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September 27, 1956
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Sanitized Copy Approved for Release 2011 /04/01 CIA-R DP81-002808000100060030-5 Sanitized Copy Approved for Release 2011 /04/01 CIA-R DP81-002808000100060030-5 Sanitized Copy Approved for Release 2011/04/01 :CIA-RDP81-002808000100060030-5 THE USE OF EXnCT FREQUENCIES IN MODERN COlOIUNICATI01(9 ENGINEERING Vestaik avyazi [Communications Herald No 12, Deceaber 1955, 1loacow, Pages 9-5 The principal areas of use of exact frequencies in aodern coautunica- tions engineering are given and definitions of the qualitative indexes which such frequencies must satisfy are formulated. Modern communications engineering is finding ever greater use for os- cillations characterized by extremely high indexes of frequency or phase atabilit~, The use of exact frequencies makes possible the creation of communications systems which permit a manifold increase in the effective power of communications channels without increasing station power and per- sit a considerable increase in the flow of communications, that is, in the traffic capacity and noise stability of lines of communications, Systems using exact frequencies for the above purposes are usually referred to as "synchronous." Modern communications systems permitting proximate realization of the maximum noise stability and traffic capacity predicted by communications theory must certainly be synchronous, Hence, communications engineering- technical workers may be interested in determining the basic areas of use o! exact trequencies in communications engineering and the results obtained from their use, In defining the advantages of single-aideband operation (OBP) i?t is usually pointed out that if transmission of the carrier frequency is eliti- nated, then for a given maximum transmitter power the voltage amplitude df the sidebanda is doubled. This is equivalent to a power signal gain o! four l'?imes (owing to best use of the output of the transmitter tubes). A narrower frequency spectrum is required for reception of single- sideband transmission than for the usual amplitude modulation (at the mcst, half as wide). Hence St is possible to obtain a corresponding re- duction in the bandwidth of radio receivers and thereby to decrease (by half) the noise received by the receivers. This is equivalent to doubling the signal strength. Thus, compared with conventional double-side band transmission, the overall gain in signal strength is 4 x 2 ~ 8 times. On short-wave systems this gain may be still greater, since in the propagation of amplitude-modulated sig:~sls the phase relationships be- L'.:yen tho components of tt~e upper and lower sldebands and the carrier fre- quency may be disturbed, which decroases Lho effectiveness of the received signals. In OBP transmission this phenomenon does not occur. In noting the advantages of OBP transmission mention of the advantage in transmitter power requirement is often omitted. Nevertheless, the gain here is still great and is explained by the decrease in power radiated by the transmitter. In fart, with 100-percent amplitude modulation the average (for power calculations) percentage of modulation in radio broadcast trans- mieaion (considering pauses and momenta of transmission of faint so~inds) is approximately 10 porcent and, consequently, the average aideband power Pside = 0.5 m2 Pcarr. = 0.005 1'carr,' that is, only about 0.5 percent of the power of the carrier wavA. Thus, in conventional modulation a radio ? r4Y ` ~~~ . ? ~ Sanitized Copy Approved for Release 2011/04/01 :CIA-RDP81-002808000100060030-5 Sanitized Copy Approved for Release 2011/04/01 :CIA-RDP81-002808000100060030-5 station not only radiates power approximately 20,000 percent greater than when transmitting on single sideband, but also occupies '.w ice the bandwith and still does not insure as high a quality of reception as single-side- band transmission. In short-wave transmission L?he latter circumstance is due chietly to a-m distortions arising from selecti?.~e fading, and in long- wave transmission it is due to the fact that single-sideband operation permits a gain in etfective power of the channel (i:~provad signal-noise ratio). Hence it is completely understandable that mod?trn communications ?+:~-?~? engineering is making ever wider use of single-sideband systems both for wire communications and for radiote'_ephone communications. Effort is being made also to solve the problem of using single-sideband transmission on long waves for the purpose of radio broadcasting. However, single-sideband transmission, as is k+ior,?n, requires restora- tion of the carrier frequency at the roceive= with r, high degree of accur- acy. In radio broadcasting, for examplo, the restor~?d car^ier frequency at the receiving station must sot differ by more than one cycle from the carrier frequency of the transmitter. Thus, or.e of the ever-expnnding arses o' applicetioa of exact fre- quencies is the single-sideband system of cr municatirns. As concerns systems of broadcasting, i' .s necesr~ary c~ ?.:rn::.s:t the ao-called eynchrrnous broadcast ne:n 1n wa?_c`? a large numbrr (often more t*.an IO) of low-power trananitters (0.5-5 kw) raual:y operate together with high-power i:ranamitters on a a=ngle treq~yncy, tt?nnsml~;ting the same program synchronously and cophaaally, ibis met'rod of b.~oadcasting ie widely employed in many Europoan countries. Its uaa hie the f?:~llow;.;:g advantages: !n daytime, with a low total radiated porar ana with extremely g?,o.7 signal- noise ratio, it permits adequate service of a 'Large territor?~ in th3 med- ium-wave broadcast bend (200-500 m); :n the e~?ering, i:i the n~urs when the sudien,e la largest, it permits considerable Improvement in the quality of broadcasting, since in the zones served by its p~opaga+:ion is chiefly by ground wave. Moreover, this method permits tt.~ vre of 'dower trequencie. for broadcasting and reduces operating costs. This reduction it operating cos `s is dui chietly to t'ca tact that the specilic radiated power of low-power t-ansmlttera Ss considerably less than for high-power tr?ansiritters (spa.:itic: radi~.ted power being the power radiated in kilowatts per square kilometer of ?;he served zone). This is ex~+lained by the fact that, in accordaii~e with la,s of propagation of medium waves, with a given minimum field s?:rength thy, area served does not increase in proportion to the transmitter power 'gut levy ~oasiderably behind. Moreover, low-power and medium-power broadest transaitters are ususlly made for unattended operation. ;hose nre the compelling reasons why, :tr..ier t~P ^ap~nhagen Inter- national Plan, frequency assignments for most of t'.r.r .iuropesn countries were secured by synchronous broadcast nets. Numere?u:i ~ESearches conducted up till World War II demonstrated that in order to +chieve synchronous broadcasting an extremely high degree of ~ynchrouis~. in the carrier-fre- quency radiation is desired. Over a period of hours the frequency phase of synchronous stations must not differ by more that several tenths of.a degree and, in any case, the frequency deviations or' synchroolzed radio broadcasting stations must not exceed one tan-millic++th of the carrier frequency (1?lU-~). Sanitized Copy Approved for Release 2011/04/01 :CIA-RDP81-002808000100060030-5 Hence, synchronous broadca~L nets ars aroLne= Smpartant eras of appli- cation of exact frequencies in cox;uaicaticns sagineeriag. Compositing the Spectra ~t Telegraph sad Ra~iotelagraph Services The third area o1 application of exact f:aquanciea is rieterained by the problems of compositing the spectra of te?e6raph and especially of radiotelegraph services. As the simplest calculation3 as well as experi- ment will show, the use of exact fregt~ancies opens extremely wide possi- bilities for further compositing the ^pactra. For example, in the voice- irequency carrier system o' radioteiagraphy of tit~N 7ChT [irequoncy-shift telegraphy) type transmission of a single teletype chan:.33 occupies a bandwidth of about 3,000 cycles, and wi::h ;car-channel t_na multiplexing the bandwidth is more than 1,300 cycled per channel (with allowance for transmitter instability). ? Transmission o1 the same inteliigeace with no less noise stability may be achieved with a bandwidth of ne more then 400 cycles per channel, and even this can be reduced by three ties i' trayemission is limited to the first harmonic of the telegraph aigna?e, which, as calculations show, is wholly permissible. Thus, in many cases Lvh etae!tcr esy F:;r~da r: 2_o commun!ca:iosa with high noise stability ~+ithin :b9 3S2r fragar'c; rarga now accuplad by a aiagle station. However, such s s4a*,r1t cac Y?s ach:rved c~`y if ?:he num:,roua aarroa bands sad ch%~r_rla e~;;cye+i fn red:c:a.e6raF~sy rrmala precisely on the assigned frsgt:arc:aa. :ht rs'_3't~+4 sh::t of the tra- quency bands o! the ind:vidca: chc?:n~+;s _or s.;ci ryr:e:s must r.aL ba sore than a taw cycles, that fa, :ea?:hs e,:~ suet Y:Lr.Gredtha o~ the shifts now permitted. Thue, the possible vast SnprovenacL3 in raLic cc_*,ncaicat:cne, the increased traffic capacity sad aoiee 2tabLi~?;y caz, oe ehtaiuaa only by the use of exact irequanc_fe a: traao~::t:us sni receiving atatiors. Phase-modulatior, telsg=achy !s c? Brest !apcrtance .or cable trunks and long-wave radio statior_s, for :: rsgLi~as a baaCw'_dth less than half that for frequency-modulation telegrazn~ with :~c ease noise stability. However, as will ba seen, the phase-sodulation ~alagraph system requires the use o1 exact frequencies of eaprcielly high quality. The fourth arc,a of u3e of aaact :r=.qusncias i.~_a special co.amunica- tions with unusual noise stability. Narrowing thA bandwidth by improving the system of keying and increasing frequency stability, it is possible by various maLhods :o increase the a:fect!va ch~r_nFl power Cto Increase noise stability). w The simplest of these is the reFoti:ioc of messages consecutively in time or in parallel along savers] chanre.s, ss well as the =athod of integrating telegraph guises for def_aite in :erva?a o: rims. Three methods, permitting a considerable iacreasa iZ the af?activf por+er o_' co.zsunications channels, require the rigid cophasal operation o_' the transmitting and receiving equipments, thnt is, the use of exact frequencies of high qual- ity. In the fifth wren of use o' exact frequencies we include the various systems permitting interm:ttert :multiplex op~retlon of communications Sanitized Copy Approved for Release 2011/04/01 :CIA-RDP81-002808000100060030-5 channels (multiplex transmission). Such systems are well known, The 9-p1exSTAT Baudot system may be cited as an example. The moat ~@nerally used methods of multiplex operation of communications lines ere psually divided into' two classes: frequency multiplex and time multiplex 'operation, The number of channels in communications line~~'1s usually relatively smell. Thus, in radio communications lines the number of channels is usually leas than ten. Under these conditions, with =requency multiplex- ing the power of any one channel decreases rapidly,Qe the number of then- nela is increased (as an approximation may be conpiQ6red that the power of-one channel decreases in inverse proportion to the square of the number of channels) . ?' In the case of time multiplexing the decrease lb power of a single channel is somewhat slower (approximately in inverd@ proportion to the number of channels). Hence, in systems with a relatively smell number of channels, especially in radio communications, time+Dl?ltiplex systems are finding increasingly wider use. These systems requil'8 cophasal operation of the transmitting and receiving equipments, hens@~they also require exact frequencies of extremely high quality. ? It must especially be noted that the achievemp~t of phase synchronism of terminal telegraph apparatus permits solution o~~n important problem of modern communications engineering, -- the electrip~l transit of tele- grams in systems with time multiplexing, - Only the principal areas of use of exact frequencies in communications engineering are listed above. Such frequencies are *lso widely used in other areas -- time service, frequency control, navigatigA, in scientific labors- tortes, etc. Qualitative Indexes of Exact Frequencies, Systems O~.Synchronism It is clearly important that in the near futur0 exact frequencies should constitute the organic basis of all the principal lines of electri-. cal 'communications. Hence it is to the interest of Communications special- ists that they become acqusi.~~ted with the principal%qualitative indexes of exact frequencies, We will here dwell only on the physical determinations of qualitative indexes without touching on problems`. Of their generation . and transmission or methods of conversion. In order to excite synchronous radio transmitt@ts, to provide the requisite synchronism of the operation of terminal t@legraph equ.':pment, to measure i:he frequencies of radio stations, etc. ?xact frequencies, distinguished by somewhat different qualities, arQ.~pcessary. In speaking of the synchronism of communications equipment usiq~,~exact frequencies it is above all necessary to distinguish the stetee?Of phase synchronism and frequency synchronism, or, so to speak, phase-eyRChronous and fre- quency--synchronous systems. We shall explain these LOrms by example. Let us assume that we are dealing with a phas@-modulation telegraph system operating et ultrasonic frequencies, for example, at a frequency of 20 kilocycles. The period of this frequency is?~Q microseconds. Tele- graphing is achieved by varying the phase of oscilldtiona in transmitting 1n time with the-Yrequency of modulation, which in amplitude modulation is equivalent to transmission of the modulation sideband frequencies only (without the carr.ier). The "carrisr wave" is restored in the proper phase at the receiving sits. In this case, as is easily s@an, the result is amplitude modulation with variations in the amplitude of oscillation twice as great as in the amplitude modulation achieved at tl:e transmitting Sanitized Copy Approved for Release 2011/04/01 :CIA-RDP81-002808000100060030-5 end. In fact, if in amplitude modulation the voltage at the receiver we~STAT to vary for example, iron 0 to 1 aillivolt, ther. in phase modulation in transmitting a signal in the positive phase the sans transmitter will add 1 millivolt to the aaplitude of the local carrier and 1a the negative phase it will subtract 1 millivolt, Thus, the amplitude of oscillations in?the receiver will vary over 2 millivolts instead of the 1 aillivolt in amplitude keying, Thia is equivalent to a channel power gain of four timeb or 8 deci- bels. In figures a, b, and c the vector OH representf the restored carrier wave at the receiving site; the vectors Hf~e;~ arg the redultias oscilla- tions of the aodulstion sideband, The value and ai~p~{direction) of these vectors depend on the amplitude and phase of the treosmitted oscillations (HA is the call vector, Hb is the rise vector). In ?i~ure a the properly phased coaponents receive a high degree of modulatiptt iron the detected oscillations. In Figure b the carrier frequency and the aodulation fre- quency are out of phase by 45 degrees; the degree o~ s~odulation is some- what reduced and the effective output of the channel decreased. Finally, in Figure c the carrier and the modulation irequsac~ are out of phase by 90 degrc~s; in this case communication is not possible. Examining these figures, we can conclude that for phase-aodulation tfle~:aphy the phase difference of the carrier frequency and Lhe modulation frsquencioa at the receiving end should not exceed one radian. Ii this difference is greater, as is seen from the i'gurea, phase modulation provide any ad- vantage in power and even becomes worse than amplitude modulation. The phase difference nay also be expressed In time. 11e have already noted above that at a frequency of 20 kc the durat:Op o! a cyclo (2 rt radi- ans) is 50 microseconds. Consequently, a phase deviation of one radian is equal to a time interval of 50 8 microseconds. Thw , it may be said that 2rt in our exasple for phase-modulation telegraphy the principal requirement for exact lrequenciea is that the phase of the exact frequencies in _ tranamiasion and reception do not differ by more than 5-7 microasconds: For phase measurements it is best to use tine units (microseconds) instead ~i electrical degrees, since in frequency conversion scheaee (multipliers, dividers) phase shifts expressed in microasconds are not changed, It the phase shifts are expressed in electrical degrees, then in multiplication and division they Hunt ba changed correspondingly. In the exa:ained case the frequencies at tha transmitting and receiving ends map be relatively unstable (that is, they may vary with time). ?he apecificatione require only their relative phase stability (OSF). In prac- tical schemes tha assigned OSF index is maintained by means of automatic phase ad~uatment. Let us suppose that we do not wish to make a ar?ecial exact-frequency transmission channel for automatic phase ad3ustmeat. Then it is necessary to seed the transmitter and the receiver from high-stability local oscilla- tors -- exact-frequency standz~ds -- providing the absolute phase stability (AuF) called for by the specifications, Since the phase of one oscillator may "move fo Huard" and the other "lag", then the ASF index of each refer- ence generator must be expressed as a value ona half that of the OSF index (that ia, 4 microseconds). This means that the "time of the standard," which io defined as the product of the rated value of the period of oscilla- tion of the standard apd the number of oscillations actually completed try the standard, must never "lead" the mean solar time or "lag" behind it by more than 4 microseconds, Simple calculation will show that this is as extremely rigid require- ment. Indeed, 1f the frequency of the reference generntor ware to deviate one one-hundred millionth from its rated value (1 x 10'-8P, then from the Sanitized Copy Approved for Release 2011/04/01 :CIA-RDP81-002808000100060030-5 UI i y formula ,~ 7 = Gf = 1 x IO-g we find thnt over a relatively short period oiSTAT ~' ~ time 'G= 1 hour 3,600 secoala the phase of the oscillations of the refer- ence generator is off by p'C = 10-g x 3600 x .l0'8 = 38 microseconds (that is, nine times greater than th% value permitted for phase telegraphy). For the sake of comparison it is intereatiag to note that the OBF index of soma modern phase-synchronized systems (synchronous broadcast nets, pulse systems of modulation, etc) is fixed within several tenths and even within several hundredth^ o! a microsecond. Of course, is these cases special chan- nels are provided for the trans asioa o! exact treQuencies in order to achieve automatic phase ad~ustm Me will nor consider another, more common task -- measuring an un- known frequency by cO~mparing it with another, exact frequency. Such measure- ment is usually pertorsied with an accuracy of one millionth (1 x 10~) of the rated frequency value, The exact frequency used for the comparison mua~ have a stability of approximately 1 x 10-~. The phase stability in thin case Sa of no interest to us. An enact frequency with ? high degree of phase stability (4 microseconds, as in the above example) may, neverthe- less, prove unsuitable, Indeed, if the automatic phase ad,justiag system corrects the phase at ? rate of one microsecond (time of the standard) per aecoad (time o1 ad~uatment), this will cause a deviation of the stan- dard frequency from the rated value by an amount determined by the equation Qf Q~ 1 x 10-8 = 1 x 10~. Thus, the frequency of the standard will 'C 1 be ten times worse than required for the measurement. Ia the case under consideration we are concerned with the absolute frequency stability (ASCh) index in relative units /4f\. Finally, the communicatioaa engineer must often deal x~ith schemes in which he is not interested in the OSF, ASF, or ASCh, but in which it is important to know the frequency deviation or relative frequency stability (OSCh). An example of ~ueh a e::heme would be a single-sideband ayatem of radiotelephone ~ransm!'ssion, 11hen the first systems of short-wave single- s?deband transmission appeared frequency stabilization techniques rsrs still inadequately developed and frequency converatoa techniques were practically unknown. Under such conditions wide use was made of schemes for automatic frequency control (in distinction irola the sutomatic pbase adjustment mentioned previously). Automatic frequency control allows a cert'Ain deviation of Qhe coaq~ared frequencies by a value of d-F', wherein the 06Ch index may be defined ns the value 2 Qf , whuro it and L2 are ri w`r2 Thus, operation with phase or frequency synchronism is determined by the requirements which the ay.stem must meet in regard to exact fre- quencies. Tho quality of phaas synchronism is determined from the ASF and OSF indexes, expressed is microseconds; the quality of frequency synchronism is determined from the ASCh and ORCh Sndexea, expressed in relative unite of frequency instability. Various communications systems must meet entirely different require- ments in regard to exact frequencies, With a high index of phase ctabil:ty, the exact-frequency stability may be far from adequate for trequency- synchronized systems and, conversely, an exr.ct frequency of extremely high stability may prove wholly unsuitable io: use in phase-synchronized systems ;phase-modulation telegraphy, pulse ayateaa of modulation, time-multiplex systems, etc). - R - Sanitized Copy Approved for Release 2011/04/01 :CIA-RDP81-002808000100060030-5 Sanitized Copy Approved for Release 2011/04/01 CIA-RDP81-002808000100060030-5 * * i'~ Sanitized Copy Approved for Release 2011/04/01 CIA-RDP81-002808000100060030-5