US2340432A - Phase modulation receiver - Google Patents

Description

R. E. SCHOCK PHASE MODULATION RECEIVER Filed May 23, l1942 3 Sheets-Sheet 1 DEAfEs RELA T/ VE PHASE ASMH' ATTORNEY Feb. l, 1944.

R` E. SCHOCK PHASE MODULATION RECEIVER Filed May 23, 1942 5 Sheets-Sheet 2 Afro/wir /PoaE/er E. ScHoc/r.

Feb. 1, 1944. R. E. scnccK PHASE MODULATION RECEIVER Filed uy 2s, 1942 3 Sheets-Sheet 3 GRAIQQ .mtwoq m. mo mw y VS me. T Rd@ Mw. my

' ATTORNEY Patented Feb. l, 1944 PHASE MODULATION RECEIVER Robert E. Schock, Riverhead, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application May 23, 1942, Serial No. 444,177

Claims.

My present invention relates to phase modulated carrier wave (PM) receivers, and more particularly to improvements in PM receivers of the type employing carrier exaltation. i One of the main objects 'of my invention is to provide a method of -PM receptionof the carrier nlter type; wherein the carrier filter may have more than 180 degrees of relative phase shift to either side of the midband frequency, and frequency division being used to insure proper functioning of an automatic frequency control (AFC) circuit when used. I

Another important object of -my invention is to provide a PM receiver of the superheterodyne type employing anl auxiliary carrier, or exalted carrier, network to provide demodulation of the PM waves; the normal modulated carrier path and auxiliary carrier path to the detector each including a frequency divider device for reducing the effective phaseshift of the auxiliary carrier to a degree such that'automatic frequency control (AFC) is readily usable in the system.

Another important object of my invention is to provide in a PM receiver of the superheterodyne type an automatic frequency control, means being employed in the carrier paths to the AFC rectifier to insure thatV the relative phase shift of the carrier filter is not in excess of 90 degrees either side of the mid-frequency of the lter. A

Other objects of my invention are to improve, and to render more eilicient'and liexible, PM receivers. v

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims; theinvention itself, however, as to both its organizationand 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 effect.

In the drawings: y v Fig. 1 shows a PM receiver system, in schematic form, embodying the invention,

Fig. 2 graphically shows the frequencyphase shift characteristics of the carriervflter path to the detector, f Y

Fig. 3 shows schematically av modication of the PM receiver of` Fig. 1,

' Figs. 4a., 4b and 4c show various characteristics useful in explaining the invention.

Referring now to the accompanying drawings, wherein likemreference characters in the figures designate similar circuit elements, the superheterodyne receiver system of Fig. 1 is generally a PM receiver of the type described on page` 131 of Proceedings of the I. R. E., for February 1939, in an article byfM. G. Crosby entitled Communication by phase modulation. The receiver, in general, comprises a superheterodyne receiver which may beoi' any conventional and well-known construction. The rectangle designated Superheterodyne receiver is to be understood as comprising the usual tuned radio frequency amplifiers, tuned first detector, and one or more stages of intermediate frequency (IF) ampliers. The rectangle designated Local oscillator is to be understood as an oscillator circuit adapted to be tuned to' a frequency different fromv the received PM' waves by the operating IF value. In this connection it is to be understood that the PM Waves may have a carrier, or midpoint, frequency locatedA in the ultra-high frequency range;

As is well known to those skilled in the art, and as described in` the aforesaid article, PM Waves are produced by deviating the transmitter carrier frequency in proportion to modulation frequency. The radio frequency deviation for the higher modulation frequencies is relatively greater than at the lower modulation frequencies.. In this respect PM Waves differ from frequency modulated carrier waves. In order to demodulate the PM Waves at the reduced, or I. F. value, there is employed a detector circuit which comprises a paid of diode rectiers 5 and 8 arrangedin opposition. The cathodes of the diodes are connected'by the series-arranged load resistors 6 and 1, and the midpoint of these resistors is connected to the midpoint on the secondary winding 3' of the, detector input transformer3. Each of resistors 6 and 'l is bypassed for I. F. currents bythe respective bypass condensers 9 and I.

The anodes of diodes 5 and 8 are connected to respective opposite ends of the winding 3. The cathode end of load resistor l is grounded. The modulation signal voltage is taken off from the cathode end4 of resistor `6, and fed through an audio amplifier to any desired type of audio output circuit; `The .lead connecting the midpoint of winding 3 to the junction` of resistors 6 and 'l includes the secondary winding of transformer 4. There is developed across the secondary winding of transformer 4 signal voltage designated Es. Y To develop this Voltage, the primary winding '4- is connected inthe output of a path which comprises a frequencydivider 2 and a phase adjuster i3. Disregarding for the time being the action of the frequency divider 2, and assuming that the phase adjuster I3 is the only network included in the path feeding winding 4', it will be understood that in the type of PM receiver shown in the aforesaid Crosby article there is fed to'transformer 4 the PM waves whose midband frequency is of I. F. value.. A second path is provided to feed the secondary Winding 3 of transformer 3. This path com prises a carrier filter I and a frequency divider I. The primary winding of transformer 3 is connected in the output of divider' l. Here,Y

again, let it be assumed that the. divider lJ is not included in the carrier lter path. The function of the carrier filter l is to remove the modulation side bands of the modulated carrier wave and leave the carrier.

In order to receive a Wave which is phase modulated, a converting circuit which converts the phase modulation. into amplitude modulation must be used. Then, in a manner similar to the process. used in frequency modulation reception, the amplitude modulation is detected by ordinary methods. As. explained in the aforesaid Crosby article, the vector relations between the carrier and side band frequencies in amplitude. modula-.. tion may be represented :by the carrier vector being at right angles toeaohof the lower and upper side bands. In phase modulation, the relation of the carrier and sideband frequenciesover a given instant of time Vis, such that, the carrier of the PM wave has been shifted 9.0? with respect to the carrier of the amplitude modulated. wave. In

other Words, the carrier vector is. in phase with 1 one of the modulation. `side-band vectors. In View of this phase relation existing between .the carrier and side frequencies of phase modulation, it is obvious that for detection it isirst necessary either to phase shift. the carrier relative to the side-bands, or phase. shift the sidebands relative to the carrier, to convert the. PM Wave into an amplitude modulated Wave.

In a. system of the type shown in Fig. 1 part ol".v

the incoming PM Waveis fed to thecarrier lter Vl Whichremoves the modulation side-bands, and

makes available an vunmodulated vauxiliary cara. rier Which may becombined with the PM wave so. as. to produce a. resultant voltage which is. am-.. plitude modulated.. The phase adjuster I3. in the normal PMV path is adjusted. sothat. the twoy componentsrv of the filtered carrier E1 and E2 are 90? out of phasewith the uniilteredPM Wave voltage Es.. This, producesfresultant voltages. between each Vdiode anode and the junction ofV resistors. 6 and 'l which are balanced in amplitude for the unmodulated condition. When the phase ofthe incoming Wave Esis shifted, one of the resultante up. A phase shift in the opposite direction pro-.

duces differential amplitude Vmodulation in the opposite sense.

When this differential modulation is detected` l. There are certain types of carrier filters (such as a band pass crystal lter) which are very suitable for this type PM receiver, but which have more than 90 relative phase shift on either side of the midband frequency.

The following discussion, in connection with Figs. 4a, 4b, and 4c, should more clearly show how it isthat. the. AFC operation is impaired by the use of a carrier filter with more than 90 degrees of phase shift-with-frequency either side l of midband, and hoW the AFC action may reverse if the carrier lter has more than 1,80 degrees of,l phase shift-withl-frequency eitherside of the midband frequency. Referring to Fig. 1, let us assume, for this discussion, that the frequency l dividers l and 2 are omitted. Let us also assume that, in order to obtain a high degree of sideband attenuation in the carrier filter path, tvvo bandpass carrier filters are used in series in the carrier lter position I. Also, assume that eachA of these two band-passrlters is of a type which has 180 degrees of phase shift-vuth-frequency either side of midband. This will. result inY an overall phase shift-With-frequency of 36.0v degrees below f midband frequency, and 360` degrees above mid- I. F. signal at midband, f of f phase shift of Es with, respect to El and vll . is modulated down in amplitude and the other f on the diode rectiiiers,y one of the diode. resistorsf is reversed With respect tothe other so as to. com-v bine the detectoroutputs in push-.pull relation.

Differential voltage for AFC action is also availf.

able, and is taken off from the cathode end of resistor 6.. The resistor-condenser H-EZ is a time constantcircuit which removes. alternating currentV components. TheY AFC. bias iS applied. to a. reaotonootube which controls. the. frequency of tno resonanttank circuit of the 'loool'.

oscillator of the receiver. This AFC. action is fvery Well known, and need not be described in network f band frequency. Such a characteristic isv represented by Fig. 4a. Inspection of *thisy characteristic shows the phase shift to be zero at midband frequency f, 360 degrees in a negative direction at; cut-off frequency je, and 360` degrees in a positive direction at cut-off frequency jc, etc.

The voltages E1, E2, and Es of Fig. 1 are shown vectorially in Fig. 4b by solidv vectors f or the oo ndition Where the incomingr I. F. signal is-y at midband frequency f in Fig. 4a. The 'amplitude and polarity ofv the` AFC voltage developed across4 diode resistors 5 and I of,V Fig. 1 overV the fre-. fluency rens@ of the carrier filter considered are shown by the curve of Fig. Y NOW' Witli the. Fig. 4a, there isnoy 4b, and' Zero AFC Volts. is developed. as; shown. by Fig. 4c. If the I. F. shifts from f to fa. 4a. shows that the filtered' Signal. Will. then be phase shifted;` 45deg1js a positive direction.. This is represented veotorially by the. dashed vector Es. in position o Fie. 4l?. and withthisphase rev lationship of Es'with; respect to Ei .and E2 a. nega-. tive. AFC voltage is. developed; across. insonne. resistors 6 endl Cif/Eis.. 1..; on the Curve of. Fie;- 4o for frequency position fa.. (The manner in which. this voltasois. developed is more fully discussed under the headingfApn diary-carrier receivers in the- Crosby paper Communiootioobvphase modulations);

Similarly a shift. of frequency to position f1, Fig. 4a, respite a. negative phase., shift. of; the filtered carrier ofY 4,5 degrees so` thatitsjvectol Es of Fig. 4b, is now representedrby.dashedjveotor Ei. in position. l.v and, asshown infisso,- o positive AFC voltage. is. developed.; From.` the foregoing discussion it may .be seen thatwhen a frequency shift. to. ahshor frequency. .takes place. inthe. I.` and; produces. a negative; voltage., the must aotinsuoh aligar-mer y as to.L lower the frequency of the I. F. signal. Con- 'lhis is.represen.ted V versely, when a frequency shift to alow'er fre#` quency of the I. F. signal takes place, thus causing a positive AFC voltage to be generated, this positive voltage must drive the AFC to raise the frequency of the I. F. signal if the AFC is to fulfill its purpose. Now, it is seen from Figures 4a, 4b and 4c that if the frequency of the I. F. signal should shift from midband frequency f down to the frequency designated as f2, the filtered carrier would experience a 90 degree shift in the negative directionl which lwould cause the maximum amount of `positive AFC voltage to be developed according to Fig. 4c.

A further decrease in frequency, say to s, would cause a decrease in the amount of AFC voltage, and at frequency f4 the filtered carrier phase shift of 180 degrees sets up the condition by which no AFC voltage is developed. At frequency f5 the phase shift experienced by the filtered carrier is more than 180 degrees, and, as seen in Fig. 4c, the voltage developed has reversed from positive to negative which will act on the AFC to drive the frequency of the I. F. signal still lower in frequency instead of driving it back to frequency f. Actually, if the carrier filter has slightly morethan the 720 degrees considered in Figures 4a, 4b, and 4c, and, if the frequency suddenly shifts to a frequency .beyond f4, the reversed AFC action will carry the frequency of the I. F. signal to fc, and the AFC will hold it at that point. From this discussion it may be seen that if the carrier filter imparts a phase shift of more than 90 degrees either side of midband frequency, there results an undesirable condition in which the amplitude of the AFC voltage, generated at a frequency beyond the 90 degree shift point, decreases; and, if the carrier filter imparts 180 degrees, or more, of phase shift it is possible for the AFC voltage to be zero or the reverse. By the use of frequency subdivision, as set forth herein, the phase shift imparted by the carrier filter may be reduced to an amount satisfactory for AFC use.

I propose by the use of frequency division to cut the effective phase shift of the filtered carrier to a degree which renders it readily usable. By passing the filtered carrier output through the frequency divider I', the carrier frequency is sub-divided to a sub-multiple of its original frequency. In this way the carrier attains a 'phase vs. frequency relation,` relative to the phase vs. frequency relation of the unfiltered PM wave, which is not more than 90 degrees either side of the filter midband, or center, frequency. The PM wave passing through the other path is transmitted through a frequency divider 2, whereupon the carrier frequency is subdivided to the same frequency as that of the subdivided filtered carrier.

The subdivided PM wave energy leaving divider 2 is passed through the phase adjuster I3. The function of the phase adjuster is to adjust the phase of Es, appearing at 4, to .be 90 degrees out of phase with the two filtered carrier components E1 and E2 appearing across 3. The curve A (full line) of Fig. 2 shows the relative phase shift with frequency of a carrier filter of the typical band-pass crystal lter type. If this latter type of filter were used at I in Fig. l, and the networks of this invention were omitted, the filtered carrier and unfiltered modulated wave energy arriving at the PM detector would have a phase relationship varying with frequency as in curve A of Fig. 2. However, if this same filter is usedw in the carrier filter position of Fig. 1,'

and if frequency dividers I' and 2 halve the midband frequency before passing them on tothe detector, the filtered and unfiltered modulated Wave energy will have a phase-frequency characteristic as illustrated by the dashed curve B of Fig. 2.

It will be seen from curve B that the phase shift of the auxiliary carrier energy is now degrees either side of the midband frequency of the filter. Of course, in the above illustration the phase deviation of the PM wave energy will, also, be halved in the frequency division process. However, in the aforesaid Crosby article it has been pointed out that relativelyv low levels of modulation are required in this type of receiver if low distortion is desired, since receivers of this type inherently distort 0n higher levels of deviation. Frequency division would, therefore, be an improvement in many cases. It will be understood, of course, that the carrier filter may be of any desired type; even those exhibiting degrees of phase shift with frequency either side of the center, or midband, frequency.

Among the various types of carrier filters which might be used, and which have more than 180 degrees of phase shift-with-frequency, are the following band-pass filters:

YConstant 1c type Mid-shunt derived m-type Mid-series derived m-type Mid-shunt derived 1mm-type Mid-series derived mm-type These, and other, filters and their characteristics are `described in the article, Extensions touthe theory and design of electric wave-filters, by O. J. Zobel, which appeared in the April 1931 Bell System Technical Journal. All the types above listed have a phase shift of minus 180 degrees below midband and plus 180 degrees above midband, or a total of 360 degrees.

The frequency dividers may be of any wellknown type. Those skilled in the art are fully acquainted with the manner of designing these dividers. Frequency dividers which may be used are the oscillating converter type (see application Serial No. 385,800, filed March 29, 1941, by M. G. Crosby), and the multi-vibrator type.

If it is desired to utilize the full phase deviation at the audio detector, the system of Fig. 3 may be used. One detector 5--8 is employed for the AFC bias production. The other detector provides the audio signals, and makes no use of frequency division. Thus, the audio detector comprises opposed rectifiers Il and 2U having respective load resistors I8 and I9. The audio detector is arranged as in the case of the AFC detector. Carrier bypass condensers 2 and 2l respectively shunt resistors I8 and I9. The output of carrier filter I feeds directly to input transformer I5, while the unltered PM energy is fed to transformer I6 through the phase adjuster I4. The latter insures impression of the filtered carrier and unfiltered PM energy upon the PM detector in phase quadrature relation.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended 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' he carried: into effect.

WhatI claim is:

1. The method of demodulatine: phase-modulated carrier energy which includes.. concurrentlyreducing the midband frequencvand degree Qf phase modulation of the modulated carrier energy, removingl from a portion of the phasemodulated carrier enerey all. modulation. side-- bands. imparting to. the carrier. as; a .result of said removal, a phase shift in excess of 90 deerees on either side of; the. midbarld. frequency.. reducing the phase shifted carrier frequency to the. frequency o f said reduced midband frequency thereby to reduce. said phase shift. so.. es not not exceed 90 degreesJV and Combining. the re sultine reduced. carrier energy with the reduced modulated, carrierv energy te provide the originel. modulation signals.

2. In a superhetercdyne receiver employing a local oscillator, means to. combine phase moduf late'zlu carrier energy with local oscillations; to produce beat. energy, a carrier filter foi` removing all modulation sidebands from the beat energy, said lter having a phase shift-frequency characteristic.. in excess of 90 degrees, means for demodulatingV the beat energy',y a iirst. frequency d ivider coupling the carrier filter to the demodulating means, a second frequency divider coupling said beat energy producing means to said demodulatng means, means for securing application of both frequency divider output energies to said demodulating means. in phase quadrature.

3. In a superheterodyne receiver employing a local oscillator, means to. combine phase moduf, lated .carrier energy with local oscillations to. produce-beat emersa. a carrier. lter for remov. ine all modulation .sidebands from theV beat er1-` Qrgyi Sdd.' filler lliVIlg. a. 912,35 Shl/rfleqlleIlCy characteristic. excess. of .9.0 degrees. Ineens Ier.- demodulatirls. the. beat eneren e first. frequency divider Couplinsthe carrier lter to the demoduell ture, and means; regulating. the Yfrequency 0f Seid.

oscillator with the demeduleted Voltage- 4.` In a superheterodyne receiver employing a. local oscillator, means to combine phase modu-l lated. Carr-ier energy- With local oscillations, to produce beat energy, a carrier filter vfor removing all modulation sidebands from the beat energy, said lter having a phase shift-frequency characteristic in excess of 90 degrees, means for demodulating the beat energy, a first frequency divider coupling the carrier filter to the demodulating means, a second frequency divider coupling said heat energy "producing means to said demodulating means, means for securing application of both frequency divider output energies to said demodulating means in phasequadrature, a second demodulating means, and means for applying the filtered carrier energy and beat energy Without frequency division to said second demodulating means.

5.. In a phase modulated carrier Wave receiver of the superheterodyne type employingan automatic frequency control circuit for the oscillator, said receiver employing a carrier filter path and an unfiltered modulated carrier Wave path to feed respective energies thereof to a demodulator in phase quadrature; the'improvement comprising a frequency divider in the` carrier lter path, said divider being locatedV between the car- I' rier lter ofk said last-named path and said deshift on either side ofv the midband frequency which is not in excess of 90 degrees.

ROBERT E. SCHOCK.

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