US2397841A - Phase modulation detector - Google Patents

Description

April 2, 1946 y lM. G. cRoslasY` 2,397,841

PHASE MODULATIONXDETECTOR Filed April 5, 194s 2 sheets-sheet 41 A TTORNE Y April 2, 1946.

M. G. cRQsBY PHASE MODULATION DETECTOR Filed April 3, 1943 2 Sheets-Sheet 2 Patented Apr. 2, 1946 PHASE `MODULATION DETECTOR Murray G. Crosby, Riverhead, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application April 3, 1943, Serial No. 481,691

14 claims. n (C1. 25o-27) My present invention relates to phase modulation detector circuits, and morepartcularly to improved forms of a phase modulation detector of the piezo-electric crystal type.

In the past I have disclosed various circuits for utilizing the inherent properties of a simple crystal lter 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 lter 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 A(l='M 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 Vmodulated carrier energy in normal tion 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 lilter to secure substantially unmodulated carrier energy, applying the unll tered PM wave energy to a pair of opposed rectiers, shifting the relative phase between the filtered and unfiltered energy to quadrature rela- V tion for the resonance condition, and applying the filtered energy to the rectiers thereby' to cause each rectifier torectify its respective vector resultant energy- Another object of my invention is to providea 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 generally 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.

The novel features which I believe to be characteristic of my invention are 'set forth with particularity in the appended claims; `the invention itself, however, as to both its organization and method of operation will best be understood by phase quadrature relareference to the following description, taken in connection with the drawings, in which Ihave indicated diagrammatically several circuit 0rganizations whereby my invention may be carried into effect. In the drawings:

Fig. 1 shows one embodiment of the invention, Fig. la graphically shows the'phase shifting characteristic of the crystal, j

Figs. 2a and 2b show vector relations between the filtered and unltered signal energy for the unmodulated and modulated states respectively, Fig. 2c vectorially explains the manner of phase detectionA from another viewpoint,

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

Fig. 3 shows a modiiication of the circuit of Fig. 1,

Figs. 4 and 4a show two further modiiications,

Figs. 5 and 5a illustrate two additional modifications.

Referring to the 'accompanying drawings, wherein like reference characters inthe different gures 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 primarycircuit 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 Y energy. the circuits prior to the primary circuit 2 are not shown in the drawings. 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.y

It will be suilicient 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 ofthe 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. rIn the superheterodyne form of` reception, a local oscillation circuit is employed to relower end of winding 1. v ergy may b e taken off from the cathode end of resistorv I4. Furthermore, that same point o f the output resistor may be tapped for VAFC voltage Ithe carrier, and not of the amplitude variations of the latter.

Considering, now, the detector circuit per se, itis first pointed out that the circuit comprises a pair of opposed rectifiers 3 and 4. These-rec` tiiiers are shown as of the diode type, since such rectifiers are simple in construction. However, the present invention is in nok wayv limited to the particular types of tubes shown, nor,/i n deed,`to

the above-mentioned specific frequencies 4which have been specified. IThe anode lof rectifier-3 'is connected to one end-of the secondary winding 1 lof input-transformeri 'The last-mem' tioned :connection includes -inseries a piezo-electric crystal P and a resistor 8.

The crystal P is locatedbetweena pair of metallic electrodes in the usual fashion, and the l crystal is tuned Vto the mean frequency of the applied signal energy. That is to say, the crystal .P istuned to the resonantV frequency of thefinput circuit 2. The electrodes of the crystal may, if

desired,be metal coats vonthe opposite faces. l The resistor 8 maybe --shunted by an'adjustable condenser 8',and the function of the latter will be .explained ata later point. The anode of rectiiier v4 is Connected to the opposite end r of the l secondary winding 1 ythrough a path comprising the capacity!! arranged in series with resistor I0. Resistor Ill is shunted by variable condenser I0.

The right-hand'terminalfof condenser 9 is conneoted by lead vII to jthe right-hand crystal electrode. Between the midpoint of Winding :'I and )the lead II thereis connected v*a resonant circuit which comprisesthe vcoil ;I2 shunted by the yadjustable condenser I3. Normally, the resonant circuit I2-A-'I3 is tuned ,to the mean frequency value of the applied-PM signal energy. The youtputload resistors of the circuit are designated byY numerals I4 and I5, and these resistors are connected inv series between the cathodes of diodes .3 ranelli. The cathode end 'ofresistor Iisestabutilized for carrier exaltation detection in the manner disclosed in my U. S. Patent No. 2,063,588, granted December 3, 1936. In such case, the pure carrier energy is fed to a separate phase modulatiomor AM, detector. It is not believed necessary to show the demodulator in such case, since those skilled in the art will readily understand that it can be a seco-nd PM detector whose input is taken from'the input circuit 2 of Fig. 1. If desired, the'ltering of the energy appliedto crystal P may be such as to leave modulation from Zero to about 200 cycles on the 'carrier'. l Y

In explaining the functioning of the circuit Vshown* inFig. 1, it is first pointed out that the condenser S'is adjusted so as to neutralize the capacity between the metal electrodes of crystal P. ,Referring tov Fig. 2a there is shown the vector relations existing between the retarded and unretardedy voltages insofar as they affectthe opposedrectiiiers 3 and 4. The PM .signal energy which passes through the crystalPis stripped of its modulation side bands,Y so that there is applied to the anode of each of recters 3 and 4 virtually unmodulated carrier energy. The vector Ep represents this crystal-filtered' carrier energy appliedto the rectifiers. The path. from the crystal to the rectier 3 is through resistor 8, while theV i path from the'crystal to the rectifier 4 is through lead I I and resistor I0. It will, therefore, be seen that the filtered carrier energy at the output electrode of crystal P is applied Ain like polarity, or in parallel, to the anodes of the respective diode .rectiers A3 and 4. The passage of lthe filtered carrier energy through resistors B and I to the respective detector input electrodes is accomplished without phase shift since the cou- -pling is totally resistive except for the effect pro- .duced by condensers 8 and I0. The effect of glished Vat ground potential, and Aeach of resistors 1 i I4 and `I5 -is shunted by a carrier bypass condenser. The junction of resistors I4 and I5 is connected by lead `lli to the midpoint of the sec- 1 ondary winding ,'I.

A second path connects the anode o f each rectifier to the respective end of winding 1.

Thus, condenser 5 connects the anode of diode y 3 to the upper end of winding 1,'whi1e the condenser ,6 connects the anode of diode 4 to the Modulation signal enin order to control the frequency of the .local OSilatOl, aS 'is Well understood. VVlhere AFC r voltage is taken'off from the cathode end .O f re- Vsister |,4f a modulation voltage filter. schermati cally represented by numeral I'I, is inserted in the. AFC outputV line. Substantially pure, or` filtered, carrier energy isV 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 ltered carrier energy may be condensersv" and I0' is compensated for by detuning I2-I3 as will be described later.

The unfiltered PM signal energy which passes to each of rectiiiers-3 land 4 through condensers l5 and 6 respectively is shifted 90 :degreesin phase by virtue of the capacity feedoto the respective 4detector input electrodes. Furthermore, since these energies are taken from the opposite ends of the winding I they are applied to the opposed 1'ecti1iers'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 frequenciesbecause the lead I6 connects the midpoint to ground through resistor I5. The

vectors Es and E5 denote the voltages applied to Y rectiers 4 and 3 respectively, and it will be seen that these vectors are in phase quadrature with the crystal-filtered carrier energy. This phase quadrature relation of the two voltages at each rectifier results from thel fact that the unfiltered signal energy is applied to the rectifiers 3 and 4 by condensers 5 and 6 respectively which are sufiiciently small to effect a degree phase shift, and are, also, of substantially equal capacities so as to produceequalphase shifts both of the un- `filtered carrier Yand of signal components. The

condensers 5 and 6 are non-selective to phase or frequency variations of lthe unfiltered carrier, and, accordingly, permit all signal components to pass to the rectiiiers 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.. TheV crystal P is, of course, selective for frequencies off resonance. This follows Afrom' the sharp selectivity characteristic ofthe crystal, as depicted ideally in Fig. 1a. f f

rhe vector representing the resultant signal energyv at each rectifier is,also indicated in Fig. 2a. Thus, the vector Eo' represents the resultant energy applied to rectifier 3. The vectorEo represents the resultant energy applied to rectifier 4. Fig. 2a depicts the situation when the mean frequency value of the applied PM signal energy is instantaneously equal to the frequency of circuit 2 and the frequency of the crystal P. The rectified outputs of each rectifier will, therefore, be equal, and the effective voltage at the cathode end of resistor I4 will, therefore, be zero. In other words, for the in tune state no .AFC bias'is developed. l

In explaining the operation of this circuit, there are two separate conditions Which'must be considered. One is the case of the demodulation ofvaf phase modulated signal, and the second is 'the case of the detectionof slow frequency variations to obtain AFC potentials. These two cases represent two diierent degrees of modulation that are acted upon bythe crystal filterin different-v manners. For the case of the relatively rapid modulation represented by the phase modulations of the signal, the lter'acts' asa device 'which selects the carrier from the side bands, and provides the equivalent ofa synchronized local carrier free of modulation. When an unmodulated carrier is received there are delivered to each of diode rectifiers 3 andV 4 voltages, Yone a filtered carrier from crystal P without phase change, and the other an unfiltered carrier substantially 9G degrees different in phase from the filtered carrier. When the 'received carrier is phasemodulated the filtered carrier remains as before, but the unfiltered signal energy-is supplied to the rectiers 3 and 4 in phases differing from the 90V degree, or quadrature,l 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 I4 and I5 due to the signal voltage increasing on oneof the rectifiers and decreasing on the other. The greater the degree of phase modulatim` the greater the combined voltage'of the unfiltered signal energy and the filtered carrier from crystal P on one of the diode rectifiers, and the less th'esuin of suchv voltages on the other rectifier. The polarity of the direct current voltage drop across the load resistors I4 and I 5 of the opposed rectiers depends on the direction of the phase change of the receivedsignal energy.

For the caseof the relativelyslow variations in frequency of the incoming signal, the crystal filter acts as a retard circuit having an output phase which varies with the' frequency of the input. For this case, thecircuit acts like a very narrow-band frequency modulation discriminator. The solid curve in Fig. la-showsthe frequency vs. phase shift characteristic of crystall P. At Fc, the center frequency, the crystal provides zero phase shift. v

The vector diagrams of Figs.v 2a, 2b andl Zcshow the conditions for the case of phase modulation detection. The ltered carrier, `which is represented by vector Ep, remains fixed in'l phase. .The unfiltered modulated signals, represented by vec. tors E5 and Ee, vary in phase tov produce differentially modulated resultanteI En and En', which are fed to the opposed detectors.-rv y which the modulated signal variesr in phase with The manner in,

respect to the filtered carrier is shown in-Fig. 2c.

A condition of'smodulation in- 'one direction is shown in Fig. 2b. The unmodulated condition is shown in Fig. 2a.

The -vector diagrams of Figs.l 2a and 2d show the conditions for the caseof AFC detection. Fig.

v2a shows the in-tune condition which is effected when the applied signal carrier frequency is in the middle of the crystal filter characteristie. Y The diagram of Fig. 2d shows the relations for an olf-tune condition. It will be noted that the carrier (or crystal output) phase shiftsvfor the 'off-tune condition. This phase shift is Y brought about by the phase characteristic of the elements.

crystal, which is similar to that of an ordinary resonance circuit, as shown in Fig. 1a. The magnitudey and sense of phase shift of the filtered 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' will not shift in phase in response to carrier frequency departures from Fc. This follows from the fact that condensers 5 and 6 are non-selective Hence-and as shown in Fig. 2d, the resultant vector voltages EnV and En will vary in relative magnitude depending on the extent and sense of theaforesaid frequency departure. rIhese relative variations in En and En' are translated into corresponding direct current voltage variations across load' resistors I4 and I5, and the differential of these direct current voltages is used asVAFC bias after filtering at I1. It is seen that the carrier phase for the off-tune condition is no longer in its proper quadrature relationship with the unfiltered signal so. that itmight be thoughtthat the detection of phase modulation would be impaired. However, this off-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 I2, I3 connects the grounded midpoint of coil l to the lead II which connects the output electrode 'of the crystal P tothe rectiflers. The circuit I2, I3 is. accordingly, in effect connected between `the outputl side of crystalP and ground. Circuit I2 I3 may be tuned to crystal frequency, or may be detunedrelatively thereto, and acts as a .coupling circuit of finite impedance between the output of crystal P and thefrectiiiers 3 and 4. The circuit I2, I3'incre'a`ses the Q of the crystal beyond what it would be if the resistance of circuit vI 3 were infinite. By detuning resonant circuit l2--I 3 the phase ofthe filtered carrier energy can be shifted to a predetermined extent. This phase shift can be compensated by each shunt capacity 'and I0'. Each of resistors 8 andl I 0 is; there- ,5, and resistor Il) and condenser Iii' may be interchanged withcondenser 6. This would in no way aiect the relative normal phase quadrature i --relationhetvreonitheireterdodand mretarsodrPM .;sier,1.al;enereya,s depicted FierZa.

a mod'caticmp-.tho

glue".

Aner as shown in Fig.'

arrangement in Fig. 1, wherein he re "Store 8 and l0, are,.rop1aoedgbr resrzeogt've o onyensers 8" f1 The unlteredtRM. signal. lenereyin that oase -is derived .from 'the primary-,Circuit ':2-

neotiondeading. tothe upperzendrof thezseoondarv tered modulated carrier., energy, is iderivedby informersecondary circuit..` SinceitheS-anodezconvnections are made to the. o ppsite .endsuof1the Since at ,resonance theprimary volta-ge hearsga Y,

phase quadraturerelation withrespect` torthe Secondary voltage. rit Will be seen :that the ,reoti-l fiers .3 and Lvvill have nropenrhase, relations vlootwoon the filtered-and nnltorodisienolenerey.

The numeral, lI s designa-tesori; ampler tube, :or

an amplitude limiter-tube, whose' platev-'oirouitfinoludes vthe resonant prima e. vplate foffamplier vl 8 -mittod through condonsersj. and respectively will'be lin polarity opposition-When;applied tothe-1 anodesiof reotiers-S and il. The selectivitycolo- 1. The anodei each rectier is connected to the jnncgtiongof load resistors .I4 andv 15 through respective;returrmesstots 414V :and I5'. f

secondary' winding.v The condenserq .connected vbetween the Vmidpoint of the secondary -`V v vinding vand the lower end ofy the primary Winding -functions to neutralize the crystal interelectrode iCarpacity. The pure carrier energy is takenirom the right-'handelectrode of the crystal.' Other` wise, Athe circuit functions -in-lthesameqmanner as described in connection with `1ig.;r1. .It is suf- -cient topoint out in connection with this modication that the filtered PM 'signa-1 energyiis applied tothe anodes of rectifier-S A'3i-and# in like polarity by virtue ofthe vconnection vofthe crystal `to the midpoint ofY the secondary winding. -On the' other hand. ythe unltered signal energy `is first shifted 90 degrees in phase :bythe rmagnetic coupling M, and-'the phase shifted energies are applied in polarity vopposition to-'the anodes-of the opposed rectiers'; -It 'mayhappen that the circuit shown in Fig. .4 will give rise to lsecond harmonics in the pure carrier energy taken off from the crystal iP. Y This -is fbroughtiahout *by full-wave rectified voltage', rectifledsfrom the secondary of' transformer 4I :and `falpearing acrOSS 12,13.

-In th at case,ltherectiers 3 :and 7,4 are reversed `in connections to .eliminate the .Afullfwave connection, and cause .therectier Ato conduct simultaneous'y instead of alternately and Vthereby suporess the production ofsooond harmonics. This `if; ldone by connecting the load resistors ,1,4 and in the manner shown in.1ig. .4a. VvThe anode of rectifier is connected to the junctiQn of resistors i4 and 15, while the veathode of diode 3 is connected to the upper .end .of resistor .1.4. In other words, the onlychange that needbe made in the circuit of Fig. .4 Ais, that indicated with respect to resistor I4 .and its associated diode rlhe diode-resistor condenser is nolonger across resistor I4, but .takes .the .form .of .condenser '.3'

ductive couplingr-fromthegprimaryrcircuit 2 ,1 and is applied; to the rrectifers3sandzl lfrom the transsecondary 1, Ywhose midpOint Lis eiectivelyfconnected 4toground 'for radio frequencies; itzwill ;be

seen that the unfiltered signal enereyig applied t0 the Opposed rectiers Y.in ;.pol arity ioppositiou. The ltered carrier ener-gm,` however, .isapplied Y to the diodesgin like phase. 'lyhehlterdcarlier energy is.'y taken Volif froin: the :leftfhafnd #terminal erges are both derived from the secondarycrcuit -oi theinput-transformer. .fHoWever,the-;required 90 degrees phase :shtis :secured fini. this zmodcatiollrio-y.v deliberately tdetuning the :resonant;.cir

cuit i3-H2. .-lhedetuningissoeadiusted-ras:to

secure the,. 90 degrees .phase retardation rIncidentally, the f crystal selectivityfisjncreasedupon detuningof-fthe resonant-circuit. `j-. In other words, with a circuitfof theiormeshown in Fig.; 5;the;net

. work Ir2-t3 not only 4functions?as@arphasexshifter element, Jb ut 'also facts; as f a gcontrpl: element over lfthe selectivity of thecrystaljlter.

In Fig. 5a thereyis shown t;he imodication fwhich is i required in Jig.V 5, such 1asztolsuppress lthe* presence; of :second harmonicszin -ztheltered carrier energy. vnithispase theffanode of diode 4 is connected tothe grQundedzrendsof load -resistor l5, while the f cathodefwill"beyconnectedto thelower endofsecondaryrwinding 1. Thefanode 0f diode 3 Willl be connected tothe upper :end 'of resistor I4, and its cathode: will fbe connected to they junction of resistors Maand slii. Itvvilliibe noted that -in vboth 'Figsfsia'and Sarthe- -rectiers are arranged to conduct .simultaneouslyrather than Valternately when -sfed hy fthe. .push-pull secondary of transformer `I.

' While I -vhaveV indicatedan'd .described several ysystemsl for carrying kmyiinvention .into effect, '-it will be apparent to one skilledzin'the-artthatrmy invention is by nomeansI limited to `theiparticular organizations `shown .:anddscribed, :but that -many -modifcations may be :.madev without sdeparting from -Athe .fscoperof :my invention, -fas Eset forth inthe appended claims.

What-Iolaim is: i Y

1. A method of .detecting'modulatedcarrier energy which comprises removing substantially. all the modulation lfroma I:portion of the energy, dividing the -unmodulated-fenergy into two portions of like polarity relation, 1dividingthe original modulated energy Avin-to two 4:additional portions of opposite polarity relation, displacing the relative phases '.loetween :said two :unrno'dulated portions andxsaid rtwo ninodulated ,portions to the vextent of substantially f90 degreesfat resonance,

separately combining in pairs an iurrmodulated portion and modulated portionin-phase ii-isplaced relation, separatelyr rectifyingl thefresfultant of each of the combined'pairsfand' combining the rectication productsof the separate rectication lStoiosin opposition. i

' 2. *Ina detector of Ipdlllatfd sig-naHingenergy,

'a vsignal input ttansormer havinga vprimary resonantcircuittuned tothegnean treguency 0iA applied modulated,signalenergy, .asecon-dary circuit including -a piezo-electric .crystal element ytuned to thas-aid mean freouenoy, anrstrtectiiierpmeans ,connecting the rectiiier electrodes .i-n .circuit :with

said crystal element thereby to'have the crystal output energy 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 rectiers in polarity opposition, separate connectiongfr'om respectively separated points of the input transformer to the respective'rectiilers for applying thereto uniilteredmodulated signal energy, said separated points being of 'opposite polarity; and means for producing a normal phase quadrature displacement between the'crystal output kenergy and the unfiltered energy.

3. 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 including a piezo-electric crystal element tuned to the said mean frequency, a rst rectifier, means connecting the rectifier electrodes in circuit with said crystal element thereby to have the crystal output energy 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 rectiers in polarity opposition, separate connections from respectively separated points of the input transformer to the respective rectifiers for applying thereto unfiltered phase modulated energy, said separated points being of opposite polarity, and a resonant circuit, normally tuned to said mean frequency, in circuit with the crystal element, said resonant circuitincluding means for adjusting its frequency whereby the phase relation between the crystal output energy and the unfiltered energy may be'varied.

4. 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 including a piezo-electric crystal element tuned to the said mean frequency, a first rectifier, means connecting the rectifier electrodes in circuit with said crystal element thereby to lhave the crystal output energy 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 rectiers in polarity opposition, Separate connections from respectively separated points of the input transformer to the respective recti'ers for applying thereto unfiltered phase modulated energy, said separated points being of opposite polarity, said separate connectionsI each including a condenser, and said separated points being on the primary resonant circuit to provide a normal phase quadrature displacement between said crystal output energy and the unfiltered energy.

5. In a detector of amplitude modulated signalling energy, a primary resonant circuit tuned to the mean frequency of applied signal energy. a secondary circuit including a piezo-electric crystal element tuned to the said mean frequency, a first rectifier, means connecting the rectifier electrodes in circuit with said crystal element thereby to have filtered output energy applied thereto, a second rectifier in circuit with the crystal element having said filtered energy applied thereto in like polarity, av common output circuit connecting said rectiers in polarity opposition, separate connections from respectively separated points of the input transformer to the respective rectifiers for applying thereto unfiltered modulated energy, and means for producing a. normal phase quadrature displacement between the n1- tered energy and the unfiltered energyat Aresonance. t f

6. In a detector of phase modulated or'amplitude modulated signalling energy, a signal input transformer having a. primary resonant circuit tuned to the mean frequency of 'applied signal energy,A a secondary circuit including a piezoelectric crystal element tuned to the said mean frequency, a first diode rectifier, means connecting the diode electrodes in circuit with said crystal.v element thereby to have the crystal routput energy applied thereto, a second diode rectifier in circuit with the crystalelement having said output energy applied thereto, a 'common output circuit connecting said rectiiers, separate connections from respectively separated points ofthe input transformer to the respective rectiiiers for applying thereto unfiltered phase modulated energy, said separate connections each including a condenser, and said separated pointsl being on the primary resonant. circuit to provide a normal phase quadrature displacement between said crystal output energy and the unfiltered energy.V

7. A methodvofdetecting phase modulated or amplitude modulated carrier energywhich comprises removing substantially all tne modulation from a portion of the energy by crystalfiltering. dividing the filtered energy into two portions, separately dividing the original modulatedenergy into two additional portions, providing a. phase shit between the filteredy and unltered original energies such that the relative phases between said two filtered portions and said two'iinfilt'ered portions is substantially degrees, combining the phase displaced portions in pairsseparately rectifying the combined pairs, andemploying the differential resultant of the separate rectifications. j

8. In a detector of modulated signalling energy, a signal input transformer having a primary resonant circuit tuned to the mean frequency of applied signal energy, a secondary circuit including a piezo-electric crystal element tuned to the said mean frequency, means for neutralizing the crystal inter-electrode capacitance, a first rectier, means connecting the rectifier electrodes in cir. cuit with said crystal element thereby to have the filtered crystal output energy applied thereto, a second rectier in circuit with the crystal element having said filtered output energy applied thereto in like polarity, a common resistive output circuit connecting said rectliiers, said rectifiers being arranged to suppress second harmonies of the filtered energy, separate connections from respectively separated points of the input transformer to the respective rectiers for applying thereto unfiltered modulated energy, said separated points being of opposite polarity, and said separated points being located on the transformer to provide a, normal phase quadrature displacement between said crystal output energy and the unfiltered energy.

9. In combination in a signal transmission network having a pair of input terminals and a pair of output terminals, a first path coupling one input terminal to its corresponding output terminal, said path including a piezo-electricv crystal element of a predetermined frequency,l a second path in shunt with the first path and. having a non-selective characteristic with'respect to signals, and a resonant circuit, tuned substantially to said frequency, in circuit with said two paths, said resonant circuit including means for adjustlng the frequencythereof thereby to control the selectivity of said crystal domi-Pnt path,

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