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MC LC

MC LC ........................................................................................................ 1 DANH MC CC K HIU V CH VIT TT ................................... 3 DANH MC HNH V V BNG BIU .................................................... 4 LI M U .................................................................................................. 6 CHNG 1: IU CH BIN .............................................................. 7

1.1 Sng v tuyn trong thng tin vin thng ............................................... 7 1.2 iu ch bin .................................................................................... 12

1.2.1iu ch bin truyn sng mang (Double Side Ban Transmitted Carrier). ................................................................................................... 17 1.2.2 iu ch DSB-SC (Double SideBand- Supperssed carrier) .......... 20

1.3 iu ch n bin (SSB: single sideband) ............................................ 24 1.3.1Mt s u im ca iu ch n bin (SSB) so vi DSB ............. 25 1.3.2 Cc phng php iu ch n bin .............................................. 26

1.4 iu ch bin di bin dt (Vestigial SideBand VSB) ................. 29 1.5 Cc mch iu ch c th ...................................................................... 30

1.5.1 Mch iu ch AM dng diode ...................................................... 30 1.5.2Mch iu ch vng ........................................................................ 30 1.5.3Mch iu bin dng transistor ....................................................... 32

CHNG 2: GII IU CH BIN ................................................. 33 2.1 Khi nim tch sng iu bin ............................................................. 33 2.2 Tch sng DSB-AM .............................................................................. 33

2.2.1 Hon iu cng. ............................................................................. 33 2.2.2 Hon iu bnh phng .................................................................. 34 2.2.3 Tch sng khng kt hp (Incoherent detection) .......................... 35 2.2.4 Tch sng kt hp (coherent) ......................................................... 37

2.3Tch sng SSB-AM ............................................................................... 37 CHNG 3: NG DNG IU BIN TRONG MY PHT HNH, PHT THANH .............................................................................................. 39

3.1 NG DNG IU BIN TRONG MY PHT HNH..................... 39

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3.1.1 Chc nng v nhim v ca my pht hnh. .............................. 42 3.1.2 S khi c bn ca my pht hnh ............................................ 42 3.1.3 c im ca loi my pht iu ch mc cng sut ln. .... 46 3.1.4 Khi ch sng hnh. ..................................................................... 47 3.1.5 Mch trn tn to cao tn hnh ....................................................... 49 3.1.6 Mch bi tn. ................................................................................ 51 3.1.7 Mch khuch i v sa m tn ..................................................... 53 3.1.8 B tin iu ch. ........................................................................... 53 3.1.9 Tnh ton mch iu ch AM ......................................................... 54 3.1.10 Chc nng v c tnh cc tng khuch i cao tn. .............. 56 3.1.11 B trung hp v mch lc hi .................................................... 57 3.1.12 Dy dn cao tn v Anten. ........................................................ 58

3.2 ng dng iu ch AM trong my pht thanh ..................................... 59 3.2.1 Kho st my pht AM mc thp .................................................. 59 3.2.2 My pht AM mc cao .................................................................. 61 3.2.3 Mt s mch in c bn trong my pht thanh AM .................... 62

Chng 4: M PHNG IU CH AM DNG MATLAB ................... 69 4.1 Gii thiu chng trnh MATLAB: ...................................................... 69 4.2 Gii thiu v chng trnh m phng ................................................... 70

4.2.1 M phng iu bin vi tn hiu sng sin ...................................... 70 4.2.2 M phng iu ch vi tn hiu file wav ....................................... 75

KT LUN .................................................................................................... 77 TI LIU THAM KHO ............................................................................ 78 Ph lc ............................................................................................................ 79

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DANH MC CC K HIU V CH VIT TT

AM

Amplitude Modulation

iu ch bin B Bng thng c s

DSB Double SideBand iu ch bin trit sng mang Fc Frequency carrier Tn s sng mang

FM Frequency Modulation iu ch tn s FSK LFP

Frequency Shift Keying Low-Pass Filter

Kha dch tn s B lc thng thp

LSSB Lower Single SideBand iu ch n bin di SNR Signal-to-noise ratio T s tn hiu trn nhiu

SSB Single SideBand iu ch n bin USSB Upper Single SideBand iu ch n bin trn VCO

W

Voltage Controlled Oscillator

B dao ng iu khin bng in p Bng thng tn hiu gc

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DANH MC HNH V V BNG BIU

Hnh 1.1: Mt tn hin nhiu trng. 10

Hnh1.2: Nhiu xuyn m 11

Hnh 1.3: Nhiu xuyn knh 11

Hnh 1.4 S khi iu ch bin 12 Hnh 1.5 dng tn hiu min thi gian vi h s iu ch m=1 14 Hnh 1.6 dng tn hiu min thi gian vi h s iu ch m

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Hnh 2.5 S khi hon iu SSB-AM 38 Hnh 3.1 Ph ca tn hiu NTSC 40

Bng 3.2 Cc thng s k thut ca cc h 41 Hnh 3.3 S khi my pht hnh iu bin mc cng sut ln 44 Hnh 3.4Mch to dao ng tn hnh 48 Hnh 3.5 Mch in trn tn 50 Hnh 3.6 Mch in trn tn s trung tn ting 51 Hnh 3.7 Mch in bi tn 52 Hnh 3.8 Mch ghim dng transistor 53 Hnh 3.9 Mch in iu bin AM cn bng dng transistor v diode 55 Hnh 3.10 Mch in tng ng ca b lc hi 58 Hnh 3.11 s khi my pht thanh AM mc thp. 60 Hnh 3.12 s my pht AM mc cao. 61 Hnh 3.13 Mch iu bin cc E dng mt transistor 62 Hnh 3.14 mch iu ch AM cng sut trung bnh 64 Hnh 3.15 Mch iu ch AM DSB-TC 66 Hnh 3.16 Mch dao ng dng thch anh vi tn s cng hng ni tip. 68 Hnh 4.1 Giao Menu ca chng trnh m phng 70

Hnh 4.2 M phng tn hiu nhiu gauss 71

Hnh 4.3 M phng iu ch DSB-TC vi tn hiu tin tc l sng sin 73

Hnh 4.4 M phng iu ch DSB-SC tn hiu sng sin 74

Hnh 4.5 M phng iu ch n bin USSB 74 Hnh 4.6 m phng iu ch v gii iu ch file .wav 76

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LI M U

Ngy nay vi s pht trin cng ngh thng tin th tn hiu s ngy cng c

s dng rng ri, v trong k thut thng tin v tuyn. m thanh m tai ngi cm nhn c l nhng dao ng c hc c tn s t

16Hz n 20000Hz. Trong k thut truyn thanh gi m thanh t u pht n u thu ngi ta phi bin i m thanh thnh tn hiu in m tn nh micro. Tn hiu ny c tn s rt nh khng th bc x ra mi trng khng gian. Do vy truyn cc tn hiu m tn t u pht n u thu vi mt c li xa trong

khng gian ngi ta phi gi tn hiu ny vo mt sng mang cao tn. Phng

thc gi tn hiu m tn vo sng mang cao tn gi l phng php iu

ch.Trong thc t tn ti nhiu phng php iu ch tn hiu, nh iu ch

ASK, FSK, PSk, QAM, FM, AM mi loi u c u nhc im ring. Vic iu ch v tch sng l khu rt quan trng trong qu trnh truyn thng

tin i vi c thng tin s cng nh thng tin tng t chnh v vy trong ni

dung n em chn ti Tm hiu k thut iu ch AM nhm tm hiu thm v nhng vn trong iu ch v gii iu ch tng t ng thi lm quen v

ng dng phn mm matlab m phng tn hiu. Ni dung n gm 5 phn: Chng 1: iu ch bin Chng 2: Gii iu ch bin Chng 3: ng dng iu bin trong my pht thanh pht hnh Chng 4: M phng iu ch AM bng Matlab

Tuy nhin, trong qu trnh thc hin ti s khng trnh khi nhng thiu st.Rt

mong s ch bo, gp , ph bnh ca qu thy c v cc bn.

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CHNG 1: IU CH BIN 1.1 Sng v tuyn trong thng tin vin thng - Tn hiu tin tc

Trong truyn thng, tin tc v d liu l tt c nhng g cn trao i chng c th l ting ni hnh nh, tp hp cc con s cc k hiu,cc i lng o lng c a vo my pht pht i hay nhn c my thu.

Tn hiu chnh l tin tc c x l c th truyn i trn h thng thng tin. V d : Tn hiu m tn c gii tn t 20Hz n 20KHz v khng c kh nng bc x thnh sng in t truyn trong khng gian vi khong cch xa.

-Tn hiu sng mang

L tn hiu cao tn c tn s trn 30 KHz tn hiu cao tn c tnh cht bc x thnh sng in t. Th d trn mt dy dn c tn hiu cao tn chy qua, th dy dn c mt sng gy can nhiu ra xung quanh, chnh l sng in t do dng in cao tn s bc x ra khng gian. -Bng thng knh truyn (Bandwidth) Bi v mt tn hiu bt k c th c xem nh l mt s kt hp ca mt chui cc sng hnh sin, nn ta c th xem rng, s truyn ti mt tn hiu bt k tng ng vi vic truyn ti cc sng hnh sin thnh phn. V tn s ca chng l khc nhau,

chng c th n ni vi suy gim l khc nhau, mt trong s chng c th

khng cn nhn ra c. Nu ta nh ngha mt ngng cn nghe c f1, th tt

c cc tn hiu hnh sin c tn s nh hn f1 c xem nh b mt. Tng t cc tn hiu c tn s ln hn f2 cng c xem l b mt. Nhng tn hin c th nhn ra c bn nghe l cc tn hiu c tn s nm gia f1 v f2. Khong tn s ny c gi l rng bng thng. Ni mt cc khc, vi mt tn hiu phc tp bt k, tn hiu ny s truyn ti c nu nh tn s ca cc sng hnh sin thnh phn ca n c tn s nm trong khong

bng thng ca knh truyn. Chng ta cng nhn thy rng, bng thng cng ln th cng c nhiu tn hiu c truyn n ni. Chnh v th chng ta thng quan tm

n cc knh truyn c bng thng rng.V d: rng bng thng ca knh truyn in thoi l 3100Hz v cc tn hiu m thanh c th nghe c nm khong tn

s t 300 Hz n 3400 Hz.

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Tn gi bng tn

Vit

tt

Bng

tn

ITU

Tn s v

bc sng trong khng

kh

ng dng

Tn s cc

k thp

ELF 1 330 Hz 100,000 km

10,000 km

Thng tin di nc

Tn s siu

thp

SLF 2 30300 Hz

10,000 km

1000 km

Thng tin di nc

Tn s cc

thp

ULF 3 3003000 Hz

1000 km

100 km

Thng tin di nc, thng tin trong hm m

Tn s rt

thp

VLF

4 330 kHz

100 km

10 km

Dn ng, tn hiu thi gian,

thng tin di nc, thit b hin th nhp tim khng dy, a vt l

Tn s thp

LF

5 30300 kHz 10 km 1 km

Dn ng, tn hiu thi gian,

qung b (sng di) AM (Chu u v mt phn Chu ), RFID, v tuyn nghip d

Tn s trung

bnh MF

6 3003000 kHz

1 km 100 m

Qung b (sng trung) AM, v tuyn nghip d, cnh bo tuyt l

Tn s cao

HF

7 330 MHz

100 m 10 m

Qung b sng ngn, v tuyn nghip d, thng tin ngoi ng chn tri, RFID, radar ngoi ng chn tri, thng tin v

tuyn thit lp lin kt t ng

(ALE) / (NVIS), in thoi v tuyn di ng v hng hi

9

Tn s rt

cao

VHF

8 30300 MHz

10 m 1 m

V tuyn FM, thng tin qung b, thng tin gia my bay-my bay v my bay-mt t. Thng tin di ng mt t v hng hi, v

tuyn nghip d v v tuyn thi tit

Tn s cc

cao

UHF

9 3003000 MHz

1 m

100 mm

Qung b truyn hnh, l vi sng, thng tin/thit b vi ba, thin vn v tuyn, in thoi di ng, WLAN, Bluetooth, ZigBee, GPS

v v tuyn hai chiu nh v

tuyn di ng mt t, FRS v GMRS, v tuyn nghip d

Tn s siu

cao

SHF

10 330 GHz 100 mm

10 mm

thin vn v tuyn, thng tin/thit b vi ba, WLAN, radar, v tinh thng tin, truyn hnh v tinh,

DBS, v tuyn nghip d

Tn s cc

k cao

EHF 11 30300 GHz

10 mm

1 mm

thin vn v tuyn, thng tin vi ba cao tn, vin thm, v tuyn

nghip d, v kh nh hng chm nng lng trc tip, my

qut sng milimet

Terahertz hay

Tn s cc

cc cao

THz

or

THF

12 3003,000 GHz

1 mm

100m

ng dng tim nng trong y hc, thay th cho tia-X, thng tin/tnh

ton terahertz, vin thm, v

tuyn nghip d

< 3 Hz

> 100,000 km

Tp m in t t nhiu v do con ngi to ra

Bng 1.1 Phn b bng tn v ng dng V tuyn AM sng di = 148.5 283.5 KHz (LF)

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V tuyn AM sng trung = 530 KHz 1710 KHz (MF) V tuyn AM sng ngn = 3 MHz 30 MHz (HF) -Nhiu trong thng tin v tuyn

+ Nhiu trng (White noise) Nhiu trng l mt tn hiu ngu nhin c mt phn b cng sut phng ngha l tn hiu nhiu c cng sut bng nhau trong ton khong bng thng. Tn hiu ny c tn l nhiu trng v n c tnh cht tng t vi nh sng trng.

Hnh 1.1: Mt tn hin nhiu trng.

+Ngun sinh ra nhiu trng:

-Nhiu sinh ra do chuyn ng nhit ca cc in t trong linh kin bn dn. -Nhng m thanh nh ting gi ting nc ca l ngun nhiu trng

-Cc vn nh thi tit con ngi.

+Nhiu ng knh (Co-Channel Interference) -Nhiu ng knh xy ra khi c hai my pht trn cng mt tn s hoc trn cng

mt knh. My thu iu chnh knh ny s thu c c hai tn hiu vi cng

ph thuc vo v tr ca my thu so vi hai my pht, v sng mang.

Nhiu ng knh thng gp trong h thng thng tin s cellular, Trong tng

hiu sut s dng ph bng cch s dng li tn s. Nh vy c th coi nhiu ng knh trong h thng cellular l nhiu gy nn do cc cell s dng cng 1 knh tn s.

+ Nhiu xuyn m (Intersymbol Inteference) Di thng tuyt i ca cc xung nhiu mc nh phng l v hn. Nu cc xung

ny c lc khng ng khi chng truyn qua mt h thng thng tin th chng s

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tri ra trn min thi gian v xung cho mi k hiu s chn vo cc khe thi gian

bn cnh gy ra nhiu gia cc k hiu (ISI)

Hnh1.2 Nhiu xuyn m

- gim nhiu xuyn m ngi ta phi lm th no hn ch di thng m vn

khng gy ra ISI

-Khi di thng b gii hn, xung s c nh trn thay v nh phng

-Mt trong nhng phng php loi b nhiu ISI l dng b lc cos nng v

b lc ngang p khng (phng php Nyquist I) -Nhiu xuyn knh (Interchannel Interference)

-Gy ra do cc thit b pht trn cc knh k nhau

Hnh1.3: Nhiu xuyn knh

-Thng xy ra do tn hiu truyn trn knh v tuyn b dch tn gy can nhiu

sang cc knh k n

- loi b nhiu xuyn knh ngi ta phi c khong bo v (guard band) gia cc di tn

Tc ng ca mi trng truyn ti vic truyn tn hiu

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iu ch AM

Tn hiu iu bin

Khi tn hiu truyn qua mi trng (v tuyn hay hu tuyn) th t nhiu s b cc tc ng lm sai lc. Cc yu t nh hng n truyn tin c th k ti l:

-Gy mo tn hiu (distortion) -Nhiu (Interference) -Suy hao

- Mo tuyn tnh v mo phi tuyn

Mo tuyn tnh l mo gy ra cho tn hiu bi cc phn t tuyn tnh trn knh truyn, trong cc phn t (trong nhiu trng hp li c th xem chng nh cc h thng - n gin l xem chng nh nhng khi/block) tuyn tnh l cc phn t m php ton biu din quan h u ra theo u vo tha mn tnh cht xp chng.

Nguyn nhn: Cc mch lc ch to khng hon ho

Do mi trng truyn:

- Dy kim loi: tn s cng cao tiu hao cng nhiu nhng tng khng u m

ln nt, mp m.

Mo phi tuyn, tri li, li gy bi cc phn t phi tuyn (khng tha mn tnh cht xp chng, c c tuyn vo-ra l mt ng khng thng) 1.2 iu ch bin Tn hiu tng t khi truyn i xa th nng lng ca chng b suy gim theo khong cch. V vy, c th truyn tn hiu i xa v thch hp vi cc dng knh truyn khc nhau cn phi thc hin qu trnh iu ch tn hiu.

iu ch bin hay cn gi l iu bin (AM) c thc hin bng cch thay i bin ca tn hiu sng mang theo bin ca tn hiu thng tin cn gi i, trong tn s v pha ca sng mang th gi nguyn

Sng mang

Uc=Uccos(wct)

Us=u0*cos(wst) Tin tc

Hnh 1.4 S khi iu ch bin

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Phng php iu ch ny gm iu ch hai bin (DSB-SC), iu ch bin truyn thng (DSB-TC), iu ch n bin (SSB-AM) v iu ch bin trit mt phn di bin (VSB-AM). S ph thuc gia tn hiu iu ch v bin ca sng mang c iu ch c th rt n gin, v d nh trong trng hp DSB-AM, hoc phc tp hn nhiu nh trong SSB-AM hay VSB-AM. Cc h thng iu ch

bin thng c c trng bi mt yu cu kh thp v rng bng thng v tnh hiu qu v cng sut so vi cc mch iu ch tn s hoc pha. Yu cu v

rng bng thng i vi h thng AM dao ng trong khong W ti 2W, trong W l k hiu cho rng bng thng ca tn hiu bn tin. i vi SSB-AM c rng bng thng l W, i vi DSB-AM v AM truyn thng l 2W cn i vi h thng VSB-AM th rng bng thng s t W ti 2W. Tn hiu m tn c th ly t Micro sau khuch i qua mch khuch i m tn,

hoc c th ly t cc thit b khc nh i Cassette, u a CD Tn hiu cao tn c to bi mch to dao ng, tn s cao tn l tn s theo quy nh ca i pht.

Tn hiu u ra l sng mang c tn s bng tn s cao tn, c bin thay i theo tn hiu m tn.

u im: ca sng AM l c th truyn i xa ti hng nghn Km

Nhc im: ca sng AM l d b can nhiu, di tn m thanh b ct sn do c im ca mch tch sng iu bin, do cht lng m thanh b hn ch.

C s l thuyt iu ch bin Tin tc s(t) l tn hiu m tn c cng thc : s(t) = Uscos2f0t Khi tin tc l m thanh ca ngi pht ra n khng c c trng ring bit no v mu dng sng .Vi tn hiu m thanh c c tnh sinh l hc c di tn s t 20Hz 15Khz ngoi di tn ny khi bin i furier :S(f)=0 vi f>15Khz. Gi s mun truyn i tn hiu audio trong mi trng khng kh vi tn s 3Khz theo tnh ton s cn anten s cn c chiu di bng di qung ng truyn sng.Mt khc tn hiu ny cn lin quan n vic truyn khng hiu qu trong

khng kh v s giao thoa do cc di tn ca di khc chn ln.Do vy ta phi ci tin tn s ,thng dng vi tn hiu khc c tn s cao hn.Tn hiu sau iu ch s t nhy cm vi nhiu so vi tn hiu gc.

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Ti tin l tn hiu cao tn c cng thc:

uc(t) = Uocos(2fct + ) Trong tn s ca sng mang fc > fs (tn s ca tin tc) Bc sng fc dc chn hp l sng mang c th c truyn i c hiu qu .V d c th chn sng c di tn trong khong gia 0,5-3 Mhz truyn xa n 250Km .Bc sng ca cc tn s tng ng c 100Mhz nh vy chiu di anten c rt ngn n mc chp nhn c.

= . 3

Tn hiu iu ch AM (theo php cng) c dng: uAM(t) = [U0 + s(t)]cos(2fct + ) uAM(t) = [U0 + Us.cos2Fst].cos(2fct + ) = U0 [1+Us/U0.cos2Fst].cos(2fct + ) t m = Us/U0 l su iu ch

uAM(t) = U0[1 + m.cos2Fst].cos(2fct + ) uAM(t) = [U0 + m.U0cos2fst].cos(2fct + ) (1.1)

H s iu ch m=UsUc

Hnh 1.5 dng tn hiu min thi gian vi h s iu ch m=1 Khi h s iu ch m>1 lc ny tn hiu sau iu ch s khng khi phc c ti bn thu chnh v vy cc h thng iu ch khng bao gi c xy ra trng hp ny.

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Hnh 1.6 dng tn hiu min thi gian vi h s iu ch m

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Hnh 1.8 m phng ph ca tn hiu sau iu ch

BWAM=2*f0 (f0 tn s ca tin tc cn iu ch) Trong iu ch n bin tn hiu gm 2 bin tn nh hnh 1.7 s c lc ch truyn i di trn hoc di vi rng BW=f0, iu ny c ngha gip gim s lng ph bng thng do hai bin tn trn v di cng cha mt lng thng tin nh nhau nhng vic thc hin kh khn hn do phi thit k mch lc c chnh xc cao.

-H s mo phi tuyn

( ) ( )

( )

++=

0

00

I....II

k2

32

2

I(c ns) (n 2) :l bin cc thnh phn dng in ng vi hi bc cao ca tn hiu iu ch;

I(c s) l bin cc thnh phn bin tn

c trng cho mo phi tuyn

trong mch iu khin, ngi ta

dng c tuyn iu ch tnh (hnh 1.9). c tuyn iu ch tnh cho bit quan h gia bin tn hiu ra v gi tr tc thi ca tn hiu iu ch u vo.

Dng tng qut ca c tuyn iu ch tnh c biu din trn hnh 1-9. ng c tuyn iu ch tnh l tng l mt ng thng t C n A. c tuyn iu ch tnh khng thng s lm cho lng bin i ca bin dao ng cao tn

A

B

C U

I0

Hnh 1.9: c tuyn iu ch tnh A-gi tr cc i B-ti tin cha iu ch.

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u ra so vi gi tr ban u (im B) khng t l ng thng vi tr tc thi ca in p iu ch. Do trn u ra thit b iu bin, ngoi cc thnh phn hu ch (cc bin tn), cn c cc thnh phn bc cao khng mong mun khc. Trong ng lu nht l thnh phn ca tn s c 2s c th lt vo cc bin tn m khng th lc c.

gim mo phi tuyn, cn hn ch phm vi lm vic ca b iu ch trong on ng thng ca c tuyn iu ch tnh. Lc buc phi gim su iu ch

1.2.1iu ch bin truyn sng mang (Double Side Ban Transmitted Carrier). - Bin iu DSB-TC l phng php iu ch bin trong tn hiu sau iu ch vn cn tn ti thnh phn sng mang.

iu ch DSB-TC, cng sut thnh phn sng mang chim phn ln. Vi +m = 1, th 66% cng sut dnh cho sng mang, ch 17% cng sut ti thng tin (mt di bin). +m < 1, cng sut ti thng tin cn gim hn na, m = , th gn 90% cng sut cho sng mang, hn 5% cng sut ca tn hiu ti thng tin. -Phng php iu bin DSB-TC:

Hnh 1.10 S iu ch DSB-TC

Trong s iu ch trn tn hiu tin tc c cho vo mch nhn vi tn hiu sng

mang sau tn hiu sau b nhn c cng chung vi tn hiu sng mang sau khi c khuch i cng sut i vi sng mang.

Vi s iu ch nh hnh trn cho ta cng thc v tn hiu thu c biu din bi

Sm(t)=S(t)cos2&' ()*2&'. (1.5) =Us (cos(wc+wt)+cos(wc-wt))+ ()*-. . ' (1.6) Phn tch biu thc (1.5) ta thy ngoi tn hiu sng mang cn c tn hiu

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c biu din bi S(t). cos(2fct) nh vy khi tin tc thay i dn ti tn hiu sau iu ch thay i theo bin ca tn hiu cn truyn .Xt v tn s tn hiu tin tc s b dch tn mt khong bng vi tn s ca sng mang theo (1.6) (di tn fc-fm v fc+fm) do thun t c mc ch truyn sng i xa. Nu gn cc gi tr tiu biu fm=15khz v fc=1Mhz ta s thy tn s b chim bi sng bin iu l t 985000 n 1.015.000Hz + Vi khong tn s ny anten c chiu di hp l c th xy dng c, l mt trong hai vn cn gii quyt.

+Vn th hai l tch knh trong mt h a hp.Ta thy nu mt tn tc bin iu mt sng hnh sin c tn s fc1, mt tin tc khc bin iu sng mang c tn s fc2 th cc nh furier ca hai tn hiu ny s khng ph ln nhau.V fc1,fc2 cch nhau t

nht 2fm.

Hnh 1.11 Bin i furier ca 2 sng AM

Thc t cc tn hiu c th tch c nu chng khng b ph ln nhau (hoc v thi gian hoc v tn s).Nu chng khng ph ln nhau v thi gian c th dng cc cng hoc cc switch tch.Nu chng khng ph nhau v tn s th cc tn

hiu c th tch ra bng mch lc thng di .Vy mt h thng nh hnh (1.11) c th tch sng mang b bin iu. Vic iu ch nhng tin tc khc nhau vi nhng sng mang cng c tn s khc

nhau gip vic tch knh trong h a hp tr ln d dng v vy c th dng iu bin trong k thut ghp knh phn chia theo tn s OFDM. Theo di tn sng mang c chia ra thnh nhiu di tn con, mi di tn con c iu ch truyn i lung d liu tc thp tp hp ca dng d liu tc thp s l dng d liu tc cao cn truyn.

19

Hnh 1.12 S tch sng iu bin hai knh hai knh Nhn xt

Qua vic biu din tn hiu thu c sau iu ch tn ti 3 di tn -

Thnh phn (1) bin U0, cn tn s sng mang fc; - Thnh phn (2) bin mU0/2, cn tn s bin trn l (fc+fm); - Thnh phn (3) bin mU0/2, cn tn s bin di l (fc-fm).

Hnh 1.13 Bin i F ca TC-AM

- Cng sut ca thnh phn sng mang: Pc =

./0 1 2()*2&'3

45

6457'

=

./0 1 (

89:;

45

?45

7' =@5

0

- Cng sut ca thnh phn bin trn:

Pb = .0 1 AB59/C5

3fm th tn hiu khng b chng ln nhau vy c th tch chng ra bng mch lc BPF thu c tn hiu Am thc t tn hiu sng mang c fc >> fm ln iu kin trn d dng tha mn.

24

Cc diode bn dn c c tuyn rt ging vi lut bnh phng (trong vng hot ng ca n)

Hnh1.19 iu bin cn bng. Xy dng b phn bnh phng th khng n gin, bt k mt linh kin phi tuyn no cng cho mt tn hiu ra tng ng vi tn hiu vo bi mt h thc m ta khai trin thnh chui ly tha

Vi y(t) l output v x(t) l input biu din mi quan h gia y(t) v x(t) s l y(t)=a0+a1x(t)+a2x2(t)+a3x+a3 x3(t)+-s hng m ta cn quan tm l a2x2(t) v phi tm cch tch ra khi cc thnh phn khc.

Trong mch trn cng sng mang vi tin tc s(t) sau a chng vo linh kin phi tuyn (bnh phng) p dng cho c -s(t).Mch tng s ly hiu s ca hai tn hiu lm loi b s hng ly tha l trong khai trin ng thi gip s hng

2Z' / )*2&' khng i du khi s(t) thay bng s(t) vy s lm tng i bin tn hiu.Vy khi s hng bc 2 b trit tiu nn tn hiu thu c s l SC-AM

1.3 iu ch n bin (SSB: single sideband) Ta bit tin tc ch cha trong bin tn, nn ch cn truyn i mt bin tn l thng tin v tin tc. Qu trnh iu ch nhm to ra mt di bin tn gi l iu ch n bin. Ti tn ch cn dng tch sng do c th nn ton b hoc mt phn ti tin trc khi truyn i. Trong cc h thng DSB ni trn khong bng tn cn thit truyn tn hiu thuc khong fc-fm v fc+fm kh bng tng cng l 2fm v vy trong phng php SSB ta ch truyn i 1 bng tn trn hoc di. Biu din trn min thi gian ca cc tn hiu ny l :

U(t) = m(t)cos(2fct)

[ (t)sin(2fct) (1.7)

Trong du tr ng vi USSB-AM cn du cng ng vi LSSB-AM.

25

Tn hiu c k hiu bng [ (t) l bin i Hilbert ca m(t), n to ra mt trc i xng qua v tr ca sng mang. Trn min tn s ta c

\@$ (f) = m(f+fc), fc |f| (1.8) 0 vi mi gi tr khc ca f

\]$ (f) = m(f-fc) |f| fc (1.9) 0 vi mi gi tr khc ca f

Hnh1.20 Biu din ph ca 2 bng cnh USSB v LSSB Ta thy bng tn ca bin iu SSB ch chim mt phn bng tn c chim ca DSB.

1.3.1Mt s u im ca iu ch n bin (SSB) so vi DSB (1) Ph ca tn hiu iu ch LSSB v USSB l mt na rng bng ca DSB v AM truyn thng do bng vi rng bng thng ca tn hiu bn tin tc l : Bt= W.

Bi vy trong cng mt di tn s th s i c th b tr tng gp i. (2) Dng tn hiu SSB s thc hin c s bo mt tt, do nu khng bit tn s sng mang th s khng thu c tin tc. Do vy my pht v my thu SSB c

s dng rt nhiu trong lnh vc qun s (3) i vi tn hiu AM trong gii truyn sng ngn, do s phn tn ca c tuyn pha m xy ra s chia pha cc dao ng trong di bin. iu lm mo tn hiu truyn v lm gim bin in p u vo b tch sng ca my thu AM. Tn hao cng sut u ra, do c nh gi l 50%. Cn i vi tn hiu SSB th mi tin tc iu c pht trong mt di bin nn khng c hin tng chia pha. (4) Do hin tng pha inh trong truyn sng m tn s sng mang f0 c th b suy gim. i vi my thu AM c lc m > 1 s gy mo do qu iu ch. Nu pha inh rt ln lm mt hn tn s sng mang th my thu s khng thu c g. Cn i vi

26

my thu SSB pha inh lm suy gim hay trit tiu tn s sng mang khng gy nh

hng g.

Tuy c nhiu u im nhng do yu cu k thut kh cao nh mch lc di phi rt hp v dc ng; vic to li tn s sng mang f0 trong my thu phi rt chnh xc mi khng mo tn hiu nn my pht v my thu hiu SSB cu to phc tp hn

so vi my pht v my thu AM. Bi vy n ch c dng trong cc my thu pht thng tin chuyn dng nh trong my pht thoi v pht tn hiu nhiu knh.

1.3.2 Cc phng php iu ch n bin V bng cnh trn v bng cnh di tch bit v tn s cc mch lc c th dng chn bng cnh mong mun bng cch thay i thng s ca cc mch lc ngi ta c th ch nhn c USSB hoc thu c c tn hiu DSB.

Cc phng php iu ch n bin trong thc t: Phng php u tin to ra tn hiu n bin SSB l t tn hiu iu bin AM ngi ta dng b lc di tch mt bin tn cn thit ca tn hiu ra. Nhng do yu cu cht lng cao nn b lc di rt phc tp. Bi vy ngi ta to hai phng php to tn hiu SSB khc nhau: phng php quay pha v phng php lc pha.

Nhng hai phng php ny li to ra mt s sn phm khng cn thit nh tn s

sng mang fc , di bin th hai Mt khc hai phng php ny khng th to ra b iu ch c ch tiu k thut cao v n nh, bi vy n cng t c dng. Ngy nay phng php to tn hiu SSB c s dng rng ri nht l phng php tng hp: to tn hiu SSB bng cc tn s sng mang khc nhau. c bit l khi b lc thch anh ra i th phng php ny c s dng rt rng ri.Tn hiu iu ch Vs v ti tin Vc c a vo b iu ch cn bng.Sau b iu ch cn bng ta thu c hai di bin (DSB). Sau dng b lc di ta s thu uc bin trn hoc bin di nh hnh 1.21.

Nhng do

27

.

Trong thc t ta chn tn s trung gian nm trong khong (100 500MHz) v di tn s ta c b lc thch anh v b lc c in tt nht. Sau chuyn fTG ln tn s ti tin f0 ta dng thm mt vi b iu ch cn bng v b lc di cc tn s khc nhau.Vic s dng tn s trung gian sau khi qua b iu ch u tin gip nng cao tn s ca tn hiu sau iu ch mt khc gip cho tng b lc mi khu c d dng hn. Mi ln lc th t s X ln hn nn d thc hin lc hn. Phng php gi l phng php tng hp.

iu ch cn bng

B lc di hp

smin smax

c - max c - min

c + max 0 +min

0

c - max c - min

4

3

1

2

0

f

f

f

4 3

2

1

Vs

Vc VSSB

Vc,Vs

VDSB

0

0

0

Hnh1.21: S khi v di ph ca tn hiu ca phng php lc

28

Phng php tng hp:

B lc 1 thng l b lc thch anh hay b lc c in cht lng cao v rt nh.

Tn s sng mang th 2 c f1>>f0 v = 1 + min kh ln nn d lc hn. V vy

b lc 2 thng l b lc L, C n gin. Nu f2 cha trong di tn s lm vic th ta li dng ti tn s th 2 : dng b iu ch cn bng 3. B lc 3 cng n gin

nh b lc 2 v ln: = 1 +2 + minC th cho n khi no ta t c tn s

lm vic f0. (Tn s ti tin).

Dao ng n

Dao ng 2 2

Dao ng 1 1

f1 f2 fn

f0 = f1 + f2 ++fn

2 = 21 + 2min

2+1+min 2+1+max 2-1-max 2-1-min

1+min 1+max

1-max 1-min 1+min 1+max

2min

min max 1

2

f

f

f

f

f

V1

V2

V3

V4

V5

Hnh 1.22: Ph tn hiu theo phng php tng hp

BCB 1

Lc 1 BCB 2 Lc 2 BCB n Lc n

29

1.4 iu ch bin di bin dt (Vestigial SideBand VSB) Trong k thut truyn hnh, truyn ti hnh nh bng iu bin AM hoc DSB-SC, ph tn s ca n qu rng W = 2 x 4,2 = 8,4 MHz.

Nu s dng SSB-AM, th gii php k thut li qu phc tp, gi thnh t. V vy, ngi ta s dng gii php iu bin rt Tn hiu VSB c th thc hin t tn hiu AM hoc DSB-SC, nh mt mch lc gii

thng c bit, gi l mch lc bin rt. Mch lc bin rt c tc dng lm gim (ct ct) phn ln ph bin tn di.

Hnh 1.23 Ph iu ch iu bin rt Mch lc trong iu ch bin rt khng lc b hon ton mt bin tn m ch lc bt mt phn di tn s trn hoc di nn khng yu cu chnh xc so vi iu ch n bin. Tn hiu VSB c biu din bng biu thc: sVSB(t) = uAM(t)*h(t) - uAM(t) l tn hiu iu bin AM, - h(t) l p ng xung ca mch lc bin rt, - Du * y ch php tch chp.

Ph tn hiu VSB xc nh bng biu thc: SVSB(f) = SAM(f) . H(f) - SAM(f) - ph bin - tn s ca tn hiu AM, - H(f) l hm truyn ca mch lc bin rt.

30

1.5 Cc mch iu ch c th 1.5.1 Mch iu ch AM dng diode

Hnh 1.23 S mch iu ch AM dng diode Phn tch

Vi c tnh V_A ca diode phn tch theo cng thc taylor ta c: iD=f(VD)=a0+a1VD+a2VD2+a3VD3 + VD=V1+V2 Ta c iD= a0+a1(V1+V2)+a2(V1+V2)2+a3(V1+V2)3= a0+ a1V1+a1V2+a2V12 + 2a2V1V2+ 2a2V22+ a3V13+

c UAM(t) = [U0 + s(t)]cos(2fct)=U0cos(2fct)+s(t)cos(2fct) =a1V1+2a2V1V2 Ta cn c b lc thng di tch hp nhm lc di bng thng ca tn hiu iu bin.

1.5.2Mch iu ch vng Trong thc t, khng c phn t phi tuyn no l bc 2 thun tu, chnh v vy to tn hiu DSB v trit tiu cc thnh phn bc 1 v bc 3, ta dng hai mch AM ghp i xng, gi l mch iu ch vng

Hai mch iu ch AM ging nhau, cng c thnh phn sng mang Accoswct. Tn

hiu tin tc x(t) c o pha v t vo hai ng vo vi pha ngc nhau Tn hiu tin tc V2 c dng V2=Uscos(wst) Tn hiu ti tin V1 c dng V1=Uccos(wct)Vi mch iu bin cn bng ta c khi D1,D2 dn ta c VD1=V1+V2

VD2=V1-V2

31

iD1= f(V1+V2)=a0+ a1(V1+V2) + a2(V1+V2)2+ =a0+ a1V1+a1V2+a2V12+ 2a2V1V2+ a2V22+ iD2= f(V1-V2)=a0+ a1(V1-V2) + a2(V1-V2)2+ =a0+ a1V1-a1V2+a2V12- 2a2V1V2+ a2V22+ a3V13+

Vy i(t)=iD1-iD2=4a2V1V2+2a1V2+2a3V22+ 6a3V12V2+ S dng khung cng hng LC u ra lc ly thnh phn 4a2V1V2, y cng chnh l tn hiu AM.

-Thnh phn 4a2V1V2= 2UsUccos(wc-ws)+ 2UsUccos(wc+ws) Trng hp 2 khi D3, D4 dn ta c mch hot ng tng t u im ca iu ch vng:

loi b c cc thnh phn c di tn khng mong mun Ngoi ra n cn gim hin tng mo phi tuyn.Mc ch ca phng php iu

bin cn bng c ng dng trong iu ch DSB.Ngoi ra c th thay th diode

D

D

D

DUb Us

CB

CB

Uc

c-

t

c +

t

c Hnh 1.24: Mch iu bin vng dng diode.

a) Mch in; b) Ph tn hiu

0

32

trong mch iu bin cn bng vi transistor vi c im u ra bin ln v mo phi tuyn nh

1.5.3Mch iu bin dng transistor V nguyn l iu bin bng Transistor cng gm cc loi: Trong trng hp Transistor lng cc, FET, n in t iu bin, ngi ta phn bit cc loi mch iu bin sau y: iu bin base, iu bin collector, iu bin ca, iu bin mng, iu bin anot, iu bin li. Cc loi mch iu bin c tn gi tng ng vi cc m in p iu ch c t vo

VCC: in p ngun cung cp trong trng hp sng mang khng iu ch

i vi Transistor, in p ca Collector khng c tng qu gi tr an ton cc i d trong thi gian ngn. Bi vy cn phi tha mn iu kin: Vs + Vc < VCemax= BVCEO

Trong :- Vc : in p cao tn cc i collector khi m=1;

- BVCEO: in p nh thng cho php cc i;

Khi in p collector thp mi ni collector c phn cc thun bi in p u vo. Do vy dao ng cao tn trc tip i qua mi ni collector phn cc thun. Sthay i ca dng collector trong vng 0- a xut hin bi iu ch qu mc khi tn hiu ln.

0

IC1

VC

Vs VAM VR

Vc - VAM+

c tnh phi tuyn BCB

Vc

VCC

Ti tng trc

Vs

Ra

L

C

C1 C2 C Lch

Hnh1.25: iu bin colector

33

CHNG 2: GII IU CH BIN 2.1 Khi nim tch sng iu bin Gii iu ch thc cht l qu trnh tch tn hiu bn tin c iu ch. Qu trnh gii iu ch ty thuc vo kiu iu ch. i vi DSB-AM v SSB-AM, phng

php gii iu ch l gii iu ch kt hp (coherent) yu cu cn phi c mt tn hiu c cng tn s v pha ging vi tn hiu sng mang ti my thu. i vi AM

truyn thng s dng cc b tch sng ng bao cho gii iu ch. Trong trng hp ny vic bit chnh xc tn s v pha ca sng mang khng phi l iu thit yu, cho nn qu trnh gii iu ch d dng hn nhiu. Vic gii iu ch kt hp i vi DSB-AM v SSB-AM bao gm vic nhn (trn) tn hiu iu ch vi mt tn hiu cosin c cng tn s v pha vi sng mang, sau cho tn hiu ca

vic nhn qua mt b lc thng thp s thu c dng tn hiu bn tin m(t) ban u. Vi s gii iu ch trn hay cn c gi l mch hon iu ng b trong c mch to dao ng Sc(t) c ng b ha c v tn s ln pha vi sng mang thu.

2.2 Tch sng DSB-AM 2.2.1 Hon iu cng. Trc ht xt mch gii iu ch DSB-SC-AM s dng mch hon iu cng

Hnh 2.1 S hon iu cng

Trong P(t) l mt hm cng gm 1 chui xung c dng P(t)= a0+ PR)*2&VRW ' Vy tn hiu vo khi LPF l

Sm(t).P(t)=S(t)cos(2fct)2 a0 PR)*2&VRW '3 Quan tm ti thnh phn bc 1 Sm (t).P(t)= a0.S(t).cos(2fct)+`a .%

`a.%#b

34

S0(t)= P. *' V hon iu c hon tt

Trn y l vic gii iu ch vi sng iu bin kiu DSB-SC cn vi mch gii iu ch kiu DSB-TC.Ta thy rng u ra mch gii m s c dng

S0(t)= P2d *'3 Biu thc trnh by tn hiu tin gc b di bi 1 hng

2.2.2 Hon iu bnh phng Ta kho st hiu qu ca phng php hon iu bnh phng: bng cch cng sng AM vo sng mang thun ty ri sau bnh phng tng

2Z' d)*2&'3 (2.1) Trc ht hy xt trng hp sng mang b nn DSB SC cng thc (2.1) s tr nn: Sm(t)=s(t).cos2&' 2Z' d)*2&'3=e)*2&'2Z' d3 f

= cos22fct+2*' d3

= 2b%83582b%835.#b

35

ha vi sng mang thu c. thc hin c vic ny my thu cn c mt mch

to dao ng ni.Nhng mt kh khn t ra l vic ng b v pha v tn s ca mch to dao ng ni v sng mang phc tp nu mt ng b u ra khi tch sng tn hiu s b mo so vi tn hiu tin tc.

Gi s vi mch dao ng ni b lch pha 1 lng g v lch tn mt lng Khi output ca mch nhn s l

Sm(t)cos2& ' g = s(t)cos2 &'.cos2& ' g

=s(t)A9:; ci%8 9:;ec8if%8

E (2.2) Biu thc trn cng l tn hiu u vo ca mch lc thng thp do vy u ra ca mch LPF s l

S0(t)=S(t)9:; ci%8 (2.3) S hng th hai ca (2.2) c thnh phn tn s 2 nn b loi b. Trong biu thc (2.3) gi s nh .nh l bin iu ch ra rng c bin i F c tn s trong khong n fm + d LPF c thit k ch cho i qua tn s ln n fm nhng n vn cho i qua fm + v fm j Gi s loi b s sai khc v tn s ri ch cn sai pha th

S0(t)=S(t)9:; (2.4) Vy khng xy ra mo vi S(t).Nhng c ch l khi g tin n 90 th tn hiu thu c s bng 0.

2.2.3 Tch sng khng kt hp (Incoherent detection) Cc khi hon iu ni trn cn phi to li sng mang my thu v vy tn s

sng mang phi chnh xc v pha phi ng ti b phn tch sng nn sng mang t i pht xem l thng tin chnh xc v mt thi gian truyn n my thu.

Nhng nu thnh phn sng mang ln trong TCAM ta c th dng kiu tch sng khng kt hp trong khng cn phi to li sng mang.

Gi s bin sng mang ln sao cho A+s(t) >0 nh hnh 2.2

36

a

Hnh 2.2 a Tn hiu TCAM vi A+s(t)>0 b Mch in tch sng khng kt hp S phn tch mch tch sng da vo 2 nhn xt:

+Input khng th ln hn output b (vi mt diode l tng) +Output khng bao gi gim vi thi gian iu kin th hai l do t in trong mch khng c ng x in nn output lun lun bng vi gi tr nh xung thi im trc

Do Diode ch lm vic vi na chu k dng ca tn hiu AM; khi t c np n gi tr in p nh, na chu k m t x, in p trn t gim theo hng s

thi gian RC.

a, b, Hnh 2.3 Biu din s phng np ca t khi gii iu ch a,khi cha c t in x

b, khi c t in x

D1

T/H ra

T/H vo

C1 R

37

vic tch sng c chnh xc yu cu hng s k l 0m l vi iu kin ny th thi gian phng np s ln hn rt nhiu ln chu k ca sng mang nn khi t s

khng b np y vi s t chu k ca sng mang.

2.2.4 Tch sng kt hp (coherent) Phng php gii iu ch l gii iu ch kt hp (coherent) yu cu cn phi c mt tn hiu c cng tn s v pha ging vi tn hiu sng mang ti my thu.

Vic gii iu ch kt hp i vi DSB-AM bao gm vic nhn (trn) tn hiu iu ch vi mt tn hiu cosin c cng tn s v pha vi sng mang, sau cho tn

hiu ca vic nhn qua mt b lc thng thp s thu c dng tn hiu bn tin m(t) ban u.

Hnh 2.4 S tch sng kt hp

C s ton hc thc hin tch sng kt hp

Trong trng hp DSB, tn hiu sau iu ch c cho bi cng thc : Ac.m(t)cos(2fct) khi tn hiu ny c nhn (hay trn) vi mt tn hiu cos(2fct) ta s c biu thc sau sau khi trn nh sau :

Y(t) = Ac.m(t)cos(2fct)cos(2fct)= m(t) +

m(t) cos(4fct) (2.5)

y Y(t) k hiu cho tn hiu li ra mch trn v bin i Fourier ca biu thc (2.5) c xc nh nh sau : Y(f) = m(f) +

< m(f 2fc) +

< m(f + 2fc) (2.6)

Nh biu thc (2.6) li ra mch trn c mt thnh phn thng thp ( m(f)) v cc thnh phn cao tn nm quanh v tr 2fc. Khi cho Y(t) qua mt b lc thng thp vi rng bng W, cc thnh phn cao tn s b lc b ch cn li thnh phn

thng thp ( m(t)) t l vi tn hiu bn tin. Qu trnh ny c trnh by nh Hnh (2.4). 2.3Tch sng SSB-AM Nhc li v dng sng bin i furier ca tn hiu SSB-AM :

38

b Hnh 2.5 a, Bin i furier ca cc bng cnh b S khi hon iu SSB-AM V phng din tn s s lm ri tn ca tn hiu Sm(t) khi bin i furier c chiu ln v chiu xung

Bin i furier ca ca tn hiu SSB:

V phng din thi gian ta thy:

FSSB(t).cos2&'=%#b5c%n%boR

39

CHNG 3: NG DNG IU BIN TRONG MY PHT HNH, PHT THANH

3.1 NG DNG IU BIN TRONG MY PHT HNH -My pht phi pht i cng sut ln cung cp t s tn hiu trn nhiu (S/N) ln cho my thu.

-My pht phi s dng s iu ch chnh xc cc thng tin c pht i, khng b bin dng qu mc. -Tn s hot ng ca my pht c chn da vo cc knh v vng ph sng theo qui nh ca Hip hi thng tin quc t (ITV). Cc tn s trung tm ca my pht phi c n nh cao.

-Cc yu cu v ch tiu k thut ca my pht l: Cng sut ra, tn s lm vic,

n nh tn s, di tn s iu ch. -Phn loi my pht: a. Phn theo cng dng: Ta c My pht thng tin c nh hoc di ng; My pht theo chng trnh nh pht thanh, truyn hnh...

b. Phn theo tn s c. Phn theo phng php iu ch

V d: + My pht iu bin (AM) + My pht n bin (SSB) + My pht iu tn (FM), + My pht iu tn m thanh ni (FM Stereo) + My pht iu pha (PM) + My pht kho dch bin ASK, QAM + My pht kho dch pha PSK, QPSK + My pht kho dch tn FSK d. Phn theo cng sut My cng sut nh Pra

40

+ Tn hiu audio: Mang thng tin v ting ng,ca nhc, li thut minh, ting ni ca nhn vt.Tn hi ting c ph rng: 0 - 20khz, tai ngi cm th tt nht trong

khong 16 hz- 16000hz.Thc hin iu ch fm i vi tn hiu audio. +Tn hiu video: Mang thng tin v hinh nh v mu sc. Tn hiu video c ph rng: t 0-6 MHz. Mt ngi cm th tn hiu en trng tt hn tn hiu mu.

Thc hin iu ch am i vi tn hiu video. + Cc tn hiu ng b: ng b mu, ng b ngang, ng b dc V d: trong truyn hnh theo tiu chun NTSC tn hiu chi c ph tn t 0-4,2Mhz.

Cng thc ca tn hiu chi: Ey=0, 30Er+0, 59Eg+0,11Eb. Tn hiu mu l hai thnh phn mu iu ch bin nn trit sng mang v vung pha vi nhau l Ii v Iq.ph tn hiu mu 1,3 Mhz . Tn s sng mang mu 3,58Mhz

Hnh 3.1 ph ca tn hiu NTSC

Qua bng 3-2ta thy cc h truyn hnh trn th gii khc nhau v cc thng s k thut c bn nh: rng ca mt knh truyn hnh; rng ca di tn s hnh; khong cch gia ti tn hnh v ti tn ting; kiu iu ch hnh v ting;

s dng trong mt mt; tn s dng v tn s mt Gii thng s dng cho truyn hnh qung b l t 48 MHz-960 MHz bao gm 5 di: Di bng tn th nht: 48-64 MHz Di bng tn th 2:76-100 Mhz Di bng tn th 3:174-230 Mhz Di bng tn th 4:470-606 Mhz Di bng tn th 5:606- 958 MHz

41

Bng tn trong h thng truyn hnh mt t

S

TT

CC THNG S K THUT

CC H TRUYN HNH BBC RTF FCC CCIR OIRT

1

rng ca ti tn

hnh(MHz)

-3

10,4

4

5

6 2 rng knh (MHz) 5 13,15 6 7 8 3 Tng s dng trong mt mt 405 819 -525 625 625 4 Tn s mt (MHz) 50 50 60 50 50 5 Tn s dng (MHz) 10125 20475 15705 15625 15625 6 Thi gian qut 1 dng (ms) 98,7 48,8 63,5 64 64 7 Thi gian qut mt (ms) 20 20 16,4 20 20 8 Xung xo dng (%H) 15 16 16 18 18 9 Xung xo mt (H) 14 41 13-20 19-30 19-30 10 Xung cn bng trc (%H) 0 7 6 4,5 4,5 11 Xung cn bng sau (%H) 0 7 6 4,5 4,5 12 Khun hnh X : Y 4:3 4,12:3 4:3 4:3 4:3 13 iu ch hnh AM+ AM+ AM+ AM- AM-

14 Mc en (% bin sng mang)

30 3

252,5

752,5

752,5

752,5

15 Mc trng (% bin sng mang)

100

100

15

10 5

10 5

16 Chn tn bin trn (MHz) Cao Thp Thp Thp Thp 17 rng bin trn (MHz) +1,25 1,75 -1,25 -1,25 -1,25 18 Khong cch gia sng

mang hnh v ting (MHz)

-3,5

+11,15

4,5

5,5

6,5 19 iu ch sng mang ting

AM AM FM FM FM

Bng 3.2 Cc thng s k thut ca cc h.

42

-CC H TRUYN HNH. Trn th gii, tu theo tng khu vc, tu theo tng nc khc nhau c cc h

khc nhau v truyn hnh en trng v truyn hnh mu.

Mi mt h truyn hnh c cc thng s k thut ring ca n.

- H FCC ca M.

- H OIRT (Organisation Intenational Radio and Television) ca T chc pht thanh v truyn hnh quc t.

- H CCIR (Colsultative Commitee International de Radio) ca U ban t vn pht thanh quc t.

3.1.1 Chc nng v nhim v ca my pht hnh. -Chc nng

My pht hnh l thit b to ra nng lng di dng sng v tuyn in c cha thng tin v hnh nh v m thanh ri qua h thng anten truyn

thng tin ny vo khng trung. Tn hiu thu c li ra my pht c gi l

tn hiu truyn hnh. Cht lng tn hiu tt hay xu ph thuc rt nhiu vo cng sut, cht lng v n nh sng ra ca my pht hnh.

-Nhim v

Tn hiu hnh v ting sau khi gia cng, x l c a vo u vo ca my pht hnh. Hai tn hiu ny tip tc c sa, b, ghim mc m bo cht lng tn hiu, khuch i mc cn thit. iu ch AM sng mang

hnh, iu ch FM sng mang ting c tn s cao tn knh pht. Cao tn hnh,

ting iu ch c khuch i ti cng sut yu cu, lc, qua cp cao tn

a ln h thng anten, bc x vo khng trung di dng sng v tuyn in.

3.1.2 S khi c bn ca my pht hnh My pht hnh (= TV Transmitter) l thit b tip nhn tn hiu video, tn hiu audio iu bin vo sng mang hnh, sng mang ting to ra tn hiu cao tn hnh AM, tn hiu cao tn ting FM tng ng vi 1 knh truyn hnh.

S khi my pht hnh rt a dng, ph thuc vo nhiu yu t nh mc cng sut dng iu bin sng mang hnh, cng sut pht sng, knh pht sng, linh kin in t tch cc c s dng cho tng cng sut, hng sn xut, mc ch s dng Phn ny gii thiu 2 dng s khi tiu biu cho 2 khuynh hng iu bin hnh: iu bin mc cng sut ln v iu bin mc cng sut nh. Vi

43

mc ch tp trung trc tin vo phn tn hiu ca s khi, mt s khi mch

nh khi cp ngun, khi iu khin hin th, khi o lng kim tra, khi

thng gi lm mt s c xem xt sau.

Xem Hnh 3.3.

Nhim v cc khi: Khuch i video vo: Khuch i v phi hp tr khng tn hiu video u vo (thng c mc in p khong 1Vpp, tr khng 75 Ohm i xng hoc khng i xng). X l video: Bao gm mt s mch x l, hiu chnh tn hiu video m bo trung thc v chun xc ca hnh nh. Mch ghim (= clamp) dng ghim cc nh xung ng b cho ngang hng, khi phc li in p DC cho tn hiu video, m bo chun xc ca cc mc xm trn hnh nh. Mch tch v sa dng xung ng b hoc mch to li xung ng b hiu chnh li xung ng b... Mch ALC dng t ng gi n nh bin tn hiu video, do n nh c tng phn v bo ho mu ca hnh nh. Mch cn bng tr nhm (= group delay equalizer) dng b tr trc tr ca cc thnh phn tn s khc nhau trong tn hiu video do nh hng ca cc mch K v nh hng ca mch trung hp pha sau, m bo tnh trung thc ca hnh nh. Mch sa trc khuch i vi sai, mch sa trc pha vi sai dng sa trc bo ho mu, sc iu ca hnh nh

44

K Video vo

X l video

K Video 1

K Video 2

K Video 3

Trung hp

Lc sng hi

Dao ng ting

iu bin FM

X l audio

Nhn tn

ting

K cao tn

ting

KCS cao tn ting

Dao ng hnh

Nhn tn hnh

K cao tn

hnh

AM v KCS

CT hnh

Tn hiu Video 1Vpp

Tn hiu Audio 0dBm

Dao ng cao tn ting c bn

T.h. cao tn ting c bn FM

T.h. cao tn ting FM

Tn hiu Video vi trm Vpp

Dao ng cao tn hnh c bn

Dao ng cao tn hnh

T.h. cao tn hnh AM

T.h. cao tn ting FM

T.h. cao tn hnh AM v t.h. cao tn ting FM ca 1 knh truyn hnh

Anten pht

Dy truyn sng

45

Hnh 3.3 S khi my pht hnh iu bin mc cng sut ln Khuch i video 1, 2, 3: Cc mch K di rng tn hiu video t mc 1Vpp ln n hng trm Vpp vi cc tnh theo yu cu (m hoc dng) cung cp cho khi mch iu bin AM mc cng sut ln. Dao ng hnh: To dao ng cao tn hnh c bn c tn s Khuch i cao tn hnh: Gm nhiu mch khuch i cao tn c nhim v nng

cao dao ng cao tn hnh ln mc in p cn thit kch khi AM & KCS cao tn hnh.

AM v khuch i cng sut cao tn hnh: iu bin AM cc tnh m tn hiu video vo dao ng cao tn hnh to ra tn hiu cao tn hnh AM (fh) v khuch i cng sut di rng tn hiu ny ln mc cng sut hnh ra danh nh ca my pht hnh.

X l audio: Khuch i v phi hp tr khng tn hiu audio u vo (thng c mc in p 0dBm, tr khng 600 Ohm) kt hp vi nng tn cao (= preemphasis) tng kh nng chng nhiu trong qu trnh truyn sng FM sau ny.

Dao ng ting: To ra dao ng ting c bn, c tn s . Bin iu FM: iu bin FM tn hiu audio vo dao ng cao tn ting c bn to ra tn hiu cao tn ting c bn FM. Khuch i cng sut cao tn ting: khuch i cng sut tn hiu cao tn ting FM

ln mc cng sut ting ra danh nh ca my pht hnh. Trung hp(= Diplexer): Phi hp tn hiu cao tn hnh AM (fh) tc l sng mang hnh AM v tn hiu cao tn ting FM (ft) tc l sng mang ting FM mc cng sut ln, to ra sng cao tn ca 1 knh truyn hnh.

Lc sng hi: Lc b cc thnh phn hi tn, cc thnh phn tn s phch khng mong mun ang c ln trong sng cao tn ca knh truyn hnh ang pht

Nhn tn hnh: Nhn tn s ca dao ng cao tn hnh c bn ln n ln to ra dao ng cao tn hnh c tn s fh.

* Phn my pht hnh.

- Ti tn hnh knh cn pht c to ra v khuch i mc cho iu ch ngay sau cc b nhn tn v cc tng khuch i cao tn RF

46

- Tn hiu hnh sau khi c sa v khuch i ti hng trm vn nh nh ri a vo iu ch.

- Sng mang hnh RFh sau khi c iu ch AM c khuch i ti mc cng sut danh nh, a sang b trung hp.

Trong trng hp khuch i cao tn RF hnh, ting chung th khng cn b trung hp na. * Phn my pht ting.

- m tn ting qua b sa c a sang b iu ch FM. - B iu ch FM l ch sng to ti tn ting c bn, ng thi thc hin

iu ch FM y lun, y gi l trung tn ting. Trung tn ting tip tc

c a sang b trn to trung tn ting. - Ch sng trn a ti b trn ting, phch vi trung tn ting to

trung tn ting knh pht. Sau cao tn ting RFt qua khuch i kch ti

tng cng sut cui cng, a ti b trung hp, qua b lc hi, cp cao tn ra anten .

3.1.3 c im ca loi my pht iu ch mc cng sut ln. * Nhc im.

Do sng mang hnh cao tn ca knh pht c tn hiu hnh iu ch

tng cng sut cui cng, do cn thit phi c bin tn hiu hnh ln tu theo cng sut ra ca my pht m phi c hng lot mch khuch i tn hiu

video va m bo di tn rng ca tn hiu hnh 6MHz. iu ny v mt k thut thc hin rt kh khn, y l nhc im chnh ca loi my pht ny.

Ngoi ra gi thnh tng thm do phi c thm mt tng cng sut cui cng ca video tng ng vi my pht hnh c 2 khi cng sut K8 trng K4.

*u im.

iu ch mc cao c u im ni bt l ch cn mt tng cng sut ra cao tn mang hnh lm vic vi di tn rng 6MHz. Sau khi iu ch mc cao ca tn hiu hnh cc tng khuch i cao tn trc n lm vic ch C vi

di tn s hp do thc hin t hiu sut cao, t c tuyn tnh cao khi cn ch iu chnh ch lm ch B va m bo tuyn tnh v m bo hiu sut ra ca my pht.So vi my pht cng sut thp my pht cng

47

sut cao cn c cc tng khuch i tn hiu hnh c mc in p co chnh v vy

kh t c tuyn tnh trong di tn s di ph.

3.1.4 Khi ch sng hnh. * Nguyn l. Ch sng hnh l b t to dao ng to ra in p hnh sin, c tn s dao ng bng tn s sng mang hnh hoc tn s c bn ca sng mang hnh. Ch sng hnh rt quan trng, n quyt nh cht lng hnh ca knh

pht. Tu theo nhng hng sn xut v tu theo th h, my pht hnh c ch

sng hnh khc nhau.

Ch sng hnh to dao ng c tn s bng tn s knh pht fn[MHz] Th d knh 6 c tn s mang hnh: fh =175,25MHz. Ch sng to dao ng c tn s c bn fo, sau bi tn c tn s sng mang hnh ca knh pht.

fh = nfo[MHz] n : s ln bi . Loi iu ch trung tn gm hai ch sng:

- Ch sng to ra dao ng c tn s trung tn hnh fT1. - Ch sng to ra dao ng c tn s cao tn fRF sau khi trn (cng hoc tr) vi tn s trung tn hnh to ra tn s sng mang ca knh pht :

fh = fRF fT1 [MHz] fRF : tn s dao ng cao tn fT1 : tn s dao ng trung tn hnh. Ch tiu c bn ca ch sng hnh : - Bo m n nh, chnh xc v bn vng ca tn s t dao ng c bn. - Bo m bin sng mang c bn mc danh nh kch thch cho tng khuch i k tip.

- Trnh khng b can nhiu t bn ngoi vo. * Cc bin php k thut t ch tiu cho ch sng hnh. - Thit k chn mch t dao ng bng thch anh, c n nh, chnh xc cao.

48

- C mch sa cho mch t dao ng, gi c n nh nht nh, khng b tc ng nhit bn ngoi lm thay i cc tham s thnh phn tham gia dao ng, nht l thch anh v cc phn t tch cc. - Bc kim thnh tng ngn v bc kim ton b ch sng trnh can nhiu ln nhau v trnh can nhiu t ngoi vo.

- Chn phn t tch cc (n, transistor ) c cng sut nh trnh b tc ng nhit .

- Chn cc linh kin phi tht ngn trnh gy cm khng, dung khng tp tn nh hng n tn s dao ng v phi c hn chc chn, nhm chng rung ng.

- Cc ch s linh kin th ng phi chnh xc, sai s khng ln hn 0,1

%.

- in p ngun cp cho b ch sng phi c n nh, phi c n p mt chiu v chng nhiu.

- Dng dy ng trc c tr khng ra nh ghp vi cc tng. * Mch in in hnh ca ch sng hnh.

Mch in hnh 3.4 l mch in c bn ca ch sng hnh. Mch t dao ng bng thch anh X. Thch anh X c tn s dao ng bng tn s cn c ca sng mang hnh knh pht.

Hnh 3.4Mch to dao ng tn hnh

R13

C9

+V

C8

C7C3C2

C1 C5

T3T2X

L

C4C6

T1

R14

R11 R12

R2

R1

R8R6

R5

R4

R10R7R3

R9

49

Nguyn l hot ng ca mch; Khi u T2 lm vic nh mch khuch i mt chiu do R5 v R6 nh thin ch cng tc. in p mt chiu emit UE p vo mt m ca thch

anh X lm cho n t dao ng tn s ca n, m kia truyn dao ng qua t C5 vo emit ca T1 khuch i cao tn mc baz chung. Ti cao tn ca dao ng ly ra mch cng hng song song L//(C2+C3), C3 tinh chnh cng hng c ng tn s sng mang. R1, R2 nh thin ch lm vic cho

T1. Sau a ti khuch i cao tn tip T2. Ti ly ra colect ri qua t

C6, R9 vo T3 mc ti emit(nh thin bi R10,R11) sau qua t C7 sang tng tip theo. Cao tn ca ch sng ra im R. Ngun cung cp Ec qua in tr R14 , t xuyn C9 v c st p trn

in tr R13 cung cp cho c 3 tranzito ca ch sng. T C8 , C1 thot cao tn

xung t.

ng t nt th hin bc kim cho ch sng trnh can nhiu t bn ngoi vo.

3.1.5 Mch trn tn to cao tn hnh Trong cc my pht hnh th h mi u c s dng cc mch trn tn s to ra tn s sng mang (hnh v ting ca knh pht). * Trn tn to tn s mang hnh knh pht.

fh = fcs - fAMTT = fCS - 38MHz

Trong : fh-Tn s sng mang hnh

fCS- Ch sng thch anh fAMTT- Tn s trung tn hnh 38MHz c iu ch

Nh vy l u vo ca b trn tn c hai in p: mt cao tn do ch sng chnh to nn c tn s ln hn tn s knh pht v mt in p trung tn

hnh c iu ch (bin ly ra t b iu ch). Hai tn hiu ny thc hin trn to ra tn hiu mi c tn s bng hiu tn s ca hai u vo - ng bng tn s ca sng mang hnh ca knh my pht. Yu cu ca b trn sao cho tn s sng mang c to ra n nh v mc cng sut cn thit.

50

Hnh 3.5 Mch in trn tn. Mch in trn tn hnh 3.5 thc t l mch khuch i cao tn. in p cao tn ch sng chnh c tn s fCS a vo baz ca tranzito, c mch L1//C2 cng hng ng tn s fCS. in p trung tn hnh AM c cng hng

mch emit bi L3//C7 ng tn s 38MHz. Di tc dng ca hai tn hiu ny lm mo dng dng ng IC m gc thng dng ng vi hiu tn s ca chng. Mch cng hng L4// C5 colect cng hng ng tn s ca sng

mang hnh knh pht fh = fcs - 38MHz.

* phn pht ting i vi my pht hnh iu ch mc cng sut thp

tn s trung tn cn thit c hai b trn: trn to tn hiu trung tn v trn to cao tn sng mang ting ca knh pht.

- Trn to tn s trung tn ting.

u vo ca b trn ny c tn s fTTh= 38MHz (h OIRT) ly t ch sng trung tn hnh.

Tn hiu iu ch FM tn s bng khong cch gia tn s mang hnh v mang ting (i vi h OIRT l 6,5MHz, h CCIR l 5,5MHz, h FCC l 4,5MHz). u ra ca b trn tn hiu trung tn c tn s l:

fTTt= fTTh- fFMf

fFMf: khong cch gia tn s mang hnh v mang ting ca

knh pht.

i vi h OIRT, PAL-D tn s trung tn ting l 31,5MHz.

fh = fcs-38Mhz

38Mhz

AMTTf

fcs

+Vc

C

C1

L1

C21uF C4L2

C3

C6

C7

+

C8C5L3

T

R2

R1

R3

R4

51

Mch in hnh 3.6 l mch trn to tn s trung tn ting. Mch in trn tn dng diode:

Hnh3.6 Mch trn to tn s trung tn ting Mch trn c cu to bi 4 diot lm vic ch cao tn mc theo kiu cu. in p xoay chiu trung tn hnh 38MHz qua t C2,in p iu ch FM

ting qua mch cng hng ni tip L1C1 cng a vo im A v B ca cu.

Cu diot D1 D4 lm vic nh tch sng pha sao cho dng thng cc diot c

pha lch so vi hai tn s a vo, tc l tn s ra bng hiu ca tn s trung tn hnh v tn s fFM.

- Trn to tn s mang ting knh pht.

B ny c nhim v trn trung tn ting vi cao tn do ch sng chnh to ra (sau khuch i ln mc cng sut ra a vo b trn) u ra ca b trn c tn s cao tn sng mang ting ca knh pht. ft = fcs - fTTt

ft : tn s sng mang ting

fcs: tn s ca ch sng hnh

fTTt: tn s trung tn ting.

3.1.6 Mch bi tn. Bi tn hay nhn tn l mch khuch i cao tn, trong s dng cc bin php k thut u ra c in p cao tn ln nht vi tn s c nhn ln n ln tn s gc.

52

Chc nng chnh ca bi tn l: nhn tn s ca tn hiu cao tn c bn do ch sng to ra t c tn hiu cao tn c tn s sng mang hnh ca knh pht.

fh = nf0 fh: tn s knh pht

n: s ln bi (nhn ) f0: tn s c bn do ch sng to nn. Tu theo tn s c bn ca b ch sng v tu theo tn s mang hnh knh pht m c bi s nhiu hay t v c cc tng nhn k tip nhau tng hp cc tng nhn li c n ln tn s gc c bn. * u im ca nhn tn.

- Dng ch sng tn s thp d n nh hn. - Trong b nhn tn, tn s ra v tn s v cch xa nhau. Do tng n ghp vi tng kia khng gy nh hng ln nhau, my n nh.

- Khng phi dng thch anh c tn s cao ch sng, m loi thch anh ny mng, va d v, va dao ng yu, ch cn ch sng n tn s gc c bn (thp) m sau khi nhn tn ta c tn s sng mang (gp n ln theo mun) n nh.

- M rng c di tn nhiu ln hn, khi ch sng di tn khng rng. - V tn s cch xa nhau, cc tng khng b gy nhiu cho nhau, khng cn

Hnh3.7 Mch in bi tn

53

3.1.7 Mch khuch i v sa m tn Tn hiu ting a ti my pht, trc khi a vo iu ch u phi c

x l. Cc mch sa, khuch i m tn ny c nhim v: - Khuch i mc in p v cng sut cp cho iu tn ting.

- Sa tn s di tn m thanh m bo c tuyn ca chng, l tng t 20Hz 20.000Hz.

Trong mch khuch i m tn c chit p iu chnh m lng, su

iu ch FM.

3.1.8 B tin iu ch. Ngn tin iu ch bao gm cc loi mch sa, hiu chnh, khuch i tn hiu hnh mu ton phn tiu chun k thut iu ch sng mang hnh.

Tn hiu hnh mu ton phn trung tm (studio) ra tiu chn k thut. Qua ng truyn dn (cp, viba...) n my pht hnh, tn hiu b tn hao, mo nn nht thit phi c x l li. Cc thng s v mc, v thi gian, v di tn video, sng mang mu, xung...u phi sa v b. Trong ngn iu ch c cc loi mch x l cc vn trn. Ngn tin iu ch l cc k quan trng, n quyt nh cht lng hnh nh truyn ti ngi xem.

-Mch in ca b tin iu ch. Mch ghim khi phc thnh phn mt chiu.

Hnh3.8 Mch ghim dng transistor Tranzito T1 l n ghim, T2 l tng ti emit. C hai tranzito u dng loi npn. Tn hiu hnh trn dng truyn dn ti my pht cng nh qua cc mch , t ngn, t ni tng b mt thnh phn mt chiu( chi trung bnh) lm cho hnh nh km trung thc. Do cn phi c hi phc li. Tn hiu ny a vo

54

u V qua t ghim C1 ti Baz ca T2. Nu khng c mch T1 th tng ti emit

T2 khng lm vic. T1 lm vic nh mt kho in t. Xung ghim cc tnh

dng c rng 3,5s, bin 7Vdd qua t C2 vo baz ca T1. Ti thi im c xung dng T1 m (in tr trong rt nh), t C1 np rt nhanh vi hng s thi gian:

m = RmC1 3,5s Rm : in tr trong khi T1 m ( rt nh ). C1: t ghim v ni tng tn hiu hnh.

T C1 c np ti in p mt chiu xc nh bng mc ghim, mc ghim iu chnh bng chit p R3. Thi gian khng xung (thi gian qut thun ca dng), T1 ng, t C1 phng in qua in tr trong ca T1 v Rd c tr s in tr trong rt ln, nn

dng phng rt chm. Ti baz ca T2 lun gi mc in p mt chiu bng mc ghim, mc in p ny ng thi cng l thin p lm vic cho n T2.

Tng ti emit T2 hot ng bnh thng, tn hiu ly ra emit (ti R5) ti im Vr c khi phc thnh phn mt chiu.

Hng s thi gian T1 ng l:

d = Rd .C1 52s Rd : in tr rt ln khi T1 ng.

iu kin mch ghim lm vic tt, bn vng l:

- Rml Rd - T1 chn sao cho nh kho tt m dt khot c thi gian chuyn

tip cc nhanh.

- Xung ghim m bo mc in p v rng. - Nu T1 l loi pnp th xung ghim c cc tnh m.

- T C1 chn tr s t 01 03F v l t thng. 3.1.9 Tnh ton mch iu ch AM B iu ch thc hin iu bin (AM) sng mang hnh ca knh pht bng tn hiu hnh ton phn c sa tng tin iu ch a sang. * Yu cu k thut.

- t su iu ch d dng. - Bo m rng knh 8MHz v di thng tn ca tn hiu hnh 6MHz.

55

- c tuyn tn s knh truyn bin di -1,25MHz t -20dB n -35dB. - Bin sng mang sau khi iu ch bo m cng sut danh nh ca su iu ch.

- Trnh can nhiu ca bn ngoi, cng nh cc tng khc trong my. - Sau khi iu ch vn m bo tuyn tnh.

Hnh 3.9 Mch in iu bin AM cn bng dng transistor v diode Tranzito T1 khuch i cao tn sng mang hnh mu qua bin p cao tn mch colect ca n, tn hiu sng mang ny c ly ra hai cc tnh ngc

pha nhau 1800 a vo b iu ch cn bng. Hay ni cch khc bin p Tr1 phi hp gia sng mang khng i xng sang i xng, m im t C l tn

hiu hnh ton phn a vo, yu cu bin p cao tn qun tht cn nhau v cun cm c m bc. Mch iu bin cn bng c to bng cc diot c tn s lm vic siu cao.

Ti hai im i xng cho cao tn, a vo tn hiu mu ton phn

(sau khi c sa, b tin iu ch) +b(t) v -b(t). Khi khng c tn hiu hnh - tn hiu iu ch ngc cc tnh nhau th

u ra khng c sng mang. Khi c +b(t), -b(t) th dn ca cc diot cao tn D1 D4 s thay i khc nhau theo bin iu ca tn hiu hnh lm cho mch

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iu ch cn bng. Do dng in cao tn bin thin qua cc diot dn ti dng sng mang n mch ra (bin p Tr2) bin i khng u v bin na m theo dng tn hiu iu ch. Tn hiu hnh ton phn mch ra b trit tiu do tn hiu a vo ngc pha nhau 1800. Nh vy iu bin sng mang hnh c thc hin tip tc qua tng m emit T2 ti tip cc tng sau. Mch iu ch AM cn bng m bo bn vng, d t c su iu ch 100%, tuyn tnh, sng mang mu khng b suy gim, cht lng mu tt.

3.1.10 Chc nng v c tnh cc tng khuch i cao tn. Do iu ch mc cng sut thp, nn sng mang hnh, mang ting sau

khi c iu ch phi c mt lot cc tng khuch i cao tn khuch

i dn ln ti khi kch thch cui cng. Cc tng khuch icao tn hnh ngoi khuch i sng mang, cn m

bo di tn Video 6,5MHz. l kh khn ca loi iu ch mc thp. Do phi c mt lot cc bin php k thut x l nhm tho mn yu cu trn, cc mch lm vic ch B hoc ch BC.

Cc tng khuch i cao tn ting d thc hin hn v di tn m thanh khng ln, c th lm vic ch C hiu sut cao.

Cc tng khuch i cao tn ting v hnh cn phi bo m:

- Khuch i mc danh nh, mi tng khuch i t 1020 ln.

- n nh sng mang.

- Bo m di tn s ca tn hiu hnh 6MHz, tn hiu m thanh

30Hz15KHz.

- Lc tt, loi tr cc hi khng c ch ngoi knh pht.

- Chng c can nhiu t bn ngoi v gia cc tng vi nhau. - Bo m phi hp tr khng gia cc tng.

- Trch mt phn cng sut tch sng kim tra.

V cu to mch, cc tng khuch i cao tn hnh v ting c bn ging nhau, ch khc cc phn t cng hng cho mi loi sng mang khc nhau

nn tr s khc nhau.

H s khuch i ca tng khuch i cao tn c xc nh bng biu thc:

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A = SZa = S /2piBtrC

Trong : A - h s khuch i.

S - dc ng( h dn) ca n. Za - tr khng mch ra ( anot hoc colect) Btr- tn s bin trn ca n hiu truyn. C - ti dung khng. T biu thc trn ta thy r rng i vi di tn ca tn hiu cn truyn cng ln th h s khuch i A cng gim. V cng rt kh khn cho vic hiu

chnh m bo ng thi sng mang v di tn tn hiu. t c di rng cn thit ca cc tng khuch i cao tn hnh lm vic ch B, ta chn mt trong cc bin php k thut sau y: - Suy gim ng b cc tng cng hng. - Cng hng ring r tng phn ring bit. - S dng cc mch ghp.

3.1.11 B trung hp v mch lc hi -B trung hp Nh ta bit, my pht hnh dc cu to gm my pht hnh v pht ting ng b vi nhau. Nu trc tip a cao tn AM ca my pht hnh v FM ca my pht ting ra cng chung mt anten th khng th c, v cao tn

u ra ca my pht hnh s gy nh hng lm mo dng c tuyn ca tng cng sut RF cui ting v ngc li. Do phi c gii php k thut l dng b phi hp sao cho ch s dng mt anten cho c hnh v ting. B phi hp ny gi l b trung hp (diflexer). - Mch lc hi B lc hi (Filter- harmonic ) c cu to nh b thng tn thp, nhim v ca n l loi tr, chn tt c cc cao tn hi c tn s bi tn s sng mang hnh v mang ting.

Trong cc mch cao tn, t tng c cng sut thp n tng cng sut

cui cng ngoi sng mang ca knh pht cn c hi bi hai, hi bi ba... gy nn nhiu sang cc knh khc nn cn thit phi c lc, chn c tuyn

suy gim t -26dB. Th d hi bc ba ca knh 2: tn s mang hnh knh 2 l 59,25MHz, bi ba ln l 175,25MHz nm ng trong di tn ca knh 6 (tn s

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mang hnh 175,25MHz ). Nu khng c lc v chn hi ny th s nhiu sang my pht knh 6.

Hnh 3.10Mch in tng ng ca b lc hi iu chnh cc hc cng hng v vn cc bu lng A, B, C, G s t c di thng cn truyn, lc v chn cc hi ngoi knh. 3.1.12 Dy dn cao tn v Anten. -Dy dn cao tn Dy dn cao tn thng gi l fid dn sng cao tn t u ra ca my pht ti anten pht. Tu theo cng, tn s knh pht, tr khng c tnh m c

loi dy dn cao tn kch c khc nhau. Dy dn cao tn l dy dn ng trc v dy dn gia v dy dn ngoi u hnh trn v c cng mt tm. Dy dn gia bng ng dn sng, dy dn ngoi l ng bc kim hnh tr trn ng tm vi dy gia. Dy ngoi c ni vi t, v my, ct anten kn mch cao tn. Gia hai dy l cht cch in cao tn, chu c in p cao tn, c th l khng kh, c th l teplon

cao tn, yu cu cch in cao. Ngoi cng l lp nha bo v.

-Anten. Anten l ti ca my pht, l lin kt gia my pht vi mi trng

khng trung m trong sng cao tn lan truyn ti my thu. Anten bc x sng cao tn ( c iu ch mang tn hiu thng tin cn truyn ) ca my pht vo khng trung.

Anten pht v thu khc nhau. Anten pht l ti ca my pht, anten thu l

ngun cp cho my thu. S khc bit c bn l chc nng, cu to. Anten pht

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yu cu nghim ngt v cc thng s ch tiu k thut, phi chnh xc di c kh, xy dng ct anten, lp t ct anten s phc tp tn km rt nhiu. Do tnh cht lan truyn ca sng cao tn vi cc bc sng khc nhau, nn c nhiu loi anten tng ng vi di tn ring bit. Trong k thut truyn hnh, mi knh c anten ring ca n, hoc anten

di rng dng cho c bng. Anten di rng dng cho bng III c tn s t 174MHz n 230MHz.

3.2 ng dng iu ch AM trong my pht thanh Gii thiu chung Tn hiu thng tin c truyn t my pht ti my thu thng qua mi trng

truyn .Tuy nhin tn hiu thng tin gc t khi ph hp vi mi trng truyn cho

nn chng ta phi bin i t dng thng tin ban u thnh dng tn hiu ph hp vi m trng truyn. my pht thanh AM s thc hin qu trnh lm thay i

bin ca tn hiu sng mang kt hp vi bin ca tn hiu iu ch(tn hiu tin tc). B iu ch bin AM l thit b c rp t nhng linh kin phi tuyn vi hai ng vo. Mt sng mang c bin c nh tn s cao v tn hiu thng tin tn s thp.Tn hiu sng mang phi c tn s cao anten bc x mt cch c hiu qu v truyn xuyn qua khng gian t do thng c gi l tn s radio hoc n gin l RF. Tn hiu thng tin c th l tn s n hay phc hp ca nhiu tn

s.Mc d mang nhng c im chung ca iu ch bin qu trnh truyn sng mang hai bin cnh DSB-TC thng c s dng nhiu nht. iu ch DSB-TC i khi cn c gi l iu ch bin truyn thng hay n gin l AM.

Bng tn c s dng trong h thng pht thanh AM Bng sng di: 300-500 KHz. Bng sng chung: 550-1650 Khz. Bng sng ngn: 1, 8 Mhz- 18, 5 MHz.

Phn loi my pht thanh AM -My pht AM mc cao

-My pht AM mc thp

3.2.1 Kho st my pht AM mc thp Hnh 3.11 v dng s khi my pht AM DSB-TC mc thp.Khi ta truyn ging ni ca con ngi hoc m nhc l ngun gc ca tn hiu iu bin. Mt cch

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tng qut n l mt b chuyn i m thanh nh microphone bngs t a CD my ht a. Mch tin khuch i phi c nhy tng i cao l mch khuch i

in p tuyn tnh loi A vi tr khng ng vo cao. Chc nng ca mch tin khuch i l tng bin ca ngun tn hiu thng tin n mc c th s dng c, trong khi li pht sinh nhiu v tuyn rt nh khng ng k ng thi phi

khc phc nhiu nhit sao cho cng nh cng tt. Mch iu khin tn hiu iu

ch cng l mch khuch i tuyn tnh nhng c th iu khin c mch iu

ch.

Hnh 3.11 s khi my pht thanh AM mc thp.

Mch do ng to sng mang RF ca my pht c yu cu phi tht chnh xc, n nh cao. Do thng s dng thch anh to dao ng trong mch in. Mch khuch i m c h s khuch i tht thp l mch khuch i tuyn tnh

ng vo c tng tr cao, nhim v ca mch khuch i m l cch ly mch dao ng vi mch khuch i cng sut cao. Mch m cung cp mt ti c nh ti

mch dao ng, mch m gip gim s bin thin tn s ca tn hiu, mch t hp opamp thng c s dng trong mch m.Mch khuch i tc thi v mch khuch i cng sut tng cui thng dng mch khuch i loi A hoc khuch i y ko loi B.H thng anten phi tng ng vi tng tr ra ca mch khuch

i cng sut tng cui. H thng pht AM mc thp c s dng rng ri trong

Khuch i cng sut

Khuh i m

iu khin sng mang

Ngun tn hiu iu ch

Tin khuch i

iu khin tn hiu iu ch

Mch iu ch

Mch dao ng RF

Mch ghp

Anten

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h thng pht cng sut thp. Nhng h thng pht cng sut thp nh h thng lin

lc v tuyn, remote iu khin, my b m phm vi gn.

3.2.2 My pht AM mc cao Hnh 3.12 s my pht AM mc cao tn hiu iu ch cng c x l tng t my pht AM mc thp ngoi tr c mch khuch i cng sut . i vi my pht

AM mc cao cng sut ca tn hiu iu ch phi cao hn trong my pht AM ln

xy ra iu ch thng thng tn hiu iu ch phi c bin tht cao t c phn trm iu ch l 100%

Hnh 3.12 s my pht AM mc cao.

Mch do ng to sng mang RF cng c kt hp vi b m v b kch thch sng mang ging my pht AM mc thp. Tuy nhin my pht mc cao th tn hiu

sng mang phi c khuch i cng sut trc khi n tng iu ch v khuch

i cng sut cui cng.

i vi my pht AM mc cao mch iu ch c 3 chc nng chnh : cung cp

mch in cn thit xy ra iu ch, mch khuch i cng sut sau cng, tc

chuyn mch cao.

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3.2.3 Mt s mch in c bn trong my pht thanh AM Ti my pht ni xy ra qu trnh iu ch phi c mch in xc nh my pht

mc cao hay mc thp. i vi my pht mc thp yu cu t ra l phn trm iu

ch phi cao.

i vi my pht mc cao s iu ch c thay cho phn sau ca tng cui, ni c

bin sng mang t gi tr cc i.V vy yu cu bin tn hiu phi ln hn t c phn trm iu ch ti a.vi iu ch mc cao tn hiu sau iu ch

phi c khuch i cng sut tt c cc di bin. Cng sut di bin phi ln hn 33% tng cng sut pht

Mch iu ch AM mc thp Mt tn hiu nh c khuch i loi A c biu din nh hnh 4.4a c th c s dng iu ch bin

Hnh 3.13 a, Mch iu bin cc E dng mt transistor b, Dng sng ng ra ca mch mch khuch i trn c hai ng vo. Ng vo th nht l tn hiu sng mang, ng

vo th hai l tn hiu tin tc. Khi khng c tn hiu tin tc th mch hot ng nh

mch khuch i loi A ng ra l tn hiu sng mang c khuch i bi li p tnh.

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Tuy nhin khi t tn hiu vo mch, mch khuch i hot ng khng tuyn tnh

v xy ra qu trnh nhn tn s tn hiu .Tn hiu sng mang c a vo cc B

ca transistor tn hiu iu ch c a vo cc E tn hiu iu ch thay i theo

li ca mch khch i di dng hnh sin v bng tn s ban u ca tn hiu iu ch.

li p ca mch iu hp cc pht c minh ha bi biu thc sau: Av = Aq (1 + m.Sin2fm.t) (3.1) Av l hs khuch i p khi c tn hiu iu ch.

Aq l h s khuch i p tnh (khng c tn hiu iu ch) Sin2fmt thay i t gi tr cc i +1 n gi tr cc tiu -1 cho nn (3.1) c th c vit nh sau:

Av = Aq (1 m) Trong : m l h s iu ch, khi iu bin 100% th m =1. Trong mch iu ch trn tn hiu tin tc c a vo thng qua bin p T1 sng mang vc c a vo cc B Tn hiu iu ch s iu khin mch trn hot ng

hai trng thi dn bo ho v ngng dn. Cho nn, cn thit phi to ra s khuch i phi tuyn khi xy ra qu trnh iu ch. T in C2 c nhim v l di chuyn tn s tn hiu iu ch t dng sng AM ra. V vy sinh ra bao hnh AM i xng ti Vout

Vi mch iu hp cc E, bin tn hiu ng ra ph thuc vo tn hiu sng mang ng vo v li p ca mch khuch i. H s iu ch hon ton ph thuc vo

bin ca tn hiu iu ch. Nhng nhc im chnh ca mch iu hp cc pht l mch hot ng ch A, m ch ny th h s khuch i khng cao.

Mch iu hp cc pht cng c nhc im l tiu th cng sut ln dng sng ng ra.

Mch iu bin AM cng sut trung bnh: My pht AM cng sut trung bnh v cao b hn ch bi vic s dng n chn khng, l nhng linh kin th ng. Tuy nhin k t gia nm 1970, my pht s dng linh kin bn dn c a vo s dng, t cng sut ra c nng cao n hng ngn Watts. Nhng thit b ny dn dn c ci tin bng cch thay th nhng mch khuch i cng sut song song vi vic phi hp pha ca tn hiu ng

ra.

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Hnh 3.14 trnh by s nguyn l ca mch iu ch AM cng sut trung bnh dng 1 transistor. Qu trnh iu ch c thc hin ti cc thu l thnh phn tn hiu ra ca transistor.

a,

b, Hnh 3.14 a, mch iu ch AM cng sut trung bnh b,Dng tn hiu iu ch khi cha c tin tc. Nh vy, nu y l tng lm vic cui cng ca my pht th l iu bin mc cao (khng c s khuch i cng sut iu ch v Anten). mch lm vic c hiu qu, t cng sut cao. Mch iu chAM cng sut trung bnh v cao thng hot ng ch C. Do cng sut thc t ca mch c th t hn 80%. S

mch in 3.14 l mch khuch i ch C vi hai ng vo: Ng vo sng mang

(Vc) v ng vo tn hiu iu ch n tn (Vm). V transistor phn cc ch C nn hot ng khng tuyn tnh v c kh nng

mch cng trn khng tuyn tnh. Mch ny c gi l mch iu hp cc thu (C)

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bi tn hiu iu ch c a trc tip vo cc thu C. RFC l cun cn tn s sng v tuyn, n hot ng nh sau: H mch i vi thnh phn DC ng thi ngn

mch i vi tn s RF.

Cho nn, RFC ngn cch ngun cung cp DC gia sng mang cao tn vi tn s

bin, trong khi vn cho php tn hiu tn s thp iu ch ti cc thu ca Q1. Hot ng ca mch in nh sau:

Hnh 3.14c Dng tn hiu sau iu ch

Khi bin ca sng mang ln hn 0.7 v vi transistor silic Q1 dn sinh ra dng cc C Ic .Khi in p sng mang nh hn 0.7 v th ng ngha vi khng c dng cc C Q1 ngng dn.Trng thi dn v ngng dn ca Q1 c iu khin bi sng mang mt khc dng cc C lun nh hn 180 chu k sng mang v hot ng

ch C. Dng sng in p cc C lp li ging nh 1/2 dng sng ca tn hiu chnh lu vi tn s c bn bng fc. Khi tn hiu c a vo cc C mt cch lin tc vi ngun cung cp DC m v dng (Vcc) th dng sng thu c s to ra sng iu ch vi bin cc i Vcc Bi v, Q1hot ng khng tuyn tnh, nn dng sng cc C ca Q1cha hai tn s ng vo ban u( fc v fm) tng v hiu tn s ca chng l fc fm. V dng sng ng ra cng cha nhng hm iu ho tn s cao hn v nhng thnh phn

bin iu tng h c gii hn li fm = fc trc khi pht i. Mt mch in thc

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t hn pht cng sut trung bnh, tn hiu iu ch AM DSB-TC c v trn hnh 3.14a v c tuyn dng sng v trn hnh 3.14b. mch in ny cng l mch iu hp cc C vi bin nh cc i ca tn hiu iu ch Vm(max)= Vcc

Hnh 3.15 Mch iu ch AM DSB-TC a, S nguyn l Hot ng ca mch cng tng t nh mch n bin hnh 3.14a ngoi tr c thm mch cng hng L1C1 ti cc thu ca Q1. Bi v transistor hot ng trng thi bo ho v ngng dn, dng cc thu khng ph thuc vo in p iu khin cc nn B. in p ca mch tng ln thng qua mch cng hng c xc nh

bi thnh phn AC ca dng cc thu, gy nn tnh cht tr ca mch cng hng ti tn s cng hng. Vi s ph thuc vo cht lng ca cun dy, dng sng ca tnhiu sng mang c iu ch v dng cc thu cng ging nh phn trc. in p ra l tn hiu AM DSB-TC i xng vi in p trung bnh l 0V, bin nh cc di dng l 2Vcc, bin nh m cc tiu l -2Vcc. Bn k dng ca sng ra c to ra trong mch cng hung bi hiu ng Flywheel. Khi Q1dn, t C1 np n gi tr Vcc+ Vm(gi tr cc i l 2Vcc). Khi Q1 ngng dn t C1x in thng qua cun L1. Khi L1 x C1 np n gi tr cc tiu l -2Vcc. Qu trnh trn to ra bn k dng ca bao hnh AM . Tn s cng hng ca mch cng hng bng tn s sng mang v bng thng c gi tr t fc- fm n fc+ fm. Thng

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thng tn hiu iu ch l mt hm iu ha. Khi phn trm iu ch t 100% th

bin nh ca tn hiu iu ch bng Vcc. Cu to chung ca mch c biu din trn hnh 3.15a. R1 l in tr phn cc cho Q1, R1v R2 l dng mch ghim to nn s phn cc trong mi ni vi hng ro th ca transistor xc nh in p m ca Q1. Thng thng Q1 c phn cc sao cho ch dn trong bn k dng ca in p song song. S phn cc ny to ra dng sng dng cc thu nh v nng cao hiu sut ca mch khuch i hng C.C3 l t in lin lc, n ngn mch i vi tn s tn hiu iu ch v ngn tn

hiu thng tin vo ngun cung cp DC. Cbe l t dn tn hiu t cc nn n cc thu ca Q1. Ti tn sv tuyn, nhng t dn tn hiu nh v trong transistor l rt quan trng. Nu dung tch ca t Cbc ln th tn hiu cc thu c th c phn hi v cc nn ca Q1vi bin va ln Q1bt u dao ng. Cho nn mt tn hiu bng nhau v bin cng nh tn s v lch pha nhau 180 s phn hi v cc nn

ca Q1hoc l trung ha gia cc ca in dung phi hi. CN l t in trung ha. Nhim v ca n l to ng phn hi cho tn hiu c bin v tn s bng

nhau nhng lch pha vi ng ra 180 c phn hi v thng qua Cbc.

C4 l t in lin lc RF hot ng ca n cng tng t nh t C3, ti tn s sng

mang C4 xem nhn gn mch ngn sng mang r vo ngun cung cp iu chnh

hot ng mch iu ch tn hiu v c phn b khp trn my pht.

Hnh 3.15b Dng sng trn cc C v ng ra

Mch to dao ng sng mang Mch to dao ng sng mang cn c chnh xc cao v phi tht n nh nh vo c tnh p in ca thch anh m thc t nhng my pht thanh thng s dng mch to dao ng bng thch anh Tc dng ca cc linh kin:

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+ L1,C1 khung cng hng

+L2 hi tip

+R1,R2 nh thin phn p cho transistor Q +R3 n nh nhit

+C2,CF hi tip dng +C3 t lc ngun.

Hnh 3.16 Mch dao ng dng thch anh vi tn s cng hng ni tip. -Nguyn l hot ng ca mch Khi c cp ngun mch dao ng vi tn s cng hng dao ng ring ca khung C1,L1. Trong khung cng hng c dao ng vi tn s cng hng ng bng tn s cng hng ni tip ca thch anh, tr khng ca thch anh nh thnh phn hi tip v cc B ln. Nh vy mch to c dao ng vi tn s bng tn s cng hng ni tip ca thch anh

69

Chng 4: M PHNG IU CH AM DNG MATLAB 4.1 Gii thiu chng trnh MATLAB: Chng trnh MATLAB l mt chng trnh vit cho my tnh PC nhm h tr cho cc tnh ton khoa hc v k thut vi cc phn t c bn l ma trn trn my tnh c nhn do cng ty "The MATHWORKS" vit ra. Thut ng MATLAB c c l do hai t MATRIX v LABORATORYghp li. Chng trnh ny hin ang c s dng nhiu trong nghin cu cc vn tnh ton ca cc bi ton k thut nh: L thuyt iu khin t ng, k thut thng k xc sut, x l s cc tn hiu, phn tch d liu, d bo chui quan st, v.v MATLAB c iu khin bi cc tp lnh, tc ng qua bn phm. N cng cho php mt kh nng lp trnh vi c php thng dch lnh cn gi l Script file. Cc lnh hay b lnh ca MATLAB ln n s hng trm v ngy cng c m rng bi cc phn TOOLS BOX( th vin tr gip) hay thng qua cc hm ng dng c xy dng t ngi s dng. MATLAB c hn 25 TOOLS BOX tr gip cho vic kho st nhng vn c lin quan trn. TOOL BOX SIMULINK l

phn m rng ca MATLAB, s dng m phng cc h thng ng hc mt cch nhanh chng v tin li.

Bn cnh MATLAB cn cung cp mt s lnh x l ha. N bao gm nhng lnh cp cao cho d liu hai chiu v ba chiu, x l nh ng v cc biu din ha. N cng bao gm nhng lnh cp thp m cho php ta ty chn s xut hin ca ha ging nh l vic xy dng mt giao din s dng ha hn ho trong nhng ng dng MATLAB ca mnh. Ngoi ra c th ni MATLAB l phn mm va cung cp cng c phn tch xy

dng x l d liu ng thi nhng th vin hm ton hc phong ph gip ngi dng thun tin trong vic m phng nhng v d mt cch chnh xc cao v d dng trin khai trong nhng d n ln vi nhng yu cu phc tp MATLAB 4.0, 4.2, 5.1, 5.2 hot ng trong mi trng WINDOWS. Cc version 4.0, 4.2 mun hot ng tt phi s dng cng vi WINWORD 6.0. Hin ti c version 8.1 (kham kho t Website ca cng ty). Chng trnh Matlab c th chy lin kt vi cc chng trnh ngn ng cp cao nh C, C++, Fortran

Vic ci t MATLAB tht d dng v ta cn ch vic dng thm vo cc th vin tr gip hay mun lin kt phn mm ny vi mt vi ngn ng cp cao.

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4.2 Gii thiu v chng trnh m phng Chng trnh m phng c vit bng phn mm Matlab. Trong phn m phng s cp ti cc phng php iu ch m cc chng trc

ni n nh iu ch bin gm c: DSB-TC, DSB-SC, SSB vi tn hiu tin tc c th chn mt trong hai loi: sng sin hoc file c nh dng .wav

Hnh 4.1 Giao Menu ca chng trnh gii thiu

4.2.1 M phng iu bin vi tn hiu sng sin Chng trnh m phng iu ch DSB-TC, DSB-SC,SSB gm c 5 phn

l :

- Phn thanh iu khin cho php ngi dng c th thay i cc gi tr ca bin , tn s ca tn hiu bn tin v tn hiu sng mang. - Phn Menu cho php ta c th chuyn i cc kiu iu ch nh chuyn sang

iu ch vi tn hiu sng sin hoc file .wav

- Phn tn hiu iu ch trong phn ny cho ta nhn thy cc dng sng tn hiu bn, tn hiu sng mang, tn hiu sau iu ch v ph ca tn hiu sau iu ch.

-Trong giao din cng c h thng cho php ngi dng chn h s tn hiu so vi nhiu gauss v c phn hin th su iu ch m.

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-Vi vic h tr vic to dao din bn cnh vic vit lnh trn comand window hoc scrip file gip cho vic tng tc gia ngi dng c d dng hn.

-Cc kt qu c a ra thng qua h thng cc nt lnh iu khin cho tng

mc ch hin th khc nhau.Vic th hin nhiu gauss (nhiu trng) s phn nh mt phn tc ng trong thc t ca vic truyn tin

Hnh 4.2 M phng tn hiu nhiu gauss

Xem hnh 4.2 ta thy tn hiu tin tc b nh hng ln nu t l tn hiu so vi nhiu l 1dB v ngc li tn hiu hu nh khng b tc ng nu t l SNR l 50dB.

S thut ton

Bt u

Sng sin File wav

Chn thng s tin tc, sng mang

Gii iu ch

iu ch

Hin th

Kt thc Tip tc

S S

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+M phng iu ch DSB-TC

m phng iu ch song bin truyn sng mang ngi dng chn cc gi tr ca sng mang v ca tin tc,loi iu ch loi tin tc l sng sin sau h thng s t

ng tr v su iu ch cng nhng cho php ngi dng chn t l tn hiu so vi nhiu kt qu s c a ra do ty chn trong menu kt qu. Vic hin th dng sng s th hin trn 5 trc ta nh hnh 4.3 Trong giao din m phng iu ch DSB-TC tn s ca sng mang, tin tc hoc bin ca sng mang, tin tc c m phng bng cc Slider tin dng cho vic thay i gi tr iu ch cng nh cho thy c s thay i tn hiu sau iu ch

theo su iu ch.

Trong chng trnh m phng iu ch v gii iu ch trn s dng code da theo m hnh iu ch bn cnh matlab cn cung cp cho ngi dng hm phn tch iu ch DSB-TC nh sau:

-ammod(x, Fc, Fs, ini_phase, carramp) Trong :

X: Tn hiu tin tc

Fc: Tn s ca sng mang

Fs: Tn s ly mu ca tn hiu ca sng mang v tn hiu sau iu ch.

ini_phase: Pha ban u ca tn hiu sau iu ch carramp: Bin ca tn hiu sng mang.

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Hnh 4.3 M phng iu ch DSB-TC vi tn hiu tin tc l sng sin

+M phng iu ch DSB-SC

S khc bit so vi m phng iu bin DSB-TC l vic khng truyn i thnh phn sng mang gy lng ph cng sut my pht ng thi th hin c s khc

bit trong s phn b ph ca tn hiu gm 2 thnh phn nh hnh 4.4. Vi iu ch DSB-SC matlab cung cp hm phn tch tng t vi iu ch DSB-TC ch khc bin sng mang mc nh bng 1 ln khng cn xut hin trong biu thc.

Mt khc gip ngi dng phn tch tn hiu sau gii iu ch bin trong communication toolbox ca matlab c hm gii iu bin c c php nh sau: amdemod(y,Fc,Fs,ini_phase,carramp) Vi gii iu ch DSB-TC sng mang c bin c biu din bi carramp, vi DSB-SC bin sng mang mc nh l 1;

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Hnh 4.4 M phng iu ch DSB-SC tn hiu sng sin

+M phng iu ch n bin USSB

Hnh 4.5 M phng iu ch n bin USSB

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C hai hm xy dng sn phn tch iu ch n bin l -SSBMOD(X,Fc,Fs,INI_PHASE,'upper') - SSBDEMOD(Y,Fc,Fs,INI_PHASE,NUM,DEN) Trong : [NUM,DEN] = butter(5,Fc*2/Fs) Ta c th chn loi iu ch bin trn hoc bin di vi lnh 'upper' hoc 'lower'. Mc nh vic gii iu ch s ly h s bc ca b lc butter l bc 5 hoc c th ty chn theo cng thc [NUM,DEN] = butter(bc b lc,Fc*2/Fs) 4.2.2 M phng iu ch vi tn hiu file wav H thng s c menu iu khin chn file wav t th mc ty chn sau s

phn tch tn hiu iu ch tng t vi sng sin.

Khi phn tch tn hiu dng file .wav matlab cung cp cho ngi dng hai cng c hu hiu l 2 hm

- wavread( tn file wav) s tr v kt qu l tn s ly mu, s bit dng m ha tn hiu,bin ca tn hiu ly mu. - wavwrite(ten file wav) ghi d liu theo nh dng wav tn s ly mu s bit biu din thng thng :8,16 hoc 32bit.

-Vic m phng iu bin vi tin tc l file wav tng t vi tn hiu sng sin nhng kt qu h s iu ch c gin lc do kh xc nh c bin ca tin tc. Mt khc cng c cng cho php ngi dng phn tch ph ca tn hiu tin tc v hnh nh theo min thi gian ca tn hiu file wav.

Hnh nh ca iu ch bin DSB-TC c th hin hnh 4.6

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Hnh 4.6 m phng iu ch v gii iu ch file .wav

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KT LUN

Vi cc kin thc c hc v vin thng, v sau thi gian thc hin ti

em tm hiu c c bn v cc phng php iu ch tn hiu cn bn nh iu ch cc tn hiu tng t v cc tn hiu s. Cc kt qu t c ca bo co p ng c yu cu ra t u l lm r v iu ch bin sao cho d hiu v c c s nh gi trc quan nht bng vic s dng cc cng c h tr vic m phng. T nhng phng php iu ch c bn ny ta c th m rng ln cc phng php iu ch cao hn mang tnh hiu qu cao hn v ng

dng nhiu hn na trong cc h thng vin thng.

-Hng pht trin ca ti : Do trnh hiu bit cn hn ch, cng nh thi gian thc hin ti v

ngun ti liu c hn nn trong ti khng cp ti tt c cc phng php

iu ch tn hiu c m mi ch gii quyt c vn iu ch bin trong cc phng php iu ch.V vy hng pht trin ca ti l xy dng c mt chng trnh m phng c y cc phng php iu ch ang c s

dng hin nay.Mt khc cc thng s c th thay i mt cch linh hot nht, ngi dng c th thao tc mt cch d dng khi s dng.

Chnh v vy em rt mong c s ng gp , ch bo ca thy c v nhng vn cn thiu st.

C c kt qu ny, mt ln na em xin chn thnh cm n hai c: INH TH KIM PHNG PHAN TH THU HNG v cc thy c trong khoa in T trc tip hng dn v gip em trong qu trnh thc hin ti.

Em xin chn thnh cm n!

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TI LIU THAM KHO

[1] Thi Hng Nh, Phm Minh Vit, H thng vin thng, Nh xut bn gio dc. [2] Phm vn Tn, Gio trnh c s vin thng, Nh xut bn khoa hc k thut.

[3] V c Th, Thng tin di ng s cellular, Nh xut bn gio dc [4] ng Vn Chuyt, Nguyn Tun Anh, C s l thuyt truyn tin, Nh xut bn gio dc. [5] Nguyn Quc Bnh, Cc h thng thng tin hin nay, Nh xut bn khoa hc k thut.

[6] Ngun internet : http://www.dientuvietnam.com http://www.dientuvienthong.com http://www.google.com

http://www.hauioline.com

http://www.vntelecom.org

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Ph lc

Code m phng iu ch v gii iu ch bin x1=get(handles.radiobutton1,'Value'); x2=get(handles.radiobutton2,'Value'); x3=get(handles.radiobutton3,'Value'); x4=get(handles.radiobutton4,'Value'); x5=get(handles.radiobutton5,'Value'); a=get(handles.popupmenu1,'value'); b=get(handles.popupmenu2,'value'); c=get(handles.popupmenu4,'value'); fc=get(handles.slider2,'value'); ac=get(handles.slider3,'value'); fm=get(handles.slider4,'value'); am=get(handles.slider5,'value'); ts=1/(fc*10);fs=1/ts; t=[0:ts:1]; vs =am*cos(2*pi*t*fm); vsc=ac*cos(2*pi*t*fc); df=0.3; if c==1 vtt=awgn(vs,1,'measured'); end

if c==2 vtt=awgn(vs,5,'measured'); end if c==3 vtt=awgn(vs,10,'measured'); end if c==4 vtt=awgn(vs,50,'measured'); end if x1==1 & b==2

axes(handles.axes1) zoom on; hold off; plot(t,vtt) axis([0 4/fm -5 5]); xlabel('Thoi gian') title('tin hieu tin tuc') end if x2==1 axes(handles.axes1) hold off;

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zoom on; plot(t,vsc); axis([0 4/fc -5 5]); xlabel('Time') title('tin hieu song mang ') end

if x3==1 & b==2 & a==2 axes(handles.axes2); hold off; zoom on; vout=(ac+vtt).*cos(2*pi*t*fc); plot(t,vout); axis([0 4/fm -10 10]); xlabel('thoi gian') title('tin hieu dieu che DSB-TC') end

if x3==1 & b==2 & a==1 axes(handles.axes2); hold off; zoom on; vsc=ac*cos(2*pi*t*fc); vout=vtt.*(ac*cos(2*pi*t*fc)); plot(t,vout); axis([0 4/fm -14 14]); xlabel('thoi gian') title('tin hieu dieu che DSB-SC') end

if x3==1 & b==2 & a==4 axes(handles.axes2); hold off; zoom on; u=vtt.*cos(2*pi*fc*t)+imag(hilbert(vtt)).*sin(2*pi*fc*t); plot(t,u); axis([0 4/fm -10 10]); xlabel('thoi gian') title('