US2397840A - Phase modulation detector - Google Patents

Description

' SIG/VAL April 2, 1946.

T0 PM M. G. CROSBY PHASE MODULATION DETECTOR Fil ed Feb. 16, 1945 2 Sheets-Sheet 1 TUNED 3/ v c DETECTED AM VOLTAGE J0 URCE AMPLITUDE 0f VOLTAGE TO DIODES/1 dndB cTizrEa.

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4M WAVE /at 0/005 A AM WA VE at 0/005 5 l/VVE/VTOR MURRA Y 61. Ckosa vw m ATTORNEY Patented Apr. 2, 1946 PHASE MODULATION ne'rro'ron Murray G. Crosby, Riverhead, N. Y... assign r to Radio Corporation of America, a corporation of Delaware Application February 16, 1943, Serial No. 476,052

12 Cla ms- My present invention relates to detectors, or demodulators, of phase modulated carrier wave energy (hereinafter referred to as PM for brevity), and more particularly to improved PM detectors employing discrimination networks of the piezo-electric crystal type.

In the past I have disclosed various PM demodulator circuits utilizing piezo-electric crystal elements for translation of the-PM-wave into a corresponding amplitude modulated (referred to hereinafter as AM) carrier wave. Reference is made to my followingpa-tents for an explanation of the fundamental theory of operation of such crystal discriminator networks: U. S. Patent No. 2,085,008, granted June 29, 1937: U. S. Patent No. 2,156,374, granted May 2, 1-939; U. S. Patent No. 2,156,375, granted May '2, 1939;U. S. Patent No. 2,204,575, granted June 18, 1940; U. S. Patent No. 2,204,574, granted June 18, 1940.

In the disclosure of my U. S. Patent No. 2,204,575 there has been disclosed a PM detector whose input circuit comprises a single piezoelectric crystal in a three-electrode crystal holder. Condensers were used to provide two filtered outputs from the single crystal. One of the filters has an over-neutralized characteristic; while the other filter has an under-neutralized characteristic. The effect of the filter characteristics is to cause a quadrature phase shift of the carrier relative to the upper and lower'modulation sidebands. In this way a conversion from PM wave energy to AM wave energy takes place, and rectification by opposed rectifie'rs follows.

In prior circuits of this type over and underneutralized crystal filter characteristics were .obtained to provide a discriminator action for automatic frequency control .(AFC) potentials, PM and AM detector, and filtered carrier output. In these circuits various methods were utilized to provide both an over and. under-neutralized output from the same crystal. In one modification, the neutralization was accomplished in a circuit isolated from the crystal by means of coupling tubes. In another, a three-electrode crystal was employed so that two separate crystal .output terminals were available for the application of over and under-neutralization, respectively. In the circuits of my present application, isolating impedances are used to separate the crystal output energy from the utilized output energy. The over and under-neutralizing energy is fed to the utilizing points to form the two different types of characteristics. v necessity of coupling tubes, and allows the use of the simpler two-electrode crystal holder.

This type of circuit eliminates the Accordingly, oneof the main objects of my present invention is to improve the crystal conversion circuits of my aforesaid patents, and in particular to accomplish in a much simpler fashion what has been shown previously in PM detectors of the over and under-neutralized type.

Another important object of my invention is to-provide a demodulator having a pair of opposed rectifiers, an input circuit supplying PM wave energy to said rectiflers through anetwork comprising a piezo-electric crystal and shunt capacity for neutralizing the crystal holder capacity, and separate condensers providing under-neutralization and over-neutralization of the crystal holder capacity thereby to provide the filter characteristics which convert the PM wave energy into AM wave energy.

Another object of the invention is to provide a simplified PM detector which employs a single piezo-electric crystal, and which detector provides automatic frequency control (AFG) voltage, modulation voltage and pure carrier voltage.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the app nded claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description, taken in connection with the drawings, 'in'which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into .In the drawin s:

Fi 1 sho s a i it em odying n m f the invention,

Fig. 2 graphically illustrates the conversion ion of the demodulat r inpu ircuit,

' Fig. Bashows the vector relations between the carrier and sidebands of the PM wave.

Figs. 3b and 3c illustrate the vector relations ofthe converted PM waveat diodes A and B respectively,

Fig. 4 illustrates a modification of the circuit of Fig. 1,

.Fig. 5 shows a further modificationof the circuitofFig. 1.

- Re er in n w t the acc mpany n d awin s. herei like e e ce charac e s in the (litte ent figure designate similar circuit elements, the circuit of Fig. '1 shows a demodulator of PM signals. The mean frequency of the applied PM.

signals is assumed to be of the order of kilocycles (ha). The tube I, which may be a pentode tube by way of example, has its input electrode coupled to a PM signal source, such as the intermediate frequency amplifier section of a superheterodyne receiver. This 'inventionis not limited to the detection of PM signals whose mean frequency is located as low as 50 kc., but is generic and can readily be used for higher frequencies in the kilocycle range, or in the megac'ycle range. Assuming, however, that the PM signals applied to the control grid 2 of tube I have been reduced to an intermediate frequency -(I. F.) value as low as 50 kc., the output transdensers 9 and 9' isolate the neutralized terminals former 3 can readily be made of the iron core 7 type. In that case the primary winding 4 of the to the 50 kc. mean, or reduced carrier, frequency. The tube I is shown as having its screen electrode established at a positive voltage which is derived, as by means of potentiometer 5, from a +125 7 volt point of .a direct current source. The plate of tube I may be established at +250 volts. Appropriate carrier bypass condensers are employedwhere required, and'a fixed trimmer condenser 5' is shunted across the primary windingid.

The amplified PM signal. energy is applied to the opposed diode rectifiers A and, B through a piezo-electric crystal network which functions to convert, or transform, the PM signal energy into corresponding AM energy. That is to say,

the function of the network coupling the secondary winding 1 of transformer 3 to the electrodes of diodes A and B is to produce a quadrature phase shift between the carrier and modulation side bands of the applied Plvlsignal energy; Before explaining the manner in which this is accomplished, the circuit details of the crystal network will be explained. r

The anode 8 of diode A is connectedto th upper end of winding 1 by a path which consists of isolating condenser 9 and the piezo-electric crystal P. The crystal P is. positioned between the metal electrodes, or holderelements, l0 and .H. The construction of the crystal, andholder elements is Well known to those skilled in the art,

n and need not bedescribed in any further detail.

The crystal P is cut so' asto be tuned to Po, the mean frequency of the applied PM signal energy. This would be, in the example of this case, the 50 kc. value. l 4 The anode l2 of diode B is connected to the opplosite end of winding 1 through a path compris ingisolation condenser'9' arranged'in serieslwith a condenser Hi. The condenser l3 functions to neutralize thecapacity between the crystal holder electrodes l8 and H. The cathode 8' andthe transformer 3 is tuned, as by tuning condenser 5,

of the crystal P.

As indicated in Fig. 1, modulation voltage, which is in the audio range, may be taken off from connection It. This connection also provides AFC voltage for the regulation of the local oscillator of the receiving system, where the receiver is of the superheterodyne type. Pure carrier energy may be derived from crystal P by way of electrode Ill. The carrier energy taken olf from this point is free of any modulation side bands, and is energy whichhas a frequency value of 50 kc.

Such pure carrier energy may be then employed another part of the receiving system to accomplish a carrier exaltation function. For example, the pure carrier energy could be fed'over connection 23 to another PM demodulator coupled to the PM signal source which feeds the grid 2-. In

such case only AFC voltage would be take off from connection It, and the circuits shown in Fig. 1 would then act as a network to provide AFC voltage and pure carrier energy. Th demodulator circuit to which the pure carrier energy would be fed would thenbe the actual demodulator of the system which provides the audio voltage output.

Such a general system is disclosed, for example; in my U. S. Patent No. 2,063,588, granted December 8, 1936.

Considering now the theoretical aspects of the circuit of Fig. 1, reference is first made to Fig. 2

wherein are shown curves A and B. 'The curve A shows the change in amplitude of signal voltage which is fed to'the diode A of Fig. 1.1 -The curve B shows the amplitude of signal voltage V ,which is fed to the diode B. It will be seen that.

both curves have acommon peak at the value Fc. This-latter value correspondsgto the mean frequency of the applied PM signal energy. In other words, the crystal network which feeds diodes A and B accomplishes in a much simpler fashion what has been disclosed previously in the PM detectorsof the over and under-neutralized types of my aforesaid patents.

Considerin the functioning of the crystal circuit' from another'viewpoint, Figs. 3a, 3b and 3c illustrate in a purely pictorial manner how 'the circuitacts to provide a relative'displacement of 90 degrees between the carrier and the upper and lower modulation sidebands of the incoming PM signal energy. Fig. 3a. shows vecto-rially the phase relations which can be depicted as existing in the PM wave applied through transformer 3. The

. carrier is marked C, the upper'modulation side by a lead I! to the midpoint iii of winding 1.

The anode 8 is connected to lead ll by a resistor l9, whilethe anode I2 is connected to lead I! by a separate resistor 20. Resistors i9 and 2t function to develop detected .AM signal voltage thereacross, shouldrAM signal energy be applied to tube 1 instead of PM signal energy. In shunt across [the series path P9 there is connected a condenser 2 I. The latter functions to provide under- 1 neutralization of the crystal holder capacity at the diode anode 8. Similarly, the condenser 22, in shunt with path 'l39', provides over-neutralization at the diode anojde l2. The conarrows attached to the side band vectors.

band is ,U, and the lower modulation side band is L. .The positive direction of rotation is takenas clockwise. The directions of rotation of the side bands with respect to the carrierare indicated by Side bands of. an order greater than unity have been neglected dueto their small amplitude and for simplicity ofexplanation. When the PM wave of Fig. 3a is passedjthrough'one side of the crystal network, theside bands are shifted 90 degrees with respect to the carrier, so thatthe side band relation shownin Fig. 3b iseffected. ,Let it be assumed that the rotation of, Fig. 3b is that of thelAM wave existing at diode anode 8 of diode A. The relation of Fig. 3bis that which may be said to'exist between the carrier and side bands in an'AM wave. g V

The opposite side of the crystal network causes the side bands to be shifted 90 degrees in phase, but in an opposite directionto theshift previously described: 'Henceat the' diode'. anode l2 there will be produced the vector relation shown in Fig. 3c. The envelope of theamplitude modulations produced by the two filter characteristics A and B of Fig. 2' are 180 degreesapart, and their detected output must be. combined in a push-pull combination to reproduce the converted modulations. Hence, the rectifiers Aand B are connected in opposition, and the differential of the rectified voltage outputs developed across resistors I4 and I5 is taken off from the cathode end of resistor I'd.

When the mean, or carrier, frequency drifts the relative amplitudes of the two filter outputs vary differentially. That is, as the frequency drifts lower, the output of thefilter whose characteristic is represented by curve A of Fig. 2 approaches the dip point, whereas the output of the other filter reduces more. gradually. This results in a differential action which produces differentially-detected energy that may be used for AFC. As is well known to thoseskilled in the art, the AFC voltage may act through any desired type of reactance tube devices upon the cal oscillator to hold the output frequency thereof such as to maintain proper tuning of the signal. Reference is made to Fig. 1 of my aforesaid Patent 2,204,575 for adetailed disclosure of said AFC circuit. Should the signal energy applied to the receiver system beAM signal energy, then the detected AM signal voltage will be. taken off from the end of resistor I9. Reference is made to the aforesaid Patent 2,204,575 for an explanation of how theAM detection occurs.

The functioning of the system of Fig. 1 may be explained from still another viewpoint. In Fig. l whenv an unmodulated carrier wave is received there are delivered to each of diode rectifiers A and B two voltages. One of the voltages is that of the filtered carrier from crystal P without phase change, and the other voltage is that of the unfiltered carrier substantially 90 degrees different in phase from the filtered carrier. The crystal output phase is zero degrees, since condensers 9 and 9' merely act as a series condenser in series with the equivalent capacity of the crystal. The substantial phase quadrature of the two voltages-at each rectifier results from thefact that the unfiltered carrier wave is applied to the rectifiers by condensers. 2| and 22 which are effective to effect a 90 degree phase shift. These condensers 2| and 22 are'non-selective to phase or frequency variations of theunfiltered carrier, and accordingly permit all signal componentsto pass to rectifiers A and B. The crystal P, however, effects no phase shift, as already explained, at the carrier, but substantially removes the phase modulation of the signal thereby restoring the carrier substantially to the phase.

and wave form which it had before modulation at the transmitter. The crystal P .is, of course, selective for frequencies off resonance. This follows from the sharp selectivity characteristic of the crystal. The filtered carrier voltage at the output electrode III of crystal P is applied in like polarity, or in parallel, to the anodes 8 and I2. The unfiltered carrier voltage is applied in pushpull, or opposite polarity, to the said anodes 8 and I2.

When the received carrier is phase modulated the filtered carrier remains as before.

The unfiltered phase modulated carrier energy is sup-plied to rectifiers A and B in phases differing from the 90 degrees, or phase quadrature, relation to an extent determined by the degree of phase modulation of the carrier. The greater nated by numeral 24.

combined voltage of thaunfilteredslgnal and illtered carrier on. one of. the 'rectifiers; and the less the sum ofsuch'voltageso'n the other rec-i tifier. The polarity of the-rectified voltage at the'lead I6 depends on the direction of the phase change of the received PM signals. The rectified voltages across load resistors I4 and I5 are differentially combined relative to ground.

The AFC voltage results from a relatively slow drift or shift of-the applied carrier wave frequency from the crystal frequency. Hence, there will occura change in the phase of the filtered carrier voltage applied to the anodes in like. polarity. This is caused by the selectivity ch'arace teristic of crystal P. Such phase-shifted filtered carrier voltage will; combine with the unfiltered signal energy. at each. rectifier toproduce rectified voltages across resistors I4 and I5. The differential voltage at lead I6 in that case is the AFC bias, and will arise only in response to relatively slow frequency shiftszof the received carrier relative to crystal frequency.

The AM signal voltage, of course, varies solely in carrier amplitude. Hence, at each rectifier there is provided the combined filtered AM carrier voltage and unfiltered AM signal voltage. The rectified AM voltage due to rectifier A appears across resistors I9 and I 4. Therectified AM voltage due torectifier B appearsacross resistors 20 and IS. The detected AM voltage taken off at the anode end of resistor I9 is the additive resultant of the rectified. voltages across resistors I9 and I5. Variationof the carrier amplitude results in variation of this additive resultant voltage.- Carrier-exaltation occurs,- bothfor AM and PM reception, due to the action of crystal P in removing modulation components from the carrier.

In Fig. 4 there is shown a modificationwhich differs from the circuit of Fig. 1 in that instead of transformer 3 feeding the crystal discriminator network,there is employed an auto-transformer. The circuit is otherwise quite similar to that shown in Fig. 1. The auto-transformer is desig- Anv intermediate point thereof is connected to the +250 volt source. A resistor 25 is shunted across the transformer to provide proper damping. Condenser 21 is arranged in shunt with auto-transformer 24 to provide proper tuning; for example, the transformer 24 could be tuned to asignal of 450 kc., if desired. The auxiliary trimmer condenser 26 is shunted across tuning condenser 21. The condenser I3 neutralizes the crystal holder capacity. The under-neutralization condenser 2I is connected in shunt with crystal P and isolating condenser 9, while the over-neutralization condenser 22 is connected in shunt with condenser I3 and isolating condenser 9'. The junction of crystal holder I0 and neutralizing condenser I3 is connected by a lead It to the junction of isolation condensers 9 and 9'. The. latter junction point is connected to ground through an adjustable, resistor 28. resistor 28 functions to adjust the Q of the crystal P. The resistor 28 may be used in any of Figs. 1, 4 or 5. The remainder of the circuit elements are similar to those shown in Fig. 1, and have corresponding reference numerals. The pure, or filtered, carrier energy is taken from crystal P.

In Fig. 5 the crystal P has its electrodes I0 and II connected to the high potential sides. of primary circuit 5'4 and secondary circuit 3ll--3.I. Coil '30 has one end coupl'edto anodel? by vconthe-degree of phasexmodulation the greater the- The denser 9; the other end is coupled by condenser 9' to anode I2; The midpoint of coil 30 is connected by lead I? to the junction of resistors l9 and 20. The holder capacity'neutralizing condenser l3 is connected fr'om' electrode H to the end of coil 30 connected to condenser 9'. The condenser 2| is connected in shunt with 'crystal P and condenser 9. The condenser 22 is arranged in shunt with condensers l3 and 9' in series. The circuit 30- 3l is tuned to either side of F to provide either positive ,-or negative reactance so as properlyto phase the crystal output energy with respect tothe unfiltered PM energy. Circuit -4 is tuned to Fe, as is crystal P. This circuit of Fig. 5 difiers from that of Fig. 1 in that neutralization of the crystal is carried out directly ahead of the secondary circuit 3il--3l of the input transformer 4-30. The neutralization is accomplished by'feeding backwards instead of'forwards. That is to'say, neutralization with the crystal and being connected between said one input terminal and like output terminal,

an under-neutralization condenser in parallel with the'neutralizing condenser and being coupled between said second input terminal and corresponding output terminal, and a pair of opposed rectifiers coupled between said output terminals.

4. In a discriminator circuit, a pair of input terminals, a pair of output terminals, a piezoelectric crystal provided with a holder consisting of the holder capacity is effected through con- I 1 will be apparent to one skilled in the art that my invention is by no meanslimited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

c What I claim is:

1. In a discriminator circuit for a phase modulation detector, a pair of input terminals and a pair of output terminals, a piezo-electric crystal, a ciystal holder consisting of an input'electrode and an output electrode, said crystal being connected by said holder electrodes between one input terminal and a corresponding output terminal, a condenser in shunt with the crystal and connected'between said electrodes to neutralize the crystal holder capacity, an under-neutralization condenser connected between said electrodes in shunt with the crystal, a second condenser in shunt with the neutralization condenser for providing over-neutralization, and said utilization condenser being connected between the second of the input terminals and its corresponding second output terminal.

2. In a discriminator circuit, a pair of input terminals, a pair of output terminals, a piezoelectric crystal provided with a holder, said hold er consisting of an input electrode andan output electrode, said holder electrodes being respectively connected to one input terminal and a like output terminal, a holder-capacity neutralizing condenser, in shunt with the crystal between its holder electrodes, coupling the second input terminal and corresponding output terminal, an under-neutralization condenser connected between said one input terminal and like output terminal and in shunt with said crystal, an overneutralization condenser in shunt with the neutralizing condenser and connected between said second input terminal and corresponding output terminal.

3. In a discriminator circuit, a pair of input terminals, a pair of output terminals, apiezoelectric crystal provided with a holder consisting of an input electrode and an output electrode connected respectively to one input terminal and a like output terminal, a holder-capacity neutralizing condenser, connected between said elecofan input electrode and an output electrode connected respectively to one input terminal and 7 5. In a discriminator circuit for a phase modu lation detector, a pair of input terminals and a pair of output terminals, a piezo-electric crystal provided with a holder consisting of aninput electrode and an output electrode respectively connected between one input terminal and a corresponding output terminal, a condenser in shunt with the crystal and connected between said electrodes to neutralize the crystal holder capacity, an under-neutralization condenser connected between said holder electrodes in shunt with the crystal, a second condenser connected in shunt with the neutralization condenser for providing over-neutralization, and a pair of opposed rectifiers coupled between said output terminals.

6. A phase modulation signal detector comprising a primary circuit tuned to the mean frequency of applied signals, a secondary signal circuit, a piezo-electric crystal resonant to said mean frequency provided with a holder consisting of an input electrode and an output electrode respectively connected to the high potential sides of the primary and secondary circuits, a holder-capacity neutralizing condenser in shunt with said crystal and secondary circuit and connected between said input electrode and the low potential side of the secondary circuit, an under-neutralizing condenser connected between said high potential sides of the primary circuit and the secondary circuit in shunt with the crystal, and an overneutralizing condenser in shunt "with said neutralizing condenser. '7. 'In a discriminator circuit lation detector, a pair of input terminals and a pair of output terminals, a'piezo-electric crystal tuned to the mean frequency of applied phase modulated carrier energy a crystal holder consisting of an input electrode and an output electrode respectively connecting the crystal between one input terminal and a corresponding output terminal, a condenser in shunt with the crystal and connected, betweenthe holder electrodes to neutralize the 'crystal holder capacity, an underneutralization condenser connected between said electrodes in shunt with the crystal, a second con-p i denser in shunt with the neutralization condenser for providing oven-neutralization, said neutralization condenser being connected between for a phase modu- V ceive a piezo-electric crystal, a capacitance in,

the second input terminal and its corresponding second output terminal, a first isolation condenser connected in series between said crystal holder output electrode and said corresponding first output terminal, and a second isolation condenser between said neutralization condenser and its corresponding second output terminal.

8. In a discriminator circuit, a pair of input terminals, a pair of output terminals, a piezoelectric crystal provided with an input electrode and output electrode respectively connected to one input terminal and a like output terminal, a neutralizing condenser connected between said electrodes to neutralize the capacity therebetween and coupling the second input terminal and corresponding second output terminal, an under-neutralization condenser coupling the one input terminal and like output terminal in parallel with the crystal, an over-neutralization condenser coupling the second input terminal and corresponding second output terminal in parallel with the neutralizing condenser, a pair of opposed rectifiers coupled between said outputterminals, and respective isolation condensers connected in series with each said crystal output e1ectrode and neutralizing condenser.

9. In a discriminator circuit for phase modulation signals, a pair of input terminals, a pair of output terminals, a piezo-electric crystal provided with holder input and output electrodes respectively connected to one input terminal and a like output terminal, said crystal being tuned to the mean frequency of said signals, a crystal-holder capacity neutralizing condenser coupling the second input terminal and corresponding second output terminal, an under-neutralization condenser coupling the one input terminal and like output terminal, an over-neutralization condenser coupling the second input terminal and corresponding output terminal, a phase modulation signal input circuit coupled in push-pull to said input terminals, a balanced rectifier system coupled to the output terminals, and means connected to the crystal for adjusting the Q thereof.

10. A discriminator network comprising a pair of input terminals and a pair of output terminals, a pair of metal electrodes, one electrode being connected to one input terminal and the second electrode being connected to a corresponding output terminal, said electrodes being adapted to reshunt with said electrodes to neutralize the capacity between the latter, under-neutralization capacitance connected in shunt with said electrodes, a third capacitance in shunt with the neutralization capacitance for providing over-neutralization, and said neutralization capacitance being connected in circuit with the second input terminal and its corresponding second output terminal.

11. In a wave detecting system, a resonant input circuit tuned to the frequency of applied waves a pair of output terminals, a piezo-electric filter coupled between one side of said input circuit and one of the output terminals, a connection between the filter output and the second output terminal whereby filtered wave energy is applied to said output terminals in like phase relation, a first path in shunt with said filter for applying said Wave energy from said input circuit to said first output terminal in unfiltered condition, a second path between the opposite side of said in,- put circuit and the second output terminal for applying unfiltered wave energy to the second output terminal in a phase opposite to the phase of the unfiltered wave energy at said first output terminal.

12. A phase modulation signal detector com prising a primary circuit tuned to the mean frequency of the signals, a secondary signal circuit tuned to one side of the mean frequency, a piezoelectric crystal tuned to said mean frequency, a holder for the crystal consisting of an input electrode and an output electrode respectively connected to the high potential sides of the primary and secondary tuned circuits, a crystal-holder capacity neutralizing condenser in shunt with said crystal and secondary circuit and connected between the input electrode and the low potential side of the secondary circuit, a respective isolation condenser in series with each of said crystal output electrode and neutralizing condenser, an under-neutralizing condenser connected from the high potential side of the primary circuit to the secondary circuit in shunt with the crystal and its series isolation condenser,.and an over-neutralizing condenser in shunt with said crystalholder neutralizing condenser and its series isolation condenser.

MURRAY G. CROSBY.

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