Method and circuit arrangement for measuring AM to PM conversion
United States Patent Baranyi et al.  
 METHOD AND CIRCUIT ARRANGEMENT FOR MEASURING AM TO PM CONVERSION Inventors: Andras Baranyl: Tamas Sarkany.  
 both of Budapest. Hungary Assignee: Hewlett-Packard Limited West Lothian. Scotland Filed: Och/3. 1973 Appl. No.: 402.926  
 Foreign Application Priority Data Oct. ll. i972 Hungary TA l2i-i Jan. it I973 United Kingdom 00825/73 US. Cl 325/67. 324/58 A. 325/363.  
  328/l63 Int. Cl G0lr 15/00 Field of Search 325/67. I33. 134. 363.  
 325/476: 324/57 DE. 58 A: l79/l5 BT. l75.3: 328/l62. 163  
 ill] 3,875,512  
 [ Apr. 1, 1975 [56] References Cited UNITED STATES PATENTS 4/1969 Custer ct al. 325/67 Prinu|r Ermniner-Howard W. Britton Assismn! L&#39;.\&#39;mnim&#39;r-Marc E. Bookbinder Attorney. Agent. or Firm-A. C. Smith I57] ABSTRACT An improved method and circuit arrangement for measuring AM-toPM conversion utilizes a phase modulated carrier generator. a two-port network having a linear delay or amplitude characteristic. a twoposition switching circuit and a phase demodulator. and is especially related to the measurement of micro&#39; wave radio relay links.  
 12 Claims. 4 Drawing Figures T-TEY&#39;HEUAPR H975 75 551 Swill 1 0r 3 21 29 4 5| 3 CA we? AMP PHASE SPECT en M00 M00 X ANAL METER -39 PHASE SHIFT 35 AMP com SEGENNAL F|g.1  
  5 49 55 5| 5s CARRIE AMP PHASE GEN MOD X DEMOD METER &#39;4&#39;? SIGNAL sew PI&#39; &#39;JENTEUAPR H975 n SHEET 2 swncn 24L 6: 19  
 NET NET 25 10 i BRANCH L i CCT PHASE oemoo Fig.3  
 PATENTEDAPR H915 1875512 3 4 .8| TONE PHASE GEN MOD NET PHASE l METHOD ASD ClRCl&#39;lT ARRANGEMENT FOR MEASL RiNG AM TO PM (TONVERSIOX BACKGROUND Oi THE lXh&#39;EN&#39;l&#39;lON in recent years. the study oi AM-to-PM conversion as an additional intermmiulation noise source has gamed m importance because of tlte increase in the cltannel capacity of microwave PM systems. it has been recognized that besides the&#39; intermodulation noise caused directly by group delay distortions. additional intcrmodulation noise is caused also by an indirect (Sn tortion mechanism. due to AM-to-PM conversionin the PM signal transmission path. (See. for instance. (i.  
 ill  
 conversion in tlte case ot&#39;a tised amplitude modulation.  
 is read off the ntcter 53 connected to the phase demodulator 5].  
 There exist several versions of the measurement ntethods according to FIGS. l and 2 with minor dit&#39;t&#39;erenccs. but all have two shortcomings. lioth methods require the realization of extremely pure amplitude modulatiott which is free from parasitic phase mmlulation. as the sensitivity of measurement is limited primarily b this unwanted phase naxlulation. Practical requiremeats canonly be met by elaborate design methods r and critical circuit adjustments. A further requirement J. Garrison. luternnalulation Distortion inFrequenc v Division Multiples PM Sgstetns. a Tutorial Summary. IEEE Trans. On Comm. Technology. April 1968. p. 289 a AM-to-PM conversion is a characteristic circuit parameter representing the transformation of a pttrc amplitude modulated signal (abbreviated AM signal) at the input-into an amplitude and phase modulated signal (abbreviated AM+PM signal) at the output. According to definition. AM-to-PM com ersion is the ratio oi the output signal PM index to theinput signal AM index. Thus the measurement of AM-to-PM conversion re-.  
 quires the .generation of an amplitude modulated signal and the measurement of the phasc modulation indes oi a signal having both AM and PM modulation.  
 of both above methods is the precise measurement of very low phase modulation. in the case of the spectrum liRlt-Il DESCRlP&#39;I&#39;lON OF Tlili FIGURES H6. 1: Block diagram of a known spectrum method of measurement of AMIPM comersion. FlG. 2: Block diagram at a known phase detector method of measurement of AMIPM conversant.  
 &#39; The growing intportance oi AM-to-PM conversion has resulted in an interest also in the measurement methods related to this parameter. This is only a recent trend. and presently ao AM-to-PM conversion nteasur ing equipment is available commercially. and relatively few methods have been published in the literature.  
 The methods used in practice maybe classified into as. groups. Methods falling in illL first group uaaztspectrum analysis. i.e. frequency domain measurement.  
 detector. i.e. time domain measurement.  
 The block diagram oi the conversion measurement by the known spectrum method is shown in FIG. I (see T. Stfrlolny. A New Method for Measuring Amplitude Modulation Compression. Proc. liiii. Part B. Mttrch 1902. p. i5! I. The signal of the carrier generator 27 is simultaneously modulated by the signal or the test signal generator M in the amplitude modulator 29 and the phase modulator 3!. The magnitude of the phase modulation is adjusted by the amplitude control 33 and the phase relative to the amplitude ntodulation is adjusted by the phase shifter 35 in a way sucltthat the input phase modulation should just cancel the phase modulaand methods in the second group make use of a phase H6. 3 Blocltdiagram ot&#39; the AM/PM conversion measurement method according to the invention herein. 1  
  FIG. 4: Block diagram of an&#39;alteruative Aid/PM conversion measurement ntethod according to the inventionherein.  
  7 SUMMARY or THE asvcts&#39;rtos&#34; The object oi the prescnt&#39;invention is the realiration of a measurement method and a circuit arrangement whichwill meet the practical measurement requirentcnts oi micrtnvave radio relay equipment and will be free from the above shortutmingstiea the rcaliration of extremely pure amplitude modulation. sensitive spectrum analyzer or phase detector. the latter having high AM suppression).  
  According to thisinv union a linear two-port network with specific frequency characteristic is utilized to gencrate an additional amplitude modulation of a phase ntoduluted signal. and the signal proportional to the AM-to-PM conversion to he measured is generated by tion originating from the test item 43.i.e. the phase The block diagram of a conversion measurement Mir up using the known phase detector method is shown in FIG. 2. The signal of the carrier generator 45 is modulated in amplitude by the signal of the test generator 47. and this modulated signal is given onthe test item 55.  
 i the output of which is connected to a phase demodulator 51. This will detect directly the phase modulation originating from the conversion. and the magnitude of this phase modulation. which is proportional to the modulation introduced by the AM-to-PM conversion tude modulation.  
 a simple subtracting circuit. in the following. the main aspects of the invention will he explained in detail.  
  The inventionis hosed on the fact that a phase modulatcd carrier transmitted on a network with linear transmission deviations is transformed into a carrier having both amplitude and phase modulations. By cascading the circuit to be measured with the linear network ntentioned above. the. additional-amplitude modulation is reconvcrted intoaphasc modulation component which is udditiv&#39;c&#39;to theinput phascmodulation already prescat at&#39;thc input. Thus. according to the invention. a g phase modulated carrier is used to measure the AM-to PMconvcrsion. hence eliminating the circuit dit&#39;ticuh ties associated with the generation oi high purity umpli According to the present phase modulation is proportional to the product of the AM-to-PM conversion to be measured and the deriva tive of the frequency response of the linear network invention. the additional mentioned above. &#39;l&#39;hcrct&#39;orc. a circuit having a linear frequency response is a cuttstttttt quantity. In practice. both the amplitude and tltc groupdclay characteristic may he t|tili7cd for performing the measurement. tints either a circuit Inninglinear group delay characteristic and constant amplitude characteristic, or a circuit hava ing linear amplitude characteristic and constant grottp delay charaetcristiecan be appliet&#39;l. ..-\ccording to this invention. ditft&#39;erent phase relations will esist in tlte two cases. in the first case. i.e. linear group delay characteristic. tltc lineareircuit will result in an AM which will be in phase with the input PM: and the same phase relaput. the test signal phase will be changed by the effect of the con \ersiott. r I  
  According to this invention. the ahove mentioned lowdevel additional phase modulation is determined as this nmdulation is proportional to the AM to lM conversion to he tttcasttt&#39;cd. According to the measurement gset-up. the network having linear transmissionjdevia tion is not inserted permanently into the signal path.  
 but is periodicallyswitched in and out between the phase modulated carrier generator and the test item.  
 Thus the additional phase modulation caused by the conversion to be measured will appear as the change of the phase modulation already present. This is indicated by a phase demodulator connected to the test item output: however. there is no requirentent to detect a very low-level phase modulation. as in the known method of FIG. 2. but the measurement of the ditl&#39;erence in phases is sutticient. r a  
  As detailed in the torcgoings. this invention is related partly to a method and partly to a circuit arrangement. Both-the method and the circuit arrangement hav two intportant versions dependent on the characteristic of the periodically inserted networkwhich may be either anetwork having linear group delay characteristic or a network having linear antplitude characteristic.  
  According to the method of this invention. the test item is driven by a carrier signal. modulated in phase by tttcsl tone signal. and thiscarrier is given along two paths to the input of the test itent. In the first halt period. the carrier is directly given onto the test itent.  
 whereas in the second hall period. a network with constant amplitude response and linear group delay response is inserted. This network has the clTect ot&#39;intrtk ducing additional amplitude modulation to the original phase modulation. in the same phase as that of the phase umdulation. The AMao-PM conversion of the test itent will re-eonvert this amplitude modulation into phase modulation. &#39;l&#39;heret&#39;ore. the signal at the output of the test item is dentodulated in phase. and the ditt&#39;erence of the test tone amplitudes in the two halt periods. proportional to tlte AM-to-PM conversion to be measured. is produced.  
 , The method as given in the foregoing may he moditied in order to incr use the sensitivity of the measurement. i.e. to increase the test-tone amplitude change originating from a given AM4o-PM convrsion. According to this modified method. networks with con- 4 stunt amplitude and linear group delay respmtses are inserted in both paths along which the carrier is given ontothe test item. the group delay responses ot these networks having equal but opposite slope \alues: i.e.. the slope of the network in path .\o. I is positive. and the slope of the network in path No. 2 is negative.  
  Another version of the method according to the invention is similar tothe foregoing. showing. however.  
 the difference of insertinga network with constant group delay and linear amplitude response in the secand hail period. thus generating an additionalamplitude naalulation which is in phase quadrature to the original phase tnmlulation. Accordingly. the difference ol the test tone phases in the two halt periods is l&#39;ormed.:  
 as this will he proportional to the AM-to- PM conve sion to he measured. The networkused in this second version has not only the desirable etTect of producing an additional amplitude modulation due totits linear amplitude response. but also the undesirableelteet of producing an additional phase modulation due -to itsconstant but non-zero group delay. This undesirable phase modulation would he added to the modulation produced by the &#34;AM-to-PM conversionto be measured. and thus would result in a measurement error. To eliminate this error. thephase modulated car-1 rier signal is&#39;not connected directly to the test item input during the first half period. but througlt&#39;a delay network having a timedelay equal to the delay responsible for the undesired phase modulation.  
  The method as given above may also be modified in order to increase the sensitivity of the measurement. According to this modified method. networks with constant &#39;gt&#39;ottpdcltty and linear amplitude responses are inserted in both paths. the amplitude responses of these networks again having equalhut opposite slope values. The network having the smaller time delay will now be amended to include the delay network as explained above in order to attain equal delays in the two paths.  
 The circuit etml&#39;tguration according to this invention andserving for the realization of said method isshown in FIG. 3. The arrangement comprises a carrier phase modulator 6|. a test-tone generator 59. a carrier generator 57. a two-position switching circuit 63. networks tor 7t and an antplitude indicator 73. The modulation 65 and 61 having constant amplitudeand linear group delay responses. the latters witheqaal and opposite slope values. a branching circuit 69. a phase demodulainput 4 of the phase modulator 6] is connected to the output .1 of the test-tone generator 59. and the carrier input 2 is connected to the output t of thecarricr gen- 4, erator S1. The output 5 of the phase modulator 6! is connected to the input not the two-pmtition switch&#39;fi}. Outputs land 8 of this switch are&#39;conneetcd to the iaputs 24 and 9 of networks 65 and 67. respectively. and outputs to and 25 of said networks are connected to inputs ti and ll of branching circuit69. Output 13 of this branching circuit 69 t&#39;eeds input 14 ot thetest item. 75. the conversion of which has to be measured. and t the output 15 of test item 75 is connected to the input 16 of phase demodulator It. with its output 17 con nected to the input ill of the test-tone amplitude indieator 73. a i y g v A version of the circuit arrangement according to this invention is shown on FIG. 4. This comprises similar circuit elements to those of the circuit in FIG. 3. ex  
 cept the networks 65 and to! with constant amplitude and linear groupdelaycharacteristics shown in FIG. 3.  
 i t t 3 gill) .311.  
 nhich in Ht 6. 4 are suhstituted hy the networks 85 and 87 uitlt constant group delay and linear amplitttde characteristics. and escept further the amplittulc indicator 73 in FIG. 3. which in H6. 4 is substituted by phase indicator 93.  
  -\s may he concluded from tlte above. the main features of this invention are as follousz&#39;a carrier. pltasc modulated by a test-tone signal. is given on tlte test item through networks with linear chamcteristics. inserted perimlically. aml the change of the test-tone ttuxlulalioth recovered at the output of a phase demodulator following the test item. and originating from the switching in and out of the networks with linear respouses. is indicated. Depending on the phase of the additional PM which may he in phase coincidence or in phase quadrature relative to the phase already prescut. the change in the test-tone amplitude or the change in the test-tone is indicatcsl.  
 As related to the application of the method. it is known that in the measurement techniques of PM systents. similar test-tone amplitude-change or phasechange will he produced during the measurement of the so-ealled dilTercntial-gain and differential-phase parameters. Thus. if measuring equipment for the measurement of the differential gain or difl&#39;crentiah phase parameters isavailahle. this equipment can he made suitable for the AM-to-l&#39;M conversion measure ment according to this invention by relatively simple means. The equipment for measuring differetttial characteristics should only he extendcdto include the nctworks with linear frequency characteristics. the branching circuit and the two-position switch shown in FR ES. 3 and 4; All further circuits comprised in the circuit arrangements of this invention attd shown in FIGS. 3 and 4 are alre: dy included in the equipment of the differential gain/phase measuring instrument. a This having a transfer function including selected group delay and amplitude parameters. where one of said group delay and amplitude parameters is frequeney imlependent and the other is linearly dependent on freqttency and the phase-nualalated carrier frequency signalis amplitude modulated hy the linearly frequency dependent parameter:  
 dentmlulating the phase of the carrier f requeney .stg:  
 nal at the output ol&#39;tlte system ttttder test during pcriodically alternating time periods: aml ttteasttt&#39;ing the phase modulation produced in the sy tem umler test because of the amplitude modulation of the carrier frequency signal. from the different demmlulattsl&#39; test-tone signals ohtained during periodicallv altcrnating&#39;time periods.  
 2. Method in claim I. wherein in the step of apply q a ing the tmululated carrier signal to the system under test. said other signal channel has a frequency independent amplitude response and a linearly frequency dependent group delay response and the difference in amplitude of the dcmmlulated test-tone signals formed during periodically altermtting&#39;time periods is measured. a  
  3. Method as in claim 2 wherein inthe step of apply ittg the nnxlulatedcarricr signal to the system under test. said one signal channel proyitles a direct eonnec- M tion. i  
 4. Method as in claim 2. whcrcinin the step of measuring. the sensitivity of the measurement is increased curly frequency dependent group dclayresmnses of both signal channels hare equal but opposite slope valmeans that the measurement method ccording to this ittt&#39;enlion may he economically rcali&#39;led by a relatively simpleextension of existing measurement gear.  
 lite version of the circuit arrangement according to i this invention comprising networks with linear group delay characteristics having equal hut opposite slopes has been realized. with the following numerical data:  
 l&#39;hase modulator and phase dc modulator carrier tretamtcy 7t! Mill test tone [auto-ac $.h MN! lhase detiatmn canned by the test-tone uppr. &#34;.l rad (to-up delay slopes of the nettwtks inserted &#34;LU mcclhllllund AM tit-I&#39;M emocrsi n cmres suuling to addt&#39;erential gain change i of t&#39;: t &#34;Idlt selected transfer&#39;fuucuon and said other channel ill by providing said onesignal channel also with a frequency independent amplitude response and a linearly f requcncy dcpeadentgroup delay res mnsc.tuul the linucs. a V H p 5.&#39;Method as in claim. 1. wherein in the step ol&#39;applying the &#39;nmdulated carrier signal to the system under test. the phase modulated carrier signalis delayed during one half time period in said one channel and. in the other half time period. is passed via the other signal channel having a frequencyindependent group dclay response and a linearly frequency dependent amplitude response. where the delay of said one signal channel compensates the delay introduced in the other signal channel by the phase modulation due to the f rcqucncy independent group delay response. and in the step of measuring. measuring the ditlerence in phase of thedemodulated test-tone signals during periodically alternating half periods. a p 6. Method as in claim 5.wherein in thestepot&#39; applying the modulated carrier signal to the system under test. saidother signal channel also has a frequency independent groupdelayrespnnse and a linearly frequenc&#39;yl dependent amplitude response and the linear amplitude responses have equal hut opposite slope \&#39;al&#39; 7. Apparatus for measuring amplitude nmdulation to phase nmdulation t AMIPM) conversion comprising:  
 a carrier frcquettcy oscillator: V i a a test-tone oscillator: 1 a t a phase modulator connected to both oscillators to provide a phase modulated carrier frequency sig nal: a a a a switching means having an input connected to the phase modulator output for applying said signal at its input in a periodically alternating sequence to each one andanother of two signal channels:  
 2 l l l i t y 7 said one signal channel having a selected transfer function. and said other signal channel has a transfer function including selected amplitude and group delay parameters. one of said antplitude and group delay paratttctcrs being frequency independeat. and the other of the parameters being linearly frequency dependent and causing antplitude modulation of the phase modulated carrier frequency signal: t means coupling each of the signal channels to the systettt under test:  
 detector means eounected to the&#39;systent under test of the demodulated signals obtained during both periodically alternatingtime periods.  
 9. Apparatus as in claim I wherein said one channel includes a direct connection.  
  [0. Apparatus as in claim 8 wherein said other signal channel also has a transfer function with a frequency independent amplitude responseand a linearly frcquency dependent group delay response and both group delay responses have equal hut opposite slope values. a  
  ll. Apparatus as in claim ;7 wherein said one signal channel includes delay means and said other signal channel has a frequency independent group delay response and a linearly frequency dependent amplitude response and the delay means compensates for the delay caused by the phase modulation because of the frequency independent groupdelay response. and the measuring means measures the difference in phase of the test-tone signals ohtainedduring periodically alternating half time periods.  
  12. Apparatus as in claim &#34;I. wherein said onesignal channel has a frequency independent group delay response and aliuearly frequency dependent amplitude response and said other signal channel also has a transfer function including a frequency independent group delay resptmse and a linearly frequency dependent amplitude response. and the slope values of the amplitude responses have equal but opposite slope &#39;alttes. I t t I a UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 875, 512 Dated A gril l, 1975 Invent0 A dras a a It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:  
 Column 6, line 48, after signals insert measuring Signer] and sealed this 17th day of June 1975.  
 (SEAL) attest:  
  C. EIARSHALL DANN RUTH C. MASON Commissioner of Patents Arresting Officer and Trademarks