CHAPTER I INTRODUCTION TO SINGLE SIDEBAND

Transcription

1 CHAPTER 1 ntrductin CHAPTER NTRODUCTON TO SNGLE SDEBAND 1. NEED FOR SNGLE SDEBAND The need fr single-sideband cmmunicatin systems has arisen because present day radi cmmunicatins require faster, mre reliable, spectrum cnservative systems. The quantity f cmmercial and military traffic is presently s great in the high-frequency (2 t 30 mc) spectrum that it has becme necessary t restrict the use f this spectrum t thse services which cannt be accmmdated by ther means. Landlines, micrwave links, and uhf scatter prpagatin are emplyed t relieve the lad frm the highfrequency spectrum. n many instances, these prvide a better and mre reliable service. There are, hwever, many cmmunicatin services which need the prpagatin characteristics btainable nly in the high-frequency range. Amng these are ship-t-shre cmmunicatins, air-t-grund cmmunicatins, and the many military and naval systems which re quire independence, mbility, and flexibility. Since high-frequency spectrum space is limited, it is essential that the best pssible use be made f the space available. This means that cmmunicatin systems must use a minimum bandwidth, that the guard bands between channels t allw fr frequency drift and pr selectivity be minimized, and that spurius radiatin be kept t a very lw value t avid interference between services. n additin t this, a mre reliable signal is desirable if nt essential. Singlesideband cmmunicatin systems in their present state f develpment prvide these assets. 2. WHAT SNGLE SDEBAND MEANS A single-sideband (SSB) signal is an audi signal cnverted t a radi frequency, with r withut inversin. Fr instance, an intelligible vice signal cntains audi frequencies ver the range f 300 t 3000 cycles per secnd (cps). f this audi signal is cnverted t a radi frequency by mixing it with a 15 mc r-f frequency, the resultant sum frequencies cver the range f 15,000,300 t 15,003,000 cps. Such a signal is an SSB signal withut inversin and is referred t as an upper sideband, because it ccupies the spectrum space abve the r-f cnversin frequency. Nte that the 15 mc carrier is nt included in the range f the SSB signal. The abve example des nt indicate the presence f a difference fre quency. Hwever, when the vice signal is mixed with the r-f frequency, a difference frequency des develp which cvers the range frm 14,999,700 t 14,997,000 cps. This signal is als an SSB signal but is an SSB signal with inversin. This SSB signal is referred t as a lwer sideband signal because it ccupies the spectrum space belw the r-f cnversin frequency. Figure 1-1 illustrates the psitin f the SSB signal in the r-f spectrum 3KC AUDO FREQ CARRER FREQ LOWER UPPER SDEBAND SDEBAND (SUPPRESSED) 3KC 3KC r-----'r : \ RADO FREQ Figure 1-1. Lcatin f SSB Signal in R-F Spectrum Frm the abve descriptin f the SSB Signal, it is apparent that nly ne sideband signal need be transmitted t cnvey the intelligence. Since tw sideband signals are btained frm the mixing prcess, it is als necessary t remve ne sideband befre transmissin. T receive the SSB signal, it is necessary t cnvert the SSB signal back t the riginal audi signal. This requires identical transmitter and receiver cnversin frequencies. n the past, a lwpwer, pilt carrier was transmitted fr autmatic frequency cntrl (afc) purpses t prvide this end. Hwever, with present day frequency stabilities (1 cps at 10 mc in grund and 10 cps at 10 mc in mbile equipment) the need fr afc and pilt carriers is eliminated. Several methds f sideband cmmunicatin are in use r under develpment. The" single-sideband" methd as the term is used thrughut this bk refers t the methd which is, perhaps, mre accurately termed "Single-Sideband, suppressed carrier." n this methd, nly ne sideband is transmitted and the carrier is suppressed t the pint f nnexistence. T demdulate the single-sideband signal requires cnversin f the signal with a lcallygenerated signal clse t the prper frequency but 1-1

2 ntrductin CHAPTER 1 with n phase relatinship required. n the "singlesideband, pilt carrier" system nly ne sideband is transmitted, but a lw-level carrier f sufficient amplitude fr receptin is als transmitted. T demdulate this signal, the pilt carrier is separated frm the sideband in the receiver, then amplified and used as the cnversin frequency t demdulate the sideband signal. n anther methd, the pilt carrier is used fr autmatic frequency cntrl f the receiver. n the "duble-sideband" (DSB) system, bth the upper and lwer sidebands f the signal are transmitted with the carrier suppressed t the pint f nnexistence. T demdulate the duble sideband requires insertin f a lcally-generated carrier f bth the prper frequency and the prper phase. This system depends upn an autmatic frequency and phase cntrl, derived frm the duble-sideband signal, fr cntrl f the lcally-generated carrier. n the "single-sideband, cntrlled carrier" system nly ne sideband is transmitted, but a carrier which varies inversely with the signal level is als transmitted. This allws an appreciable average carrier level fr autmaticfrequency-cntrl withut reducing the sideband pwer belw the full transmitter rating. 3. HSTORCAL DEVELOPMENt OF SNGLE SDEBAND COMMUNCATON SYSTEMS Althugh SSB transmissin has nly received publicity in the last few years, the knwledge f the sideband and the develpment and use f SSB techniques have prgressed ver the last 40 years. The acustical phenmenn f cmbining tw waves t prduce sum and difference waves carried ver int electric-wave mdulatin. The presence f the upper and lwer sidebands in additin t the carrier frequency were tacitly assumed t exist but were nt cncretely visualized in the earhest mdulated transmissins. Recgnitin that ne sideband cntained all the signal elements necessary t reprduce the riginal signal came in t was then, that at the Navy Radi Statin at Arlingtn, Va., that an antenna was tuned t pass ne sideband well, even thugh the ther was attenuated. Frm 1915 until 1923, the physical reality f sidebands was vigrusly argued with the ppnents cntending that sidebands were mathematical fictin. Hwever, the first trans-atlantic raditelephne demnstratin in 1923 prvided a cncrete answer. This system emplyed an SSB signal with a pilt carrier. Single Sideband was used in this system because f the limited pwer capacity f the equipment and the narrw resnance bands f efficient antennas at the lw frequency (57 kc) used. By 1927 trans Atlantic SSB raditelephny was pen fr public service. The first verseas system was fllwed by shrtwave systems, 3 t 30 mc, which transmitted duble sideband and carrier because SSB develpment did nt 1-2 permit practical SSB transmissin in this frequency range. Hwever, SSB techniques were emplyed in varius telephny applicatins and in varius multiplexing systems. t has nt been until recently that equipment develpments have permitted the advantages f SSB cmmunicatin t be fully explited. These develpments have been in the fields f frequency stability, filter selectivity, and lw-distrtin linear pwer amplifiers. These develpments have led t military and cmmercial acceptance f SSB cmmunicatin systems. There are presently available several radi amateur and cmmercial SSB radi sets, fixed-statin SSB exciters up t 45 kw linear pwer. amplifiers, and airbrne transceivers capable f reliable cmmunicatins with unlimited range. Sme f these equipments, especially the military equipments, are prvided with autmatic frequency selectin and autmatic tuning t further enhance their value as reliable, easily perated systems. 4. BASC FUNCTONAL UNTS OF A SNGLE SDEBAND TRANSMTTNG SYSTEM Sme f the basic functinal units f an SSB system have been previusly mentined. Figure 1-2 shws these units in their functinal relatinship fr an SSB transmitter. The audi amplifier is f cnventinal design. Audi filtering is nt required because the highly selective filtering which takes place in the SSB generatr attenuates the unnecessary frequencies belw 300 cps and abve 3000 cps. t shuld be nted that a vice signal is used nly as a cnvenience fr explanatin. The input signal may be any desired intelligence signal and may cver all r any part f the frequency range between 100 and 6000 cps. The upper limit f the input audi signal is determined by the channel bandwidth and the upper cutff frequency f the filter in the SSB generatr. The lwer limit f the input audi signal is determined by the lwer cufff frequency f the filter in the SSB generatr. The SSB generatr prduces the SSB signal at an i-f frequency. The mst familiar way t prduce the SSB signal is t generate a duble-sideband (DSB) signal and then pass this signal thrugh a highly selective filter t reject ne f the sidebands. The SSB signal is generated at a fixed i-f frequency because highly selective circuits are required. The highly selective filter requirements fr the filter methd f SSB generatin are met by either crystal r mechanical filters. Bth f these filters have been imprved in perfrmance and reduced in size and cst t make their applicatin practical. The generated SSB signal at a fixed i-f frequency then ges thrugh mixers and amplifiers where it is

3 CHAPTER 1 ntrductin 10 3KC : 3KC '300KC r , MXERS AND v-\ 11.7 TO 31.7 MC AUDO SSB AMPLFERS LNEAR D- AMPLFER GENERATOR (DOUBLE P.A.... CONVERSON) 300KC FREQUENCY CARRER MULTPLERS n-. GENERATOR X 0,1,3,7 L EC.!TE f '\ 17 FREQUENCY - STANDARD 100KC SMO 2-4MC NOTE": SGNAL NVERSON, DUE TO SUBTRACTVE MXNG N FRST STAGE OF SSB EXCTER, MAKES T NECESSARY TO USE THE LOWER SDEBAND OUTPUT, FROM THE SSB GENER ATOR, TO PRODUCE THE FNAL UPPER SDE BAND SGNAL. Figure 1-2. Functinal Units f ad SSB Transmitting System ( cnverted up in frequency t the transmitted r-f frequency. Tw stage cnversin is shwn with the secnd cnversin frequency being a multiple f the first cnversin frequency. The frequency cnversins required t prduce the r-f frequency prduce sum and difference frequencies as well as higher rder mixing prducts inherent in mixing circuits. Hwever, the undesired difference frequency r the undesired sum frequency, alng with the higher rder mixing prducts, is attenuated by inter stage tuned circuits. The SSB exciter drives a linear pwer amplifier t prduce the high pwer r-f signal. A linear pwer amplifier is required fr SSB transmissin, because it is essential that the plate utput r-f signal be a replica f the grid input signal. Any nnlinear peratin f the pwer amplifier will result in intermdulatin (mixing) between the frequencies f the input signal. This will prduce nt nly undesirable distrtin within the desired channel but will als prduce intermdulatin utputs in adjacent channels. Distrtin in the linear' pwer amplifier is kept lw by the design chice f pwer amplifier tubes, their perating cnditins, and use f r-f feedback circuits. The lw distrtin btainable in mdern linear pwer amplifiers is nt essential t the SSB system nr is it essential fr gd vice transmissin, but it is essential t minimize the guard band between channels and thereby permit full utilizatin f the spectrum space. Because an SSB system withut a pilt carrier demands an extremely stable frequency system, the frequency standard and stabilized master scillatr (sm) are extremely imprtant. The sandard frequency is btained frm a crystal scillatr with the crystal hused in an ven. Since the' stability f the crystal frequency depends directly upn the stability f the ven temperature, stable thermal cntrl f the ven is necessary. This thermal cntrl f the ven is btained by using heat-sensitive semicnductrs in a bridge netwrk. Any variatin in the ven temperature, then, is indicated and crrected by an unbalance in the cntrl bridge. This system will limit changes in ven temperature t C. Such ven stability will prvide a standard frequency which will vary n 1-3

4 ntrductin CHAPTER 1 mre than 1 cps in 10 mc per day when used in fixedstatin equipment and n mre than 10 cps in 10 mc per day when used in mbile statin equipment. The carrier generatr prvides the i-f carrier used t prduce the fixed i-f SSB signal, and the sm prvides the necessary cnversin frequencies t prduce the r-f SSB signal. The frequencies develped in these units are derived frm r phase lcked t the single standard frequency s that the stability f the standard frequency prevails thrughut the SSB system. Chice f the fixed i-f frequency and the cnversin frequencies t btain the r-f frequency is an extremely imprtant design cnsideratin. Optimum perating frequencies f the varius circuits must be cnsidered as well as the cntrl f undesirable mixing prducts. The frequency scheme shwn is the result f extensive study and experimental verificatin. t prduces minimum spurius utput in the high-frequency range (2 t 32 mc). The use f harmnically related cnversin frequencies in the mixer permits the frequency range t be cvered with a single 2 t 4 mc scillatr, a very practical range fr btaining high scillatr stability. Use f the 300 kc fixed i-f frequency is the ptimum perating frequency fr the mechanical filter required in the SSB generatr. The freging discussin may give the errneus impressin that nly Single channel cmmunicatin is pssible with an SSB system. Quite the ppsite is true. T add additinal channels t the SSB system requires nly additinal circuits in the SSB generatr. One methd is t use the upper sideband f ne signal and the lwer sideband f the ther signal. Figure 1-3 shws the circuit fr prducing these tw channels and the lcatin f each channel with respect t the carrier frequency. t shuld be nted that with this methd a twin sideband is transmitted, and that the signal in the lwer sideband is inverted. Anther methd f adding channels is shwn in figure 1-4. Different fixed i-f frequencies, ne raised 4 kc frm the riginal, are injected int separate SSB generatrs, and the upper sideband is filtered frm each utput. This prduces tw channels bth using the upper sideband. t shuld be realized that as additinal channels are added t the system, less transmitter pwer utput is available fr each channel. The SSB transmitter is designed fr linear peratin frm the audi input amplifier thrugh the utput pwer amplifier. That is, the transmitter faithfully transmits the riginal input intelligence with negligible distrtin. This distrtin-free system is ideally suited fr the transmissin f multiplex and Kineplex signals, because the riginal pulses are transmitted withut distrtin f their wave shape. CHANNEL A --(UPPER SSB GENERATOR SDEBAND), 3KC,, 300KC 3KC : 3KC, 300KC 3 OOKC.--- TWN SDEBAND TO MXERS NOTE: CHANNEL B SGNAL S NVERTED. CHAN NEL A SGNAL S NOT. SSB GENERATOR CHANNEL B --(LOWER SDEBAND) 3KC 300KC Figure 1-3. Generatin f the Twin-Channel Sideband Signal 1-4

5 CHAPTER 1 ntrductin SSB GENERATOR CHANNEL A (UPPER SDEBAND) : 3KC VA\ 300KC : 3KC 3KC 296KC 300KC 300KC 296KC TWO CHANNELS TO MXERS NO TE: NETHER CHANNEL A NOR B SGNAL S NVERTED. CHANNEL B SSB GENERATOR (UPPER SDEBAND) 3KC v--y-\ 296KC Figure 1-4. Generatin f Tw Channel SSB Signal 5. BASC FUNCTONAL UNTS OF A SNGLE SDEBAND RECEVNG SYSTEM T receive the SSB signal requires a heterdyning system which will cnvert the radi-frequency signal back dwn t its riginal psitin in the audi spectrwn. The basic functinal units f such a receiver are shwn in figure 1-5. t can be seen that the SSB receiver is almst identical t a cnventinal heterdyne receiver except fr the detectin circuit. The r-f signal is amplified and cnverted dwn in frequency t a fixed i-f frequency. Then a final fixed i-f injectin frequency is required t bring the signal dwn t its riginal psitin in the audi spectrum. Many f the units f an SSB receiver are identical with units f the SSB transmitter as can be seen by cmparing figure 1-2 with figure 1-5. The frequency standard, carrier generatr, and sm are identical. The duble cnversin mixer and amplifier unit f the receiver can be made identical t the duble cnversin mixer and amplifier unit f the transmitter. This similarity f functins permits the cnstructin f transceivers with much f the circuitry used fr bth receiving and transmitting by merely adding switching t reverse the directin f signal flw. By using dual purpse units and adding switching t reverse the directin f the signal, equipment size, weight, cst, and pwer cnswnptin are substantially reduced. 6. COMPARSON OF SSB WTH AM. a. POWER COMPARSON OF SSB AND AM. There is n single manner which can be used t evaluate the relative perfrmance f AM systems and SSB systems. Perhaps the mst straightfrward manner t make such a cmparisn is t-determine the transmitter pwer necessary t prduce a given signalt-nise (sin) rati at the receiver fr the tw systel1s under ideal prpagating cnditins. Signal-t-nise rati is cnsidered a fair cmparisn, because it is the sin rati which determines the intelligibility f the received signal. Figure 1-6 shws such a cmparisn between an AM. system and an SSB system where 100 percent, single-tne mdulatin is assumed. Figure 1-6A shws the pwer spectrwn fr an AM. transmitter rated at 1 unit f carrier pwer. With 100 percent sine-wave mdulatin, such a transmitter will actually be prducing 1. 5 units f r-f pwer. There is.25 unit f pwer in each f the tw sidebands and 1 unit f pwer in the carrier. This AM. transmitter is cmpared with an SSB transmitter rated at 1-5

6 ntrductin CHAPTER 1.5 unit f peak-envelpe-pwer (PEP). Peak-envelpepwer is defined as the rms pwer develped at the crest f the mdulatin envelpe. The 88B transmitter rated at.5 unit f peak-envelpe-pwer will prduce the same sin rati in the utput f the receiver as the AM. transmitter rated at 1 unit f carrier pwer. The vitage vectrs related t the AM. and 88B pwer spectrums are shwn in figure 1-6B. The AM. vltage vectrs shw the upper and lwer sideband vltages f.5 unit rtating in ppsite directins arund a carrier vltage f 1 unit. Fr AM. mdulatin, the resultant f the tw sideband vltage vectrs must always be directly in phase r directly ut f phase with the carrier s that the resultant directly adds t r subtracts frm the carrier. The resultant shwn when the upper and lwer sideband vltage are instantaneusly in phase prduces a peak-envelpevltage (pey) equal t twice the carrier vltage with 100 percent mdulatin. The. 5 unit f vltage shwn in each sideband vectr prduces the.25 unit f pwer shwn in A,.25 unit f pwer being prprtinal t the. square f.5 unit f vltage. The 88B vltage vectr is a single vectr f. 7 unit f vltage at the upper sideband frequency. The. 7 unit f vltage prduces the.5 unit f pwer shwn in A. The r-f envelpes develped by the vltage vectrs are shwn in figure 1-6C. The r-f envelpe f the AM. signal is shwn t have a PEY f 2 units, the sum f the tw sideband vltages plus the carrier vltage. This results in a PEP f 4 units f pwer. The PEY f the 88B signal is.7 unit f vltage with a resultant PEP f.5 unit f pwer. When the r-f signal is demdulated in the AM. receiver, as shwn in figure 1-6D, an audi vltage develps which is equivalent t the sum f the upper and the lwer sideband vltages, in this case 1 unit f vltage. This vltage represents the utput frm the cnventinal, dide detectr used in AM. receivers. 8uch detectin is called cherent detectin because the vltages f the tw sidebands are added in the detectr. When the r-f signal is demdulated in the 88B receiver, an audi vltage f.7 unit develps which is equivalent t the trans.mitter upper sideband signal. This signal is demdulated by heterdyning the r-f signal with the prper frequency t mve the 88B signal dwn in the spectrum t its riginal audi positin. f a bradband nise level is chsen as.1 unit f vlt"age per 6 kc bandwidth, the AM. bandwidth, the same nise level is equal t.07 unit f vltage per 3 kc bandwidth, the 88B bandwidth. This is shwn in figure 1-6E. These values represent the same nise pwer level per kc f bandwidth; that is,.12/6 equals.072/3. With this chsen nise level, the sin rati fr the AM. system is 20 lg sin in terms f vltage, r 20 db. The sin rati fr the SSB system is als 20 db, the same as fr the AM. system. The 1/2 pwer unit f rated PEP fr the 88B transmitter, therefre, prduces the same signal intelligibility as the 1 pwer unit rated carrier pwer fr the AM. 3 KC v-==\ 3 KC r===\! 1.7 TO 31.7MC 300KC L MXERS AND MXER AND RF AMPLFER AMPLFERS SELECTVE AUDO AMPL (DOUBLE +-- F L TERNG CONVERSON) 300KC FREQUENCY MULTPLERS X 0,1,3,7 CARRER GENERATOR SMO 2-4 MC FREQUENCY STANDARD 100 KC 1-6 Figure 1-5. Functinal Units f an 88B Receiving 8ystem

7 CHAPTER 1 ntrductin AM SNGLE TONE, SNE-WAVE MODULATON SSB SNGLE TONE, SNE -WAVE MODULATON -- RATED POWER RATED CARRER POWER LSB C USB- A C.5 USB RATED PEP POWER=.5 VOLTAGE VECTORS 100% MODUATON LSBUSB, \ LSe1tuSB N B > lu Q. T.7 USB C C RF ENVELOPE PEV=.7 PEP=.5 RCVR AUDO SGNAL VOLTAGE f\ f\ 0 UUSB+LSBO 1\0 V.7 NOSE VOLTAGE [ARBTRARY NOSE POWER PER KC OF BW EQUAL N AM AND SSB.. E., VOLTAGE=... "'" _.1 PER 6KC E BANDWDTH VOLTAGE=.07 PER 3KC BANDWDTH (.1 )76 = (.07n3] SN RATO + 20 LOG = 20DB F 20 LOG L. =.07 20DB Figure 1-6. SSB and AM. Cmparisn with Equal Signal-t-Nise Rati 1-7

8 ntrductin CHAPTER 1 transmitter. This cnclusin can be restated as fllws: Under ideal prpagating cnditins but in the presence f bradband nise, an SSB and AM. system perfrm equally (same sin rati) if the ttal sideband pwer f the tw transmitters is equal. This means that an SSB transmitter will perfrm as well as an AM. transmitter f twice the carrier pwer rating under ideal prpagating cnditins. b. ANTENNA VOLTAGE COMPARSON OF SSB AND AM. Of special imprtance in airbrne and mbile installatins where electrically small antennas are required, is the peak antenna vltage. n these installatins, it is ften the crna breakdwn pint f the antenna which is the limiting factr in equipment pwer. Figure 1-6C shws the r-f envelpes f an SSB transmitter and an AM. transmitter f equal perfrmance under ideal cnditins. The peakenvelpe-vltage prduced by these tw transmitters is shwn t be in the rati 2 fr the AM. transmitter t.7 fr the SSB transmitter. This indicat.e that f.1 equal perfrmance under ideal 'cnditins, the peak antenna vltage f the SSB system is apprximately 1/3 that f the AM. system. A cmparisn betwe.en the SSB pwer and the AM. pwer which can be radiated frm an antenna f given dimensins is even mre significant. f an antenna is chsen which will radiate 400 watts f peak-envelpepwer, the AM. transmitter which may be used with this antenna must be rated at n mre than 100 watts. This is true because the PEP f the AM. signal is fur times the carrier pwer. An SSB transmitter rated at 400 watts f PEP, all f which is Sideband pwer, may be used with this same antenna. Cmpared with the 50 watts f sideband pwer btained frm the AM. transmitter with a 100-watt carrier rating. c. ADVANTAGE OF SSB WTH SELECTVE FADNG CONDnONS The pwer cmparisn between SSB i:ljld AM. given in the previus paragraph is based n ideal prpagatin cnditins. Hwever, with lng distance transmissin, AM. is subject t selective fading which causes severe distrtin and a weaker received signal, At times this can make the received signal unintelligible. An AM. transmissin is subject t deteriratin under these pr prpagatin cnditins, because all three cmpnents f the transmitted signal, the upper sideband, lwer sideband, and carrier must be received exactly as transmitted t realize fidelity and the theretical pwer frm the signal. Figure shws the deteriratin f an AM. signal with different types f selective fading. The lss f ne f the tw transmitted sidebands results nly in a lss f signal vltage {-cm the demdulatr. Even thugh sme distrtin results, such a lss is nt basically detrimental t the 'signal, because ne sideband cntains the same intelligence as the ther. Hwever, since the AM. receiver perates n the brad bandwidth necessary t receive bth sidebands, the nise level remains cnstant even thugh nly ne sideband is received. This is equivalent t a 6 db deteriratin in sin rati ut f the receiver. Althugh the lss f ne f the tw sidebands may be an extreme case, a prprtinal deteriratin in sin rati results frm the reductin in the level f ne r bth sidebands. The mst serius result f selective fading, and the mst cmmn, ccurs when the carrier level is attenuated mre than the sidebands. When this ccurs, the carrier vltage at the receiver is less than the sum f the tw sideband vltages. When the carrier is attenuated mre than the 'sidebands, the r-f envelpe des nt retain its riginal shape, and distrtin i& extremely severe upn demdulatin. This distrtin results upn demdulatin because a carrier vltage at least as strng as the sum f the tw sideband vltages is required t prperly demdulate the signal. The distrtin resulting frm a weak carrier can be vercme by use f the exalted carrier techniq\le whereby the carrier is ampliffed separately and then reinserted befre demdulatin. n using the exalted carrier, the carrier must be reinserted clse t the riginal phase f the AM. carrier. Selective fading can als result in a shift between the relative phase positin f the carrier and the sidebands. An AM. mdulatin is vectr ally represented by tw cunter-rtating sideband ve.ctrs which rtate with respect t the carrier vectr. The resultant f the sideband vectrs is always directly in phase r directly ut f phase with the carrier vectr. n an extreme case, the carrier may be shifted 90 frm its riginal psitin. When this ccurs, the resultant f the sideband vectrs is ±90 ut f phase with the carrier vectr. This results in cnverting the riginal. AM. Stgnal t a phase mdulated signal. The envelpe f ilie phase mdulated signal bears n resemblance t the riginal AM. envelpe and the cnventinal AM. detectr will nt prduce an intelligible signal. Any shift in the carrier phase frm its riginal phase relatinship with respect t the sidebands will prduce sme phase mdulatin with a cnsequential lss f intelligibility in the audi signal. Such a carrier phase shift may be caused by pr prpagating cnditins. Such a carrier phase shift will als result frm using the exalted carrier technique if the reinserted carrier is nt clse t its riginal phase, as previusly mentined.

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10 ntrductin CHAPTER 1 12 D z '"..J ;! 56 "'" -'S.'00 >... 3 ",\' in <5 ::; ;j ,.\l S\\l sse REFER NeE AM TRANSMTTER-OQW CARRER 3 ldeal ( NOSE) ALONE GOOD PROPAGATON CONDTONS POOR SEVERE FADNG AND NTERFERENCE Figure l-s. Relative Advantage f SSB ver AM. with Limiting Prpagating Cnditins An SSB signal is nt subject t deteriratin due t selective fading which varies elther the amplitude r the phase relatinship between the" carrier and the tw sidebands in the AM. transmissin. Since nly ne sideband is transmitted in SSB, the received signallevel des nt depend upn the resultant amplitude f tw sideband signals as it des in AM. Since the receiver signal des nt depend upn a carrier level in SSB, n distrtin can result frm lss f carrier pwer. Since the receiver signal des nt depend upn the phase relatinship between the sideband signal and the carrier, n distrtin can result frm phase shift; Selective fading within the ne sideband f the SSB system nly changes the amplitude and the frequency respnse f the signal. t very rarely prduces enugh distrtin t cause the received signal r vice t be unintelligible. d. COMPARSON OF S5B WT'" AM. UNDER limtng PROPAGATNG CONDTONS One f the main advantages f SSB transmissin ver AM. transmissin is btained under limiting prpagating cnditins ver a lng-range path where cmmunicatins are limited by the cmbinatin f nise, severe selective fading, and narrw-band interference. Figure l-s illustrates the results f an intelligibility study perfrmed by rating the intelligibility f infrmatin received when perating the tw systems under varying cnditins f prpagatin. 1 The tw transmitters cmpared have the same ttal sideband pwer. That is, a 100 watt AM. transmitter puts 1/4 f its rated carrier pwer in each f tw sidebands, while a 50 watt SSB transmitter puts its full rated utput in ne sideband. This study shws that as prpagatin cnditins wrsen. and interference and fading becme prevalent, the received SSB signru will prvide up t a 9 db advantage ver the AM. signal. The result f this study indicates that the SSB system will give frm 0 t 9 db imprvement under varius cnditins f prpagatin when ttal sideband pwer in SSB is equal t AM. t has been fund that 3 f the pssible 9 db advantage will be realized n the average cntact. n ther wrds, in nrmal use, an SSB transmitter rated at 100 watts (PEP) will give equal perfrmance with an AM. transmitter rated at 400 watts carrier pwer. t shuld be pointed ut that in this cmparisn the receiver bandwidth is just enugh t accept the transmitted intelligence in each case and n speech prcessing is cnsidered fr SSB transmissin. e. COMPARSON OF ARBORNE HGH-FREQUENCY SYSTEMS Figure 1-9 shws a cmparisn in weight, vlume,. input pwer, effective utput pwer, and peak antenna vltage between Radi Set AN/ ARC-3S and Radi Set AN ARC-5S. These sets are bth airbrne transceivers perating in the 2 t 30 mc, high-frequency range. The AM. set, AN/ARC-3S, is rated at 100 watts r-f utput, and the SSB set, AN/ARC-5S, is rated at 1000 watts r-f utput. The effective u1put pwer f the SSB transceiver is shwn t be 16 db higher than the AM. transceiver. This 16 db is equivalent t a pwer advantage f 40 t "1, which is an enrmus advancement in the cmmunicatin ability f an airbrne system. n additin t the pwer advantage f the SSB system f significance in airbrne equipment is the mre efficient use f the antenna with the SSB system. f. SUMMARY Fr lng-range "cmmunicatinsin the lw-, medium-, and high-frequency ranges, SSB is weli suited because f its spectrum and pwer ecnmy and because it is less susceptible t the effects f s-alective fading and interference than is"am. The principal advantages f SSB result frm the "eliminatin f the high-energy AM. carrier and frm imprved perfrmance under unfavrable prpagating cnditins. On the average cntact, an SSB transmitter will give equal perfrmance t an AM. transmitter f fur times the pwer rating. The advantage f SSB ver AM. is mst utstanding under unfavrable prpagating cnditins. Fr equal perfrmance, the 1 J. F. Hney, "Perfrmance f AM. and 8SB Cmmunicatins," Tele-Tech, September

11 CHAPTER 1 ntrductin AN/ARC-38 _ AN/ARC W-AM \4-- ODB+3DB+3DB=16DB(EFFECTVE) -+ OUTPUT POWER 1000 w- 55 B AN/ARC LBS AN/ARC-58 WEGHT 163 LBS AN/ARC-38 AN/ARC-58 VOLUME 3.5 CU FT 3.8 CU FT AN/ARC W AN/ARC W NPUT POWER REQURED AN/ARC- 38 _... _ 100% AN/ARC-58_ % PEAK ANTENNA VOLAGE Figure 1-9. AN/ARC-3S and AN/ARC-58 Cmparisn size, weight, pwer input, and peak antenna vltage f the SSB transmitter is significantly less than the AM. transmitter. 7. COMPARSON OF SSB:WTH FM Althugh much experimental wrk has been dne t evaluate the perfrmance f SSB systems with AM. systems, very little wrk has been dne t evaluate the perfrmance f SSB systems with FM systems. Hwever, figure 1-10 shws the predicted result f ne such study based n a mbile FM system as cmpared t a mbile SSB system f equal physical size. 1 The tw systems cmpared als used the same utput tubes t their full capacity s that the final r-f amplifiers dissipated the same pwer during nrmal speech lading. The study is cmplicated by evaluating the effects f speech prcessing, such as clipping and preemphasis, with its resultant distrtin. ch speech prcessing is essential in the FM system but has little benefit in the SSB system.., " 50r r a: '" 20 f z f z iii '" DETECTED SPEECH EQUAL TO NOSE -OL- ---.L ---L ---:L J ATTENUATON BETWEEN TRANSMTTER AND RECEVER. DB Figure SSB Perfrmance Cmpared with FM 1 :H. Magnuski and W. Firestne, "Cmparisn fssb and FM fr VHF Mbile Service," Prceedings f the RE. December

12 ntrductin CHAPTER 1 Figure 1-10 shws the signal-t-nise rati in decibels n the y-axis and the attenuatin between transmitter and receiver in decibels n the x-axis. This graph indicates that with between 150' t 160 db f attenuatin between the transmitter and receiver, a strng signal, the narrw-band FM system prvides a better sin rati than the 88B system. Under weak signal cnditin, frm 168 and higher db f attenuatin between transmitter and receiver, the sin rati f the FM system falls ff rapidly, and the 8SB system prvides the best sin rati. This fall-ff in the FM sin rati results when the signal lhel drps belw the level required fr peratin f the limiter in the FM receiver. The cnclusins which can be drawn frm figure 1-10 are as fllws: (1) Fr strng signals, the FM system will prvide a better sin rati than the EBB system. Hwever, this is nt an imprtant advantage because when the sin is high, a still better sin rati will nt imprve intelligibility significantly. (2) Fr wak signals, the 88B system will prvide an intelligible signal where the FM system will nt. (3) The 88B system prvides three times the savings in spectrum space as the narrw-band FM system. 8. NATURE OF SNGLE SDEBAND SGNALS a. NTRODUCTON As defined in paragraph 2, chapter 2, a singlesideband signal is an audi signal cnverted t a radi frequency, with r withut inversin. T facilitate i.llustrating the manner and the results f this cnversin, it is necessary t use pure sine-wave tnes, rather than the very cmplex wavefrms f the human vice. Fr this reasn single tnes r cmbinatins f tw r three tnes are generally used in the fllwing discussin. b. THE SSB GENERATOR The mst familiar SSB generatr cnsists f a balanced mdulatr fllwed by an extremely selective mechanical filter as shwn in figure The balanced mdulatr prduces basically tw utput frequencies: (1) An upper sideband frequency equal t the injected i-f frequency plus the input audi frequency. (2) A lwer sideband frequency equal t the injected i-f frequency minus the input audi frequency. Theretically, the injected i-f frequency is balanced ut in the mdulatr s that it des nt appear in the utput. t shuld be especially nted that the generatin f undesirable prducts ccur in any mixing peratin as well as the generatin f the desired prducts. The equipment must be s designed t minimize the generatin f undesirable prducts and t attenuate thse undesirable prducts which are generated. This is accmplished by designing gd linear perating characteristics int the equipment t minimize the generatin f undesirable frequencies and by choosing injectin frequencies which will facilitate suppressin f undesirable frequencies. 30lKC USB 299KC LSB AUDO S GNAL KC BALANCED MODULATOR 301 KC USB PLUS 299KC LSB MECHANCAL FLTER KC PASSBAND USB 30lKC EM 1 F SGNAL 300KC '0..- CARRER RENSERT Figure Filter-Type 88B Generatr 1-12

13 CHAPTER 1 ntrductin Figure Single-Tne Balanced Mdulatr Output After Filtering Out the LSB Figure Single-Tne, Balanced Mdulatr Output t shuld als be nted that the i-f carrier injected int the balanced mdulatr is nly theretically canceled frm the utput. Practical design cnsideratins determine the extent t which the carrier can be balanced ut. Present balanced mdulatrs, using cntrlled carrier leak t balance ut uncntrlled carrier leak, result in carrier suppressin f frm 30 db t 40 db belw the PEP f the sidebands. Further suppressin f the carrier by the SSB filter results in an additinal 20 db f carrier suppressin. Ttal carrier suppres sin f frm 50 db t 60 db can, therefre, reasnably be expected frm the transmitter system. c. GENERATNG THE SNGLE-TONE SSB WAVEFORM The mst fundamental SSB wavefrm is generated frm the single audi tne. This tne is prcessed thrugh the SSB generatr t prduce a single i-f frequency. As pinted ut in paragraph 4, chapter 1, the SSB signal is actually generated at an i-f frequency and is subsequently cnverted up in frequency t the transmitted r-f frequency. t is the generatin f the SSB signal at the i-f frequency with which we are cncerned. Figures 1-12 and 1-13 shw the wavefrms btained in a filter-type SSB generatr. The audi tne injected int the balanced mdulatr is 1 kc and the i-f frequency injected is 300 kc. The utput frm the balanced mdulatr cntains the 299 kc lwer sideband and 301 kc upper sideband frequencies. These tw sideband frequencies, being f equal amplitude, prduce the characteristic half sine-wave envelpe shwn in figure The repetitin rate f this envelpe with a 1-kc tne is 2 kc, the difference between the tw frequencies represented by the envelpe. This i-f signal, which cntains bth the upper sideband and lwer Sideband Signal, is called a dublesideband signal (DSB). By passing the DSB signal thrugh a highly selective filter with a 300 kc t 303 kc passband, the upper sideband signal is passed while the lwer sideband Signal is attenuated. The 301 kc signal which remains is the upper sideband signal and appears as shwn in figure Nte that the SSB Signal remaining is a pure sine wave when a single-tne audi signal is used fr mdulatin. This SSB signal is displaced up in the spectrum frm its riginal audi frequency by an amunt equal t the carrier frequency, in this case 300 kc. This SSB signal can be demdulated at the receiver nly by cnverting it back dwn in the frequency spectrum. This is dne by mixing it with an independent 300 kc i-f signal at the receiver. d. GENERATNG THE SSB WAVEFORM OF A SNGLE TONE WTH CARRER Frm the single-tne SSB signal withut carrier it is a simple step t generate the Single-tne SSB ' signal with carrier. This is dne by reinserting the carrier after the filtering peratin, as shwn in figure When the carrier reinserted is f the same amplitude as the SSB signal, the wavefrm shwn in figure -14 results. Nte that this wavefrm is Similar t the duble-sideband sigaal btained directly ut f the balanced mdulatr, as shwn in Figure Single-Tne SSB Signal with Carrier- Carrier Equal in Amplitude t Tne 1-13

14 ntrductin CHAPTER 1 f" \ 1\1\ 1\/\/\ /\ vi.-' Figure Single-Tne SSB Signal with Carrier- Carrier 10 DB Belw Tne figure Hwever, the frequency cmpnents f the tw wavefrms are nt the same. The frequency cmpnents f the SSB signal with carrier are 301 kc and 300 kc when a 1-kc audi signal is used. The SSB signal with full carrier can be demdulated with a cnventinal dide detectr used in AM. receivers withut serius distrtin r lss f intelligibility. r- the tw-tne SSB signal cntains a different tw frequencies than either f the ther tw. n the tw-tne SSB signal shwn in figure 1-16, 1 kc and 2 kc audi signals f equal amplitude are injected int the balanced mdulatr. After filtering, this results in a tw-tne SSB signal cntaining frequencies f 301 kc and 302 kc. f a pilt carrier is reinserted with the tw-tne test signal, the pilt carrier will be indicated by the appearance f a sine-wave ripple n the twtne wavefrm. This wavefrm is shwn in figure: The generatin f this tw-tne envelpe can be shwn clearly with vectrs representing the tw audi frequencies, as shwn in figure When the tw vectrs are exactly ppsite in phase, the envelpe value is zer. When the tw vectrs are exactly in phase, the envelpe value is maximum. This generates the half sine-wave shape f the tw-tne SSB envelpe which has a repetitin rate equal t the difference between the tw audi tnes. f the reinserted carrier is such that the carrier level is less than the level f the single-tne SSB signal, the wavefrm shwn in figure 1-15 results. T successfully demdulate this signal, the carrier must be separated, amplified, exalted, and reinserted in the receiver, r lcally supplied. The separate carrier amplificatin shuld be sufficient t raise the reinserted carrier t a level greater than the level f the sideband signal. The wavefrm shwn in figure 1-15 represents the wavefrm used in the SSB with pilt carrier systems. The exalted carrier technique is used t demdulate such a signal. e. GENERATNG THE TWO-TONE SSB WAVEFORM The tw-tne SSB wavefrm is generated by cmbining tw audi tnes and then injecting this tw-tne signal int the balanced mdulatr. One sideband is then suppressed by the filter, leaving the SSB wavefrm shwn in figure This tw-tne SSB signal is seen t be similar t the single-tne DSB signal as well as the SSB signal with full carrier. Hwever, 1-14 Figure Tw-Tne SSB Signal Tnes f Equal Amplitude Figure Tw-Tne SSB Signal with Small Reinserted Pilt Carrier The tw-tne SSB envelpe is f special imprtance because it is frm this envelpe that-pwer utput frm an SSB system is usually determined. An SSB transmitter is rated in peak-envelpe-pwer utput with the pwer measured with a tw equal-tne test signal. With such a test signal, the actual watts dissipated in the lad are ne-half the peak-envelpepwer. This is shwn in figure When the half sine-wave signal is fed int a lad, a peak-reading, rms-calibrated vtvm acrss the lad indicates the rms value f the peak-envelpe-vltage, This vltmeter reading is equal t the in-phase sum f e1 + e2' where e1 and e2 are the rms vltages f the tw tnes. Since in the tw-tne test signal e1 equals e2, the PEP equals (2e1)2/R r (2e2)2/R. The average pwer dissipated in the lad must e'hual the sum f the pwer represented by each tne, el /R + el/r, 4e12R r 4e22/R. Therefre, with a tw equal-tne SSB test signal, the average pwer dissipated in the lad is equal

15 CHAPTER 1 ntrductin e 2 _ -e l TWO TONE SSB S GNAL Vvtvm (e 1 + e 2 ), PEP P average Therefre: (1) with el and e2 in phase and rms values Vvtvm /Rlad 4e 1 /R r 4e2 /R, where e 1 = e 2 = e 2 /R + e 2 /R = 2e2 /R r 2e2/R PEP 'vtvm /R (2) Paverage = 112 PEP (3) Ptne lr Ptne2 = ]/.l PEP Figure 1-18, Pwer Measurements frm Tw-Tne SSB Test.Signal t 1/2 f the PEP, and the pwer in each tne is equal t 1/4 f the PEP. peak-envelpe-,f0wer can be determined frm the relatinship "PEP = V vtvm/r;" the average pwer can be determined frm the relatinship lip average = 1/2 V2 vtvm/r." This is true nly where the vtvm used is a peak-reading, rms-calibrated vltmeter. Similar measurements can be made using an a-c ammeter in series with the lad instead f the vtvm acrss the lad. The abve analysis can be carried further t shw that with a three equal-tne SSB test signal, the pwer in each tne is 1/9 f the PEP, and the average pwer dissipated in the lad is 1/3 the PEP. These relatinships are true nly if there is n distrtin f the SSB envelpe, but since distrtin is us ually small, its effects are usually neglected, f. GENERATNG THE SQUARE WAVEFORM Transmitting an audi square wave at a radi frequency impses severe requirements n any transmitting system. This is true because the square wave is cmpsed f an infinite number f dd-rder harmnics f the fundamental frequency f the square wave. Therefre, t transmit such a signal withut distrtin requires an infinite bandwidth, an infinite spectrum. This, f curse, is impssible because tuned circuits will nt pass an infinite bandwidth, The idealized SSB square wave, where all frequency cmpnents are present, shwn in figure indicates that the SSB signal requires infini te amplitude as well as infinite bandwidth, This ccurs because the harmnically related SSB cmpnents will add vectrally t infinity when the mdulating signal switches frm maximum psitive t maximum negative and vice versa. This infinite amplitude is nt present in an AM. envelpe, because the AM, envelpe cntains bth sidebands with the frequency cmpnents in ne sideband cunter-rtating vectr ally frm the frequency cmpnents in the ther sideband. The result is, then, when the resultant amplitude f ne sideband is plus infinity; the resultant amplitude f the ther sideband is minus infinity, which prduces a net amplitude f zer. The Significance f the SSB square wave lies in its rlatinship with cnventinal clipping techniques used t limit the mdulatin level Figure 1-20 shws the SSB envelpe which results frm severely clipping a 300 cps sine wave. The clipping level is such that the mdulating signal is essentially a square wave. n generating the SSB envelpe frm the mdulating signal, all harmnics abve the ninth are redved by the highly selective SSB filter. Figure 1-19 shws that speech clipping, as used in AM., is f n practical value in an SSB transmitter because the SSB envelpe is s different frm the audi envelpe. n an SSB transmitter, autmatic lad cntrl, rather than clipping, is used t prevent verdriving the pwer amplifier by hlding dwn the mdulatin level. t is pssible t use a Significant amunt f clipping in an SSB SGNAL AUDO SGNAL Figure Square Wave 8SB Signal--All Frequency Cmpnents Present 1-15

16 ntrductin CHAPTER 1 SSB ENVELOPE FROM CLPPED S:_V:7 /""/..., " ' / " ', / ' / / ', /, / ' Figure SB Envelpe Develped frm 300 CPS, Clipped Sine-Wave (Harmnics abve 9th Attenuated) Over-all transmissin efficiency depends upn the average pwer transmitted, while transmitter pwer is limited t the peak pwer capability f the transmitter. Therefre, fr vice transmissin, it behves the transmitter designer t use speechprcessing circuits which will increase the average pwer in the vice signal withut increasing the peak pwer. This can be dne in three different ways: (1) by clipping the pwer peaks, (2) by emphasizing the lw-pwer, high-frequency cmpnents f the speech signal and attenuating the high-pwer, lwfrequency cmpnents f the speech signal, and (3) by using autmatic-gain-cntrl circuits t keep the signal level near the maximum capability f the transmitter. Figure 1-23 shws a pwer vs frequency distributin curve fr the average human vice, after filtering belw 200 cps and abve 3000 cps. This curve shws that the high-pwer cmpnents f speech are cncentrated in the lw frequencies. Frtunately, it is the lw-frequency cmpnents f speech which cntribute little t intelligibility since these frequencies are cncentrated in the vwel sunds. The lw frequencies, therefre may be attenuated withut undue lss SSB transmitter if the clipping is perfrmed n the i-f SSB signal rather than n the audi signal. f clipping were perfrmed at this time, additinal filtering wuld be required t remve the harmnic prducts caused by the clipping. Hwever, clipping at this stage is satisfactry, because the harmnic prducts prduced are nt in the passband f the filter and nly small intermdulatin prducts are generated in the passband. g. GENERATNG THE VOCE WAVEFORM The human vice prduces a cmplex wavefrm which can be represented by numerus frequency cmpnents f varius amplitudes and varius instantaneus phase relatinships. N human vice is exactly like anther vice, but statistical averages cncerning the frequencies and amplitudes in the human vice can be determined. The average pwer level f speech is relatively lw when cmpared t the peak pwer level. An audi frequency wavefrm f an ii sund is shwn in figure This same ii sund, raised in frequency, is shwn in figure 1-22 as it appears as an SSB signal. Frm the "Christmastree" shape f these wavefrms, it is evident that the peak pwer, which is related t the peak vltage f a wavefrm is cnsiderably higher than the average pwer. Figure Vice Signal at Audi Frequency--a Sund Figure SSB Vice Signal--a Sund 1-16

17 CHAPTER 1 ntrductin CD 1..J > '" '"..J " \ "- '- "- "'" FREQUENCY-CPS maintain a cnstant signal level t the single-sideband generatr. (3) Highly-selective filters used in filtertype SSB exciters attel).uate sme f the high-pwer, lw-frequency cmpnents f the vice signal. There are als several speech prcessing circuits under investigatin which, if effective and practical, will be used t imprve the efficiency f vice transmissin. These circuits include (1) increased audi clipping with additinal filtering t remve the harmnics generated, (2) reductin f the pwer level f frequencies belw 1000 cps by shaping the audi amplifier characteristics fr lw-frequency rll-ff, and (3) use f speech clipping at an i-f level where the generated harmnics can be mre easily filtered. See paragraph 2-2a fr input signal prcessing circuits used in an SSB exciter. Figure Pwer Distributin in Speech Frequencies--Lw and High Frequencies Remved f intelligibility f the speech. The lw-pwer, highfrequency cmpnents present in a vice signal can be pre-emphasized t increase the average pwer f the signal. Since it is the high-frequency cmpnents which are predminate in the cnsnant sunds, sme emphasis f the high frequencies will imprve intelligibility. Hwever, t emphasize the high frequencies sufficiently t raise the average pwer level significantly wuld require cmpatible de-emphasis at the receiver t prevent lss f fidelity. Clipping pwer peaks results in flattening the wavefrm at the clipping level, and with severe clipping the vice signal becmes a series f square waves. Since an SSB square wave envelpe requires infinite amplitude as well as infinie bandwidth, clipping the audi signal must be dne with discretin. n the SSB transmitter, autmatic lad cntrl is used t cntrl the average pwer level input, rather than clipping, t prevent verdriving the pwer amplifier. Clipping then is used nly t remve the ccasinal pwer peaks. Speech-prcessing methds are being reinvestigated in relatinship t SSB transmissin t determine the mst suitable methd r cmbinatin f methds. Several circuits are presently used in SSB transmitters which effect sme speech prcessing, althugh the primary purpse f mst f these circuits is t prcess the input signal t prevent verdriving the pwer amplifier. These circuits include the fllwing: (1) Autmatic-lad-cntrl t maintain signal peaks at the maximum rating f the pwer amplifier. (2) Speech cmpressin, alng with sme clipping, t 9. MECHANCAL FLTERS Bth SSB transmitters and SSB receivers require very selective bandpass filters in the regin f 100 kc t 500 kc. n receivers, a high rder f adjacent channel rejectin is required if channels are t be clsely spaced t cnserve spectrum space. n SSB transmitters, the signal bandwidth must be limited sharply in rder t pass the desired sideband and reject the ther sideband. The filter used, therefre, must have very steep skirt characteristics and a flat bandpass characteristic. These filter requirements are met by LC filters, crystal filters, and mechanical filters. Until recently, crystal filters used in cmmercial SSB equipment were in the 100-kc range. These filters have excellent selectivity and stability characteristics, but their large size makes them subject t shck r vibratin deteriratin and their cst is quite high. Newer crystal filters are being develped which have extended frequency range and are smaller. These newer crystal filters are mre acceptable fr use in SSB equipment. LC filters have been used at i-f frequencies in the regin f 20 kc. Hwever, generatin f the SSB signal at this frequency requires an additinal mixing stage t btain a transmitting frequency in the high-frequency range. Fr this reasn, LC filters are nt widely used. The recent advancements in the develpment f the mechanical filter have led t their acceptance in SSB equipment. These filters have excellent rejectin characteristics, are extremely rugged, and are small enugh t be cmpatible with miniaturizatin f equipment. Als t the advantage f the mechanical filter is a Q in the rder f 10,000 which is abut 100 times the Q btainable with electrical elements. Althugh the cmmercial use f mechanical filter::, is relatively new, the basic principles upn which they are based is well established. The mechanical filter is a mechanically resnant device which receives electrical energy, cnverts it int mechanical 1-17