Phase modulation receiver

March 5, 1940.

2,192,684

M. G. CROSBY

PHASE MODULATION RECEIVER Filed Nov. 26, 1938

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ATTORNEY.

March 5, 1940.

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FREQUENCY

M. g. crosby 2,192,684

PHASE MODULATION RECEIVER Filed Nov. 26, 1938 2 Sheets-Sheet 2

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INVENTOR bV

ATTORNEY

Patented Mar. 5&lt; 1940

2,192,684

UNITED STATES PATENT OFFICE

2,192,684

PHASE MODULATION RECEIVER.

Murray G. Crosby, Riverhead, N. Y., assignor to Radio Corporation of America, a corporation of Delaware

Application November 26, 1938, Serial No. 242,469 (Cl. 250—20) resonant effect described above. This is of material advantage in reducing the distortion due to fading of the character during transmission.

This disclosure concerns a phase modulation receiver of the back-to-back type in which a 5 series connected crystal filter is used for the under-neutralized characteristic and a shunt connected crystal filter is used for the over-neutralized characteristic.

The present receiver utilizes a combination of *0 the principles described in my above-mentioned patents. In the present application, one of the crystal filters of the conversion circuit feeding the back-to-back demodulator arrangement is in a series connection and the other is connected in ¹⁶shunt with respect to the output electrodes of the filter. This arrangement provides a circuit in which there are no neutralizing circuits and in which it is not necessary to connect a reactance across any of the crystal holders. In the types of crystal filters such as, for example, of the prior applications, in which an inductance is connected across the crystal electrodes, there is a tendency to reduce the selectivity of the crystal. For this reason, it is desirable to avoid circuits of this type, e. g. wherein the crystal holder is shunted by an inductance, if possible. The elimination of neutralizing circuits, as is accomplished in the present disclosure, simplifies the circuit and the circuit adjustments. ⁸⁰

In describing my invention, reference will be made to the attached drawings wherein:

Figure 1 illustrates the essential elements of a phase modulated wave energy receiving, amplifying and converting means with automatic fre- « quency control means, all arranged in accordance with the present invention. Certain of the elements of the receiver are shown diagrammatically by rectangles since these elements per se form no part of the present invention. The filter cir- 40 cuit for converting phase modulations on the wave energy to characteristic amplitude modulations are illustrated in detail, although even these circuits do not include all known refinements which applicant might use in a practical applica- 45 tion.

Figures 2α to 27t, inclusive, are resonant curves, reactance curves, and wave component vector diagrams used in illustrating the operation of the converting circuit. ⁸⁰

Referring to the drawings, Figure 1 shows a somewhat complete receiver employing the principles involved here. In this receiver, wave energy to be demodulated is intercepted by an aerial system A and supplied by A to a radio frequency 55

Claims.

This application concerns a phase modulation receiver of the crystal filter converting type, in which two separate crystals are used, one in a series connection and the other in a shunt con5 nection, to obtain the required over- and underneutralized filter characteristics with demodulating means of the back-to-back type coupled with the output of the filters. The characteristic of the filters comprising the under- and over10 neutralized crystals is such that when phase modulated wave energy is passed by the filters, it is converted to waves having corresponding amplitude modulation and the filter outputs are so arranged and coupled to the back-to-back de15 modulators that the modulation envelopes resulting from the conversion have a 180 degree relation so that the detector system coupled to the filters detects the amplitude modulations resulting from the conversion to render the signal.

-° Undesired amplitude modulations reaching the filters have similar effects on the output thereof, and are, due to the back-to-back effect in the demodulators, cancelled or substantially cancelled by differential action.

In my United States Application Serial No. 167,344, filed October 5, 1937, Patent No. 2,156,375 dated May 2, 1939,1 have described a receiver of the general type involved here. In the said application, energy is fed from an impedance or re30 actance to the crystal through a resistor and the output of the filter comprises the drop across the crystal. In said application the crystal is shunted by the output. In a modification in said application and in my United States application 35 Serial No. 178,655, filed December 8, 1937, Patent No. 2,156,376 dated May 2, 1939, the crystals in their holders are connected in a series circuit so that at the resonant frequency of the crystal a maximum amount of wave energy is passed 40 thereby, while at the anti-resonant frequency of the piezo-electric crystal a minimum amount of energy is passed. The anti-resonant frequency characteristic of the crystal is due to the holder capacity and the inductive effect of 45 the crystal per se. With the crystals connected in series as in the said latter application, a more efficient transfer of energy from the input reactance or impedance to the filter outputs is obtained due to the fact that there is no loss in the 50 resistor which has been used, heretofore in some cases for feeding the crystals in the shunt connection. I have also found that the series connection as used in the said invention gives a greater exaltation of the carrier with respect to 55 the side bands by virtue of the resonant and anti2,102,884 amplifier 4 wherein the energy is selected and amplified as desired. Energy from the output of 4 is supplied to a first detector 6 also coupled to a local oscillator O of the controllable type. Osβ dilations from O beat with the wave energy in 6 to produce intermediate frequency energy supplied to intermediate frequency amplifier 8. The output of 8 supplies potentials to the primary winding .of a transformer 10, the secondary wind10 ing of which has one terminal connected to ground and the other terminal connected to electrodes of piezo-electric crystals 24 and 25. Crystal 24 is connected in series with the secondary of 10 by an adjustable portion of potentiometer IS resistance PR, while crystal 25 is connected in shunt to the secondary of 10 by the resistance R. The primary and secondary windings of 10 are tuned to substantially the mean frequency of the intermediate frequency energy by condensers 12 20 and 18, respectively. In order to pass substantially Uniformly all of -the-frequencies involved, the primary and secondary windings are shunted by damping resistors 9 and PR, respectively, the latter, also serving to adjust the connection of 24. 2® An: electrode of crystal 24 is connected to the control grid 35 of a coupling and amplifying tube 34, while an electrode of piezo-electric crystal-25 is connected as shown to the control grid 37 of a coupling and amplifying tube 36; The crystal 25 30 has its two holders or electrodes connected in shunt to the input electrodes of 36. Crystal 24 is in series with the secondary of 10 and with the input grid 35 and cathode of coupling tube 34. Grid leak resistors 39 and 30' are connected ^w between the control grids 35 and 37, respectively, and ground G, while self-biassing resistors shunted by by-passing condensers 40 and 41 are connected between the cathodes of the tubes 34 and 36, respectively, and ground.

The anode-electrode of 34 is connected with the primary winding of a coupling band-pass transformer 50, while the anode of 36 is coupled to the primary winding of a band-pass transformer 52. The band-pass transformer 50 » may be tuned to the desired frequency by tuning condenser 55 and may be damped by damping resistor 51 to give it the desired band-pass characteristic. In like manner and for the same purpose, the primary winding of trans50 former 52 may be tuned by condenser 56 and dan. ^ed by resistor 53: The secondary windings of 52 and 50 are connected in series between the anodes of diode rectifier 60 and 64. The cathodes of diode rectifiers 60 and 64 are connected 65 together by resistors 63 and 65, the resistors being shunted by by-passing condensers as shown. The cathode end of the resistor 65 is connected to ground so that potentials at the cathode end of 63 vary with respect to ground in 00 accordance with the combined outputs’of 60 and 64, which in turn are excited by the energy passed by 50 and 52. The latter energy, in a manner which will be described hereinafter, is amplitude modulated energy resulting from the 06 conversion of the phase modulations in the crystal filters. Moreover, the amplitude modulations have opposed envelopes. The resulting amplitude modulations are impressed by condenser 70 on the input electrode 72 of audio fre70 quency amplifier 73, the anode of which is coupled to a transformer 74 having a secondary which may supply the amplified modulation potentials to any utilization circuit. Potentials from the resistors 63 and 65 are also supplied by 76 way of a time constant control circuit 61 to the control grid 82 of a modulator tube which controls the frequency of the oscillator O supplying oscillations to the first detector for beating purposes.

The modulator M comprises a reactance tube 5 80 having an anode 84 coupled to the anode 93 of the oscillator 94. The anode to cathode impedance of modulator tube 80 is in shunt to the reactance 96 in the oscillation generating circuit 94 so that it in part controls the frequency 10 of the oscillation generated. The anode 84 of tube 80 also is connected by way of a phase shifting condenser 88 and resistor 90, and blocking condenser 91, to its grid 86. Resistor 89, which is by-passed by a condenser, furnishes self-bias 15 for tube 80. The grid 86 of tube 80 is coupled to the connection between 90 and 88. Oscillator tube 94 has its grid 98, anode 93, and cathode 100 coupled in a frequency determining and stabilizing circuit 96 which circuit is coupled to the 20 first detector 6. This reactance circuit and oscillator are as described in my copending Patent NO; 2,156,375.

The value of resistance 90 is high as compared to the reactance of condenser 88 for the fre- ²⁵quency used, that is, generated at O, so that the current through this circuit is largely resistive and is in phase with the voltage. However, the voltage drop across 83 leads the current by 90 degrees and the phase quadrature relation between ³⁰the radio frequency potentials on 84 and 86 necessary for the reactive effect is obtained. The reactance of tube 80 shunts the frequency determining circuit 96 and consequently the reactance tube controls to some extent the frequency ³⁵of the oscillations produced in 96 and tube 94. This reactive effect, which may be considered inductive or capacitive, is, in turn, controlled by the potential supply from the time control circuit 61 to the grid 82 of reactance tube 80. Po- 40 tential supplied to 82 is a function of the mean frequency of the intermediate frequency energy supplied by 50 and 52, changing when the intermediate frequency drifts in either direction from this mean frequency. Such drift may be caused 45 by the change in frequency of the received wave or the frequency of the oscillator O, or both, and results in a change potential developed in 63 and 65, a change of the potential of 82 and a correcting change in the reactance reflected bv 50 the reactance tube 80 into the circuit 86 or a part thereof. Since the plate 84 is connected to the plate 93 and the grid 86 is excited by voltage displaced in phase relation to that of the circuit 96, the plate current in 80 is likewise out of phase 55 with the voltage of 96 and the control tube 80 looks like a reactance to the circuit 96.

The operation of my receiver and in particular of the converting circuits including the crystals 24 and 25 will be more clearly understood by 60 reference to Figures 2α to 2Λ, inclusive, of the drawings. In the series connected crystal filter including crystal 24, the carrier is tuned to the resonant point of the crystal at which point maximum energy is passed by the crystal. On the high frequency side of this point the crystal holder capacity resonates with the inductive effect of the crystal and causes an anti-resonant point which rejects side band energy in the intermediate vicinity of that point. This combi- 70 nation of a peak point and a rejection point affects the desired Characteristics of a capacitive effect on- both sides of the carrier. In other words, 24, when so connected and arranged, provides a crystal filter effect having a frequency 75

3,192,884 against output characteristic as illustrated in Figure 2α of the drawings.

Tn the shunt connected crystal filter circuit including crystal 25. the intermediate frequency 5 carrier is tuned to the anti-resonant point of the crystal holder and crystal, at which point maximum energy is passed. This' is opposite to the series connected crystal due to the shunt connection which makes the energy transfer maxi10 mum at the frequency at which the filter is of maximum impedance. Due to this opposite situation, the rejection point of the shunt connected crystal filter appears on the low frequency side of the carrier instead of on the high 15 frequency side, as is the case of the series connected filter. This, accordingly, provides a filter including crystal 25 which has . a frequency against output effect as illustrated in Figure 2e of the drawings. This makes the shunt connected 20 filter equivalent to an over-neutralized filter and. the series connected filter crystal equivalent to an under-neutralized filter. Consequently, by. using both types fo connections and grinding the crystals so that the resonant point of the series 25 connected crystal synchronizes with the anti-resonant point of the shunt connected, crystal, the required pair of filters with over- and underneutralized chaarcteristics are obtained. A fortunate part of this arrangement is the fact that 5® the anti-resonant point of the shunt connected crystal may be varied to a slight extent by varying the capacity of the holder of crystal 25 which is made adjustable. This fact facilitates the synchronizing of the filter carrier frequencies.

³⁸ The reactance characteristic of the filter effect of crystal 24, which is capacitive, is shown in Figure 2b. The voltage from transformer 10 is fed directly to this circuit and the drop across the crystal is inductive or capacitive, depending '40 on the frequency.of the wave supplied by IS, and, in accordance with the curve of Figure 2b, it can be seen that in the region where the side bands are disposed, the crystal is. capacitive on both sides of the carrier frequency except for a small 45 interval (below) near the carrier frequency. Due to this fact, the phase of the side bands is shifted 90 degrees with respect to the carrier as shown in Figures 2c and 2d. Figure 2c represents the phase modulated wave supplied from 10 to be 50 converted, while' Figure 2d represents the wave to which the phase modulated wave has been converted and has the characteristics of an amplitude modulated wave. Tire amplitude characteristic of this filter, including crystal 24, has 55 been shown at Figure 2a.

Crystal 25, which is shunt connected, has a reactance frequency characteristic as shown in Figure 2/. In this figure, the filter is inductive on both sides of the carrier except for a small 60 portion of the frequencies adjacent (above) the carrier. Due to this reactive effect, the phase of the side bands is 90 degrees lagging instead of leading as was the case with crystal 24. Consequently, the phase modulated wave of Figure 2&amp; 65 js converted to the amplitude modulated wave of Figure 2h.

The amplitude characteristics of the filter 25 are as shown in Figure 2e. From an examination of Figures 2d and 2ft, it can be seen that the 70 phases of the amplitude modulation envelopes resulting from, the conversion by the two filters is 180 degrees out of phase. Consequently, the detection of this amplitude modulation in combination by the senes connection of the diodes 68 and 75 64 and diode resistors 63. and 65 combines the phase modulation outputs additively. The resulting potentials of modulation frequency are amplified in 73 and utilized from 74, while the resulting potentials due to slow changes or drifts in the mean frequency act through 61 to correct 5 the oscillator frequency.

Briefly, during operation intermediate energy produced in the circuits 4, 6, 8 and oscillator is supplied to the primary winding of 18 and from 10 through 24 in series with the control grid 35 10 and from 25 in shunt to the control grid 37. Potentiometer PR feeds series crystal 24, which in turn feeds coupling tube 34 to form the underneutralized crystal filter. Shunt connected crys- . , tai 25 is fed through resistor R and the drop 15 across the crystal 25 is fed to the tube 36 to form the over-neutralized crystal filter. Transformers 5-3 and 52 feed the two filter waves to diode detectors 60 and 64. The phase modulation detected ., output appears across diode resistors 63 and 65 20 which are connected so as to add the modulation components resulting from the conversion of the phase modulation components and to cancel the amplitude modulations on the original wave which are unconverted by the crystal filter cir- 25 cuits. This arrangement also cancels the even harmonic distortion resulting from the detection of the phase modulation. The resulting potentials are supplied by capacity 70 to the control ,. grid 72 of an electron discharge device 73 and ³⁰from there to a utilization circuit such as 74. Automatic frequency control energy is also supplied from the upper terminal of 63 by way of line 75, to the reactance tube described above, to control the frequency of the oscillator to main- ⁸⁶tain the receiver in tune and insure an intermediate frequency carrier of the proper frequency to operate through the crystal filters in accordance with the prior disclosure and the , _tcharacteristic curves in the diagrams of Figures 40 2a to 2ft, inclusive.

Amplitude modulation, both noise and signal, being received by this receiver balances out due to the differential connections of diodes 60 and S4. This is because the amplitude modulations 45 affect the filters in like manner and since the characteristics of the filters oppose, the effects of the undesired amplifier modulations passed by the filters compensate or cancel. Even harmonic detector, distortion is also cancelled or compen- 50 sated in the output.