US2192684A - Phase modulation receiver - Google Patents

Description

Mam]? 1940- M. G. CROSBY ,192, 84

PHASE MODULATIN RECEIVER Filed Nov. 26, 1.938 2 Sheets-Sheet 1 vvvvv 1w INVEN TOR.

i 7kg? 6. mossy BY M ATTORNEY.

Mar 5, 19 M. e. CROSBY PHASE MODULATION RECEIVER Filed 'Nov. 26, 1938 2 Sheets-Sheet 2 FREQUENCY Y m U m m Elly-2g Fiy.2]l

' ligaflc 12 2a I INVENTOR MURRA fROS/SY BY AIJTRORNEY Patented Ma 5, 1940 UNITED STATES 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

Claims.

This application concerns a. phase modulation receiver of the crystal filter converting type, in

tic of the filters comprising the underand overneutralized 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 demodulators 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 difierential action.

In my United States Application Serial No. 157,344, filed October 5, 1937, Patent No. 2,156,375 dated May 2, 1939, I have described a receiver of the general type involved here. In the said application, energy is fed from an impedance or reactance 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 Serial N o. 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 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 efiect of the crystal per se. With the crystals connected in series as in the said latter application, a more eificient 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 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 the side bands 'by virtue of the resonant and antiresonant 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 10 the principles described in my above-mentioned patents. In the present application, one of the crystal filters of the conversion circuit feeding the ba'ck-to-back demodulator arrangement is in a series connection and the other is connected in 1 shunt 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 reas'on, itis 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.

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 frequency 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 filtercir- 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 2a to 2h, inclusive, are resonant curves, reactance curves, and wave component vector d1- agrams used in illustrating the operation of the converting circuit.

Referring to the drawings, Figure 1 shows a somewhat complete receiver employing the pr1nciples 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 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. Oscillations from beat with the wave energy in 6 to produce intermediate frequency energy suptal 24 is connected in series with theseconda-ry of H] by an adjustable portion of potentiometer resistance PR, while crystal-25is connected in shunt to the secondary of ID by the resistance R. The primary and secondary windings of II] are tuned to substantially the mean frequency of the intermediate frequency energy by condensers l2 and 16, 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 latteralso serving to adjust the connection of 24.

Anjelectrode of crystal 24 is connected to the control grid 35 of a coupling and amplifying tube 34, while an electrode of piezo-electric crystal25 is connected as shown to the control grid 31 of a coupling and amplifying tube 36-. The crystal 25 has its two holders or electrodes connected in shunt to the input electrodes of 36. Crystal 24 is in series with the secondary of It! and with the input grid 35 and cathode of coupling tube 34. Grid leak resistors 36 and 39 are connected between the control grids "35 and 31, respectively, and ground G, while self-biassingv resistors shunted byby-passing condensers 40 and 4| are connected between the cathodes of the tubes 34 and 36, respectively, and ground.

rThe anode-electrode of 34 is connected with the primary winding of a coupling band-pass transformer 56, while the anode of 36' is coupled 'to the primary winding of a bandpass transformer 52. The band-pass transformer 56 maybe tuned to the desired frequency by tuning condenser 55 and may be damped by damping resistor to give it the desired band-pass characteristic. In like manner and for the same purpose, the primary winding of transformer 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 66 and 64. The cathodes of diode rectifiers 66 and 64 are connected together by resistors 63 and 65, the resistors being shunted by 'by-passin-g 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 accordance-with the combined outputsof 60 and 64, which in turnare excited by the energy passed by 59' and 52. The latter energy, in a manner which will be described hereinafter,- is amplitude modulated energy resulting from the conversion of the phase modulations in the crystal filters. Moreover, the amplitude modulations have opposed envelopes. The resulting amplitude modulations are impressed by condenser Won the input electrode 72 of audio irequency amplifier 13, the anode of which is coupled to a transformer 14 having a secondary which may supply the amplified modulation potentials towany utilization circuit. Potentials from the resistors 63 and 65 are also supplied by way of. a time constant control circuit 6| 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 86 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 of the oscillation generated. The anode 84 of tube 80 also is connected by way of a phase shifting condenser and resistor 96, and blocking condenser 9|, to its grid 86. Resistor 89, which is by-passed by a condenser, furnishes self-bias for tube 80. The grid 86 of tube 89 is coupled to the connection between 90 and 88. Oscillator tube94 has its grid 98, anode 93, and cathode I60 coupled in a frequency determining and stabilizing circuit 96 which circuit is coupled to the first-detector 6. I'hisreactance'circuit and'oscillator-are as described in my copending Patent No.-2,156,3'75.

The value of resistance 96 is high as compared to the reactance of condenser 88 for the frequency used, that is, generated at 0, so that the current through this circuit is largely resistive and is in phase'withthe voltage. However, the voltage drop across 83 leads the current by 90 degrees and the phase quadrature relation between the radio frequency potentials on 84 and 96 necessary for the reactive eflect is obtained. The reactance of tube 90 shunts the frequency determining circuit 96 and consequently the reactance tube controls to some extent the frequency of 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 6| to the grid 82 of reactance tube 80. P0- 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 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 by 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 with the voltage of 96 and the control tube 89 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 reference to Figures Za to 2h, 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 combination 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 acrystal filtereffect having ,a frequency against output characteristic as illustrated in Figure 2c of the drawings.

In the shunt connected crystal filter circuit including crystal. the intermediate frequency 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 connect crystal due to the shunt connection w -s the enemy transfer maximum at the qucncy at which the filter is oi maximum nce. Due to this opposite sitnation, the rejection point of the shunt con-- nected crystal on the low frequency side of the carrier instead of on the high frequency side, as is the case the series con-- nected filter. provides a filter including crystal which has frequency against output effect as illustrated in Figure oi the drawings. This makes the shunt connected filter equivalent to over-neutralized filter and the series conn ed fi er crystal equivalent to an undermeo c. filter. Consequently, by using both types fo connections grindiuig the crystals so that the resonant point of the series connected crystal synchronize with the anti-resonant point of the t conrected crystal, the required pair of with over-- and underneutralized chaarcter t .ined. A fortunate part of ngerncnt s the fact that the anti-resonant point of the nt connected crystal may be ied to a sl a tent hy varying the capacity oi the h e r f crystal 25 which is made adjustable. is i ct facilitates the synchronizing oi the filter c ,r frequencies.

The reactance characteristm of the filter effect of crystal 2%, which is capacitive, is shown in Figure 2?). voltage from transformer H] is fed directly to this circuit and the drop across the crystal is inductive or capacitive, depending on the frequency of the wave supplied by ill, and, in accordance with the curve of Figure 2b, it can be seen that in the re on where the side bands are disposed, the crystal is capacitive on. both sides of the carrier frequency except for a small interval (below) near the carrier frequency. Due to this fact, the phase of the side bands is shifted degrees with respect to the carrier as shown in Figures 20 2a. Figure 2c represents the phase modulated wave supplied from it to be converted, while Figure represents the wave to which the phase modulated wave has been converted and the characteristics of an amplitude modulated wave. The amplitude characteristic of this filter, including crystal 24, has been shown at Figure 201 Crystal 25, which shunt connected, has a reactance frequency characteristic as shown in Figure 2,1. In this the filter is inductive on both sides of the carrier except for a small portion of the frequencies adjacent (above) the carrier. Due to this reactive efiect, the phase of the side bands is 98 degrees lagging instead of leading as was the with crystal M. Consequently, the phase modulated wave of Figure 2g phase modulation outputs additively. The resulting potentials of modulation frequency are amplified in l3 and utilized from 14, while the resulting potentials due to slow changes or drifts in the mean frequency act through 6| to correct the oscillator frequency.

Briefly, during operation intermediate energy produced in the circuits 3, 6, 8 and oscillator is supplied to the primary winding of Ill and from is through in series with the control grid 35 and from 25 in shunt to the control grid 31. P0-

tiometer PR feeds series crystal 24, which in turn feeds coupling tube 34 to form the under ieutralized crystal filter. tel 35 is fed through resistor R and the drop across the crystal 25 is fed to the tube 35 to form the over-neutralized crystal filter. Transformers 5? and 52 feed the two filter waves to diode detectors and iil. The phase modulation detected output appears across diode resistors 63 and 65 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 circuits. This arrangement also cancels the even harmonic distortion resulting from the detection of the phase modulation. The resulting potentials are supplied by capacity 16 to the control grid '52 of an electron discharge device 13 and from there to a utilization circuit such as 14. Automatic frequency control energy is also supplied from the upper terminal of 63 by way of line "iii, to the reactance tube described above, to control the frequency of the oscillator to maintain 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 sated iu the output.

What is claimed is:

l. in a wave filtering system an impedance on which wave energy, of wave frequency, to be filt,red may be impressed, pairs of output electrodes, a crystal connected with said impedance and in ser'es with a pair of said output electrodes, said crystal being (dimensioned) ground to resonate at the frequency of the wave energy impr ssed on said impedance, and a second crystal connected with said reactance and in shunt with another of said output electrodes, said second crystal being (dimensioned) ground to be antiresohant at the frequency of the wave energy im- A ressed on said impedance.

2. In a wave filtering system an impedance on which wave energy, of a selected frequency, to be filtered may be impressed, pairs of output electrodes, a crystal connecting said impedance in series with a pair of said output electrodes, said crystal being (dimensioned) ground to resonate at the frequency of the Wave energy impressed on said impedance, and a second crystal connected in series with said impedance and in shunt with another pair of said output electrodes, said Shunt connected cryssecond-crystal being ground to be anti-resonant at the frequency of thewave energy impressed on said impedance.

3. In a wave filtering system an impedance on which wave energy, of a predetermined frequency, to be filtered may be impressed, pairs of output electrodes, a first crystal connected to said impedance and in series with a pair of said output electrodes, a second crystal connected with said impedance and in shunt with another pair of said output electrodes, said first and second crystals being respectively resonant and anti-resonant at the frequency of the wave energy impressed on said impedance whereby filters having underand over-neutralized characteristics respectively are formed by said system.

4. In a wave filtering system a tuned reactance on which wave energy, of an ascertainable frequency, to be filtered may be impressed, pairs of output electrodes, a first crystal connecting said tuned reactance in a series circuit including a pair of output electrodes, a second crystal in series with said tuned reactance and in shunt with another pair of said output electrodes, said first and second crystals being respectively resonant and anti-resonant at the frequency to which said tuned reactance is tuned, whereby filters having underand over-neutralized characteristics respectively are formed by said system.

5. In a system for converting phase modulations on wave energy, of a selected frequency, into characteristic amplitude modulations, an impedance on which said wave energy to be converted is impressed, a pair of output electrodes, a first crystal, resonant at the mean frequency of said wave energy, connected to said impedance and in a series circuit including a pair of said output electrodes, and a second crystal, anti-resonant to the frequency of the wave energy impressed on said impedance, connected with said impedance and in shunt to another pair of said output electrodes.

6. In a system for converting phase modulations on wave energy, of a selected mean frequency, into characteristic amplitude modulations, 2 tuned reactance on which said wave energy to be converted is impressed, a pair of output electrodes, a first crystal, resonant at the frequency to which said tuned circuit is tuned, connected to said tuned reactance and in a series circuit including a pair of said output electrodes, 2. second crystal, anti-resonant to the frequency to which said tuned circuit is tuned, connected in series with said impedance and in shunt to another pair of said output electrodes,

"7. In a system of demodulating phase modulations on Wave energy, of a selected mean frequency, an impedance on which said fave energy to be demodulated is impressed, a pair of output electrodes, a first crystal, resonant at the mean frequency of said wave energy, connected to said impedance and in a series circuit including a pair of said output electrodes, a second crystal, antiresonant at the meanfrequency of the wave energy impressed on said impedance, connected with said impedance and in shunt to another pair of said output electrodes, and means for differentially detecting the potentials appearing across said output electrodes.

8. In a system for demodulating wave energy modulated in phase at signal frequency, an imtially at the mean frequency of the phase modulated wave energy. I

9. In a system for demodulating wave energy modulated in phase at signal frequency a tuned reactance on which said phase modulated wave energy is impressed, a pair of detectors having differential output circuits, a first crystal connected to said tuned reactance and coupled in a series circuit with the input electrodes of one of said detectors, a second crystal connected with said tuned reactance and coupled in shunt to the input electrodes of the other of said detectors, said first and second crystals being resonant and anti-resonant respectively substantially at the frequency to which said reactance is tuned.

10. In a receiver system, wave amplifying means including tuning means, a tuned reactance coupled to said amplifying means, a pair of detectors having differential output circuits said detectors having input circuits, a first crystal resosin nant substantially at the frequency to which said 4,5

MURRAY G. CROSBY.

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