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Nguyn Thnh Chung

Lp :13B.KTTT.KH

SHHV: CB130532

Bo co paper mn phn tch thit khthng vin thng

Bi bo co da trn bi bo Understanding the sampling process By R.N.Mutagi, cng bthng 9 nm 2004.

Bi bo xoay quanh qu trnh ly mu l mt trong bn bc qu trnh bin iADC (analog digital convert ) gm lc,ly mu gimu, lng tha, m ha nh(hnh 1). u tin tn hiu a qua blc thng thp ( cho tn hiu tn sthp iqua), u ra blc l tn hiu tng tc di thng hn ch. Tip tc ri rc hatn hiu thu c vmt thi gian chnh l tin hnh qu trnh ly mu,ktip rirc ha tn hiu sau khi ly mu vmt bin chnh l qu trnh lng tha.Saulng tha tn hiu cng coi l tn hiu snhng vn l tn hiu thay i a mctrong thi gian cnh. Do cn thit m ha di cc bit 0 bit 1,y l nhdng tht stn hiu ssdng trong cc ng dng.Smc lng tsquyt nhsbt cn m ha. V dlng tha 8 mc cn dng 3 bt m ha. Bn cnh m ha gip qu trnh truyn, sa sai c tt hn nhcc phng php m hakhc nhau. i su vo cc k thut ly mu sdng hin nay nh oversampling

(ly mu d ),a ra khi nim ly mu, ca sph, gip ta hiu cc tn hiu qutrnh xl tn hiu.

Hnh 1: S cc bc chuyn i tn hiu tng tsang tn hiu s.

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Hnh 2: Dng sng tn hiu cc giai on tng ng hnh 1.

Tc gia ra v d lin hqu trnh ly mu trc quan. Hy tng tng bnang i xe p t nhin tri ma. Bn khng thgicho mt mtrong ma,mt bt u nhp nhy,vic m mt thng xuyn quan st cnh vt pha trccng nh qu trnh ly mu. Bno ca bn xy dng hnh nh y tcc muquan st c khi i mt l mtrong thi gian ngn. Hay khi bn xem phim bnthc sang xem nhng mu hnh nh chiu ln mn hnh (16-25 khung hnh trngiy). Bno ca bn nhn thc c cc khung hnh ring lnh mt hnh nhlin tc m khng nhp nhy vi tc ny.

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Vi thit bsnh my nh k thut s,CD,DVD,in thoi di ng k thut svn c ly mu. Ging ni con ngi, m thanh nhc c,tn hiu video mynh u c chuyn t tn hiu tng t ca tnhin sang dng s lu trtrong cc thit bsnh trn.Khi m knguyn cng nghbng nnh hin nay,

th vai tr ca tn hiu sl cn thit, phbin c nhiu u im hn tn hiu tngt. Qu trnh chuyn i cng chnh xc th tn hiu cng ging vi ban u.

Ty ara khi nim tn hiu tng tl tn hiu m bin , thi gian u lintc. Tn hiu sl tn hiu m bin , thi gian u ri rc gi trhu hn.

Hnh 3: Tn hiu tng t, tn hiu s.

u im ca tn hiu sl lm n gin ha cc hthng khi m hnh hatheo bit 0 v 1, c thxl tng bng my tnh, gim c nhiu ddng canthip ci thin bng cc phng php xl s.Tn hiu cho php sao lu nhiu lnm cht lng khng i.Cc bxl tn hiu sDSP khi sn xut hng lot cho

cht lng xl ng nht, cht lng xl thay i theo thi gian c bn khngng k, ddng xl lu trtruyn cc nh dng skhc nhau thun tin hiuquhn. Tuy nhin khng v vy m ta phnhn thay thhon ton tn hiu tngtbng tn hiu s. Nu so snh hai tn hiu ti ngun ( khng c suy gim thmc a vo cc tn hiu ) th tn hiu tng tchin thng. Hay ni vmtcht lng tn hiu tn hiu skhng bng tn hiu tng tsong nhcc k thut

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khc nhau m tn hiu sstin gn hn tn hiu tng tgip n ph hp nht cth. Khi tn hiu c xl , lu tr, truyn nu tn hiu tri qua mt ssuy gimth tn hiu sli li thbi diu ch, dtruyn ti vi khong cch xa vi mcsuy gim chp nhn c c khnng sa sai. Trong khi cng iu kin nh cng

sut, bng thng th tn hiu tng tssuy gim nghim trng theo khong cchdo thi gian v yu t bn ngoi.Nn hiu sut thp n mc khng chp nhnc. Tn hiu scng chnh xc khi smu ly cng nhiu. Vy qu trnh ly mul g thc hin nh thno.

nh ngha : Ly mu l bc u tin trong qu trnh bin i tng tsang s,mc ch ca bc ny l t tn hiu tng t ta to nn dy xung ri rc rngbng nhau, bin xung bng vi gi tr tn hiu tng t ti thi im lymu.Tn s ly mu cng ln( ngha l cc xung cng st nhau) th c thkhi

phc li tn hiu ban u mt cch trung thc, t bnh hng ca nhiu v sai strong qu trnh ly mu.

Hnh 4: Ri rc mt thi gian bng cch nhn tn hiu tng t(lin tc) x(t) vitn hiu sinh ra tkhi pht xung S(t).

Xs(t)=x(t).s(t)=x(t) =

.

Khi pht xung l chui hm delta nhn gi trduy nht ti thi im t v bng 0trong khong cn li.

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nh l ly mu Shanon a ra gii hn di ca tn s l fs 2fmax hoc . Trong fsl tn sca tn hiu cn ly mu, fmax l tn scc ica phtn hiu tng t. Vi fs=2fmaxc gi l tn sNyguist.

Trng hp tn hiu tng tl tn hiu thng di c phtfLn fH th tn slymu c chn nh sau:

vi n l phn nguyn ca

Hin tng chng phhay mp mph(aliasing) l hin tng xy ra khi tn sly mu khng tha mn iu kin nh l ly mu shanon. Khi cc phtn hiuknhau schng ln ln nhau gy sai lch mt thng tin.

Hnh 5: Quan st qu trnh ly mu trong min tn s(a) phtn hiu tng t(b)Phca tn hiu ly mu vi tn sln hn tn sNyguist (c) Phtn hiu ly mu

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b hn tn sNyguist (d) Ph tn hiu ly mu vi tn s ln hn nhiu tn sNyguist.

T=1/fs: gi l chu k ly mu.

Ca sph: l p ng t0-0,5fsHz l quan trng v nhng g xy ra trong ca ss lp li ti cc phkhc. Chng ta chnhn thy nhng g xy ra trong ca sny. Khi hin tng chng phxy ra l do cc tn hiu c trong ca sphn xlica s. Do lun c blc thng thp t trc qu trnh ly mu m boloi bthnh phn 0,5fskhng nm trong mu c chn.

Qu trnh lng tha

Ri rc ha mt bin tn hiu. Hay sxp xha cc gi trca cc mu tng

tbng cch sdng smc hu hn M. M=2n

n l sbt nhphn dng m haM mc.

Hnh 6 : Lng tha xp x cc mu

Skhc nhau gia m(t) v mq(t) nhiu hay t ty thuc vo khong cch gia hai

mc cnh nhau gi l kch thc bc S( step size). Cht lng ca tn hiu khiphc c thc ci thin nu gim kch thc bc, tuy nhin lc smc tngln dn n sbt cn m ha smc tng ln.

V=fs.n : tc ca knh

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Bc lng tha

=

=S tng ng 1 LSB(Least significant

bit :bit ngha t nht.)

Sai slng tha x=X(t)-X*(t) hay tp m lng tha

Cng sut tp m lng t Pq=

nh gi S/N= SNqR N: mi c tp m lng tcha c trn ng truyn

SNqR=Ps/Pq* vi Pq* gi trtrung bnh.

: LTH tuyn tnh ( LTH u)

: LTH phi tuyn ( khng u )Oversampling ( ly mu d hay ly mu tn scao)

L k thut xl li lch trt ng btrong qu trnh ti to mu, ly mu, truyntn hiu mu( trong mi trng PAM). Bin php ny y tn sng bng hln cao gp 4,8,..24 ln v tin hnh bt li sa li. Sau dng blc hn chatn hiu vtn hiu gc ti u ra.Ging nh khi ta chnh sa nh chng ta phngi nh ln sa chi tit cn thit sau mi trvin nh ra. Mc ch gim nhiutng chnh xc.Tn sly mu tlvi tn sln nht ca tn hiu.

Fs=2**B

Fs l tn sly mu

B l tn scao nht ca tn hiu

l trsoversampling.

Oversampling

Chng ta c ththay thblc thng thp tng ttrong hnh 1 bng blc ssaukhi tn hiu c c lng tha v m ha ? iu ny c vllng nhng xtvmc ch ca blc th blc sc ththay thtng ttrong 1 vi ng dng.Tuy nhin nhng yu cu trong ly mu tn hiu th tn s ly mu phi cao hntn sNyquits, trch vic chng ln tn hiu analog v gim tc ly mu saublc snh hnh 7.

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Hnh 7:Ly mu d ca tn hiu tng tcn thit c blc hn chhmu xungtn sNyguist.

Kthut ny lm cho vic thit kca cc nh thit kddng hn, bi v hnhdng cut-off ko cn cn thit trong b lc tng t na. Hy tng tng victhit k1 blc audio vi nghing ca c tnh suy gim t15khz ti 16khztng ng khong 60-80 dB.y l mt yu cu in hnh trong i kthut sFM vi tc ly mu l 32khz. Khi 1 tn hiu c ly mu ti fs phca nhiulng t sy ra di band ny. Khi tn hiu c ly mu tc cao hn same

noise (ci phthuc vo sbit sdng) tri ln tn smi fs nh hnh 8. Nh hnh8, khi c ly mu tai fs mt nhiu N,khi c ly mu ti 2fs gim xung N/2nh hnh 8b. V vy tng nng lng l nh nhau.Trong hnh 8c tn sly mu ctng ti 4fs v mt nhiu gim N/4 gicho tng cng sut nhiu ko i. Chngta lu rng, nhiu trong bandNyquist c gim nh chra trong ca shnh 8a.Gim nhiu trong band Nyqist c thc lm bng vic sdng cc bit lng tha. Chng ta bit rng theo biu thc

(S/N)= 6n + 1.8 dB vi n l sbt nhphn m ha M mc.

Gim 1 bt stng nhiu ln 6db. y l kthut cbn c sdng trong iu chdelta- signma bi vic dng 1 bt lng tha ti tc ly mu cao c so snhvi tc Nyquist, c theo bi hnh dng nhiu v b lc gim tc lymu v tng slng bit trong m ha k t.Tstn hiu trn nhiu lng tcbiu thc

(S /N)= 6.02n + 1.8 +10log (fs/2)

Do , mi ln tc ly mu tng gp 2 ln th tstn hiu trn nhiu trong ditn hiu stng ln 3dB.

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Hnh 8 : nhiu lng tvi cc tn sly mu khc nhauNhiu ng dng x l tn hiu yu cu thay i tn s ly mu ca tn hiu. V d,trong m ha trong di bng con v m thanh, tn hiu s c ly mu ti tn s

Nyquist c chia vo di tn s khc nhau v mi band c chuyn i xung tn hiu c bn v ti ly mu ti mt tc thp hn v n c di thng thp hnso vi tn hiu ban u.Mibng con c m ho ring bit bng cch lng tvi scc bit khc nhau ty thuc vo bin tn hiu. V d, hnh 12 (a) cho thymt ph tn hiu c chia thnh tm di band con v Hnh 12 (b) cho thy vicchuyn i bng tn ss4 ti baseband. Tt ccc band t hai n tm c

downconverted tng t. Cc tn hiu trong mi band c ti ly mu v tilng tvi sbit khc nhau. Cc bandsplitting(bchia band ) c thc hinvi cc blc bandpass k thut s. u ra ca blc l ti tc ly mu banu. S dng down-sampling ,tc ly mu c gim. KhiKhi cn gim t l ly mu, chng ti dng ti down-sampling hocdecimation. Khi tn hiu c ly mu ti khong T chng ti c mt chui ccmu x [nT] nhthhin trong hnh 13 (a), vi phtng ng thhin trong hnh13 (b), tn s ly mu l bng 1 / T. y c th l downconvertedtn hiu baseband ca hnh 12 (b). Nhchng ti nhn thy tlly mu cho tnhiu ny l qu ln v chng ti dnh gim bt. Nu chng ta mun gim tl ly mu bi mt s nguyn ca M sau chng ti gim (M-1)mu v ly tt c cc mu MTH trong trnh tban u. Trnh t mi, trongdexed nhx [NMT], c thhin trong hnh 13 (c). Tlly mu mi fs/ M =1/TM niTM = MT. Chng ti sdng M = 2 trong hnh 13. Quang phtngng, th hin trong hnh ure 13 (d), hin ang c s dng ti u. nDng nh chng ti c th tn mt sam-tn s pling ch khi tn hiu chim

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mt bng tn sdi y /2M. y lmnh lc u chnh xc. Chng ta c thtn st ly mutlngay ckhi tn hiu bng thngvt qu 1/2TM nhngchngti chquan tm n tn scc thnh phn di y fs/2M. Trc khi decimat-ing tlly mu, tuy nhin, chng ta cn phi sdng mt blc thp qua, trong hn chsig-bngti no di fs / 2M Hz.UPSAMPLING

Nu chng ta mong mun tng tc ly mu sau upsampling. Cc upsamplerbao gm mt bgi mu c theo sau l mt blc thng thp vi tn sct Lfs/ 2 nhthhin trong hnh 14 (a). ln mu mt tn hiu x [nT], thhin tronghnh 14 (b), do mt snguyn L trc tin chng ta chn (L-1) mu khng c gitrgia cc muban u lin tip nhtrong hnh 14 (c) v sau lc cc tn hiu.

thay i tc ly mu bi yu tK = L/M . chng ta kt hp kthut ni suy

v kthut decimation c m ttrn.V dthay i tn hiu t10MHZ ti15mhz (K= 3/2) . chng ta upsampling tn hiu bi 3 thnh nhn 30MHZ , sau down- sampling kt quxung 2 ln t c kqu

PHC HI TIN HIU TNHNG MU CA N

a nhng mu tn hiu qua 1 blc thng thp,c tnh blc thng thp vi tnsct 0,5fs= T , n stwuwowmg ng vi xung ng bnh hnh 15. p ngXung ca 1 blc l u ra ca blc cho 1 xung u vo, khi c 1 xung hp c

a vo u vo blc

Nh chng ta thy khi mt xung, l xung rt hp (l tng khng chiu rng,chiu cao v hn v hu hn nng lng) c p dng cho cc u vo ca blcu ra bt u xy dng theo cch hnh sin, t gi trnh v sau cht itrongmt cch dao ng mt ln na nhtrong hnh 15 (b). Hnh dng ca u ra cdng hnh sinc, sin (x) / x. cc hm i qua gi tr khng ti khonh khc nT,trong n lmt snguyn dng hoc m. mt iu quan trng cn hiu l mcd v tr ca cc xung u vo sc hin thy (v trong hu ht cc vnbn) trng vi khng ngay lp tc ca u ra, trong thc t, n khng phi l nhvy. Lm th no cc b lc c th bt u c mt u ralu trc khi u vo c a n?

Trong thc t, c l mt s chm tr gia u vo v u ra. chomt blc tng gch l (ideal brick wall filter) tng schm trny l v hn

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nhui ca sinc chc nng u ra cng lv hn. Ngoi ra, c c mt bctng gch hon ho p ng tn s ca b lc chng ta cn phi s dng RCvhn hoc cc phn LC. y lL do ti sao chng ta khng thxy dng mt blc l tng, v thm ch nu chng ta c thxy dng mt, chng ta khng th

sdng n. Gi trnh cao ca dng sng sinc l tlthun vi nng lng xung.Ktkhi tn hiu ly mu l mt lot cc xung ca chiu cao tlthun vi gi trtn hiu ngay lp tc ly mu, i qua hthng qua mt blc sn xut tn hiu sincchng cho nh trong hnh 15 (c). Cc dng sng thm ncung cp mt phong b kt qulban u lin tc tn hiu x (t)

Undersamplingl k thut chn thm cc gi trkhng vo mu gc bng phngphp ni suy.Sau khi ni suy chiu di tn hiu ri rc tng ln. Sau sdng blc thng thp

Hnh 8:undersampling chn thm gi trkhng vo gia cc mu gc.

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Kt lun

Bi bo ca tc gi R.N. Mutagi cho ta ci nhn trc quan qu trnh ly mu bini tn hiu tng tsang tn hiu s. a ra cc khi nim , k thut ly mu c

bn hin nay, cc gii php k thut nng cao cht lng cng nh ti u qutrnh ly mu. Vi su thmi p dng k thut undersampling kt hp DSP shatrc tip trong my thu mm SDR sdng bbin i ADC tc cao. c ngdng v dtrong thit bGPS/Galileo cho chnh xc cao ti u vthit k, githnh tng hiu nng. Thc cht ca k thut l xdng bphn chia v ni suykt hp thnh bthay i nhp ly mu.