US2375577A - Phase modulation detection - Google Patents

Description

f Milly 8, 1945. L, E, NORTON 2,375,577

' PHASE MODULATION DETECTION 4 Filed May 22, 1945 V,2 sheetsheet 2 E /nfaaa t) Jas.

120106Z an l i Cittomeg `Patented May 8,1945

PHASE MODULATION DIEI'LEC'LIBI` LowellE. Norton, Princeton, N. J., assg'nor to Radio Corporation of America, a corporation of Delaware i I Application May 22, i943, serial No. 488,018`

` sclams. 01; 25o-20)'` VThis invention relates to phase ,modulation detection, and moreparticula'rly to the detection of phasemodulation in which the' time rate of change of phasel-is'verylowas compared to carrier frequency so that frequency selective cir cuits are incapable of providing detection. t.

Ordinarily the time rate of change of phase in phase modulation systems is largeenough as compared to the carrierfrequency so asto provde an appreciable.` frequency shift, and high Qselective circuits ordiscriminators may be used forldetection. However, in certain applications, for example direction finders of the `type described in copending U.S. application Ser. No. 487,678, -led by L. E. Norton on May 20, :1943, ,it is necessary to detectphase modulatiomwhich comprises a small shift in phase'` of a relatively highfrequency carrier at a very low4 modulation frequency; A I f c, .13

l It is the principal object of this invention to provide amethod of `and means forfdetecting small phase shifts in arelatively high frequency carrier-which is lmodulated ata low-frequency.

`This andother objects'will become apparent c to those skilled inthe art upon consideration of the following description, lwithreference to the accompanying drawings, vof "which: l

- Fig. 1 is a schematic blockl diagramof aphase detector system accordingftol the `instant invention,` v "c H Fig. 2 is a schematicblock diagram of a `modication of Fig. 1, l

Fig. 3 is a schematic circuit diagram of a balanced modulator of thetype employed in the circuits ofFigs. 1and2,and c l Fig. 4 is a schematic circuit diagram of a modulator also employed in the circuits of Figs.

Referring" to Fig. 1, a radio frequencyamplier lis connected througha modulator 3 to a lter 5 and thence through an automatic frequency control circuit I5 to a radio frequency .oscillator 1. The oscillator is arranged for VvariableV frequency operation, and is connected to a `balanced modulator 9. The `tuning of both the amplifier" I andthe oscillator VI `may be varied nzlanually.` 'Ihe `two controls are mechanically ganged, as indicated' by the dashline II. The

.oscillator `l is arranged so thatits frequency may i be `varied over a relatively narrow range by variations in the magnitude of a DC. control voltage applied thereto. I

An oscillator `I3 arranged to operate at a xed intermediate frequency is coupled to the balanced modulators.` The output of the balanced modulator 9 is applied to the modulator 3,. The lter 5 is tuned to pass the frequency of `the IF oscillator I3.` The automatic frequency control circuit I5, which is connected to the output of the filter 5, is alsoconnected to the" oscillator I3 through a 90 phase shifter Il.` `The Dl-C. output of theautomatic frequency control circuit I5 `is applied to the frequency control input circuit of the oscillator l. I

A detector I9 comprising a diode rectifier or the like is'coupled through resistors ZI and 23 to the output circuits of the IF lter 5 and the `IF oscillator I3, respectively. A resistor 25 is shunted across the inputcircuitof the detector I9.

The resistance of the resistor 25 is much lower than that of the resistors 2l` and 23.

t The operation of the above described sys-tem is as follows: I

The tuning controls of the amplifier I and the oscillator 'I are adjusted to the carrier frequency of `a phase modulated signal applied to the input of the amplifier I; The phase modulated car-'- rier may be `represented by the expression eo=sin [wt-WGH in which w/21r is the carrier frequency. The oscillator l will operate initially at a frequency wo/21r, approximately vequal tothe carrier frequency w/21r. I 1 i Designating the frequency of the operation of the IF oscillator I3 as a/21r, voltages e1=E1 1+m1rcos at) and ez=E1(1-m1 cos at) produced thereby are applied to the `balanced modulator `9. voltages e3=Es(1}-m2 cos wat) and e4=Ea(1-m2`cos wot) from the oscillator 'I are also applied ,to the balanced modulator. The resulting `output from the balanced modulator 9 is `This may be rewritten as:

cos (aofi-pdt] fg'j f 2,375,577

The voltage e5 from the balanced modulator 9 frequencies. When the frequency of the oscil and the voltage eo from the RF amplier I are lator 'I'is equal to that of the input voltage applied to the modulator 3, as shown in Fig.4. (wo=w), the output from-the modulator IIB cor- The resulting output from the modulator 3 is responds to the original phase modulation, pro- The voltage ee is applied to the filter 5 Which is vided as before that WU) does not depart apturned to pass the frequency u/21r. The output preciably from sin [ll/Kt) of the lter 5 is Referring to Fig. 3, a suitable circuit for the The voltage es is applied to automatic frequency balanced modulator 9 of Figs. 1 and 2 comprises to V=a frequency exactly equalto that of the input 001'117101 Circuit l5. The Output Of the IF Oscillaa.' pair of multi-,grid convertor type tubes 22 and tor I3 is applied through the 90o phase shifter 2.4, Such as RCA 61;.75, one input is appned I'I to the automatic frequencycircuit ihwhel'e 20 through av` transformer 26 in push-pull to the it S Compared With the Voltage ef's to 'produce e inner control grids Y2'I and 2'9 of the tubes 22 and D.C. control voltage Output which is Plopol- 24. The otherihput is applied through e tienstlonal 1n megnltude and related m polatelty' to former 3| vinpush-pull to the outer control grids the dliiereece 111 frequency 0f the two inputs: as and 35 of the tubes 22 and 24 The eh'odes of The .control voltage from the automatic fre- 25 the tubes 22 and 24 are con'nected to ether quency control circuit I5 tunes the oscillator I through a Commn 10a d resistor ,31 to agD C `to the amplifier I so that wo=w.l.The output Source 39- Self'bas is provided by 'a comme? voltage from the filter 5 then becomes Cathode resistor lll bY-PaSSeCu Capacitor 43- g einigt- .ifm-mg Sie [-ai+(t)]}=k2EE5{12mm3.ein.[.pf(t)] toont; The voltage H6 may be Wrillten for Convenience Screen potentials are .provided .fromthe .source as 7c3Ee{1-Imo sin llbf(t)l cos u}. 39 through a Voltagedropping resistor 45..

The voutput Voltage` of the 1F oscillator I3 may 35 Flg- 4 Shows a Circuit Suitablefr the modulube represented as e1=E1` cos ut. This voltage tOTS 3 and H9 0f FlgS- Yl and 2. Armultl-Brld is-oomhined with the lvoltage efe in the resistor f convertor tube '41. Similar t0 the tubes'22 and-24 2'5.' Denoting the resistance of the resistors 2l, O f the System of Fig. 3 1S connected OI applica- 23 and 25 as R1 R2 and Re respectively;v the voltt1011 0f 011e input Voltage t0 Vthe Ainner control age across R3 is 40 grieta. Theother input voitege is eppned to the outer control grid 5I. A.D.C.isource .55 :is

, w I -ef/,Jfiseu l connected to the anode of the tubeas throuehe 1 3 2 3 load resistor 453. The modulation productsof and 1S 0f the-form L the two inputs appear across the load resistor V5.3.

ev=Evl+m1vSiI1.[sf/f(t l GOS .ati 4 Thus the invention has been described as a where the .modulation lfactor .mq .is determined method vof phase detection capable of `providing by the relative sizes fof. R1 and Re. The l response to 10Wfrequencyplowrate,.phase modu- Voltage eiis `applied to the detector ILS, providlationofa high frequency carrier.' A.local uning an output including a voltage ,0 modulated-oscillator is-arranged to be automatieszEa sin www] l i l cally maintained in operation .at the frequency f of the phase modulated carrier `by means Acof 7an Where W (t) 1s the ongme'l phase medule'tlon '.Of automatic frequency control circuit-arranged .to the carrier frequency w/21r. If the phase modurespond to the Cumullof a balanced modulaton Ylationis small so that the departureof sin [ltfdll The Gulput of the local oslllator is Compared from 1S Smau then e8 1S apprexlmetely 55 with the modulated carrier to provide a voltage equal to `Es (il/f?) i Thus the Output. 'the de' corresponding to the original phasemodulation. tector I9 corresponds to the original phase modu- I claim as my invention: lationf 1. A method Yof detectingphase .modulation of Fig. 2 yshovvs a modified circuit in wh1ch the a high' frequency Can-ler comprising the sl-eps detector I9v and the `combining network I2|, 23, 60 of locally generating voltage .equl'jm jff- 25 are replaced by a modulator l|9 The p of quenoy to the frequency df said carrier, 'locally tions of the circuit '0f Fig. 2, WliiCh are 5119011911" generating a'second voltage of intermediate lfre-- to correspondingV elements of Fig. 1, `are indiquency, combining 'said"iocally generated will, cated by similar lreference numerals.Y The methages to provide modmationproducts, mddulating Od'rof Centrvlline the frequency 0f the radio fre- 65 Suid phase .mocmeteuoerrier with `sei-dimo-"dunquency oscillatorfi lis the same as that used in tion pmduCtstO-provide-a Sec-0nd group of mqduthe'systemof Fig. 1,.aud the mOdulatOI 3,'iilte1` lation products, ltering said second "group `of oscillator I3 and automatic .frequency'control modma'l-,ionl-products "to vremove@therefrom all Circuit' l5 perform aS in the circuit 0f Fg- 1- components having frequencies equal *to `rand In this case, detection is accomplished by apply- 70 higher than said carrier frequency-,g additivly ing the output l0f the radio frequency ampliei combining saidfiltered modulation-pro'ductsiwith I` and that of the oscillator 'I to the modulator said 1oca1ly --generated voltage #of intermediate H9, which is Similar to` the modulator 3 but prov"frequency, and reotify'ing the resuitaritff seid vided With a capacitor shunted across itsoutput -vadditive vI;011110111ation,V 4 r circuit 'to remove 'voltages crcarrier and `higher 75 '2. lThe invention Neseet rff-urth v| in piedini-1,A 'fin- *2,375,577 cluding the method of controlling the frequency of said locally generated high frequency voltage which comprises the steps of combining said iiltered modulation components with said locally generated intermediate frequency voltage to produce a direct current voltage related in magnitude and polarity to the difference in frequency of said combined voltages, and controlling the y y `3 modulate said carrier with the` output of said balanced modulator, lter means connected to said second modulator and arranged to pass the intermediate frequency components of the output thereof, an automatic frequency control cir-` cuit connected to said lter andto said intermediate frequency oscillator to provide a direct current output related in magnitude and polarity to the difference in frequency of the inputs to said automatic frequency control circuit, means for applying said direct current voltage to said local high frequency oscillator to control the frequency of operation thereof, a rectifier, and means for applying the output of said lter and the output of said intermediate frequency oscillator additively to said rectifier.

LOWELL E.` NORTON.

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