US2425924A - Phase modulation detector - Google Patents

Description

vAug. 19, i947. M. G. CROSBY v PHASE MODULATION DETECTOR Original Filed April 3, 1943 Y fP/af I Arme/VU.

Patented Aug. 19, 1947 UNITED STATES PATENT OFFICE PHASE MODULATION DETECTOR Murray G. Crosby, Riverhead, N. Y., assigner to Radio Corporation of America, a corporation of Delaware (Cl. Z50-27) Claims.

My present invention relates to phase modulation detector circuits, and more particularly to improved forms of a phase modulation detector of the piezo-electric crystal type. This application is a division of my copending Patent No. 2,397,841, dated April 2, 1946.

In the past I have disclosed various circuits for utilizing the inherent properties of a simple crystal filter to convert phase modulation of carrier energy into amplitude modulation for detection. For example, in Communication by phase modulation, Proceedings of the I. R. E. for February 1939 (pages 126 to 136), I have shown a crystal filter phase modulation translating network, and have explained the operation thereof on the basis of over and under-neutralization of crystal holder capacitance.

It is one of the main objects of my present invention to provide improved and modified types of crystal filter phase modulation (PM hereinafter for brevity) detectors, in each type, or form, of circuit the basic functioning being considered as involving the application to each of a pair of opposed rectiers the resultant of crystal-filtered carrier-energy and modulated carrier energy in normal phase quadrature relation at resonance.

Another important object of my present invention is to detect PM wave energy by a process which involves passing the PM energy through a piezo-electric crystal lter to secure substantially unmodulated carrier energy, applying the unfiltered PM wave energy to a pair of opposed rectiers, shifting the relative phase between the filtered and unfiltered energy to quadrature relation for the resonance condition, and applying the filtered energy to the rectiiiers thereby to cause each rectifier to rectify its respective Vector resultant energy.

Another object of my invention is to provide a PM wave energy detection network for PM wave energy or AM (amplitude modulation) wave energy, the network being capable of providing automatic frequency control (AFC) voltage, substantially pure carrier energy for carrier exaltation, and modulation signal voltage,

Yet another object of my invention is to provide a device for controlling the selectivity of a crystal lter circuit feeding opposed rectiers of PM carrier wave energy.

Other objects of my invention are to improve enerally the construction and operation of PM or AM detectors of the crystal filter type, and more especially to provide such detectors in a simple and economically-manufacturable form.

Other features will best be understood by reference'to the following description, taken in connection with the drawing, in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

In the drawing:

Fig. 1 snows one embodiment of the invention;

Fig. la graphically shows an ideal phase shifting characteristic of the crystal;

Figs. 2a and 2b show vector relations between the filtered and unfiltered signal energy for the unmodulated and modulated states respectively;

Fig. 2c vectorially explains the manner of phase detection from another viewpoint;

Fig. 2d shows the vector relations between the filtered and unfiltered signal energy for a condition of off-tune, or oil-resonance; and

Figs. 3 and 3a show two further modifications.

Referring to the accompanying drawing, wherein like reference characters in the diiferent iigures designate similar circuit elements, there is shown in Fig. 1 one form of a detector circuit which embodies the features of the present invention. The detector network is provided with an input transformer l whose primary circuit 2 is resonated to the center, or mean, frequency of the applied modulated carrier wave energy. Since the present invention is not in any way concerned with the source of the signal energy, the circuits prior to the primary circuit '2 are not shown in the drawing. The signals applied to the input transformer I may be PM, or AM, signal waves. Those skilled in the art are fully aware of the various networks which could be employed prior to circuit 2. Particular reference is made to my aforementioned publication for a more specific disclosure of such prior networks, where the receiver is of the PM type.

It will be sunicient for the purposes of this application to assume that the detector circuit is employed in a PM receiver of the superheterodyne type, and that the carrier frequency of the transmitted PM wave energy will be some predetermined frequency in the high frequency band. For example, the band below 25 megacycles (mc.) is particularly desirable for the radiation of PM signals. In the superheterodyne form of reception, a local oscillation circuit is employed to reduce the mean frequency of the received PM carrier energy to an intermediate frequency (I. F.) and it is the PM signal energy at the I. F. mean value which is applied to the primary circuit 2. Hence, the circuit 2 is resonated to the I. F. mean value of the incoming PM Wave energy. If desired, an amplitude modulation limiter stage may be employed prior to the circuit 2. The modulation signal output of the detector will then be truly representative of the phase deviations of the carrier, and not of the amplitude variations of the latter.

Considering, now, the detector circuit per se, it is first pointed out that the circuit comprises a pair of opposed rectifiers 3 and f5. These rectiers are shown as of the diode type, since such rectiers are simple in construction. However, the present invention is in no way limited to the particular types of tubes shown, nor, indeed, to the above-mentioned specific frequencies. The anode of rectifier 3 is connected to one end of the secondary winding I of input transformer I. The last-mentioned connection includes in series a piezo-electric crystal P and a resistor 8.

The crystal P is located between a pair of metallic electrodes in the usual fashion, and the crystal is tuned to the mean frequency of the applied signal energy. That is to say, the crystal P is tuned to the resonant frequency of the input circuit 2. The electrodes of the crystal may, if desired, be metal coats suitably provided on the opposite faces of the crystal. The resistor 8 may be shunted by an adjustable condenser 8', and the function of the latter will be explained at a later point. The anode of rectifier 4 is connected to the opposite end of the secondary winding 'I through a path comprising the capacity 9 arranged in series with resistor IB. Resistor I is shunted by variable condenser I. The right-hand termina1 of condenser 9 is connected by lead I i to the righthand crystal electrode. Between the midpoint of winding 'i and the lead II there is connected a resonant circuit which comprises coil I2 shunted by the adjustable condenser i3. Normally, the resonant circuit I 2-I3 is tuned to the mean frequency value of the applied PM signal energy. The output load resistors of the circuit are designated by numerals Ill and i5, and these resistors are connected in series between the cathodes of diodes 3 and 4, The cathode end of resistor I5 is established at ground potential, and each of resistors Id and I is shunted by its respective carrier bypass condenser. The junction of resistors I4 and I5 is connected by lead I5 to the midpoint of the secondary winding l.

A second path connects the anode of each rectier to the respective end of winding 1. Thus, condenser 5 connects the anode of diode 3 to the upper end of winding 1, while the condenser 6 connects the anode of diode l to the lower end of winding l. Modulation signal energy may be taken off from the cathode end of resistor Id. Furthermore, that same point of the output resistor may be tapped for AFC voltage in order to control the frequency of the local oscillator, as is well understood. Where AFC voltage is taken off from the cathode end of resistor Iii, a modulation voltage filter, schematically represented by numeral I'i, is inserted in the AFC output line. Substantially pure, or filtered, carrier energy is taken off from lead I8. The filtered carrier energy will have a frequency equal to the mean value of the applied PM signal energy. Such filtered carrier energy may be utilized for carrier exaltation detection in the manner disclosed in my U. S. Patent No. 2,063,588, granted December 8, 1936. In such case, the pure carrier energy is fed to a separate phase modulation, or AM, detector. It is not believed necessary to show the dernodulator in such case, since those skilled in the art will readily understand that it can be a second PM detector whose input is taken from the input circuit 2 of Fig. l. If desired, the filtering of the energy applied to crystal P may be such as to leave modulation from zero to about 200 cycles on the carrier.

In explaining the functioning of the circuit shown in Fig. 1, it is first pointed out that the condenser 9 is adjusted so as to neutralize the capacity existing between the metal electrodes of crystal P. Referring to Fig. 2a there is shown the vector relations existing between the retarded and unretarded voltages insofar as they affect the opposed rectifiers 3 and 4. The PM signal energy which passes through the crystal P is stripped of its modulation Side bands, so that there is applied to the anode of each of rectifiers 3 and l virtually unmodulated carrier energy. The vector Ep represents this crystal-filtered carrier energy applied to the rectifiers. The path from the crystal to the rectifier 3 is through resistor 8, while the path from the crystal to the rectifier 4 is through lead i I and resistor IG. It will, therefore, be seen that the filtered carrier energy at the output electrode of crystal P is applied in like polarity, or in parallel, to the anodes of the respective diode rectifiers 3 and 4. The passage of the filtered carrier energy through resistors 8 and I0 to the respective detector input electrodes is accomplished without phase shift since the coupling is totally resistive except for the effect produced by condensers 8' and I. The effect of condensers 8 and I0 is compensated for by detuning circuit IZ-IB as will be described later.

The unltered PM signal energy which passes to each of rectifiers 3 and 4 through condensers 5 and 6 respectively is shifted 90 degrees in phase by virtue of the capacity feed to the respective detector input electrodes. Furthermore, since these energies are taken from the opposite ends of the winding 'I they are applied to the opposed rectifiers 3 and 4 in polarity opposition. It will be noted that the midpoint of winding 'i is effectively at ground potential with respect to radio frequencies, because the lead I5 connects the midpoint to ground through the condenser in shunt with resistor I5. The vectors E6 and E5 denote the unfiltered voltages applied to rectifiers 4 and 3 respectively, and it will be seen that these vectors are in phase quadrature with the crystalfiltered carrier energy. This phase quadrature relation of the two voltages at each rectifier results from the fact that the unfiltered signal energy is applied to the rectifiers 3 and Il by condensers 5 and 6 respectively which are suficiently small to effect a degree phase shift, and are, also, of substantially equal capacities so as to produce equal phase shifts both of the unfiltered carrier and of signal components. The condensers 5 and 6 are non-selective to phase or frequency variations of the unfiltered carrier, and, accordingly, permit all signal components to pass to the rectifiers 3 and 4. The crystal P, however, effects no phase shift 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, as depicted in Fig. la.

The vector representing the resultant signal energy at each rectifier is also indicated in Fig. 2a. Thus, theV vector En represents the resultant energy applied to rectifier 3. The vector En represents the resultant energy applied to rectifier d. Fig. 2a depicts the situation when the mean frequency value of the applied PM signal energy is instantaneously equa-l to the frequency of circuit 2 and the frequency of the crystal P. The rectiiied outputs of each rectifier will, therefore, be equal, and the effective voltage at the cathode end of resistor Iii will, therefore, be Zero. In other words, for the in tune state no AFC bias is developed.

In explaining the operation of this circuit, there are two separate conditions which must be considered. One is the case of the dernodulation of a phase modulated signal, and the second is the case of the detection of slow frequency variations to obtain AFC potentials. These two cases represent two different degrees of modulation that are acted upon by the crystal iilter in different manners. For the case of the relatively rapid modulation represented by the phase modulations of the signal, the filter acts as a devic which selects the carrier from the side bands, and provides the equivalent of a synchronized local carrier free of modulation. When an uninodu lated carrier is received there delivered to each of diode rectiers 3 and il voltages, one a filtered carrier from crystal l: without phase change, and the other an unfiltered carrier substantially 90 degrees different in phase from the ltered carrier. When the received carrier is phase-modulated the iiltered carrier remains as before, but the unfiltered signal energy is supplied to the rectiiiers 3 and il in phases differing from the 90 degree, or quadrature, relation to an extent determined by the degree of phase modulation. If the degree of phase modulation is small, a relatively small direct current voltage is built up across rectifier output resistors and l5 due to the signal voltage increasing on one of the rectiers and decreasing on the other. The greater the degree of phase modulation the greater the combined voltage of the unfiltered signal energy and the filtered carrier from crystal l; on of the and the less the s un of such voltages on the other rectifier. rhe polarity of the direct current voltage drop across the load resistors lil and l5 of the opposed rectiers depends on the direction of the phase change of the received signal energy.

For the case of the relatively slow variations in frequency of the incoming signal, the crystal filter acts as a retard circuit havino` an output phase which varies with the frequency of the input. For this case, the circuit acts like a very narrow-band frequency modulation discriminator. The solid curve in TEig. la, shows frequency vs. phase shift characteristic of crystal P. At Fc, the center frequency, the crystal provides zero phase shift.

The vector diagrams of Figs. 2a, 2b and 2c show the conditions for the case of phase modulation detection. The filtered carrier, which is represented by vector Ep, remains fixed in phase. The unfiltered modulated represented by vectors E5 and EG, vary in phase to produce differentially modulated resultants Eo and En' which are fe to the opposed detectors. The manner in which the modulated signal varies in phase with respect to the ltered carrier is shown in l'iig. 2c. A condition of modulation in one direction is shown in Fig. 2b. The unmodulated condition is shown in Fig. 2a.

The vector diagrams of Figs. 2a and 2d show the conditions for the case of AFC detection. Fig. 2a shows the in-tune condition which is effected when the applied signal carrier frequency is in the l f middle' of the crystal lter characteristic. The

diagram of Fig. 2d shows the relations for an oif-tune condition. It will be noted that the carrier (or crystal output) phase shifts for the olf-tune condition. This phase shift is brought about by the phase characteristic of the crystal, which is similar to that of an ordinary resonance circuit, as shown in Fig. la. The magnitude and sense of phase shift of the ltered carrier energy are respectively dependent on the amount and direction of frequency departure of the modulated carrier energy at circuit 2 relative to the predetermined frequency Fc. The latter is, of course, the resonant frequency of crystal P. The signal energy passing through condensers 5 and t will not shift in phase in response to carrier frequency departures from Fc. This follows from the fact that condensers 5 and e are non-selective. elements. Hence, and as shown in Fig. 2d, the resultant vector voltages En and Eb will Vary in relative magnitude depending on the extent and sense of the aforesaid frequency departure. rhese relatives variations in Eo and E0 are translated into corresponding direct current voltage variations across load resistors it and l5, and the differential of these direct current voltages is used as AFC bias after iiltering at il. It is seen that the carrier phase for the off-tune condition is no longer in its proper quadrature re iationship with the uniiltered signal so that it might be thought that the detection of phase modulation would be impaired. However, this olf-tune condition is never allowed to exist to any appreciable degree, since the AFC circuit functions to correct the tuning and maintain it in the in-tune condition represented by Fig 2a.

Winding l' has its midpoint grounded, as pointed out above, and parallel resonant circuit l2, I3 connects the grounded midpoint of coil 'I to the lead l l which connects the output electrode of the crystal P to the rectiiiers. The circuit l2, I3 is, accordingly, in effect connected between the output side of crystal P and ground. Circuit I2, i3 may .be tuned to crystal frequency, or may be detuned relatively thereto, and acts as a coupling circuit of finite impedance between the output of crystal P and the rectifiers 3 and It. The circuit l2, i3 increases the Q of the crystal beyond what it would be if the resistance of circuit IE, i3 were innite. By detuning resonant circuit l2, i3 the phase of the ltered carrier energy can be shifted to a predetermined extent. This phase shift can be compensated by each shunt capacity 8 and lil. Each of resistors 8 and l is, therefore, shunted by a respective compensation condenser. In this way, the selectivity of the crystal P may be improved without af.. footing the zero phase shift state of the ltered carrier energy. In other words, detuning the circuit l2, i3 not only will cause a phase shift of the filtered carrier, but will also act to increase the selectivity of the crystal filter. If desired, resistor 8 and condenser 8 may be interchanged with condenser 5, and resistor lll and condenser lll may be interchanged with condenser l. This would in no way affect the relative normal phase quadrature relation between the retarded and unretarded PM signal energy as depicted in Fig. 2a.

In the arrangement of Fig. 3 the filtered carrier energy is derived from the primary resonant circuit 2. lThe unltered signal energy is shifted degrees by virtue of the magnetic coupling M existing between the windings of the input transformer l. In this case, the crystal P is connected between the upper end of the primary winding' of the input transformer and the midpoint of the secondary winding. The condenser 9, connected between the midpoint of the secondary Winding and the lower end of the primary winding, functions to neutralize the crystal inter-e1ectrode capacity. The pure carrier energy is taken from the output electrode Vof the crystal. Otherwise, the circuit functions in the same manner as described in connection with Fig. l. It is sufficient to point out in connection with this modification that the filtered PM signal energy is applied to the anodes of rectifiers 3 and 4 in like polarity by virtue of the connection of the crystal to the midpoint of the secondary Winding. On the other hand, the unfiltered signal energy is first shifted 90 degrees in phase by the magnetic coupling M, and the phase shifted energies are applied in polarity opposition to the anodes of the opposed rectiiiers. It may happen that the circuit shown in Fig. 3 will give rise to second harmonies in the pure carrier energy taken off from the crystal P. This is brought about by full-wave rectified voltage, rectified from the secondary of transformer I and appearing across circuit I2, I3.

In that case, the rectiiiers 3 and 4 are reversed in connections to eliminate the full-Wave connection, and cause the rectifiers to conduct simultaneously instead of alternately and thereby suppress the production of second harmonics. This is done by connecting the load resistors I4 and I5 in the manner shown in Fig. 3a. The anode of rectifier 3 is connected to the junction of resistors I4 and I5, while the cathode of diode 3 is connected to the upper end of resistor I4. In other words, the only change that need be made in the circuit of Fig. 3 is that indicated with respect to resistor I 4 and its associated diode 3. The diode-resistor condenser is no longer across resistor I4, but takes the form of condenser 3 inserted from the cathode of diode 3 to the connection leading to the upper end of the secondary winding of the input transformer.

While I have indicated and described several systems for carrying my invention into eect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention.

What I claim is:

l. In a detector of phase modulated signalling energy, a signal input transformer having a primary resonant circuit tuned to the mean frequency of applied signal energy, a secondary'circuit tuned to said mean frequency, a piezo-electric crystal element tuned to the said mean frequency, means connecting saidv crystal element between one side of the primary circuit and the mid-point of the secondary circuit, a first rectier, means connecting the rectifier electrodes in circuit with said crystal element thereby to have the crystal outputenergy applied-thereto, a second rectifier in circuit with the crystal element having said output energy applied thereto in like polarity, a common output circuit connecting said rectifiers, separate connections from respectively separated points of the input transformer to th'e respective rectifiers for applying thereto unltered phase modulated energy, said separated points being pointsof opposite polarity on said secondary circuit, a resonant circuit, normally tuned to said mean frequency, in circuit withI the crystal element, said resonant. circuit including means for adjusting its frequency whereby the phase relation between the crystal output energy and the unfiltered energy may be varied, and means for utilizing the crystal output energy for carrier-exalted demodulation.

2. In a frequency-sensitive system, a first resonant circuit adapted to have frequency-variable signals applied thereto, means establishing a point of the circuit as a reference potential point, a second resonant circuit coupled to the first circuit, said circuits being tuned to a desired frequency, a first rectifier circuit connected from one side of said second resonant circuit to an intermediate point thereof, a second rectifier circuit connected from the opposite side of the second resonant circuit to said last named intermediate point, a piezo-electric crystal, tuned to said desired frequency, connected from a point on said first resonant circuit on one side of said reference potential point to said intermediate point, a third resonant circuit, tuned to substantially said desired frequency, included in both of said rectifier circuits, said third circuit having an input terminal connected to said intermediate point, and means adapted to derive from said rectifier circuits the differential resultant of the rectified signal voltages thereof.

3. In a frequency-sensitive system, a rst resonant circuit adapted to have frequency-variable signals applied thereto, means establishing a point of the circuit as a reference potential point, a second resonant circuit coupled to the first circuit, said circuits being tuned to a desired frequency, a first rectifier circuit connected from one side of said second resonant circuit to an intermediate point thereof, a second rectier circuit connected from the opposite side of the second resonant circuit to said last named intermediate point, a piezo-electric crystal, tuned to said desired frequency, connected from a point on said first resonant circuit on one side of said reference potential point to said intermediate point, a third resonant circuit, tuned to substantially said desired frequency, included in both of said rectifier circuits, said third circuit having an input terminal connected to said intermediate point, and means adapted to derive from said rectifier circuits the differential resultant 0f the rectified signal voltages thereof, means for neutralizing the inter-electrode capacity of said crystal comprising a condenser connected from a point on said first resonant circuit on the opposite side of said reference potential point to said intermeditae point.

4. In a frequency-sensitive system, a first resonant circuit adapted to have frequency-variable signals applied thereto, means establishing a point of the circuit as a reference potential point, a second resonant circuit coupled to the rst circuit, said circuits being tuned to a desired frequency, a first rectifier circuit connected from one side of said second resonant circuit to an intermediate point thereof, a second rectifier circuit connected from the opposite side of the second resonant circuit to said last named intermediate point, a piezo-electric crystal, tuned to said desired frequency, connected from a point on said first resonant circuit on one side of said reference potential point to said intermediate point, a third resonant circuit, tuned to substantially said desired frequency, included in both of said rectifier circuits, said third circuit having an input terminal connected to said intermediate point,

9 and means adapted to derive from said rectier circuits the diierential resultant of the rectied signal voltages thereof, a signal amplifier tube preceding said rst resonant circuit, means connecting th'e tube output electrode to said point on said rst resonant circuit to which said crystal is connected, and means connected to said refer- 10 ence potential point adapted to apply a positive direct current voltage to said output electrode.

5. In a system as defined in claim 2, said rectier circuits including respective diodes, one of the diodes being directly shunted across its respective load resistor.

MURRAY G. CROSBY.

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