low power factor coupling the input electrodes of each of said demodulator tubes to the output circuit of said resonant circuit, means for tuning each of said circuits of low power factor to resonance at frequencies on opposite sides of the mean frequency of said Wave energy, filter circuits con necting the output electrodes of said demodulators in push-pull relation and a load circuit coupled with said filter circuits.

9. In a phase modulated wave energy demodulating system, an oscillator, a first detector having tuned input and tuned output circuits of low power factor, a coupling between said oscillator and one of the circuits in said first detector, means for impressing phase modulated wave energy on the input circuit of said first detector, an intermediate frequency amplifier tube having input and output circuits of low power factor, said input circuit being coupled to the output circuit of said first detector to derive therefrom phase modulated wave energy of intermediate frequency, said first low power factor circuits being sharply resonant to the mean frequency of the phase modulated Wave energy and said second loW power factor circuits being sharply resonant to the mean intermediate frequency whereby the side frequencies are attenuated in accordance with their frequency spacing from said carrier, a pair of demodulator tubes, a circuit of low power factor coupling the input electrodes of each of said demodulator tubes to the output circuit of said intermediate frequency amplifier, means for tuning each of said last named input circuits to resonance at frequencies on opposite sides of the carrier frequency of said intermediate frequency energy, filter circuits connecting the output electrodes of said demodulators in push-pull relation and a load circuit coupled with said filter circuits.

10. In a phase modulated wave receiver, an amplitude modulated wave demodulator having an input, a band pass filter having a highly selective characteristic such that it attenuates frequencies passed thereby in accordance with their spacing from the mean frequency of the band passed by said filter and a filter having a sloping characteristic such that the amplitude of the waves passed vary in accordance with their frequency, connected in cascade, means for impressing phase modulated energy on the input of said filters, and means for applying modulated wave energy from the output of said filters on the input of said demodulator.

11. In means for converting a frequency modulated wave receiver including a filter of sloping characteristic having an input and having an output connected to an amplitude modulated Wave detector, to the reception of phase modulation, a second filter having a highly selective characteristic such that it attenuates Waves passed thereby in accordance with their frequencyspacing from the mean frequency of said second filter, said second filter having an input and having an output connected to the input of said first filter, and means for impressing phase modulated wave energy on the input of said second filter.

HALLAN EUGENE GOLDSTINE.

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