principles of communcation

8/22/2019 Principles of Communcation

1/23

ffik,'ffift,r1r

ALJ

&t$2+3u+5IIIItIIIIIIIItl;rf

oruecrrvesDefiln a"tnplitadi dulationDescribc thc AM envclopcDcscribe the AM freguncy $pctnlm rmd bandwidthErplain tbc phalor representation of so AM waveDefinc and erplain the foltowing terms: cocficient of modulation ard prce* mdulationDcriw AM voltagc disrributionAnalyzc AM in rhc titr* domailDerivc AM powcr distributiodExplain AM currcnt calculationsDcscribe AM with a cornplex informarion signalbcwribe AM modulator sircuits and the difference betwcen low- and higfi-level nrodulationDescribe linear-integrared circuit AM rnodulalorsDescribe AM transmittec gnd tbe differcnce betx'een low- and bigh-levcl trammitter$Dqcribe trapezoidal pstterns ard thcir signilicancc in analyzing AM rvavefsmsDcfine carrier shifrDe$qibc qudrature anplitude modulation

Amplitude ModulationTransmission

OilPTEN OUTLn|EInaoductionPrinciplcs of Amplitudc ModuluionAM Modulating CircuitsLincar-Integratcd-Circuit AM ModulatorsAM Transmincrs

4-6 TrapczoidalPatterns4-7 Canier $hift4-8 AM Envclopcs hoduced by Complex Nonsinusoidal Signals4-9 Quadraturc Amplitude Mcdulation


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1'll $:*ff signals rre transportcd bctlvccn a transmitrer and a rcceiver over sorue form ofIransrusslon mediunl However, the original information signals are seldom in a form thatis suitable for ransmission Ilt"*f"*, ;t;t must bc transformed ftom their uiginal forminto a form that is more suitable for transmiision. The process of impressing low-frequencyinfmation signale onto a higb-aequency cc nier sigtwl is callef. nwdulatiott Demadulationis the reverse proccls where ttre receiveo signals arc transformed back to their originalforn' Thc Purpose of this chapter is to introduce the reader to the furxtamental corrcepts of*ttp I iude rnodu Iat i on (At il).f OF AIvlPffiUoE MOOULAnoNAtrttvlitude modulation is the process of changing the arnplitude of a relatively high fre-lueocy carrier signal in proportion with the instantaneous value of the modulatinglignat' (inforn*ion). Amplitude modularion is a relatively inexpensive, lowquality form of med-ulation that is used for commercial broadcasting of both auriio and video signals. Ampli-ndc modulation is also used fnr two-way mobile radio communications, such as citizensbond (CB) radio.AM modulators are nonlinear tleviccs with two inputs and one output. One input is adnglo, high-frcquency carrier signal of constant amplitude and the second input is com-prisad of rclativcly low-liequency information signnis that may ue a single frequency or acoruplex waveform made up ofmany frequencies. Frequencies that are iigh enough-to becfficiently radiated by an antenna and propagated through free space are commonly called

radio fruqucncies, or sirnply RFs. In thc mtxtulator, ttre infornratlon acts on or madulatesthe RF carrier prrxJucing a modulated waveforrn. The infonnation signal may be a singlefteqr*ncy or more likely consisr ofa range of frequencies. For exarnpli typical voice_gradcoommunicetions systems utilize a range of information hequencies Uet*een 3CO Hz and3000 Hz' The modulated ourFut rvavef(rrm flom an AM modulator is otien calleel an A.\{covelope"&-1 The AM Envetope ;:Although there are several types of amplitude mo


3/23

CarlrrtSliltr+3Str

lQHt+5llt lgtirrffiA'i \i\

97 kH:Lsr

1@kHztc

0 = l0l*lr103 ldt 106 t*t-

fusF

nqrRE 4-g efrpui Epcfrrrn fu. Bm* SiEreltph 4tFor an AM DSBrc rrduY::r tth a carrier frequcncyt = lm kHz 11d r Errrimro modulsdng sig-rl ftcquwyJ**, = 5 tllA detcrrdoea. &tqucncy &iloib fm thc uppa rnd lowcr ridcbands.L Brndwidtr.i"ff lowrr sidc frcq*encics produced whcn the moduraring signsr is a singlc-hcqucncyT16 .4 Dnrw tb oupu euclcy spectruraSohnion r' Thc lowcr sideband er.terds &om tlre b$E$ Fsribb brrcr sidc ftcqrrey to hc c{rlrtrqrreya. tSB _ [i _r**)] lo.f,= (lO0 - J) kHz ro lm kHl = 95 llfu to lm kHuItc uppcr sideband cxtends hom thc carricr frtqucncy ro tbc higlrst possiblc upper side fruquency orUSB *t ro tf, + f_t*i- = 100 kH, r,o (100 + j) kHz : lm kfiz !o loj kflzL Thc bandwidth is cqual to the differeacc bctwccn thc maximum upper si& fuequency and tbe Frin-lann lowcr ci& frequcncy or I = T.


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-l$f Codttdsa ot Ms*daterr urd ftrccc moff*mo'dn;** d r *tl.'n.ir e un osca J-A;u,E-;*;-qb,*nr rm+ddno)pacatinrnAMwavcform.&ruanf."At r-b&ryll), ilofno+**ton *arcd g r perccgtsgc. Molt spocincalli, pcrratrl ,ro*'iioo gltr tG-pctccottgcchqt' in thc arnpritude of rhc urpuiw"". **Jm *rr- ir.""a"iTil-arr"""gsignal. Mathcmatically, the moOutition co< fhcicnt isE^=E (+r)wherc nr = modulation coefficient (unitless)& = p""t change in the amplitu& of rhc orput warrcform voltase (voh)d'. = pea& amptitude of the unmodulatcd carrier voltryc (voh$) - '

Equation 4-I can be rearranged to solve for E- and f" asE^= mE", :8,'mard

ad perccar modularion (M) is

@2t({*3)

(a)Thc rehtioaslrip lmong lrl E* andE is shown in Figure 4_5.If thc mdulating signat is a pure, singrc-frequency sinc wave aod frc modurationprccc$$ is syrrnreuical (i.e., the positit" ana-negativc cxc'rsions of the enverope,s ampri-Td"-T 1*t), rhen percent modulation can ue-ocaveo as folrows rir*,, ili* 4-s fshe followirg dcrivation) ;

M = *x l00orsimptyar x lfr)

IE:t(V*-y,,i,) (,1-5)

+V-rt"+E- +V,rarE.-f.

-Vm--E"+E^

-v;'-9.-5-FAJRE {-5 Modrlistion coeflicierrt, F*, anrl fc

CrrbrpfrrrF lldbrt.ttblnqrandc

.od

Thcreforc, Ma-|rr.+Y;)l/4v* - v"r.)= Ia6;+161**ffit*'*

({-6)

(s7)where V* * E. + E-V :F -F|m ec e^

lb pcali chflrsc in tbe anplitrdc ddre ouput wave (fJ is rhc sum of &e voitages honr&e upperand lclrroskle teqrrrrcies. Therefcc, $ire r'n * f.* * Euand f'x = Era,, drnE6= E6=!t=ry= |tn* - r;) {+E)

wherc Ea, - peak anplinrdc of tbe upper ri& firqrrcncy (votts)f,6 = pcak arplitudc of tlrc lower si& @ucwy (volts)Fmm Eguation 4*1, ir can bc seen thtt the pcrcent modulation goes to 100% whenE- = E..Ttb coodtion is showo in Figurc 4-&t. It can also be sccts rhsr at 100% modula-tiotu ttc qirrmum unplihdc of thc cnvclopc Y,oi, = 0 v. Figrrc 4{c $ows r 50d6 modr-

ht

lt' ,- - a.r,lr.

i,"Jr.

E-tEc

FlBt"sl 4"6 Peruent moduleum d m AM GtsFC envelope: lal rnodulabng siqnel: {b! unmo$r:lar;ai canr*.; tel 505:rtoduleed rmw: (dl 1ffi?6 rnaduicr+s wave


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lrtd corclqci &c pa* +*r in rb qfu& of rb cwtqc L osrr nebtrrb@ir* of 6a mmoailcd srw. Tbc oedann pi$ald "ro&r-q 6* cs bc hrpocd withnrt eurciry cxcarciw dfumdon b 100s. soreor'a nswl uoftla*n b cx-pt"r$ al $ ncat chansc $ tbc volrrgs of ihe modulard rnrw wlt rlfpccr ro ltc pcaksllplitrrde of tb undulardd carricr (i.e., pcrcnt changc = AEJE. x lUD.Erampn aCFor ftc.AM *$form rbc.! in f{urre +7, daenire- Fcrt rnplindc of 6o rrycr rod h*cr sftto fioElcrrcice.b. kaf ssnplitrdr of 6e unrno&rl*d carris.e. hat cbrnge in thc rnplinrdc drbc cnvclopc.d. Cocl6ciotof drhrioa.a Ferocntdul,*imSolu{*rn r. FronEqrurion4-8.

E'-* E6= ftrt - 2) * tvr.*jtrt+2)etQyf,.=i(ls-2)*8v

L FbooEqnrioo+6

c. Itom Equfio *5,

FIGUFE 4-7 AM enrre&:pa lor Erampae $p

vrt'2 Vr

r'*ttJ = E. sin{lrLr) * +/.)rl +

d. FooBqdm+1,

a homEqutioo4-f,rad frmEgtioo+7,

,lr'* * o.sild=0.EXtfi)=80%

r*iHxroo=Bo%4 AMVbhqp Oias{rutionAn unmodulatcd carricr cau bc darcribcd mathematically as

v.(r) * f,.sio (2*fi) (&$)rrtcrc y"(0 = timc-vrrying voftagc wavcforrn for tbc csderf" .r peakcrrkr srylihdc (vohl), .1, * cu*rfuqucry (bstz)In a plrkxrs raioo. it wg pointcd ogt rhat th rcpctitiou ra6 of an AM cnvclopc ico$sl b Sc ftE$Frcy of &c mo&liling signal, the amptiMc of &c AIvl wavc vrics pra,porticd !o tb esrplitdc of tbc modul*ing sigrral, sDd tbc rnaximum anplitr* of lhmodilold Ylvc is cqual to 4 + E-. Thcrefore, tbc instrntrncous anplittdc of the rnodu-

hrcd wavc can bc cxprcssod a*y-(r) : [E + g_ sin(2nfi]llsirdzxflll (4-r0]shcle [4 + E'- sin{2r"r,J] = amplinrdc of tlre modulatpd wave8- = pcr& chansc itr &c amplitudc of tlx envelope {voltr)J! = fteguercy of tbc nndrlating simal (hcrtz)ffm.8" is substitutcd for E-

v-(r) = [(4 + ,trf,q ein(2tl-l)][sin(2*f$twhclc (8. I mE" sin(2xf*r)J equals the amplitu& of the modulared wave.Factoring E, frorn &luation 4-l t and rcarranging givesu*(f) = [l + m sin(?r/;)][f," sin(2nl.r)l (4-12)

wbrp [ +msin(2nfil)J = ronstatrt + nrodulatingsignal[8" sin(lzf,r)] = unmodulated c&rrierIn F4uation 4-l2,il Eaa be seen that tbe modulating signal contains a consranr componsnr(l) ard r sinusoidal component ar the moduiating signal frequency (ar sinflnfu]]. Ths fol-lowing analysis will show how the constant cornponcnt prcduce* the carrier comF)nent inthc modulated wave and the sinusoidal component produces the side frequencies. Multr-plying out Eguation 4-l I or c yrelds

r'",,i(r) = l', sin1:nl.r) * lmE. sin(2nfir)l[sin(21tf.t]1 r.t-HI

-"L,}tl tJ'ldiTheretore,

(&il)

mE^-;*cos[Zn(f,mE..-;-cosl2r(/"


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Fnqurctlxrl mJE 4 \brEgsgpsctrrrrI brsrA[4GSCmrp

where E" sin(?;ltfJ) = carrier signal (volts)*(nEt2)cosI?tr (f.. + fstl = upper sld neqrmcy signat (volts)+{nfrhos[2r (f" * f)tr = rower sidc frequ"ocy rigrar (vorrs]severat interesting cha*caristics about doubre.sidcbrnd frrn*rnisr ampritu&odulation can bc pqhtGd o.il from Equatioa + r+. pinr, nok *_d **ru of the car_ierafter moduration isthe same as it was bcfore modur*ioo (EJ rtrcrir. oc amplitudcf the carrier is unaffectd b1 thc ryodutati";e-;;: Sccon4 rrh aryliado of tu upcrnd lower sidc frcqucncics depen& oo uott o" o,,'* anplinrdc strd &c cocfficicat ofodurarion- For r 00% modurarioo, 1,, * r, *o o"-.*pritrdcc of thc uppcr cnd lower sidct,equencics are crch equrl to onc_batf * "*Or** of thc carrier (El). Therefore, at00% modulation,

andYt*) ={+V{,oio) * 4 *

*"**u"E.Et - T = 0vFrum tle relarioashipc *rorn above rnd using &rutisr +r4, it is evidcat drag e*long as we do not exceed rm* npdolciou n" or*ri*our peat amplitrede of sn AM cnvc-oPe Vr*r = 28- and dre :plrnun pcak ir*i*t tlf * AM cnvclope Vrmiar o 0 v. Thirelationship was shown infi$ +6* ng; +slh"ws the vorrage sp'ctnim for an Ai{SBFC wave (norc that {li rhc volag*ali giu"o'ii'peat valucsy"Aiso, from Equation 4-14, ttrc tratveirase reretionship betweco tbc carrier and rFplrr end rower side frequencies is evident. The ""..ia, "o*ponenr is a * sine funcdor''e upper side frequency a - cosine functio", *a tJro*er siJe fiequency a + cosine func-tion' Als',

'neenvelope is a repedtive waveform. Tbus, ar the beginning of esch cycrc ofthe envelop, the ca*ier is 90' out of pha* *lrri.irrt *r" upper and rower side frequencie*.and the upper and iower side frequencies u.* iio" *, orphase with each odrrr. This pha*crelationship ran be seen in Figure 4_9 for i ; ;-5ff|*a; = 5 Hz.Erample 4-3

orc input ro a conve.tioilal AM mcduramr is a Stxrkr{z carricr with an arnp!.itu{e of l0 vo. The scc,ond inpu is a l0-kliz moririlaiing sigr*rl drar i, oirorfr.i,pnt wave of X7.3 Vo. n"*r*ioa nf amplitrr'lc lo carrs a chorge in dra' rruf-r Uppcr an.! lower sirir ticquencie s.I Modulation crrfficient a*d percenr modulatjon.c. PeaI amptirude of dre *:,",,S:*T oJ* upper anrl lrwer eide herloency 'otages.. Marimun and minimum iunpiitrdcs "f rfr" _"*1"[.' 'Chcptar 4

'if,llhr,.r- r'-,t - i00 x AJ7S = 3-t.S%

tfioatrr*f

.&,j,J;;JIe,-:l;,p$$

.il:,'l

;'1,I n0.tlltrrrnJ*FretS +g Generatirr d s,Atvt m,Fc eruebpe o,{rin h s,6 ,,,.6 doruin:el -t co


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DV, e.ht hl|fitr:fHgn

flELnG il.f O Orn* apeclrrrn torE6mdo4.S

-0v

FTGUR il-11 At\,t nvdops for Exarnple rll

c. The peak arnplitude of ttrc modulrted carricr and thc ugpcr aod lowcr sidc ftEqucsirs is'.(modulatcd) = Q(unmodulatcd) = 20 Vp

Es = E,.1 = &'=E+(il} = 3.75 vpd. The maximum and minimum ampliardes of thc cnvclopc arc dctcnnirod as fo[orve:Vr*r - Ee + Ed= 20 + 7.5 : 275Yo

Vr,u"r = E" * En = 2O - 7.5 = 12.5 Vpe. The expression for tlre modulated wavc folloys tlrc format of Equation zl-14:v^(r) = 2Osin(2d00tr) - 3.75 cos(2r5t0tr) + 3.7J c$s(2rt4g0tr|f. The ouput sf,e{trum is shown in Figurc,t-10.& The mrrlulared envclope is shown in Figuc +ll.dt AM Time0omeih AnaitsisFigure 4-9 shows how an AM DSBFC envclope is pr.rduced from rhe atgekaic addition oftlrs wavetbrms for the carrier and the upper and tower side fiequencies. For si:rrplicity, tlrcfollorving wavetbrms are used for the modulating and carrier inprt signals:

*ttt+0.15t+olffi-&{s-ortit+oJ-0.ttt-0.{ct+g{ct+Oltt-0J+o155+0.{:t-0.,W-or55+ot-{u5t-o.a&t+O.{I'+0.t5:t-0J+o.l5t+0.406*0.,1{E+o.{qt+0.15,!i

+O.5+0.ts5-0.a{E-{1..$t+0.15i+0-5+0.r5J-0.,l(It-oatlt+0.t55+0.5+0.155-0.rot-o.{xt+0.1J5+0J+0.t55-0.40at-0.{6+0.t55+0.5+0.t55-0.{05."0.4{xt-0.405+o.t55

0+ l.3l0-1.il0

20-t.fi0+ l.3l0*0.600+0.t9000+0.190

-0.600+ I.3l(l* t.Er0+ l.3l

0lo2A30d0v)60?0to90

100lt0120r30t4{)150r60170180r902cNr0270230240250

0+lo*lo+l0-l0+l0-l0+l0-l0+l0-t0+l0-l0+l

cafrier = Y"{t) = f" sin(2tt2st}modulating signal = y-{l) = f- $in(2*5r)

@t't(-t6t

in l0-ms intcrvals" arc substituted into Equation 4-17, The unmodulated carrier vohagrE = I V" aod 100% rdulartion is achicved. The corresponding waveforms are shown i!Figurc 4-9. Note that the maximum envelopc voltage is 2 V (2f.) and that the minimur;r cnvelopc voltage is 0 V.ln Figure 4-9, note thar dre dmc betwcea.similar zero crossings within the envelopisconstant(i.c.,71 =72= 13,andsoon).Alsonote thattheamplitudcsof successive pcrk,within &c envelope are not egud. This indicates that a cyclc within the envelope is nct ipure sine wave and, thus, the mo


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.rg l ,-ir.l- j j,i!f ,, :1rhc P. = crnrrportr(rs,f,. = pedrcarirrctqgliy*) lX - MruirEn*tolar;,, :', _llhc rypcr ud burw_sidcband pdrs! sG crtrtlicd nrtbcnatically sr*tcrc nr'l2 is hc pcak vorragr : *;ffi *;* Rearranging yierdr=r.*# (l-re)whcrc P6 = uppcrskhbrdpocrcr(vrm)Put - lowcr sidcbo{d potv(r*ts}Rcarranghg Equrtion,1. 19 gives

Substituting Eguation +lg fu,ro Equari$ &19 givcsP"o=P*=*

It is cvidcnt fro' Equadon {-2r rtar for a rnoduration coefficienr m : o,the powerin tlre upper and lower sidebads is zcm,8d the ,orur *n orio"t po,o.. i* sinipty the car-ricr power.The to..l power in an amplinxtc-nodulatcd wave is oqual to tlrc sum of the powersof the carrier, the uppcr sidebad" and &c rower sideband. Ivtathematicaily, the totar powcrin an AM DSBFC envelope isP,=Pr+P,!b+p,"bwhere P, : totat powcr of an AM DSBFC envelope (watts)P" = carrier power (watb)Po"u = upper sideband powcr (.worr)P,* = lowersidebond pfi,cr(rd4

Subetituting tguarion 4-21 into Egration 4-22 yictds

Combining terms gives

Pr.= P, - +whGrc (a2p"yJ ic the rotal ridehrdpowr.Facnring P. gives us a=a(r.il)

' Frorn thc pnieding analysir, ir can bc scca thst tie carrier pover in thc modularcd*rvo ir rhe srmc as the carricr po*,cr tn oc urno*rrarca wlvc. Tbus, it is evident that the

Po=PH=f{*}

f,*P..+.+

5!lfrfnctq0t, ffi.nE +1e punr spectru{n for an Ail! t}SgFCnave uiJr a *lglc.fuequncy rpa*Urq *gn6l

t00=lo =t*

powcr of thc crria is unaffectod by tbc rdslrtio prces* Also, becausc rhe toral po*cin lhs AM wave is tbc sum of thc carrier urd sidcbend powerl, ttre total power in an {lcavcJopc incrcrs with modulation (i.c., as ra ircrcascs, p, incrcascs).ngu! $12 rbows thc powcr spectrum for rn AM DsBFc wave. N,te that witl100% spdutation thc maximum power in the up,per or lower sideband is equal ro only oncfourtb thepowcr in the carrier. Thus, the raximum tota! sideband po*"ri, equai to r:nehalf thc carricr powcr. onc of the most cignificant disdvaatages of AM DSBFC rransmissitrn is the fact that tlrc information is contained in the sideband although most of the pos,eris wasted in the carrier. Acnrally, the power in the canier is not touly wasted because idoes dl'o$' for thd usc of rclatively simple, inexpensive dcmodulator circuin in the recei verwhich is tlrc predominanr advantage of AM DSBFC.

(4-2t)

(4-221

\4-23)

(4-21'

(4-2s)

Erample 44For'an AM DSBFC wavc with a peak unmodulatcd carrier voltage /" - l0 Vo, a load resistanccRr = l0 (1, and a modulation coeflicient m * I, detcrmrnea. Powcn of thc carricr rnd thc uppcr md lower si&bands.b. Total sidcband power.c. Tntal lnwerof tlt modulated wave.. Thcnd. Draw lhc pourer sprtrum.e. Rcpeat stcpe tr) through (d) fir a modutation index n : 0.5"Solution a. Ttrc canier powq is found by substituting into Equati


9/23

!!

rI5a

Ltlr*rqurttqy ltt0

LF f. f,rFrrt|rrq gtrl

FEURF rL13 Power gpctrurn for Exafiph 4-ld

FIGURE 4-14 Porrwr spactrum ftr Ersrnpl 4-4d. Th Fowcr spctrum is shown ir Figure +ll.e. the carrier p*wcr is found by subsrituting irrto Equaricn .1- l8;

P"=-lr!:4=ru.,,' 2( l0) 2cThe uppei ar:d iower sideb:rnd power is iound try sulrsrituting into Equatirn .t,J !

{0.5 )2f5 )P*,0 - Poo = -- = 031?5 W

-, m:p. (0.5 )r{i rl'q = --i- -. 0.615 Wlhe total sideband niwrr is

Th* t*ra! power of the niodulated wave is found by rubstiruring inio Equalitrn J-2-5P'= 5.625 W

Thc por+er spectrurn is shr:wn in Figure 4*14.Frr:m E,rample 4-"i, ir cnn b seen why il is ifirFcrtnnt ru use as high a percentagcol modulation es pxrssiblt while still being sure uot to ovenncldriliirl. As the e.runrl;lcshows, the carrier power remains tir srme as n changes. H*wever, rhe sideban


10/23

: . ..t ,:.wher ,il, - barldcfu*of modulatirnlal1, nt2, n4s, 6nd no = cocffifuts of moerktim for inprr rig[alB t, {3, md rThc co*nbincd cocfficic'ntof m&rladmcan.bc urcd to&rsmfuE tttc totat sidobsdand transmit powers ss follows:h fu fiad eJcrm db nd Fl8 u,hcrt tb cgrirr sigtal b st ie maximum ampliu* an4thw,rqdtlsrnur*tUftcrrnpliu*rroerhtingtignal brchiertarcasonabbpercentmod-ulalbu. Wltb higtFbrcl modrl*ion, ftc nnat modulrtiry signal amplifier mut supply all tttesi&band powcr, which @td bc as rurch a.s 33% of dtc toal tran$mit powcr. An obvious disad-vn*ageof low-lcvd rnodul*ion is in higb-poureralplications when all tln amplifim thet fol-low rlte rnnduhtsstrgomustbe limam$inen which isexrerrly incfficienr4-$l tew"l-td AM ModulatorA smal sigltst, chcs A amplifier, such as thc orc *own in Figurc :1- l 5a, can bc used to per-form amplitudc modulatioq howcver, tbc rrnplifier nust have two inputs: orrc for the car-rier signal urd drc rsrd for the modulating sigrrat. ltfrth no rnodulating signal prcsent, thecircuit opcratrs as a tincar clasr A amplifier, aod drc uput is simply tlrc carrier amplificdby the quicsr:cnt voh&gc gain. However, wlpn a dulnting rignal is applied, the aruplifrcropcrttes nonlineadn anrl $i8n8l multipticalioa as doscfibed by Equ4ion 4-10 occurs- InFigrne ,1-t5a fic csris is applied to tlre bass srd tltc modulating signal to the emitter.Thercfsra, &is circuit configuration is cdld emitter npdubtion. The moduladng signalvaries the gain of the rrnplifier at a sinusoidal rnte egual to the frequency of the no,"lulat-ing signal. Tbe depth of modulation achieved is proportional to the amplitude of the mod-ulating signal. "fhe voltage gain for an emitrcr modulator is expre$sed mathematicaily asA,-Aqll+msin(zrfu)l (4-34)where A, = arnplificr voltage gain with moduletion (unittess)A" = amplificr quicsccnt (without modulation) voltage gain (unitless)Sin(2rJ"l) gocs from a nraximum value of + I to s minimum value of - l. rfhus, Equation4-35 reduces to

A,, == Ar(l t n) (.1"351where ra equals the modulation coeffrcient. At l0O% nroclulation, m = l, and F-quatii:n {-35 rcduces to

.{,a,n*, = 1{o'\r,^,n,- 0

Frgurc 4-t5b shows the waveforms frrr tlre circuit shown in Rgutc 4-15a. The mtxJulat-ing signal is appiied through isolation transfirrmer fr to the emitter r.rf Qs, and the currier rs lpplie"J diru"tly ur rhe base. The modularing signall drives dre circuil into boih saturdtion and cut-off, rhus pnducing the nonlinear amplification necessary tbr modulation li.r r:ccur. The cal!ect


11/23

Ycr. !o!*

Collistotr,ohga(16l

FHrnE +16 AtU arwotoo for&grpb a,6Thcnc" Skr*ch tbc antputAM cnvctopc.8du&m r. Suhtituting inro Equatioo 4-3.A*=100(l+0.8)=lS6b A-- = l0(l - 0.S; = 26o(nu) * lq0.fl)s) = 0.9 VYorlnia) = 2q0.m5) = 0.t Vc. Thc AM cnvclopc ia shown in Figurc 4- I 6.4Se Metliunrpower AM ModulatorEarly mcdium- and high-power AM transminers werc limitd to those that us*ci vacuumtubes for thc active dcvices. However, since thc mid-1970$, sotid-state transrnitters havebeen available with output powers as high as several rhousand *"* rh. i, ".comprishedy placing several final power arnprifiers in parailel .".h ,hJrt;; "",orirL,*,, combinein phase and are, thus, additive.Figure 4- l7a shows thc schematic diagrarn for a singlc-transistor metliurn-gxrwer Alulmodulator.'[he moduration takes place in tie colrector, *ti"t is the output ,I.*"nr ur rnutransistor' Thereforc, if this is the final acrive stage of the transmitter (i.e., rhere are *. ini-lifien between ir ancl the antenna), ir is a high-[vel modulator.To achieve high power e'iciency, *Ju-- ard high-p


12/23

llcn.!oV*

FEtnE .L16 Atll anvrtope forFrsrph a

Tb.oc. Skach &c qnpntAM envclopc.Sc*utbn r. Subotituting into Equation 4-3.h

A*=100(l+0.E)=l8OA.-=100(l-0.E1=29o(ru) = lt0(0.m5) = 0.9 VVq(m6)-2Q0.m5)=0.1V

&lFq.E 415 {al SirFe tnrft*dr, en*tter rrrodutstor: {b} output,^row'orrfi

c" Thc AM cnvclope is shown in Figurc 4- I 6.+3-2 Mediumponnr AM ModulatorEarly rnedium- and high-power AM ransmincrs were litnitpd to those that usecJ vacuumtubes for the active devices. However, sirrce thc mid_lgOs, solid_state transminer.s havebeen available with ouput r'wers as high as several thousad watts. This is accomplishe*by placing severar finar power amprifiers in pararer ru.n trrut rt"i. *r0", ,L,r", c.mbincin phase and a-re, thus, atlditive;Figure 4- l7a shows the schematic diagram for a single-transistor mediurn-trxrwer.\i\,tmodulator' The modulation takes place in the collector, ,"rricr, ls the output e irnen{ of rherransistor' Thereforc. if this is the final active srage of the transmitler (i.e., rher" urr n{r .ir!r-plifiers between ir and the anrenna), ir is a high_[vel modulator.achieve high power elliciency, *.diu*- and high-power AM modurerors sener-ally operate ctass c. Thercf3re, a praclicar efficiency of asliir as ror i, ;;il;. ffi:;"uit shown in Figure 4-l7a is a crass c amplilier with two inputs: a carier (v.) and a ringre-fieq-uency mtxtulating signal (r,, ). Because thc transistor is biased "la*'C, ir op1,pn1e5nonlinear and is capable of nonlinear nrixing (modularion). This circuir is called a tullerktrmodalaturbexause the nrodulating signal is appta directly fo thc collector. The RFC is aradio-frequency choke rhar acts as a shorr ro dc and an op"rto t [i f,"q;;;;;.:. Therefirre.T.f9 M*"s thc dc pnwcr suppry from ttrc higb-frequency "irri*, *o .i.t t ".1u"n"i.,hile still allowing the row-frcqueniy intertigerJe signars to modurate the coilect*r of g,." +&i. t cirruit opration. For dre following erpranatinn, refer to dre circuirshown in Figure 4-r7a and the waveforms shown in.Figurr-&trb. when rhe ampriru+lc .rthe carrier exceeds the barrier potentiai of ttw barc-*rniccr junction lupp-*.*u,"r.u o. i vfor a siticon transistor), Q, turns *n, and coileetor current flows. wh-; th; ampliturlc ,.,l.rhccarrier drops berow 0.7 V, Qr tums off, ana collecror cufrlcot ce.iues. consequcnriv, Q,witches between eaturstion and cutoff conuullql by the carrier signal, collector c'rr*rrllows ibr iess thin 180" ofeach carrier cycle, ancl class c opcration is achievctl. Hucrr suc,cessive cycle of thc carrier tums Q1 on firr an instant anrl altows current to flow trir r sh.ntime' protlucing a negative'grring wavefbrm ar the c'llecror. The c'llecror cur(,nt iind ri)lt-age wavelonn.\ arc stxrwn in liifure .t-17h. 'I]tr (.ollcrrnr v.h*ar *".ri&i"a..Jie,rrtle. A".fr , :, hatt-,rove rrc,{tfied clq"oi -

Crt cbrtonrgr{16l

tio.ftrbtng dgni{rrl

A$ osSFClmnbp.P*l--L-_


13/23

V, r Yg6Vr t YccMtr"lgbtqncrralldn9-t* ?1 r t;l

-- o.rvOV

btd mfliuftpci,r transistor *l .ry11mod{t8tor:.(al sctremstic diagrsn; {bJ coilemr naawfofmsl; [cJ eo{ecbr vrerrtrrrns,.,tt u,rnO1ialng-ffi'" "

wren a modurnting signal is appried to rhc collcctof in scrics with 6c & supply volr-gc' it ds to atrd suhac*8 fu v.t rrr *"""r* shorrn in Fig'oe 4-l?c are ptoducedhen fts maximum * s{s signat unpriauc cqualr v".- Ir can be seen *'at rheutput vortage waveform swing' fron " or"*i-uo, ydry of zvcc toapprroximatcly 0 vt'@r.dJ. The pcat *191 l:ry* *ft"gl, "g*l m 1,.._ Again, tbe waveform rcsem-les a harf-wavercc*'ficd crnissupua-po**a *rro a tow-&'Jncy * ioaurg"* rignul.ecausp p1 ir opcratiag nonlincar,'oc *rit*. wavefonn conrains G *o originalnput fteqrrcrrcics (f, aac,[.] anu rclr r,-;;;;;;;#ffi:.t . *"u." *"rutput wavefqm also coatdnr bE higr*-o.d;rrarmonics od int"hJ,Iuti* compc.enrs, it must bc brdlinitod o/. tf,U"for U"_g tra$smitcd.

Qrt:t.ffi16)

ffirnE +18 trlerfrrrpo|g" translctor At\' SAFC modulator: (el schernatic*rgun ttutttd)Amrp practicel circuit for producing a mediurn-powereU Off[ signal is shown

in Frgure 4-1Ea *n "*rop"Jog *u".fit*. shown in Figure 4-18b. This circuir is alsoa collcctor mdulator *ilh; ;;;"m Peak moduladng signal amplitude,v-i*^; = v66'irp.*,iri"r,ne circuit iralmost identicat ro the circuit shown in Figure 4-l7a except fortbcadditionof atanf circuiiiC,"oaf'r)inttrecollecrorof Q'' Because thetransistorisop.(ttirybctlvoensanlr$iorsodcutofr,collectorcurrcntisnotdependeatonbasedrivevolt.agc.Tbcvolragedcvclopcd-acrosstbetankcircuitisdcrcrmincdbytbcsccomponentofthecol*.a currrnt and thc;p"d-". of the tank circuit at resonance' which depends on thequality frctu (g) of tbc ;. Tb" waveforms for the modulating signal, carrier" and col-lcctor crrrent uc i&nticd ; tboc'e of the previous example' The ouput voltaEe is a sym-mcqical AM DsBFc *ipur *io * """.i" voltage of 0 y a nnximum positive peak arn-ditdc '*rl ts ZVco "'f " *"*i*r. iegsCve peah amplitude equal

to -zvcc' ,\eiJ,iri"" fu'*y"r Jr t', ;S., ";;.r. i3r"a,t"o in the tank circuit bv thc flvwhccl{ccr" Wkn gr is cooductini, C, "h"'g"t to-!n-* '-(a mdximum value of 2V6'6')' andwhcn Or is off, Cr di*h4;thfughi,- Whto L1 discharges' C1 charges to a minimumvdrr cf -LVcc.rtue ffics miositi"e half-cycle of &e AM cnvelopc' The resanantft"Sr.& "f t *f iit*i it "q""f to.the cary{ @ucncy' and dre bandwidth extcndskosJl * f- to.f" + I' i-*t qt"ntln rh9 modulating signal' the harmonics' end all thchighcr.gdcrcroesprocuctsu*,*'on"ofromlhcwavcforrqleavinglsymrrreuicalAMDsBnC wavc. Onc nooot"a p"ront rnodulation occurs whn ttp pcsk arnplitudc of thcno&rl*lng siPal cquals Y6'6'

rlrlrltl

llrlllrlrlrl-l'ii-- **ilt vcc

vcrra;ovvcrvcr:0v

IIIIII


14/23

vr.t:

{ Ycc

&afthg lnro fu pounr aryfly s thc moduh,ting signal circuitry and being dirtributcdtksnslort ltc uarrnirm.tf-{l}{l Snrdtarmrrs 8s36 ard Go{ector ltllodul*ionCdbaor modulalors pfofucc a rnore rymmctrical envclope tban low-powcr emitter mod-uldora rnd colhcfor nodulatons rrc mort powcr efticienl Howevcr, collector rrodulatorsmryirt a highs uuglitdc-modulating signal, and thcy canno{achieve a full saturation-tocutoffmput vdtagc swing, thus prevcnting 10096 modulation from occurring. Thercforc,b rchicvc aymrnFJtrial modulatios, opcrrtc at maximum cffrcicncy, dcvelop a high outputpowcr, rnd rcquirc as linle modulating signal &ivc power as possible, cmitter and collec-rtr nodulatiotrs src c.ilneti!trcs used simulAneously..l-3.$1 Clrqdi opcnathn Figure 4-19 slrows an AM modulator that uscs a com-biutim of bo$ blsc td collctor modulations. The nrodulating signal is simultaneouslyfcd into thc collectors of the push-pull modulators (Qz Nd 0J and to the collecror of tbe&iwamdificr(Q1). Collccto'rmodulationmcun in Q,; thus, the carriersignalon the bascof gt snd 9t has alrSdy bcen panially moOulaiea, and the modulating signal power eanbc rduccd. Also, the modulstors arc not rcquired to operate over their entire operatingcurve to achieve 100% modulation.

44 UNEAR INTEGRATEDCIRCUIT AM MOOUTATORTiIincar integruted-circuit function gencratorr use a unique arrangement of transisrors andFETb to perform signal multiplication, which is a characteristic that makes them ideallysuited for generating AM waveforms. lntegrated circuits, unlike their discrete counrerparts,can precise ly match currcnt flow, amplifier voltoge gain, and temperature vlr!-iatir'rns. l-inearintcgrated-circuit AM modulators also offer excellent tiequency stability, syrnmetricalmodulation characteristics, circuit miniaturization, I'ewer cnmponents. temperature rrnrnu-nity, and simplicity of design and roubleshooting. Their tlisadvantages include low ciurputpower, a reladvely low usable ttequency range, and susceptihility to lluctuarions in rhe dcpower supply.

The XR-2206 monolithit f'utrt'tion generator is ideally surted for perti;rrning enrp{i-tude modulation. Figure 4-20a shows the block diagrarn lbr rhe XR-2206. anJ lirgule .1-?0b shows the schematic diagrarn. The XR-2206 consisls of four functional bltuks: a r olt-age-controlled oscillator {VCO). irn analog multiplier and sineshaper, a unirv-gain buffer.and a s*t of curre nt swi(ches. 'fhe VCO irequency of osciliatir:n/. is determined b1' the ex-temal timing eapacitor (C,) hctween pins,5 and 6 and by timing resistor (R,) connectedbctween either pin 7 or 8 and ground. Whether pin 7 or 8 is selectcd is rictcrminetl hr thevoltage levcl on pin 9. If pin 9 is open circuited or connected to an extemal volrage > ll V.pin 7 is selected. Ifthe voltage on pin 9 is 3 I V. pin 8 is selected. The oscillator frequencyis girrcn by

several componc'.* shown in Figurc 4-lga have not bccn expraincd. Rq is thc biasresistor for Ql. R1 and c2 form a ctamper circuit that produces a ,vers ..selFbias and, inconjunction with thc barrier potcotial of the transistor, tletermines the rurn-on volragc fo,rQ1. Consequently, e, can be biased to turn on only during ,rr" ,*t p"iiti". p.rrc ormcarrier voltage. This producu r aarros coltectc curr.nt wavcfonn rnd cnhances clsss cefficiency.c3 is a bypass capacitor that tookr litc a short to thc modulaeing sigqar @ucn-ies, prcventing the informuion sigonb from cntcring thc dc power;;;:; Cb. ii thcbase'to'collcctor junctiod .".ry*erro of e1. AtraoiJ frequenci"i, .th" firil""ry smalrjunction capacitances xrithin thc transigtor are insignificant. rroc capaciti"e rractalccof c6. is significant, thc collector signal may be rcturned to ths basc with suffcicnt an-plitudc to causc Q1 to bcgin oscillating. Tbireforc, a signal or cquat "*pu*d" and frc-quency and 180" out of pha$c nust be fed back to the base ro clcet oi neutralize tltcinterclectrode capacitawc feedback clv is a neutralizing capacitor. rts purposc is toprovide a feedbact p"rh fT: {F{ thar is equat in amplirudc ana ncquency bur lEO.out of phase rvith the signar fd back through co.- co is a RF uyp*r L]Litor. Irs pur-pooc is to icola* thc dc po,'"" rupfly from tdiJLqucncics. Its opcration is quirc sim-ilar; at thc cTrier freguency, Crloofs $ke a short circuit. p*rr*i"g ,b. o*io fn*

r = ;fH, {4-36)The output amplitude on pin ? can be rnodulated by applying o dc bias anrl a modu-lating signal to pin t. Figure 4-10r" shows the normalired output amplitude-versus-dc bias.A normalized output of I corresgrnds to maximum output voltage, a nonnalized value trf0.5 corresponds to an output voltage equal to half the marimum value, and a normalizedvalue of0 corresponds to no outpul signal. As the figurc shoivs. the output amplitude varieslincarly with input bias for voltages within +4 volts of V'12. An input voltage equal to V'/2cruses thq output amplitude to go to 0 V, and an input voltage either,4 V abr:vc or brlow

V+F mv*oec m*timrrn ntltnrlf lmnlitntle


15/23

tfl Y1Amnfr'ril.J-I$lxr$mdl-lGfi$d$Fo'ddFByFn$FTilChnt

stfl t'oEI668 0.5xE! 0

lFdi:-on.

,ry

otlDtltt lltct

\,+rlHtetI gf 'l rlu|lrl.*nL?:tr|;#f# OCw.becttpan I (v.12)(c)

FALhE.rFaO )ffiPO6: ia) Elocjt diagrarn; {b) schemaic diagrm; {cl orrsut 'roesgetssreinput voltage cunve

Figwc 4-21 shows &c lchematic diegrap fa a lioeer integrarc*cin:uit AM mtxiulatorusing the l(R-2?06. Ttlc VCO otgut @ucncy il the carriq eignal. Thc mxlulating signaland bias voltagc are ap'plicd to ttte inrcrnal muhipliq (stodulats) circuit through pin l. Themndulating signal mixes with thc VCO *ignal producing an AM wavc at V*. The output uraveio a syqgretric*lAM env*iopecontainrngtlreerrricr aml the qpaandlower sidc fraqucrlcrcs-

Le6EE!f;'6c66EJtIoIb!lFE,pI&+ltrcgtfi

Eg5i $,


16/23

FGURE rt1 Linar ;ntegufiBd*luit At\, rnodj4or

Example dt-7For an XR-2206 LIC moduratnr such as tbc onc shown in Figure 4-2r with a power suppry voltagcf.: ]2 Y-e,a modulating si8naf gnfi$c V" 1_2 V" a modutating signat ticquency !_ = 4WIza dc bias foi- = +4 V dc, riming rcsisrm x, = io frf "ia mioe *di,.; c, = dmiur. ****"a. Carrier fmquency.b. L;pper and lower sidc frequencies.Thenc. Skerch ihe ourput wa"e.d' From th output wavefonn, daermim rhe cocfficient of modulation anrl lxrcenr mqlulat cn.Solution a. The carricr frequcncy is &trmi&d from Equation 4_36:

/' = (lo koxo"oot,,'F) = loo kHzlr Tte upper and lower ride hcquenciss rrc siqly tre sur! ard differcnce frequcccies bctrveen tlrccanbr ud thc modulating rignal."{,.r = 16 kfU + 4 tHz * 104 kllz. tw* 100${r - 4kHz* 96kHze' Figsre 4-22 *hows how rnAM cnvelope is prodrred for the outErr vort"ge-versus-irir*rr volragccharaci.ristica of tbc XR-2X)6"d" The perent moduration is dctsmirrcd frfin tt* AM en'elope showa in rigure 4..22 usi'gEquation 4-7:

f*=10V, *V,*=gyr0-ot't=---.=l10*oM=lxt00=loo%

+10 V

FIGURE +22 AM eneiope lor Exemple 47+5 AM TRANSMTTTERS

Y*r 13YuI -"--*

\2V, Modulating signal input

4-5-1 LourLevel TransmittersFigure 4-23 $hows a block diagram for a low-level AM DSBFC transminer. For voice ormusic transmisrion, the source of ttrc rnodulating signal is generally an acoustical trans-ducer, such as r mimophonc, a magnetic tape, a CD, or a phonograph rerard.\\e preamp-lificr is typrcally r *nsitive, class A lincar voltage amplifier with a high input impedance .The function of thc prcamplifier is to raise the arnplitudc of the source signal to a usablelevel while pnodrrciag minimum nonlinear distonjon and adling as little thermal noise aspossible. Tbe driver fo'r the modulating sigual is also a lincar amplifier that simpl-v unpli-fies the infomution signal to an adquate level to sufficieutly drive the morlirlator. Ilorethan one &ive amplifier nray be required.The RF carrier ascillator can bc any of the oscillator configurations dixusred inCtraper 3. Thc FCC has stringent rrxluircmeats on aansmitter accuracy and stability; rhere-fore. crystal-controlled oscillirtors are thc rnost common circuits u-d. The bdla r anpli"ficr isa low-gain, high-iaprt impodance lincar amplifier. Its fuircdon is to isolatc rh oscillator fromthe high-power anplifien. Thc buffer provide* a relativcly consrant load to *re osc'rllator"which hetpc to reduc.e the occ-uncrrce and magninrde of short-term frequ,:nc;, va.ndtrtrn\.Egritter f*llowerq nr intesraftxl-cinrlit ng**nrpc :rr* nfte'n uq'd fs'th* hrfFer '$ri' nrr" i,. i:,r,,r

Vnialov

1r!3r4 nZma 13tunctfi6 ilrrrbt 12atrr .0.01 tFRr'l0 l0


17/23

*c6sc-tItgaET-pgocIb.G{xot

c'(!rdlrlt:olt

;lIg

$*

m.nE 4e4 Bbct dapam of a hqh.levd AM lElffioarurntnrcan use either emim oq collocta uo&rlation. The intermediatc asd fiaet pr:wer anplifiersareeithetlinearclassArclassB pn*h-grl rdulaton. This is reguired*'itb low-ler.el rrans-mittts t$ maiftrtit s]frmtry in dre AM envelope. The anterna cti+&ES network nratctresthe output impedance of tlc final power amplifier to the transmission ?k a*d ananna.l,ow'level transmittrrs snch ag drc onc shown in Figure 423 a:t l:ed perloruiaantlyfor low-power, low-caprity systcrns, such as wireless intercoms. 't:;*te-congo! units,pagers, and short-range waltie-talkiesIt-$2 HigtrLevol Transrnittar"BFigure 4-2.1 shows thc block diagram for a high-level AM DSBrc lracvrjrlrr. Ttr rmciular-ing signal is proccssocl in the satne manrrr as in thc low-level transmj*sf ercept far *r ad-dition of a power amplifier. With high-level transmitters, the power of '..!x *rdulurag signa!must be considerably higber than is neccssary with low-level u"nsrn nr-r: . Thr:i i s hecau;e rhecarrier is at full power at the point in the transmitter where rnodulaiil":: cccun iieJ- cr:nse-quently, requires a high-arnplituda modutating signal to produce l{ti4 rxrJelatian.The RF carrier oscillator, its associated butTer, and lhe carrier,:-i\er crrr ;r,i* es$etr-tially the same circuits used in low-level uansmitters. However, *ilh hg!:-lerrl r:r-nsinit-ten, the RF carrier undergoes additional power amplification pri,_,r rc r_b* niodui;il,.ln lrage,and the final power nmptifier is also the rnodulator. Consequently. *r* mtrduiaror r! gener-ally a drain-, plate-, or collector-modulated clats C amplifier.$/ith high-level transmitten, rhe modulator circuit has threa Frnar.v frrnct*ns. Itprovides the circuitry necessary for modulation to occur (i.e., narrllr*an{r.i. it i.{ *c i'inalpower amplifier (class c for eficiency), rnd it is a frequency upc{rnve ner. An up-cacvenersimply translates the low-@uency intelligence signals to radio-f'rcgue*cy signals rhar canbe efficiently radiated hom sn antenna ard propagated through free space.

'I TRAPEZOIDALPATTERNSTrapcztsidttl panerns are used fo obscrving the rncdulation charreterigrcs of ,{lr{ r*nsmit-ters (i.e., coefficient of rmdularion a*d modularitn symmetr,v). Aldrugh thr rns-rJularioncharactcristics can be examined with an gscillo$coFJe. a trapezoidal pnern is more c*lit andaccurately inrerpreted. Figure 4-25 shows tlre basic *st sefup for prtxlucing a rr..ipr:,.,:ilal p:rt-tern on the CRTof a standard oscilloscopc. The AM wave is applied to rlrc venreal rr:;vl ul rheoscilloscope, and tln modularing signal is applierl to ttre external horizi:ntal inpur * rrh *re in-tenral borizonral sweep disabled. Therefoe, thc horizontal swecp ralc is ,Jet*nnir*i b,_"- tlremodulating signal ftequency, and thc magni0lrde of the hmizrmtal tleflenior rs pn.r.;ryiry1;! 1sthe amplirude of thc modulating signal. The verticrl deflcrtion ir i,:r:rl'. Ji:rrc.:rJ;1r i rlr{


18/23

tlo*S.tingttqnal lntut

frc4$SiiqbaahPrr

AI'dFdrdcdhtrt

tnprroflStta.f!

FGUR 46 Producint a Fapzordal patternIf the modulation is sylnrnerical. the top half of the niodulated signal is a minor im-age ofthe bonom half, rnd n rlaperoid*l patiern, such as the one shown in Figure 4-27a, isproduced. At 100% mottulatiln, the minimum amplitude of the modulated signal is zero. and

the trapezoidal panem comes to a point at one end a-s shown in Figure 4-27b. If the modula-tion cxcceds t00%, the pa$ern shown in Figure 4-27c is Thc panern shown inFigure 4-27a is a 50% modulated wave. If the modulating signal ard drc rnodulated signalare out of phase, a Fanem simiiar to the one shown in Figun 4-27d i$ Foduced" If the mag-nitude ofthe poaitive anr! :regative alteniations ofthe rnodulated sigaal art notequal, &e pat-tem $hown in Figure *-27* results. lf ttre phase of the rnodulating signal is shiftsd l8O" (in-verted), the trapezordal pattnis wuukJ sirnply point in rlre opgxitc dircction..d$ you can see,perceot modulation and modulsti$o s,yrnnitry are mone easily wLcd with r rapezcicialpatrern than with a s,tandard oe-ill{-\scope drsplay of the modulated signal.

Catier shifi isa teim tltat is often rnisundcrstcxrd or mi*interpretcd. Crrrier shift is snnre -rimes called npwa nl or downward nodulation and has absolutely nothing to do with the i:'*-quen{.'y of.the uarrier. Clu"rier shill is a form nf amplitude distortion intmducerl *heii il}r

AM liggratircrrt

FlCYnE 4-?5 Te* sefirp b disgaying a trrspezoidal parEenn on anoscilbscopeampliarde and rale of change of &e rnodulatcd signal. In essenee, the eiecFon beam emittcdfrom the cathode of the CRT is acted on simultaneously in both the horizontal and yerticalplanes.Figure rl-26 shows how the mslulated signal and the modulating signal prcJuce atrapezoidal panern. With an oscilloscope, when 0 V is applied to th. .xtJ.nj horizontd in-put. the electron beam is centerecl horizontally on tlie CRT. When a vohage other than 0 Vis applie{ lo the vertical or honzontal inputs, the beam will deflect venicilly and horizon-I tally, respectively. If we begin with both the modulate


19/23

tnocno,.ffixro rnoctr,Ir-!.roy_+O"'@fOD

-IYr

_1

-Vo > +V,

IFES PRSOUCEO 8Y COMPLE( NONSINT.TBOIDAL SETUALS.ffii:i'::Siil::"Iffi;"fi'* #::i::T:jlTpj3 or rwo or m{,re r-requcncies.;il'h;r:r;;;:F*x"::r;i:iTJ"'.T.#:TJ,ly:J;nT:,.ilffiTt-a.lly related sine waves and i'rrrt waves -anO include sguare, , ' rrr('rs llilrm(tnl-conrplex modulating signats can arso F^h,ai- ....^ ,*tungulur' anr! rriangui:u waves.-r'-'r 'rrvesr4rtug srgtrflts-on also conlain two r'rdixu,as voice signals originaiing ;1om diffem,r "^,,----"l,To'" unrelated fre,;uencies, suchiffercnt source$, wt"n ,: --' !s trsqucrlctes' sucnile a *rir. +rr^ _-,_,, . ttgnals o*ter than prrre siner costne waves arnpliiude nrodulatc;;.;. il wnen slgnals otier rhan prrre sineand lowcr sitr'ehsn.i ir".]u.n",u* corrmens'rnrp .,,,,I"1:1":_o 11vel9pc conuins upper

Quadrature ampritude maduration is a fcrm of ampritude modurarion where signalsium l'*o separdte inlbrmadon sources fi.e., tw., lfrannels) modulare the:,arnc crrrrcrrequency ar the ssr$e time withour lnterrtring-ollieach other. The informadon rourceso'dulate the sarne carricr after it has.been duJ; into two carrier signars rhar are 90o::[i,$:-- with each other' ?his schcle i*;;;";"t"* calred sncdraiiiilh(euAMA.:implifiedblrrkdiagramofaquadrarure.4MmrrrdularorirshuwninFigurul iii.i\r the tigure sho*s. rhere is a q;,!,,r*..*i.,, ;";,il.C;;ha;1'";Uf.cJ. ".n-phqss. ,

TIl/,IIt--l't,frKrffi:if 'ffi j;*ffi#,Wrn on; rcr,no,r rhan

[:'i"::' il#:il:;it.;il::fiil:ff#,frf*,r signar are no, equa, (i e , nonsym.nation of the modulated sien;;;;#,:":::,:p.os,:ve or negative. If rhe positive atrcr_carrier shift resurrs. ,,' *Jfl1.lT: largeramptitude rhan rhe ne'gati;;il;fi*. posiriveshift occurs. u.J negauve alcrnation is la4ger n* ,r,Jp.rr,ri.ln[luu. "*",. . Carri*r shift is an indication of fte averaFe vollositive and negalive rnr*, Jil. #f;;-5ff-1o,l18e of an AM mrdulareci signaj. Iithe*il1,ru|'", {trTi.ffi -'ffi ,t;*:l*;#lll,il:..:***$Li;:;.;d;:;";;:-##,'fl #ffi "'#m,TX;*"f*iffi

FIGURE r}pg Carrier shdt:_[a] tinear. modutebon; fbj posjuv carrrarshft; (cJ neganc cmer shilt;il};:|.:Hrhe shape of the envelope rerembles &e shape of the originai modu-

Figure4-29 shows threecompiex modurating siguars andtheir rcspoctiveAM enveropes.'I.9 oUAoRATuR AMPIITuT}E MooUT.aTIoN

an d r ow c r s i,r'e hs nd r'" q ". n"i i* ". 'o*,i"'nr,ifi Tifl,fl:rri il:f tr ;:",T$: ,l?ffi


20/23

oAllOt{tr/t

Odlamslhhrmafnn

oBCano.d- modab,RGURE +3CI cuadr*rc AM: tal rr**Sl,o, **a.lot''

(.)

oBannceOtrodadabrv{ml -------

v{aml

GUR 4S A$d e/it$ srrnp{* ft6cl*:{errs s;,-*-r .-.1 !-: -,


21/23

earricr to rh l.mfut$or and 6co &ns *! *Fo O nd rupplia r rccmd qurd-arurccrrrier ro rhc ''modutllg, ru *tp",, ft"- rlc two;;;r";"r [ncrrry:}ffi}tr uodcrgoing Edditi*at r"r* 'i t"n""*r ;o*'o,fr;iJo and powerFigu a-30b -*::,-.Tg*ed bloct diagnam for a quadrarunrAM dcmdutatorAs ttrc figue shows, demodututir.g q-"at-** iu ,isn*r$ renroirc*-;;* ,ccoycryircuit rq rcproducc tu :1s".*r *fo;; turd;r;aad ptrase aud two bal&ccd modura-o* to *tualy dcoodutatc the signnrs. ttir ir ""ma ry"rar"** t#u","od rhrcocss makes denodulating.quadrarure AM signals qui " ""p"*i*" *u, c'mpar'dirh convcntionat AM dcmoOulator "io.ulrr. e.",morecomprexn-"*""olii.Jffi ffi 'J:i:iffi "Xf ff ffi #llrH:ately thc sanre-quarity 1"-1"r"r10!i-r"r. n" u--y sdvanrsgc, howe"r, of quad-arurc AM is conscn'atiou of bandwidri. a""d*'lr* AM requires only harf as muchandwidth as coavcntionar.ly'yd *o r"ir*.banoers can rnodularc dre sarne car-ier. QAM ir somstitncs caila pnasc-aiviiiiltiprcxing andwas oo. of ttre modura-ion hcbniqucs considered for '-"*"- u*"i"*t"s of AM signats. For now. quadrarureAM is rhs moduration scheme useo ror ;;;g coror signals in anarog rerevisionroadcasting systenu.Today' quadrature AM is uscd almost excrusively for digitar modulation of anarogarriers in data modems to.go.1v-e' oua *,-"grr-the pubric telephone network. Quad-ature AM is also used for digiral sarelrirc "o*i-*r"u,io* il;.;;;quadrature;f;}iflglsynchronous derecrion * *pi", .o"ercd ir more o"oii i, 1"t", .h"p-

*l,l Wbacftadoor ffid*abrre oa 0rc r4litodc of eG csrricrcompodmr of fu no&rhraddgdryoemmf+lt Dr$ribc 6r dgfrfrcrre of dro following formula:r,=e(r.f;)

{.lG Wh.r ds,{tl I\SDfC *rnd for?+17. D?$dbG tbc rctetioarlip betwecn the carricr and sidcband porrcn in rn AM DSBFCSNYC.+r&tl-19.+torl.2l.

Whar is eo pr*minant dicdva$tage of AM DSBrc?Wh* ie fu prcdominanl rdvNrtrye of AM DSBrc?Wh.t b thc F.iE rt dissdvsntste of low-lcvcl AM?slh; & eny anplifien thgt follow the modulsor circuit in atr AM DsBFc tr'trs?nitr.r hlve lobc lie.s?*fa Dcscribc &d diffarnccs brwcen lrrrc- ud high-level ndslaan.

'l'23. Lisa ll'e dvantagca of lo\p-tcver moduration and high-lever rnoduration"il'2iL what rrc tic rdvEntages of using &near-in rcgrttcd circui, ,rlodrrarrr for AM?+2s wb.t i0 tl* rdvaotage of us ng r tmpezaidal panern to evaruate an AM cnvcrope ?

Define amp i t ude m


22/23

4ll. Fo.6c AM cotidoF {orrl tcfoq, a.-;rcr, Featarndiur&of{b' Pcrt snptirr& "a# m-d br'trddc fitgcncinr'c.. Peat changc itr rhc uptiurdc of tbc [email protected]. Modulation crcfrcicnt. .e. pcrceot modulatioo.

+12' oneinputtoanAMDSBFCnrcdurar


23/23

i. Midoupodirc**h;;il.!C St rch rb cnvc@ turprobkr +fJ.* ff;ft:ffi'ffi F* vohsr o{iiv.oa. niuin- pcrr{crar ,,ortr. krccnt oodrladon, 'L &al voltagcc of thc canicr ud ri& &uqucncies.c Maximum posiri"e cnvclopc *olugc.--'-tty'*e'Minimun positi"" *".top",*fr "sa' ooc inpr{ ro rnAM DS'FCmo*:aroris r 50euL go-*F "p*t anplih{c of 32 y. Thcccord hput is r l2'rctlz rrpruhtinr "rco.t rG ao,prihrrc ir ruffcicrrt to mbange h rhe amptinrdc of ,t'" *";;;;;;;; thc foltouir3: ducc r tit-.vor. Upperend lowerri& Acqreocicsb,- Jr{odulation coemcicnt arg perocot noeddi6-c. Mrxtunun and nnirimum ampti*d"";;;;r"p".Tfrcrrd. &aw fu ouerut cnvelopc._ c. Draw tb sutput ftcqucncy $poctrum,4'2f' For a modulation cocfficient of-o.q rra n .*lr power of 4{D w, dc{cnnircI TouJsidcbandpower.b, Torat &nsmitbd power."* r;i#m wrve witb ar unrnodurarcd carrier vorragc of t$ vn and a road resisrsncee. Unmodulared carrier power.b. Modulatcd carri.r power.c. Totalsidehadpower.d. Upper aud lower sidcbanC powcrs.e. Iixal trrasmit&d power.o3l' For a low-power AM modularor with a rrodurarion coefficieot of 0.g, a quiesccnt gain of g0.nd an input carrier amplitudc "f l0 *V;:;;;;r. Maximum and minimum voluge gails.b. Marimurn ard minimum envciopc voltagcs.Thcnr. Skctch tlrc AM cnvelopc.

4-64-74-84-9

Ttapczoidal PattcrnsCarrier $hiftAlvl Envclopcs h,oduced by Complex NonsinusoiQuadnaturc Amplitude Modulation

Iillnonplitudc duhtionotcrlbe the AM cnvel'opeprribo thc AM froqueoca spectrum and bandwidth,{Dhln thc phasor repesentation of a.n AM wave'dlr| aod crplain the following tcrrns; cuficient af madiation ard percent mdulationtrr AM voluge ditibution,t$mAM in thc tinrc domaintflvr Atrl pwcr distributiortilfH! AM cwrcnt cakulations

haltlba AM with a complex iafirrmatinn *ig*alAM nrodulaor circuits and thc difference between loir- and higb-level modulatir-rnlincar-integraled circuit AM nodularorsAM uansmitte'rs and tlle dillerence betrl,

principles of communcation

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