LI NI U

Gio trnh truyn hnh cp

GIO TRNH TRUYN HNH CPMC LC

Ni dungTrang

Li ni u1

Mc lc3

Chng I. c im, tiu chun yu cu i vi mng truyn hnh cp H Ni7

I. Tiu chun truyn hnh v bng thng7

1.Tiu chun truyn hnh tng t

2.Quy hoch tn s

II.Tiu chun giao din RF10

1.c tnh truyn dn RF chiu xung

2.c tnh truyn dn RF chiu ln

3.Thng s vo/ra cable modem

Chng II.Cu trc mng HFC14

1.H thng trung tm Headend

2.Mng truyn dn v phn phi tn hiu

3.Mng truy nhp

Chng III.c tnh k thut truyn dn22

I. Nhiu v mo hi trong phn mng cp ng trc22

1.Nhiu v cc nh hng ca nhiu

2.Mo hi

3.Phng php cn bng nhiu v mo hi

II.Tnh ton nhiu, mo hi trn mng xui chiu29

III.Nhiu trn mng ngc chiu31

1.Nhiu nhit

2.Tp m ng truyn dn

3.Nhiu thm nhp

4.Khc phc nhiu trong h thng mng ngc chiu

Chng IV.Phng php kho st, thit k mng 36

I. Kho st, thit k mng ng trc xui chiu36

1.Thit k node quang

2.Thit k mng ng trc

3.Ngun cung cp

4.Tnh ton suy hao

5.Thit k u cui thu bao v cc khu chung c

II.Kho st, thit k mng ng trc ngc chiu43

1.Thit k s thu bao trn node quang v CMTS

2.V thit k mng ng trc

3.Tnh ton suy hao

4. rng bng thng knh ngc chiu

III.Kho st, thit k mng quang54

1.Ngun cng sut quang

2.Suy hao cp

3.Suy hao vng cp

4.D phng cp

5.Suy hao ca cc mi hn gia nhit v hn c hc

6.S cc mi hn ni

7.Suy hao quang

8.Suy hao d phng

9.Loi cp quang

10.V d thit k h thng mng quang

11.Thit b chia quang

12.Suy hao connector

13.Tnh ton suy hao tng th mng

IV.H s thit k60

1.i vi h s thit k mng ng trc

2.i vi h s thit k mng quang

Chng V. gain n v, phng php thit k v cn chnh khuch i62

I. Th no l gain n v62

II. gain n v hng xui chiu62

III. gain n v hng ngc chiu65

IV.Cn chnh khuch i67

1.Lp t v cn chnh khuch i hng xui chiu

2.Cn chnh khuch i hng ngc chiu

Chng VI.Nhng vn ch trong thi cng76

I. Nhng ch trong thi cng76

1.Phn mng quang

2.Phn mng ng trc

3.Phn mng thu bao

4.o kim, cn chnh tn hiu

5.Yu cu an ton trong thi cng

II.Mt s vn thng gp trong thi cng mng 2 chiu78

Chng VII.H thng k hiu thit b mng HFC82

I. Ch 82

II.Thc hnh v82

III.Cc k hiu 82

1.Ct in

2.Cc thnh phn h tr cp

3.Mu neo, mc v no

4.K hiu khc

5.Thit k i ngm

6.Cc v tr x l tn hiu

7.Cc loi khuch i

8.Cc b chia ng trc

9.Thit b ngun

10.Cc thit b trn ng dy

11.B chia thu bao

12.B chia u cui

13.Cc loi cp ng trc

14.Cc thit b quang

15.Cc k hiu v im hn ni quang

16.Cc k hiu quang khc

17.Cc khi d liu

Chng VIII.Thng s k thut tiu chun mt s thit b, vt t 91

I. Cp91

1.Cp ng trc treo QR540

2.Cp phn phi RG11

3.Cp thu bao RG6

II.Khuch i94

1.Khuch i trc

2.Khuch i nhnh

III.Thit b quang97

1.Node quang

IV.Thit b th ng99

1.B chia u v nh hng mng trc

2.B chia nhiu cng ra mng trc

3.B chia u v nh hng mng nhnh

4.B chia nhiu cng ra mng nhnh

V.Connector108

1.5/8 connector cho cp QR540

2.F Male connector cho cp QR540

3.Connector ni cp QR540

4.5/8 adapter

5.3/8 adapter

6.3/8 connector chocp RG11

7.3/8 connector chocp RG6

8.u PHTK 75Ohm mng trc

9.u PHTK 75Ohm mng nhnh

VI.Ngun cung cp

Ti liu tham kho110

CHNG IC IM, TIU CHUN K THUT YU CU I VI

MNG TRUYN HNH CP H NI

I. Tiu chun truyn hnh v bng thng:1. Tiu chun truyn hnh tng t:

Cn c theo c im k thut Ngnh Pht thanh-Truyn hnh Vit nam, mng truyn hnh cp H ni khai thc dch v truyn hnh tng t theo tiu chun PAL D/K vi cc thng s k thut nh sau:

STTTn thng sGi tr

1S lng dng/nh625

2Tn s qut dng (Hz)50

3Tn s qut mnh (Hz)15625

4Di thng/knh (MHz)8

5Bng thng sng mang video (MHz)6

6Khong cch gia sng mang video v audio (MHz)6.5

Hnh I.1: M t tiu chun PAL D/K

2. Quy hoch tn s:

Nhm p ng yu cu v mc tiu, cht lng k thut v nh hng pht trin, mng truyn hnh cp H ni s dng quy hoch tn s vi phn b cc di tn nh sau:

STTDi tn s (MHz)Mc ch s dng

15 65Tn hiu ngc dng cho cc dch v gia tng

nh: Internet, VOD, IP phone..

265 85Bng thng cch ly chiu ln/xung

388 108Tn hiu truyn thanh iu ch FM

4110 862Tn hiu truyn hnh 1 chiu tng t v s

Quy hoch chi tit:

+ Tn s truyn hnh theo tiu chun OIRT PAL D/K:Di thngKnhDi thngSng mang hnh nhSng mangm thanhEoCTn s pht miOff-AirGhi ch

[MHz]

5.0 - 48.5internet5.0 - 65.0 MHz

I148.5-56.5VHF-L

258-66

II376-84

484-92Radio FM Band (88-108) MHz

592-100

100 110

AA1110-118114CATV Programs

A2118-126122

A3126-134130

A4134-142138

A5142-150146

A6150-158154

A7158-166162

A8166-174170

III6174-182178VHF-H

7182-190183.25189.75

8190-198191.25197.75

9198-206199.25205.75

10206-214207.25213.75

11214-222215.25221.75

12222-230223.25229.75

FF1230-238231.25237.75

F2238-246239.25245.75

F3246-254247.25253.75

F4254-262255.25261.75

F5262-270263.25269.75

F6270-278271.25277.75

F7278-286279.25285.75

F8286-294287.25293.75

F9294-302295.25301.75

SS21302-310303.25309.75S-Band

S22310-318311.25317.75

S23318-326319.25325.75

S24326-334327.25333.75

S25334-342335.25341.75

S26342-350343.25349.75

S27350-358351.25357.75

S28358-366359.25365.75

S29366-374367.25373.75

S30374-382375.25381.75

S31382-390383.25389.75

S32390-398391.25397.75

S33398-406399.25405.75

S34406-414407.25413.75

S35414-422415.25421.75

S36422-430423.25429.75

S37430-438431.25437.75

S38438-446439.25445.75

S39446-454447.25453.75

S40454-462455.25461.75

S41462-470463.25469.75

IV21470-478471.25477.75UHF

22478-486479.25485.75

23486-494487.25493.75

24494-502495.25501.75

25502-510503.25509.75

26510-518511.25517.75

27518-526519.25525.75

28526-534527.25533.75

29534-542535.25541.75

30542-550543.25549.75

31550-558551.25557.75

32558-566559.25565.75

33566-574567.25573.75

34574-582575.25581.75

35582-590583.25589.75

36590-598591.25597.75

37598-606599.25605.75

38606-614607.25613.75

39614-622615.25621.75

AV40622-630623.25629.75

41630-638631.25637.75

42638-646639.25645.75

43646-654647.25653.75

44654-662655.25661.75

45662-670663.25669.75

46670-678671.25677.75

47678-686679.25685.75

48686-694687.25693.75

49694-702695.25701.75

50702-710703.25709.75

51710-718711.25717.75

52718-726719.25725.75

53726-734727.25733.75

54734-742735.25741.75

55742-750743.25749.75

56750-758751.25757.75

57758-766759.25765.75

58766-774767.25773.75

59774-782775.25781.75

60782-790783.25789.75

61790-798794

62798-806802

63806-814810

64814-822

65822-830

66830-838

67838-846

68846-854

69854-862

Khi thit lp tn s cho knh mi trn mng cp cn trnh t tn s trng vi tn s cc knh dch v sng mt t v nhng knh sng mt t s to nhiu xm nhp rt ln qua cp, cc mi ni gy nh hng n cht lng tn hiu knh ..

Bng tn Internet cng ngh EoC t: 5 65 MHz.

II. Tiu chun giao din truyn dn RF:

Hin nay trn th gii c rt nhiu tiu chun truyn dn cho mng truyn hnh cp. Vic chn la, tun th h thng thng s tiu chun ca th gii cho mng truyn hnh cp l rt quan trng, c bit i vi lnh vc vin thng, truyn hnh, cng ngh thng tin v lin quan n s ng nht trong vic m rng, nng cp mng, mua sm thit b v kt ni vi nhng mng vin thng khc.

Cn c theo tnh cht, kh nng p dng, mc ph bin ca cc tiu chun, mng truyn hnh cp H ni la chn tiu chun Euro-DOCSIS do hip hi k s vin thng cp quc t-SCTE ban hnh trong tiu chun ANSI/SCTE 79-1 2003 DOCS 2.0 Part 1: Radio Frequency Interface.1. c tnh truyn dn RF chiu xung cho truyn hnh analogue v tn hiu m thanh:STTThng sGi trGhi ch

1Di tn s hot ng65 MHz 862 MHz

Tuy nhin di tn s hot ng thc t:

108 862 MHz

2 rng bng thng/knh8 MHz

3Tr truyn dn t Headend n khch hng xa nht( 0.8 ms

4C/N

(trong bng thng 8 MHz/knh)( 39 dBo ti

H thu bao

5CTB( -57 dBco ti

H thu bao

7CSO( -57 dBco ti

H thu bao

8 dc tn hiu ti u cui thu bao

(trong di 85 862 MHz)( 14 dBTheo c hai chiu dc(+/-)

9Bin thin bin do p ng tn s( 2.5 dBTrong 8 MHz

10Mc tn hiu sng mang m thanhNh hn sng mang hnh nh cng knh trong khong:

13-17 dBo ti

HeadEnd

11Mc tn hiu bin thin theo thi tit ln nht 8dB

12Mc tn hiu ln nht ca 1 knh bt k trong di tn o ti h thu bao80 dB(V

13Mc tn hiu nh nht ca 1 knh bt k

trong di tn o ti h thu bao60 dB(V

2. c tnh truyn dn RF Internet cng ngh CMTS:

STTThng sGi trGhi ch

1Di tn s hot ng5 n 65 MHz

2Tr truyn dn t Cable Modem xa nht n CMTS( 0.8 ms

3T s sng mang trn nhiu nhit

trong knh hot ng( 22dB

4T s sng mang trn nhiu xm nhp trong knh hot ng( 22dB

5T s sng mang trn nhiu

(tng ca tp m, mo hi, bin iu cho)

trong knh hot ng( 22dB

6Bin thin bin do p ng tn s2.5dBtrong 2 MHz

7S bin i mc tn hiu theo maT mc thp nht n mc cao nht khng ln hn 12dB

3. c tnh truyn dn RF Internet cng ngh EoC:

STTThng sGi trGhi ch

1Di tn s hot ng5 n 65 MHz

2Tc Down/Up ti a200Mbps

3Cng sut pht RF ca b EoC Master104 dB(V

4Suy hao trn ng truyn ti a70dB(V

5Tn hiu nhn Modem EoC ti thiu34dB(V

4. Thng s tn hiu vo STB (u thu HD):STTThng s Gi tr

1Tn s trung tm114 ti 858 MHz 30KHz

2Mc tn hiu thu

(01 knh)47 n 67 dBV: iu ch QAM 64

3Dng iu chQAM- 64

4Tc m ha k t6.956 Msym/sec vi QAM 64

5Bng thng/knh8 MHz

6Tr khng u vo75

7Suy hao phn x u vo> 6dB (85 862MHz)

8Connectoru F chun ISO16924

CHNG IIH THNG MNG CATV

Mt mng truyn hnh cp CATV c cu trc c bn nh sau:

H thng trung tm Headend

Mng truyn dn v phn phi tn hiu

Mng truy nhp

Thng thng cu trc mng cn tu thuc vo nhiu yu t: a l, mt dn c, lin quan n vic nng cp v nhiu yu t khc na. Tuy nhin hu ht mng truyn dn v phn phi tn hiu u c cu trc theo kiu vng hoc sao, cn mng truy nhp thng c cu trc kiu cy v nhnh (Hnh II.1).

Cn c vo loi thit b s dng trong mng truy nhp, ngi ta chia mng CATV thnh 2 dng: HFC v HFPC.

Mng truyn hnh cp hu tuyn kiu HFPC: l mng lai gia cp quang v cp ng trc m trong phn mng truy nhp ch dng cc thit b th ng chia tn hiu m khng c bt c mt thit b tch cc no.

Mng truyn hnh cp hu tuyn kiu HFC: l mng lai gia cp quang v cp ng trc m trong phn mng truy nhp c dng cc thit b tch cc( cc b khuch i cao tn).1. H thng trung tm Headend:

L ni thu nhn tn hiu t nhiu ngun khc nhau: tn hiu qung b, v tinh, sn xut chng trnh ti ch, chn tn hiu sn xut ni b Sau khi qua cc bc x l nh gii m, gii iu ch, iu ch, phn knh, m ha, trn), tn hiu c a ra ngoi mng truyn dn v phn phi ti khch hng thu bao.

i vi Headend pht trin cc dch v tng tc nh: Internet, VOD, in thoi Headend s nhn tn hiu ngc dng t cc h thu bao sau a ti cc h thng b phn lin quan nh CMTS, Telephone Switch kt ni vi mng vin thng bn ngoi. Trong qu trnh ny, b phn tnh cc (Billing) tnh cc dung lng trao i ca khch hng xc nh ph s dng hng thng.

Hnh II.2: S tng qut h thng Headend HCATV2. Mng truyn dn v phn phi tn hiu:L mi trng truyn dn tn hiu t Headend n cc node quang FN(Fiber Node) nh cc tuyn cp quang. in hnh l mt hay nhiu mch vng cp quang kt ni gia HE s cp v cc HUB s cp, trong mt s trng hp khc th cc vng th cp li lin kt gia cc HUB s cp vi cc HUB th cp. T y cc node quang FN c lin kt vi cc HUB hoc HE theo dng cu trc hnh sao. nng cao hiu sut mng, ngi ta xy dng mng quang theo cu trc FTF (cp quang ko n tn nhnh), FTTC (cp quang ko n tn vng ngoi ), FTTB (cp quang ko n tn to nh), FTTH (cp quang ko n tn nh thu bao), thm ch l HTTD (cp quang ko n tn bn lm vic). Phng chm thit k mng quang: FAFAYCA (Cp quang ko i xa nht c th).

Trong khi thit k s dng cu trc cy v nhnh cho mng cp ng trc th mng cp quang li dng cu trc mng sao hoc vng. Trong cu trc mch vng Ring c d phng 1+1 cho tin cy ca h thng cao hn. Tc l cu trc gm 2 mch vng chy theo chiu ngc nhau. 2 vng ny chy trn 2 si vt l ring bit: 1 si hot ng cn 1 si ch ch hay ch bo v, c kh nng i tuyn. Nh vy c ti mi 1 trm (HUB/ node quang) c 4 ng cp quang ko n (mi si quang cho hng xui v ngc u c d phng). Khi mng c s c th s thc hin chuyn mch t ng/ nhn cng sang h thng d phng m bo tn hiu truyn c thng sut.

Mch vng Ring c th c cu trc khp kn hoc theo cu trc m. Xu hng hin nay thng xy dng theo cu trc m, mang li nhiu tin ch hn.Tuy nhin cn c vo mng thc t a ra s lng cp d phng cho hp l (=50-100% s si cp hot ng), va m bo ng truyn d phng, va m bo kinh t.

Bn cng ngh s dng trong xu hng pht trin mng quang:

S dng my pht quang cng sut cao hot ng bc sng 1550nm nhm ko di khong cch truyn dn. S dng bc sng 1550nm ti u cho cc knh iu ch bin cu phng (QAM) v mng phn phi vi chi ph thp.

Trong truyn s liu, s dng phng thc ghp knh theo chun SONET lm in hnh xy dng mng a phng tin tc cao.

Ghp knh phn chia theo bc sng (DWDM) khng ch c tc dng tng dung lng truyn dn t 1 n 16 knh m cn ko di tuyn quang. (S dng si cp quang n mode v loi connector c phn x thp APC cho php h thng c th truyn i xa hn 60km m khng phi dng khuch i quang). Do lm gim chi ph mng.

Cng ngh s dng cc thit b quang tch cc tr thnh in hnh trong xu th pht trin mng. Vic dng cc b khuch i quang si c pha tp cht Eribium (EDFA) lm tng khong cch truyn dn n hn 100km, thm ch c th ti 200km.H thng thng tin si quang s dng 4 vng ca s quang nh hnh v sau:

Hnh II.3: Bn vng ca s quangTrong hnh v th hin 3 ng cong: ng trn cng, nt gch, tng ng vi si quang nhng nm trc nm 1980, ng cong gia, nt chm, tng ng vi sau nm 1980, ng cong di tng ng vi si quang hin i.

Trc kia, h thng thng tin quang thng khai thc bc sng hot ng 850nm, cn gi l vng ca s th nht. Tuy nhin khi cng ngh pht trin th vng ca s ny b gim tnh hp dn bi mc suy hao kh ln: 3dB/km.

Hu ht cc cng ty chuyn sang vng ca s quang th 2(1310nm) c suy hao thp hn nhiu(khong 0.35dB/km). Sau mt thi gian cc nh nghin cu li pht hin ra vng ca s th 3 (bc sng 1550nm) cho suy hao thp hn (khong 0.2dB/km). Trong khi bc sng 1310nm, khong cch truyn dn cp quang n mode xa nht cho php khong 60km th vi bc sng 1550nm, khong cch l 150km.

Cc h thng thng tin si quang hin nay, nht l cc h thng tc bit cao, phn ln hot ng vng bc sng 1550nm nhm dng cc b khuch i quang si pha tp Erbium (EDFA) tng c ly truyn dn. Tuy vy, si quang n mode tiu chun( si G.625) c h s tn sc ti vng bc sng ny l rt ln(18ps/nm*km @1550nm) trong khi ti bc sng 1310nm th h s tn sc ch l 3ps/nm*km. Tn sc ln lm mo tn hiu v to ra hin tng giao thoa gia cc k t( ISI Intersymbol Interference) do dn xung ti cc khe thi gian, lm gim cht lng truyn dn v hiu qu. Nhn chung, nh hng ca tn sc n nng lc truyn dn ca h thng l phc tp, iu ny gy kh khn cho vic thit k cc h thng thng tin quang tc cao, c ly xa. Ca s quang th 4( bc sng 1625nm) cng ang c a vo trin khai. Tuy n c mc hao tng ng vi bc sng 1550nm nhng li thch hp khi s dng ko di tuyn v ghp knh theo bc sng. Song cn cn nhc gia hiu qu v chi ph.

S cu trc mng truyn dn tn hiu quang(Hnh II.4) trn cho bit s b chi tit mt s thit b trong mng quang. Tn hiu sau khi c x l ti Headend c a ra ngoi mng thng qua cp si quang, n cc trm lp HUB. Ti y tn hiu c a vo b chia quang sau cp cho khuch i quang EDFA cng ra ca khuch i quang cp tn hiu cho b chia quang theo t s cp n cc node quang FN. Node quang lm nhim v chuyn i tn hiu quang thnh tn hiu in phn phi ti cc nh thu bao qua mng cp ng trc.Cng c trng hp my pht quang TX c t ngay ti H thng trung tm Headend cp n cc node quang FN tuy nhin t s dng do my pht quang ch yu c cp vo cc b khuch i quang. (Tu thuc vo a hnh, a bn, phn b dn c m quyt nh thit k c cn t trm lp quang hay khng).

3. Mng truy nhp:

Thng thng chia ra lm 2 kiu: mng truy nhp th ng v tch cc.Mng truy nhp kiu th ng (HFPC)

c im ca mng HFPC:

+ p ng c cc yu cu xy dng theo mng 1 chiu hay 2 chiu.

+ S dng node quang c cng sut ln.

+ Mng quang chim t trng ln trong ton b mng tng th.

+ Mng ng trc ch c cc tuyn trc chnh v tuyn cp thu bao vi cc thit b chia th ng

+ Kh nng phc v t 400-600 thu bao/node quang.

u im:

+ Cht lng tn hiu tt do khng s dng cc b khuch i.

+ Do khng s dng cc b khuch i cao tn nn vic thi cng lp t, vn hnh d dng hn.

+ Cc thit b th ng c kh nng truyn 2 chiu nn n nh mng vn cao khi trin khai mng 2 chiu.

+ S lng thu bao/node quang nh nn c kh nng cung cp tt dch v 2 chiu vi tc cao.

+ Gim chi ph cp ngun cng t in, bo dng, thay th cc thit b tch cc.

Nhc im:

+ Kh nng bao ph ca 1 node quang nh do khng s dng khuch i.

+ Yu cu node quang s dng phi c cng sut ln, cht lng cao, n nh.+ Ph hp vi khu vc c quy hoch tp trung, khng ph hp vi a hnh Vit Nam.

+ Yu cu v chi ph cao.

Mng truy nhp kiu tch cc (HFC):

c im ca mng HFC thun tu:

+ p ng c cc yu cu xy dng theo mng 1 chiu hay 2 chiu.+ Mng ng trc chim t trng ln trong ton b mng tng th.

+ Mng ng trc c chia thnh 3 cp:

Cp trc chnh: bao gm cp ng trc trc chnh, khuch i trc, b chia tn hiu ng trc.

Cp trc nhnh: bao gm cp ng trc trc nhnh, khuch i nhnh, b chia tn hiu ng nhnh.

Cp mng thu bao: bao gm cp ng trc thu bao, TV.

+ Kh nng phc v t 1500-2000 thu bao/node quang. u im:

+ Phm vi bao ph ca 1 node quang ln nh ko di mng ng trc bi s dng cc khuch i cao tn.

+ Ph hp vi a bn Vit Nam.+ Chi ph ban u thp nh s dng t node quang. Nhc im:

+ C ch thi cng, vn hnh, bo dng phc tp, khng thun li khi trin khai thnh mng 2 chiu.

+ Yu cu cht ch v ngun cp in trung gian. Nu im cp ngun no mt in th ton b tuyn pha sau cng mt tn hiu.

Ngy nay mng CATV thng dng cu trc theo kiu HFC nhm gim chi ph trong khi vn thu c hiu qu cao v phm vi bao ph tn hiu ln. Ngay c khi vi nhng khong truyn ngn th s kt hp gia cp quang v cp ng trc cng cho ngi thit k h thng a ra gii php v chi ph thp nht cho mi phn ca mng truyn hnh cp CATV.CHNG IIIC TNH NHIU TRONG MNG CATVI. Gii thiu:

Nhiu (Noise) theo IEEE c nh ngha l: Nhng tn hiu tp, nhiu khng mong mun chng ln nhng tn hiu mang thng tin lm m, suy gim ni dung, cht lng thng tin. Trong thc t, ngoi nhng ngun nhiu, tp lun tn ti trn mng nh nhiu nhit, nhiu do giao thoa cc ngun tn hiu(mo hi), do iu ch ca ngun thu/pht quang, do khng phi hp tr khngcn c nhng ngun nhiu khch quan nh nhiu xm nhp, nhiu cng nghip. V vy, vic tnh ton, hn ch ti thiu tc ng ca nhng ngun nhiu l v cng quan trng.

Trong truyn hnh cp, thut ng nhiu-noise c dng ch cp n nhiu nhit. Cng vi nhiu nhit, mo hi l nhng ngun nhiu c nh hng ln nht n cht lng tn hiu m c th xc nh v hn ch ti thiu nh hng. Cn ch rng, nhiu sinh ra t mt thit b hoc ti mt thi im hon ton khng lin quan n nhiu trong cc thit b khc hoc ti mt thi im khc. Tuy nhin, mc cng sut nhiu trung bnh c th xc nh c. Lu : Nhiu do nhiu ngun tc ng bng tng nhiu n t cc ngun c lp.II. Nhiu- Noise:

Nhiu c xc nh bng t s tn hiu trn tp m( Carrier-to-noise:C/N) vi cng thc nh sau:

C/N (dB) = 10log ();

Trong : c v n l gi tr cng sut v hng ca sng mang hnh nh v nhiu.

1. Nhiu trong mng ng trc:

a. Nhiu nn:

Do s chuyn ng t do ca cc in tch trong cc cht dn in, h thng mng sinh nhiu. Nhiu ny pht sinh do nhit, lun tn ti trong h thng mng, c o trong bng thng ca knh v gi l nhiu nn (noise floor). Nhiu nn trong mng truyn hnh cp h PAL c gi tr:

NP= -58,2 dBmV

b. C/N qua mt khuch i:

Khuch i sinh ra nhiu ti mi im trn mch in ca n, tuy nhin, thun li, nhiu do khuch i sinh ra c coi nh t mt ngun gy ra v t u vo khuch i. T s cng sut nhiu ti u vo trn cng sut nhiu nhit c gi l ch s nhiu (noise figure) ca khuch i. Do vy, mt khuch i c ch s nhiu (noise figure) FA (dB) s c cng sut nhiu u vo NA bng:

NA (dBmV) = NP (dBmV) + FA (dB)

Nhiu ti u ra khuch i (Nout) s bng nhiu ti u vo tng thm G(dB)

Nout (dBmV) = NP + FA + G

Tng t, mc tn hiu vo Ci s c khuch i G ln, do vy, mc tn hiu mang thng tin u ra Cout bng:

Cout = Ci (dBmV) + G (dB)

Do , t s tn hiu trn tp m khi tn hiu i qua mt khuch i s l:

C/N (dB) = Cout - Nout = (Ci + G) (NP + FA + G)

= Ci Np FA

Trong mng truyn hnh cp h PAL, C/N c o trong bng thng 5MHz ca 1 knh, v vy, C/N qua mt khuch i c tnh nh sau:

C/N (dB) = Ci (dBmV) + 58.2 FA (dB)

Ch : Trong h thng ghp knh theo tn s (FDM), C/N c nh gi theo mi knh c lp v l mc tn hiu sng mang tng knh so snh vi nhiu trong bng thng knh . V vy, C/N ca tng knh s khc nhau do mc tn hiu cc sng mang trong bng thng khc nhau.

c. C/N qua cc tng khuch i:

Trong thit k mng cp phn phi ng trc, vic tnh C/N qua cc tng khuch i l rt cn thit, cng thc tng qut tnh C/N ca nhnh c n tng khuch i nh sau:

CNR (dB) = -10log

y l cng thc tng qut khi tnh C/N trong mt nhnh bao gm cc khuch i c ch s nhiu (FA) khc nhau, mc khuch i khc nhau. Nu nhnh gm nhng khuch i ging nhau( c ch s nhiu ging nhau), suy hao qua on cp ni gia hai khuch i bng mc khuch i (gain) ca khuch i trc th cng thc tng qut s c gin ha nh sau:

CNRd (dB) = C/NA 10log n

Trong :

C/NA = C/N qua mt khuch i = Ci (dBmV) + 58.2 FA (dB)

n = S khuch i ging nhau c lp cch nhau mt khong cp c suy hao bng mc khuch i (gain) ca cc khuch i.

C th thy rng khi s lng khuch i tng gp 2 th CNR gim i 3dB

S lng khuch i trn nhnhCNR gim (dB)

23

46

89

1612

Cng thc trn rt tin dng v c ch c dng khi:

Cc khuch i trong chui c ch s nhiu (noise figure) ging nhau, suy hao qua on cp gia hai khuch i trong chui bng mc khuch i (gain) ca khuch i trc.

Mc gain ca mi khuch i phi ln (nh nht l 20dB)

u vo khuch i ch bao gm nhiu nhit.

Nh vy, khi thit k lun cn quan tm n 3 thng s sau m bo mc C/N yu cu ti im cui thu bao: Ch s nhiu ca tng khuch i, mc tn hiu vo tng khuch i v s lng khuch i trong nhnh.

2. Nhiu trong mng quang:T s tn hiu/nhiu trong mng quang c xc nh bng cng thc di y:CNRs(dB) = -10log;CNR1,2,..n thnh phn l CNR ca cc my pht, khuch i quang, node quang.

3. Nhiu trong Headend:Ti Headend, thit b gy nhiu(noise) ch yu l nhng b iu ch Modulator. Nhng thit b ny lun gy ra 2 ngun nhiu:

Nhiu trong bng tn hot ng( In-band noise) gy nh hng cht lng tn hiu ca chnh knh thit b iu ch. Nhiu ngoi bng tn hot ng( Out-band noise) gy nh hng cht lng tn hiu cho ton b cc knh khc trong di thng. Tng s lng knh pht cng l tng s lng Modulator, v vy, nhiu ngoi bng tn tng theo v nh hng v cng ln. Nh vy, nhiu ca mt knh sau khi ra khi HE l tng nhiu trong bng tn v nhiu ngoi bng tn do cc khuch i khc gy ra. V vy, cng tng knh th nhiu trn cc knh cng tng.T s sng mang trn tp m ca 1 knh sau khi ra khi HE c xc nh nh sau:CNR(dB) = -10log

CNRoutband-sum= C/Noutband 10log(n-1)Trong :

CNRinband :T s sng mang trn nhiu trong bng tn hot ng do 01 Modulator gy ra.

CNRoutband:T s sng mang trn nhiu ngoi bng tn hot ng do 01 Modulator gy ra.

CNRoutband-sum: T s sng mang trn nhiu ngoi bng tn hot ng do cc Modulator gy ra.

n: S lng modulator iu ch tn hiu ti HETrong thc t, c th lm gim nhiu trn 1 knh ti HE bng cch s dng cc b lc ti u ra cc Modulator hn ch nh hng ca nhiu ngoi bng tn.( Phng php ny ang c s dng ti HE ca Truyn hnh cp H ni v mang li hiu qu rt cao)III. Mo hi:

Khng ging nh nhiu nhit ch ph thuc c tnh h thng truyn dn v bng thng knh, mc mo hi ph thuc vo s lng tn hiu knh trn mng. Mo hi sinh ra khi tn hiu knh qua cc phn t khng tuyn tnh (phn t tch cc) c th l khuch i. Mo hi l kt qu ca s tng tc (tng v hiu) gia cc sng mang khc nhau khi i qua khuch i. Chnh v vy, mc mo hi ca thng s k thut khuch i thng c xc nh di mt hoc hai iu kin v ti knh. Trong cc loi mo hi, mo hi bc 2,3 l tn hiu mo c cng ln nht v xut hin thng xuyn trn di thng h thng, nh hng n cht lng tn hiu v vy c quan tm xem xt tm cch khc phc. Di y l phn gii thiu tng quan tnh cht, phng php tnh mo hi bc 2 (CSO) v mo hi bc 3 (CTB)

Ghi ch: n v ca CSO, CTB l dBc, dBc c ngha l t s mo hi trn sng mang. Mt s ti liu m t gi tr dBc dng ngha l t s sng mang trn mo hi (C/CSO, C/CTB). Trong ti liu ny s dng cch m t tiu chun theo SCTE, v vy, CSO, CTB lun c gi tr dBc m.

a. Mo hi bc 2 (CSO):

Mo hi bc 2 l tng v hiu gia hai sng mang bt k, f1 + f2 v f1 - f2, khi tn hiu knh qua cc phn t khng tuyn tnh to ra sng mang th 3 c bin nh hn f1 , f2. Mo hi bc 2 tun theo nguyn tc 10log - (nguyn tc cng sut), vi thay i 1dB mc tn hiu u ra ca khuch i dn n thay i 1dB ca mc tn hiu mo hi bc 2. C th, nu mc tn hiu u ra khi khuch i tng 1dB th mo hi tng 1dB, nu mc tn hiu u ra gim i 1dB th mc mo hi c ci thin 1dB.

Trong thit k mng cp phn phi ng trc, vic tnh CSO qua cc tng khuch i l rt cn thit, cng thc tng qut tnh CSO ca nhnh c n tng khuch i nh sau:

CSO (dBc) = -10log

Trong :

CSO1 l mc mo hi bc hai khi tn hiu ch i qua khuch i th 1

CSO2 l mc mo hi bc hai khi tn hiu ch i qua khuch i th 2

..

CSOn l mc mo hi bc hai khi tn hiu ch i qua khuch i th ny l cng thc tng qut khi tnh CSO trong mt nhnh bao gm cc khuch i c ch s CSO khc nhau, mc khuch i khc nhau. Nu nhnh gm nhng khuch i ging nhau (c ch s CSO ging nhau), suy hao qua on cp ni gia hai khuch i bng mc khuch i (gain) ca khuch i trc, mc tn hiu ti u ra cc khuch i ging nhau th cng thc tng qut s c gin ha nh sau:

CSO (dBc) = CSOA + 10log n (php cng do dng CSO vi n v dBc m)Trong :

CSOA l mc mo hi bc hai khi tn hiu ch i qua khuch i.

n l s lng khuch i trn nhnh cn tnh.

Mi khuch i trc khi xut xng u c ghi ch s CSO c o theo ti knh c nh. Tuy nhin ch s ny cha th p dng c vo ngay cng thc trn do khng ng vi ti knh thc t ang dng trn mng. CSOA l CSO cn c theo ti knh thc t v c tnh theo cng thc sau:

CSOA = spec + (actlev reflev)-

Trong :

spec: Gi tr CSO do nh sn xut a ra

actlev: mc tn hiu thc t ti u ra khuch i (dBmV)

reflev: mc tn hiu tham kho ti u khuch i (dBmV) do nh sn xut a ra

actslp: dc (slope) c s dng thc t (dB) trong khuch i

refslp: dc (slope) tham kho trong khuch i (dB) do nh sn xut a ra

hifreq: tn s ln nht trn di thng thit k

lowfreq: tn s nh nht trn di thng thit k

analog: tn s knh analog cui trn di thng

b. Mo hi bc 3 (CTB):

Mo hi bc 3 l tng v hiu gia ba sng mang bt k, f1 ( f2 ( f3, s to ra cc sng mang c bin nh hn f1 , f2 , f3. Mo hi bc 3 tun theo nguyn tc 20log - (nguyn tc in p), vi thay i 1dB mc tn hiu u ra ca khuch i dn n thay i 2dB ca mc tn hiu mo hi bc 3. C th, nu mc tn hiu u ra khi khuch i tng 1dB th mo hi tng 2dB, nu mc tn hiu u ra gim i 1dB th mc mo hi c ci thin 2dB.

Trong thit k mng cp phn phi ng trc, vic tnh CTB qua cc tng khuch i l rt cn thit, cng thc tng qut tnh CTB ca nhnh c n tng khuch i nh sau:

CTB (dBc) = -20log

Trong :

CTB1 l mc mo hi bc hai khi tn hiu ch i qua khuch i th 1

CTB 2 l mc mo hi bc hai khi tn hiu ch i qua khuch i th 2

..

CTB n l mc mo hi bc hai khi tn hiu ch i qua khuch i th ny l cng thc tng qut khi tnh CTB trong mt nhnh bao gm cc khuch i c ch s CTB khc nhau, mc khuch i khc nhau. Nu nhnh gm nhng khuch i ging nhau (c ch s CTB ging nhau), suy hao qua on cp ni gia hai khuch i bng mc khuch i (gain) ca khuch i trc, mc tn hiu u ra ti cc khuch i ging nhau th cng thc tng qut s c gin ha nh sau:

CTB (dBc) = CTB A + 20log n (php cng do dng CTB vi n v dBc m)Trong :

CTBA l mc mo hi bc hai khi tn hiu ch i qua khuch i.

n l s lng khuch i trn nhnh cn tnh.

Mi khuch i trc khi xut xng u c ghi ch s CTB c o theo ti knh c nh. Tuy nhin ch s ny cha th p dng c vo ngay cng thc trn do khng ng vi ti knh thc t ang dng trn mng. CTBA l CTB cn c theo ti knh thc t v c tnh theo cng thc sau:

CTB A = spec + 2 x (actlev reflev) -

Trong :

spec: Gi tr CTB do nh sn xut a ra

actlev: mc tn hiu thc t ti u ra khuch i (dBmV)

reflev: mc tn hiu tham kho ti u khuch i (dBmV) do nh sn xut a ra

actslp: dc (slope) c s dng thc t (dB) trong khuch i

refslp: dc (slope) tham kho trong khuch i (dB) do nh sn xut a ra

hifreq: tn s ln nht trn di thng thit k

lowfreq: tn s nh nht trn di thng thit k

analog: tn s knh analog cui trn di thng

IV. Phng php cn bng nhiu v mo hi:

S nh hng ln nhau gia nhiu v mo hi d dng thy r khi phn tch mt chui nhng khuch i trn nhnh. C th d dng thy rng, mng hot ng tt nht khi mc suy hao gia cc khuch i phi c b bng ng mc tng ch (gain) ca khuch i, khng tha khng thiu. Nu mc suy hao nh hn th mc tn hiu vo mi khuch i v, v vy, mc tn hiu ra tng ng s ln hn mc tn hiu ra ca khuch i trc, do , mo hi s tng cao. Nu mc suy hao ln hn mc tng ch, u vo mi khuch i nh hn khuch i trc dn n t s C/N gim.

Khi suy hao gia cc khuch i c thit k bng ng mc tng ch ca cc khuch i th iu quan tm l mc tn hiu u ra khuch i bng bao nhiu m bo cc yu cu v cc thng s C/N, CTB, CSO. Di y mt v d m t cch thit k mc tn hiu u vo khuch i m bo cc thng s ng tiu chun:

Gi thit yu cu mt nhnh t mc c C/N = 49.6 dB, CTB = -63.4 dBc. Mi khuch i c ch s nhiu FA = 8dB, G = 30dB, CTB = -73dBc vi iu kin o: 77 knh NTSC, mc tn hiu u ra 48 dBmV, di tn hiu ra c dc dng 8dB.

( Ta phi tnh gi tr CTB cho cc knh h PAL. Di tn cho 77 knh NTSC tng ng cho 60 knh PAL, ta thy rng s lng sng mang knh PAL gim nn gi tr CTB s c ci thin. Theo tnh ton thc t th gi tr CTB c ci thin 3dB -gi l h s chuyn i khi chuyn i gi tr CTB tnh trong h PAL. V vy, gi tr CTB = - 76dBc)Xt nhnh ch bao gm 1 khuch i th mc tn hiu vo nh nht m vn m bo C/N bng:

Ci (minimum) = C/N + Np + FA = 49.6 58.2 + 8 = - 0,6 dBmV

Khi Ci > 0,6dBmV tng ng Cout > 29,4dBmV th t s C/N s cao hn 49.6dB tuy nhin nhiu do mo hi gy ra cng tng cao. Mc tn hiu u ra khi khuch i tng 1dB th mo hi tng 2dB, vi khuch i c CTB = - 76dBc ti Cout = 48 dBmV, khi mc CTB = 63.4dBc th lc mc u ra khuch i tng n mc ln nht bng:

(76 - 63.4)/2 + 48 = 54.3 dBmV.Nh vy mc tn hiu u ra khuch i c th bin thin trong khong t 29,4 dBmV n 54.3 dBmV th cc thng s C/N v CTB vn m bo yu cu t ra.

Xt nhnh c nhiu khuch i, theo cng thc trn, ch s nhiu s tng 3dB mi khi tng gp i s lng trn nhnh, do , mc Ci (dn n Cout) cng cn tng vi mc tng ng gi mc C/N c m bo. Tng t, mo hi tng 6dB mi khi tng gp i s lng trn nhnh, v vy, mc Cout ln nht cho php phi gim i 3dB m bo mc mo hi, theo di bng di y:

S lng khuch i/tngKhong mc tn hiu ti u ra cho php (dBmV)

Nh nht

(C/N ti thiu)Ln nht

(CTB ti thiu)

129.454.3

232.451.8

435.448.8

838.445.8

Nh vy, thit k mt h thng ng trc hot ng ti u, n nh, mt s thng s, c im sau cn c tun th:

Cc khuch i trong chui c cc thng s chnh nh ch s nhiu (noise figure), CTB, CSO ging nhau. Suy hao qua on cp gia hai khuch i trong chui bng mc khuch i (gain) ca khuch i trc. Mc gain ca mi khuch i phi ln (nh nht l 20dB).

Lp bng quan h iu kin nhiu-mo hi (bao gm C/N, CTB, CSO) cho nhnh cn tnh xc nh khong iu chnh mc tn hiu ti u ra khuch i cho php ti cc s lng khuch i khc nhau trn nhnh nhm m bo cc thng s t tiu chun.

V. Tnh ton nhiu, mo hi trn mng xui chiu:

a. Cng thc tng qut:

C/N:

T s nhiu/tp m (C/N) ti thu bao c tnh tng qut nh sau:

=

C/Nttl = -10log

Trong :

C/Nttl = C/N ton h thng, tnh t Headend ti h thu bao

C/Nh = C/N ti Headend (khi tnh coi nh ch ng mt mnh, khng kt ni vi ton mng)

C/Ns = C/N phn mng quang (khi tnh coi nh ch ng mt mnh, khng kt ni vi ton mng)

C/Nd = C/N phn mng ng trc (khi tnh coi nh ch ng mt mnh, khng kt ni vi ton mng)

C/Nt = C/N ca thit b u cui nh Set-top-box (khi tnh coi nh ch ng mt mnh, khng kt ni vi ton mng)

CTB v CSO:

Mo hi hu ht ch xut hin ti phn mng quang/ng trc v thit b u cui, ti HE mo hi xut hin nh, khng ng k.

CSOttl = -10log

CTBttl = -20log

b. Bi ton thc t hin nay:

C/N:

Cn c theo c tnh truyn dn v tuyn chiu xung ca tiu chun Euro-DOCSIS:

C/Nttl phi ln hn hoc bng 44dB

C/Nh ti HE HCATV hin nay trung bnh bng 48dB. Hin nay, cha s dng set-top-box nn khng c nhiu do thit b ny gy ra, do vy coi nh khng c thnh phn C/Ntp dng cng thc ta c:

= -

10log () = 10log ( - )

- 10log () = 46.2 (dB)

Gi thit phn mng quang bao gm 01 my pht quang v 01 trm lp (thu quang/pht quang) ca C-Cornet, C/N mng quang c tnh nh sau:

C/N my pht quang: 52 dB, C/N my thu quang: 63.5 dB

p dng cng thc cng 10log, C/N ca tuyn quang = 48.85 dB

Nh vy, C/N yu cu ca phn mng ng trc ti thiu phi bng: 49.6 dB

CTB/CSO:

Cn c theo c tnh truyn dn v tuyn chiu xung ca tiu chun Euro-DOCSIS:

CTBttl phi nh hn hoc bng -57dBc

Hin nay, cha s dng set-top-box nn khng c nhiu do thit b ny gy ra, do vy coi nh khng c thnh phn C/Ntp dng cng thc ta c:

CTBttl = -20log

Gi thit phn mng quang bao gm 01 my pht quang v 01 trm lp (thu quang/thu quang) ca C-Cornet, CTB mng quang c tnh nh sau:

CTB my pht quang: -69dBc, CTB my thu quang: -92dBc

p dng cng thc cng 20log, CTB ca tuyn quang = -62.7dBc

Nh vy, CTB ca phn mng ng trc ti a bng: -63.4dBc

T y, ta p dng phng php cn bng nhiu-hi xc nh mc tn hiu t ti u vo khuch i v s lng tng khuch i m bo phn mng ng trc c C/N ti thiu bng: 49.6 dB v CTB ti a bng: -63.4dBc.

c. Phng php tra bng tnh C/N, CTB, CSO:Di y, cm nang gii thiu cch tnh tng, hiu C/N, CTB, CSO theo cch tra bng khng cn s dng my tnh:

Bng tra tnh CTB:

Bng tra phc v tnh C/N, CSO:

CHNG V GAIN N V, PHNG PHP THIT K

V CN CHNH KHUCH I

Trong mng HFC, khuch i l thit b c tm nh hng ln nht trong h thng mng. Vic thit k, thi cng khuch i hp l s lm hn ch ti a nn nhiu v mo do hi bc cao gy ra. Nu mc tn hiu vo khuch i t cao c th lm tng t s C/N nhng mo hi cng s tng theo, nu gim mc tn hiu khc phc nh hng ca mo hi th li lm gim t s C/N. V vy, xc nh c mc tn hiu a vo cc khuch i ra sao l v cng quan trng. Chng ny gii thiu nguyn tc gain n v-unity gain- nguyn tc chnh, xin nhn mnh l rt quan trng trong thit k mng HFC hai chiu.I. Th no l gain n v:

Nhiu v mo hi l hai vn nh hng ln nht n vic vn hnh mng truyn hnh cp. Nu mc tn hiu cao tn qu thp, t s sng mang trn tp m gim, nu mc tn hiu cao tn qu cao th t s sng mang trn mo hi li gim. iu ny c ngha l mc tn hiu hot ng phi c thit lp nm trong mt khong gi tr nht nh m bo cho mng c m rng mt cch ti a m khng lm nh hng n cht lng tn hiu.

Ngay t thi k u lch s truyn hnh cp, cc k s khm ph ra cch s dng khuch i m mang li hiu qu cao nht: Mi khuch i phi c mc gain bng vi tng suy hao ca tn hiu qua on cp v thit b th ng nm sau khuch i . Khi nim ny c tn l gain n v- unity gain v vn l phng php then cht i vi vic thit k v vn hnh mng truyn hnh cp hai chiu hin i.

II. gain n v trong hng xui chiu:

Trong hng xui chiu, gain n v c ngha l: mc tn hiu vo on cp phi bng mc tn hiu ra ca khuch i theo sau on cp (tr trng hp khuch i trc c thm cng nhnh m c mc tn hiu ra thp hn so vi cng chnh- bridge amplifier).

Hnh trn m t mt chui gm 4 chic khuch i ging nhau (ging v mc gain, ch s nhiu-Noise figure, CTB, CSO). Trong v d ny, khi mc tn hiu vo on AB bng mc ra ca khuch i B, mc tn hiu vo on BC bng mc ra ca khuch i C, mc tn hiu vo on CD bng mc ra ca khuch i D v c tip tc nh vy. y cng chnh l im gain n v. Khi gain n v khng c thc hin, mng nhanh chng ri vo tnh trng khng th chp nhn c.

V d:

Nhm nhn mnh tm quan trng ca nguyn l gain n v, chng ta ly v d mt chui gm 10 khuch i ging nhau c mc gain bng 22dB, suy hao gia cc khuch i l 23dB. Nu mc ra ca khuch i u tin bng 32dBmV, lc mc vo khuch i th 2 bng 9 dBmV. Mc tn hiu ra ca khuch i th 2 s bng 31dBmV, mc vo khuch i th 3 s bng 8 dBmV v mc ra s bng 30dBmV. C nh vy n khuch i th 10 th ta thy rng mc tn hiu vo s ch cn bng 1 dBmV v mc ra l 23dBmV. Lc ny C/N b nh hng mt cch nghim trng.

Hnh trn m t v d chi tit v gain n v mc xui chiu bao gm mt chui nhng khuch i ging nhau vi mc gain 22dB. Mc tn hiu ra ca khuch i A cng l mc tn hiu vo on cp AB bng 32dBmV. on AB suy hao 22dB ti 750MHz v nh vy mc vo khuch i B bng 10dBmV, do khuch i B c mc gain 22dB nn mc ra ca n bng 32dBmV nu ATT u vo t mc 0dB. y ta c th thy r rng rng gain n v c xc lp do gain 22dB ca khuch i B bng mc suy hao ca on cp AB hay mc ra 32dBmV ca khuch i A bng mc ra 32dBmV ca khuch i B.

on BC bao gm cp v thit b chia th ng, thit b chia ny c mc suy hao cng in-out bng 2dB (insertion loss) v suy hao cp bng 14dB, tng suy hao l 16dB. on BC hin nhin c mc suy hao khng bng mc gain ca khuch i C, do vy, cn t ATT u vo khuch i C c suy hao mc 6dB nhm lm tng mc suy hao on BC ln ng 22dB bng mc gain 22dB ca khuch i C. Nh vy mc ra 32dBmV ca khuch i B bng mc ra 32dBmV ca khuch i C, gain n v c xc lp. y, cn hiu r mc tn hiu c khuch i l mc tn hiu vo tng khuch i u tin ca thit b khuch i sau khi i qua cc modun ATT (nh suy hao) v EQ (to dc).

on BD c tng suy hao do cp v cng chia bng 19dB, iu ny c ngha rng cn gi tr suy hao 3dB thm ti u vo khuch i D. Nh vy, gain n v c thit lp do 22dB suy hao on BD bng mc gain 22dB ca khuch i v mc ra 32dBmV ca khuch i B bng mc ra 32dBmV ca khuch i D.

Nh vy, ta c th thy rng im gain n v ca hng xui chiu l im u ra (output) ca khuch i. Trong mt chui khuch i ging nhau, nhim v l phi t c mc tn hiu ging nhau ti im gain n v ca cc khuch i.

Chng ta hiu c th no l thit lp gain n v cho mng hng xui chiu. Vy, im gain n v s c thit lp gi tr bao nhiu? Mi cc bn quay v phn tnh ton nhiu, mo hi trn mng xui chiu (mc II) v phng php cn bng nhiu- hi (mc I.3) chng II tm cu tr li.

III. gain n v trong hng ngc chiu:

i vi hng ngc chiu, gain n v cng c tm quan trng tng ng. hng ngc chiu c hot ng tt, gain n v nht thit phi ng nht trn ton mng.

V d:

Hnh trn m t chi tit v d v gain n v trong hng ngc chiu. S thit b ny chnh l s v d trong mc II, ch khc suy hao y c tnh ti tn s 30MHz thay cho 750MHz. Cc khuch i c modun khuch i ngc dng ging nhau v cng mc gain 15dB. Ging nh trng hp gain n v ca hng xui chiu, mc gain ca khi khuch i ngc chiu phi bng mc suy hao ca cp v thit b chia trong on cp nm sau khi khuch i ngc chiu . im gain n v ca hng ngc chiu l ging im gain n v ca hng xui chiu v mt v tr vt l. Theo chiu tn hiu chy th im gain n v hng xui chiu nm u ra khi khuch i xui chiu, ngc li, im gain n v hng ngc chiu nm u vo khi khuch i ngc chiu. Khi gain n v hng ngc c thit lp, mc tn hiu vo khuch i bng mc tn hiu ra khi on cp nm sau khi khuch i , y cng chnh l mc tn hiu vo khuch i ngc dng tip theo.

Trong v d ny, chng ta cn 15dB suy hao b cho 15dB gain ca mi khuch i. on AB ch suy hao 4dB ti 30MHz, do , cn ATT ti u ra ca khuch i B c gi tr suy hao 11dB. (Nhng khuch i c khi ngc dng hin nay u c ATT-nh suy hao v EQ-to dc c ci t ti u ra ca khi khuch i ngc chiu). Nh vy, on AB c mc suy hao 15dB bng mc gain ca khuch i B. Do , mc tn hiu 20dBmV vo khuch i B bng mc 20dBmV ra on AB, hay chnh l mc vo khuch i A.

on BC bao gm suy hao 5dB ca c thit b chia th ng v cp. Do vy, cn ATT ti u ra ca khuch i C c gi tr suy hao 10dB to ra tng suy hao mong mun 15dB. Nh vy, khuch i C c mc gain bng mc suy hao on BC v mc tn hiu 20dBmV vo khuch i C bng mc 20dBmV ra on BC.

on BD c tnh ton v ATT ti u ra khuch i D c xc nh theo cch tng t. Nh vy, i vi ton b cc khuch i ngc dng, gain n v c xc lp, cc im gain n v u c xc lp vi mc gi tr ging nhau v l u vo ca cc khuch i ngc chiu.

gain n v l mt nguyn l quan trng xuyn sut t vic thit k mng, lp t thit b, cn chnh v bo dng mng hai chiu.

IV. Cn chnh khuch i:

nm c phng php cn chnh khuch i, trc ht ta cn hiu s cu to c bn ca mt khuch i hai chiu nh hnh v di y:

1. Lp t, cn chnh khuch i hng xui chiu:

B1: Trc khi cp ngun cho khuch i cn chc chn rng mc tn hiu vo khuch i khng qu cao hoc c th gy hi n khuch i, ta c th t ATT-pad u vo mc cao nht trc khi cp ngun.

B2: t mc 0dB cho mc EQ-pad u vo v 0dB cho EQ plug-in nm gia hai tng khuch i.

B3: Cp ngun cho khuch i v o mc tn hiu u ra ti tn s cao nht cng test point -20dB. Cn c theo nguyn tc gain n v v im gain n v nu trn, xc nh gi tr ATT-pad u vo m bo mc tn hiu ti tn s cao nht ra khi khuch i bng tng suy hao do cp, b chia, ATT-pad gy ra ti tn s cao nht. Lun nh rng, gi tr o c thp hn gi tr thc t ti cng ra 20dB.

B4: Chnh EQ-pad u vo cho n khi mc tn hiu ra ti cc knh xp x bng nhau trn ton di thng, lc ny mc tn hiu vo tng khuch i th nht cng bng phng trn ton di. Trong iu kin ny, t s C/N t mc tt nht trn ton di thng.

B5: Ci t EQ plug-in to dc cho mc tn hiu ti u ra theo yu cu. Mc tn hiu u ra c dc hay phng c quyt nh bi ngi qun l mng.

B6: Tinh chnh mc tn hiu u ra v dc vi ATT-pad v EQ-pad u vo. Lun lun iu khin ATT-pad trc sau mi n EQ-pad.

B7: Nu c vn trong vic t mc tn hiu u ra nh mong mun th cn kim tra li mc tn hiu ti testpoint u vo.

B8: Ghi li mc tn hiu vo/ra, gi tr cc pad, EQ plug-in vo ming giy dn trn np trong ca khuch i tin cho vic theo di, bo dng sau ny. B9: Tip tc B1 n B8 vi nhng khuch i tip theo.

2. Cn chnh khuch i hng ngc chiu:

a. im bm tn hiu test:

Trong qu trnh cn chnh khuch i hng ngc chiu c hai gii php v im thit lp mc tn hiu kim tra hng ngc chiu. Gii php th nht xc nh im gain n v hng ngc l im ti u vo modun khuch i (gain stage) v gi l im gain n v trong-internal unity gain reference point. Gii php cn li xc nh im gain n v hng ngc l im ti u ni-connector u vo khuch i v gi l im gain n v ngoi-external unity gain reference point.

C nhiu kin tn thnh cng phn i cho migii php trn, nhng gii php sau c mt s nh sn xut khuch i tin c. iu quan trng l gii php no ph hp vi h thng ca chng ta v duy tr n trong sut qu trnh cn chnh khuch i.

u tin chng ta xem xt gii php im gain n v trong-internal unity gain reference point. Gi thit mun mc tn hiu ngc dng bng 10dBmV ti im ny. Vy mc tn hiu test (do my pht tn hiu test ngc dng pht ra) cn t vo u v bng bao nhiu ti u vo modun khuch i ngc dng t mc 10dBmV?

Nhn s khi khuch i ta c th ngh rng s kt ni my pht tn hiu test ngc dng vo im test point u vo khi khuch i ngc dng (Reverse amplifier input test point). Tuy nhin, nu nhn k hn na ta s thy rng im test point ny l cng ca mt b chia (dng directional coupler) o tn hiu ngc dng vo modun khuch i (gain stage) ch khng phi im bm tn hiu test do nu bm tn hiu test vo im ny th tn hiu s i theo chiu xui dng ch khng phi chiu ngc dng ta cn.

Nh vy, im tt nht ni my pht test ngc dng vo l im o tn hiu u ra xui dng-Output test point. Khi ni my pht test ngc dng vo im ny, tn hiu i theo cng ca 01 b chia, qua 01 b lc phn hng v 01 b chia na mi ti u vo khi khuch i. Nu khuch i c Output test point -20dB th my pht test ngc dng phi c mc bng: 10 (dBmV)_mc yu cu + 20dB + (0.5 1)dB_suy hao do qua b lc phn hng + 1dB_suy hao qua b chia + dB_suy hao do ATT u vo khi modun khuch i ngc dng (ATT ny thng ch c mt s khuch i v khng dng cn chnh cho hng ngc chiu). Nh vy, mc pht ca my test ngc dng phi ln hn hoc bng 32dBmV.

Nu ta s dng gii php im gain n v ngoi-external unity gain reference point v im bm tn hiu test l im o tn hiu u ra xui dng-Output test point. Nh vy, mc tn hiu ti im gain n v ngoi bng: mc tn hiu test + 20dB. y l phng php tin dng v ph bin.

b. Cn chnh khuch i ngc dng:

Thit lp mc tn hiu cho my pht test v kt ni vo im bm tn hiu, v d l khuch i A s trn. Cn chnh ATT u ra khuch i ngc dng mc tn hiu vo khuch i ny bng mc tn hiu vo node quang v bng ng mc tn hiu ti u ra my thu quang ngc chiu kt ni vi CMTS. Nh vy, gain n v c thit lp. Lc ny, nu mc tn hiu ti my pht test l X (dBmV) th mc ti CMTS s l: (X +20)dBmV.

Chuyn ti khuch i tip B, bm tn hiu test v cn chnh ATT mc tn hiu vo khuch i B bng mc vo khuch i A v cng chnh l bng mc tn hiu vo node quang v CMTS. Nh vy, gain n v c thnh lp.

Cc khuch i tip theo cng c thc hin tng t.

Nh vy, cn 2 k thut vin thc hin cng vic cn chnh, mt ngi ng ti node quang o mc vo ti cng test, mt ngi thc hin cng vic cn chnh ti khuch i, hai ngi lin lc bng b m hoc in thoi. Nu vic cn chnh t khuch i u tin p ng nguyn l gain n v, th ti khuch i th n, nu mc tn hiu vo khuch i bng mc vo node quang ngha l cng bng mc vo cc khuch i trc n.

C mt cch m ch mt ngi cng c th cn chnh bng cch, t mt camera trc my o tn hiu u vo ti CMTS, camera ny c iu ch v pht ra mng ti mt knh ring khng s dng trn hng xui chiu. K thut vin mang theo mt tivi b ti, u vo mng ti im cn chnh, d theo tn s knh ring v theo di mc ti CMTS.

3. ATT v EQ, tnh ton v s dng:

Trong thit b khuch i, node quang, vic khai thc, iu chnh ATT (attenuator) v EQ (Equalizer) l yu t rt quan trng lin quan n cht lng tn hiu, nhiu, mo hi ca ton mng. Vic iu chnh hai linh kin ny quyt nh mc tn hiu ra cao hay thp v dc tn hiu cc knh ton di thng. Trong truyn hnh cp ngi ra quy c: + Dc dng: Knh tn s thp c mc tn hiu thp hn cc knh tn s cao.

+ Dc m: Knh tn s thp c mc tn hiu cao hn cc knh tn s cao.

Qua cc chng trnh by trn, ta bit rng mc tn hiu ra khi khuch i qu thp s nh hng n ch s C/N gy hin tng knh truyn hnh b nhiu, mui; nu qu cao li nh hng n ch s CTB v CSO gy hin tng knh b xc trn mn hnh ti vi ca khch hng thu bao. Mt khc, tn hiu cc knh trn di thng ra khi node quang, khuch i cn to mt dc nht nh vi tiu ch cc knh tn s thp c mc tn hiu thp hn cc knh tn s cao( dc dng). Vic to dc dng trc ht lm dung ha c tnh suy hao do cp. Nhng l do quan trng hn l nhng sn phm mo hi bc hai, bc ba ch yu do cc knh tn s thp khi i qua cc cc phn t tch cc trn mng gy ra cho ton gii thng. V vy, vic to dc dng s lm gim bin ca nhng sn phm mo hi gip lm gim nh hng ca mo hi n cht lng cc knh. Do vy, mng truyn hnh cp hin i ngy ny thng t chnh lch mc tn hiu ti u ra node quang v khuch i gia knh thp nht 110MHz n knh cao nht 862 MHz trong khong 8 ( 12dB theo dc dng.

Di y, ti liu s gii thiu cc loi ATT, EQ v phng php tnh ton s dng:

a. Khi to suy hao- ATT:

Khi ATT dng to suy hao, lm gim mc tn hiu cc knh trn ton b di thng vi cng mt gi tr. Chng phi hp vi EQ to ra mc tn hiu nh mun ti u ra b khuch i. C th ni, vic iu chnh ATT l iu chnh mc khuch i ca thit b.

Cn c theo phng php iu chnh, thit lp gi tr, ngi ta phn ATT thnh hai loi:

+ Plug-in Attenuator Devices (vit tt l PADs) hay Fixed Attenuator l dng module cm c nhng gi tr c nh. Cc module cm thng c gi tr to suy hao t 0dB ( 20dB vi bc nhy 0.5dB, 1dB, 1.5dB, 2dB ty theo cc nh sn xut khuch i. Di y l bng gi tr PADs ca nh sn xut khuch i ACI chng ta tham kho:

Gi tr suy haoM linh kinGi tr suy haoM linh kin

0SXP-011SXP-11

0.5SXP-0.511.5SXP-11.5

1SXP-112SXP-12

1.5SXP-1.512.5SXP-12.5

2SXP-213SXP-13

2.5SXP-2.513.5SXP-13.5

3SXP-314SXP-14

3.5SXP-3.514.5SXP-14.5

4SXP-415SXP-15

4.5SXP-4.515.5SXP-15.5

5SXP-516SXP-16

5.5SXP-5.516.5SXP-16.5

6SXP-617SXP-17

6.5SXP-6.517.5SXP-17.5

7SXP-718SXP-18

7.5SXP-7.518.5SXP-18.5

8SXP-819SXP-19

8.5SXP-8.519.5SXP-19.5

9SXP-920SXP-20

9.5SXP-9.520.5SXP-20.5

10SXP-1021SXP-21

10.5SXP-10.5

Vi vic s dng ATT dng PAD, mi ln cn iu chnh mc nh suy hao ta phi thay th cc PAD c gi tr thch hp.

+ Ajustable Attenuator hay Variable Attenuator l dng iu chnh bng nm vn c khong iu chnh mc suy hao thng t 0dB ( 20dB. Mi ln cn iu chnh mc nh suy hao ta ch cn vn nm chnh v o mc tn hiu u ra cho n khi t gi tr mong mun.

Trn thc t thi cng, bo dng mng, vic iu chnh ATT bng phng php nm vn linh hot hn cht so vi vic s dng dng pad cm. Tuy nhin, ATT dng nm vn li d b bin i bi nh hng ca thi tit v c n nh km hn nhiu so vi ATT-pad. Mt khc, trong qu trnh thit k, cn b k thut c tnh ton, xc nh gi tr ATT thit lp cho khuch i. Trong qu trnh thi cng, cn c theo bn v thit k, nu s dng ATT-pad, ta ch vic chn loi pad c gi tr nh suy hao theo nh bn thit k thit lp cho khuch i sau o mc tn hiu v tinh chnh. Nu s dng ATT dng nm vn, ta kh xc nh gi tr to suy hao ca ATT hin ti bng bao nhiu v iu chnh bao nhiu cho ph hp ngoi cch phi o mc tn hiu vo/ra thit b lin tc. Ngoi ra, vic iu chnh ATT bng nm vn rt d b qu tay gy nhanh hng khi iu chnh ATT . Chnh v vy m cc nh sn xut thng khuyn ngh mng t c s n nh cao nn s dng loi khuch i c iu chnh khi ATT dng pad cm.

b. Khi to dc tn hiu:

Khi to dc tn hiu dng to suy hao, lm gim mc tn hiu ti cc tn s khc nhau nhng gi tr khc nhau. C hai loi khi iu chnh to dc l EQ (Equalizer) v CS (Cable Simulator) hay CEQ (Cable Equalizer):

+ Khi iu chnh EQ dng to dc suy hao vi gi tr suy hao rt ln cc tn s thp v gi tr suy hao rt nh cc tn s cao- to dc dng nhm cn bng cho di tn hiu c dc m. + Khi iu chnh CS hay CEQ c tc dng ngc li, dng to dc suy hao vi gi tr suy hao rt ln cc tn s cao v gi tr suy hao rt nh cc tn s nh- to dc m nhm cn bng cho di tn hiu c dc dng. Trn thc t, khi iu chnh CS hay CEQ thng ch dng trong phng th nghim hoc phc v cng tc o, kim tra thit b. Vic iu chnh EQ nhm dung ha c tnh suy hao ca cp, m bo mc tn hiu bng nhau ton gii thng trc khi vo khi khuch i u tin ca thit b tun th nguyn l gain n v.

Cn c theo phng php iu chnh, thit lp gi tr, ngi ta phn EQ thnh hai loi:

+ Plug-in Equalizer hay Fixed Equalizer l dng module cm c nhng gi tr c nh. Cc module cm thng c gi tr t 0dB ( 20dB vi bc nhy 0.5dB, 1dB, 1.5dB, 2dB ty theo cc nh sn xut khuch i tng ng iu chnh dc gia 110Hz v 862Mhz trong khong 0 ( 12dB. Do mi module cm to nhng gi tr suy hao khc nhau ti nhng tn s khc nhau, mi dc cn cn bng tng ng vi cc loi module khc nhau. V vy, nht thit cn tra bng thng s EQ chn gi tr thch hp. Do cc bng thng s gi tr EQ ca cc hng sn xut c gi tr tng ng sai s (0.5dB nn di y l bng gi tr EQ dng module cm ca nh sn xut khuch i ACI chng ta tham kho:

M ph kin EQGi tr suy hao (dB) ti cc tn s

557291550650750870

EQ/00.00.00.00.00.00.00.0

EQ/1.5-1.7-1.6-1.6-0.8-0.6-0.7-0.7

EQ/3-2.8-2.7-2.6-1.1-0.8-0.9-0.7

EQ/4.5-4.3-4.1-4.0-1.6-1.3-0.9-0.7

EQ/6-5.2-5.1-5.0-2.3-1.8-1.5-0.8

EQ/7.5-6.2-6.1-6.0-2.2-1.6-1.2-0.7

EQ/9-7.4-7.2-7.0-2.6-1.9-1.2-0.9

EQ/10.5-8.7-8.3-7.9-2.7-1.9-1.1-0.6

EQ/12-9.8-9.3-8.9-3.4-2.2-1.4-0.6

EQ/13.5-10.9-10.4-9.8-3.4-2.3-1.5-0.7

EQ/15-11.6-11.2-9.9-3.4-2.2-1.2-0.6

EQ/16.5-13.4-12.8-12.3-4.3-3.0-2.1-1.1

EQ/18-14.5-14.1-13.5-4.7-3.3-2.2-1.1

EQ/19.5-15.9-15.1-14.4-5.4-3.7-2.6-1.3

EQ/21-17.0-16.2-15.5-5.4-3.9-2.4-1.1

V d: chnh lnh mc tn hiu vo khuch i ti tn s 231.25MHz v 743.25MHz bng 8dB v gi thit l dc dng tuyn tnh th mc tn hiu qua khi EQ cn bng ton gii thng th phi chn Module EQ c gi tr EQ/13.5 hoc EQ/15.

c s iu chnh mang tnh tng th, n nh, lu di trn ton mng. Ngay t lc thit lp h tng, cc nh qun l mng thng pht ngay trn mng 2 tn s sng pilot tn s gii hn thp nht v cao nht trong di thng lm cn c iu chnh mc tn hiu v dc cho ton b cc knh nm trong di gii hn . Hin nay ti HE ca Truyn hnh cp H ni pht 02 sng pilot ti tn s 96MHz v 752MHz c mc tn hiu bng mc tn hiu cc knh truyn hnh tng t trn mng. Cn c theo s pht trin cc dch v gia tng ca mng tn s ln hn 750MHz. Cm nang xut vi hng xui dng, ti HE nn s dng b pht cc knh pilot cc tn s 110 MHz, 862 MHz nhm phc v cho vic cn chnh tn hiu v thit lp dc tn hiu.

+ Ajustable Equalizer hay Variable Equalizer l dng iu chnh bng nm vn c khong iu chnh t 0dB ( 20dB tng ng iu chnh dc gia 110Hz v 862Mhz trong khong 0 ( 12dB. Mi ln cn iu chnh mc nh suy hao ta ch cn vn nm chnh v o mc tn hiu u ra cho n khi t gi tr mong mun.

Tng t nh ATT, trn thc t thi cng, bo dng mng, vic iu chnh EQ bng phng php nm vn linh hot hn cht so vi vic s dng dng module cm (plug-in hay fixed). Tuy nhin, EQ dng nm vn li d b bin i bi nh hng ca thi tit v c n nh km hn nhiu so vi dng module cm. Mt khc, trong qu trnh thit k, cn b k thut c tnh ton, xc nh gi tr EQ thit lp cho khuch i. Trong qu trnh thi cng, cn c theo bn v thit k, nu s dng EQ module cm, ta c th chn ngay loi c gi tr theo nh bn thit k thit lp cho khuch i sau o mc tn hiu v tinh chnh. Nu s dng EQ dng nm vn, ta kh xc nh gi tr to dc ca EQ hin ti bng bao nhiu v iu chnh bao nhiu cho ph hp ngoi cch phi o mc tn hiu vo/ra thit b lin tc. Ngoi ra, vic iu chnh EQ bng nm vn rt d b qu tay gy nhanh hng khi iu chnh EQ . Chnh v vy m cc nh sn xut thng khuyn ngh mng t c s n nh cao nn s dng loi khuch i c iu chnh khi ATT dng pad cm.

i vi hng ngc dng, vic s dng, iu chnh ATT, EQ cng tng t. Tuy nhin, do c tnh suy hao ca cp c chnh lnh v mc suy hao cc knh di tn ngc dng t nn vic iu chnh dc cc knh ngc chiu t quan trng hn so vi hng ngc chiu.

c. Phng php tnh ton EQ: Bc 1: Tnh dc tn hiu s(EQ) = OUT + LL LH - (iEQ)

s(EQ) : dc tn hiuOUT : lch u ra yu cu(iEQ) : lch trung gian gia 2 khi khuch i

LH: mc tn hiu vo khuch i ti tn s cao

LL: mc tn hiu vo khuch i ti tn s thpCng thc trn thng ch p dng cho khuch i cui cng ca tng khuch i to dc thch hp a ti h thu bao. i vi cc khuch i trong tng, nhm p ng mc tiu a tn hiu bng phng ton di thng vo moun khuch i u tin nn lch u ra yu cu l do iu chnh EQ gia hai moun khuch i.(iEQ) V vy OUT =(iEQ) nn cng thc c ti gin nh sau: s(EQ) =LL LH Bc 2: Tnh gi tr EQ cn s dng tng ng vi dc s(EQ) Tham kho bng tra suy hao ca EQ theo tn s:EQ(dB)Tn s( Mhz)

306587110174550750862

01.001.001.001.001.001.001.001.00

22.632.452.362.292.101.401.131.00

44.253.903.733.573.201.801.271.00

65.885.355.094.864.302.211.401.00

87.516.806.466.145.412.611.541.00

109.138.257.827.436.513.011.671.00

1210.769.709.198.717.613.411.811.00

1412.3911.1610.5510.008.713.821.941.00

1614.0212.6111.9211.289.814.222.081.00

1815.6414.0613.2812.5710.914.622.211.00

2017.2715.5114.6513.8612.015.022.341.00

Bng gi tr EQ tng ng vi cc mc s(EQ) trong di tn s 110-862MHz:s(EQ) 0123456789101112

00.001.563.114.676.227.789.3310.8912.4514.0015.5617.1118.67

0.10.161.713.274.826.387.939.4911.0512.6014.1615.7117.2718.82

0.20.311.873.424.986.538.099.6511.2012.7614.3115.8717.4218.98

0.30.472.023.585.136.698.259.8011.3612.9114.4716.0217.5819.14

0.40.622.183.735.296.858.409.9611.5113.0714.6216.1817.7419.29

0.50.782.333.895.457.008.5610.1111.6713.2214.7816.3417.8919.45

0.60.932.494.045.607.168.7110.2711.8213.3814.9416.4918.0519.60

0.71.092.644.205.767.318.8710.4211.9813.5415.0916.6518.2019.76

0.81.242.804.365.917.479.0210.5812.1313.6915.2516.8018.3619.91

0.91.402.964.516.077.629.1810.7312.2913.8515.4016.9618.5120.07

VD: s = 2.3 ( gi tr EQ cn dng l 3.58dB ( chn EQ = 4dB Bc 3: Tham chiu gi tr EQ yu cu vi cc module EQ c sn chn ra gi tr EQ thch hp.

Bc 4: Tng ng vi EQ chn, tra bng gi tr tnh mc tn hiu thc t vo khuch i sau khi cm module EQ .

Bi ton v d:

VD1: lch knh u vo khuch i hot ng trong di tn (110-870) l s = 3.5dB( dc m). Coi mc u ra ca cc knh bng nhau (OUT=0dB) v lch trung gian (iEQ)=0dB. Do gi tr EQ cn dng l 5.45dB. Tuy nhin theo hng sn xut khuch i ch c loi EQ/4 hoc EQ/6 ( chn EQ/6.

Tra bng suy hao EQ/6, tn hiu vo khuch i ti tn s 110MHz thc t sau khi lp EQ s b tr i 4.86dB v ti tn s 870MHz s b tr i 1dB.

VD2: Gi s mt khuch i c mc tn hiu vo l 93dBuV@ 110MHz, 82dBuV@862MHz.

G = 30dB, lch trung gian (iEQ) =8dB.

Mc suy hao u vo (Loss in) l 0.6dB@ 110MHz, 1dB@862MHz.

Cn phi s dng gi tr ATT, EQ no c mc tn hiu ra ti khuch i l 106dBuV@862MHz v lch u ra OUT= +8dB?

Hnh v m t qa trnh bin i mc tn hiu khi hiu chnh khuch i

Sau khi i vo khuch i, tn hiu b suy hao nh sau:

@110Mhz: 93 0.6 = 92.4dBuV

@ 862Mhz: 82 1 = 81dBuV

p dng cng thc trn, tnh c lch tn hiu l:

EQ = 8 + 92.4 81 8 = 11.4dB

Tra bng tng ng vi lch trn th gi tr EQ = 17.74dB ( chn EQ=18dB.Tng ng vi EQ=18dB, tra bng tm c suy hao do EQ gy ra l 12.57dBuV@110MHz, 1dBuV@862MHz.Nh vy, mc tn hiu qua khuch i sau khi thit lp gi tr EQ = 18dB nh sau:

@110Mhz: 92.4 12.57 = 79.8 dBuV

@ 862Mhz: 81 1 = 80 dBuV

( Mc tn hiu vo khi khuch i th nht gn bng phng, m bo yu cu k thut.

Nu mc ATT = 0dB th tn hiu ra khi khuch i c gi tr nh sau:

@110Mhz: 79.8 - 8 + 30 = 101.8 dBuV ( tr 8 dB l do suy hao ca iEQ to ra)@ 862Mhz: 80 + 30 = 110 dBuV

Cn c vo yu cu ca bi ton, chn gi tr ATT= 4dB. Nh vy mc tn hiu ra khi khuch i s l 97.8dBuV@110MHz, 106dBuV@862MHz.(m bo yu cu ca bi ton).Ch : Hin nay cc ph kin ca khuch i (EQ v ATT) thng thng c gi tr t 0-18dB, vi bc nhy =2dB.Thc t cng c trng hp tn hiu khuch i vo c dc dng ngha l mc tn hiu ti tn s thp thp hn mc tn hiu ti tn s cao. Nh vy, khng th dng EQ cn bng mc tn hiu ton gii thng trc khi a vo modun khuch i u tin ca khuch i. Nguyn nhn l khuch i trc t tn hiu u ra c dc dng nhng khuch i sau t qu gn nhau( v d: cp tn hiu vo mt khu vc no nm gn khuch i trc..) hoc trn on ni hai khuch i c qu nhiu b chia ng trc nn khng t c s suy hao theo c tnh ca cp.

V vy, cc nh k thut khuyn ngh khng t cc khuch i qu gn nhau v s gy chi ph ln. Cc khuch i c thit k c nhng khong cch suy hao vi nhau nht nh sau ko cp trc chy ngc li vo cc khu vc, thc t l gi tr on cp ko chy ngc li r hn nhiu vic s dng cc khuch i qu gn nhau.CHNG VIPHNG PHP KHO ST, THIT K MNGI. Kho st, thit k mng quang:

Cng vic u tin khi thit k h thng mng cp quang l phi la chn, tnh ton k lng cc thng s hot ng ca mi thnh phn trong h thng pht quang. l nhng vn lin quan n tc d liu, t s li bit trong h thng s, tuyn tnh, t s S/N trong h thng tng t, khong cch truyn tn hiu, loi si quang s dng, cng sut my pht quang, loi ngun quangc cp trong bng di y:

Ni dungLa chn

Khong cch truyn dn tn hiu phc tp ca h thng tng theo khong cch truyn

Loi si quang s dngn mode hay a mode

tn scKt hp vi my pht tn hiu hay l b tn sc

Si phi tuync tnh si, bc sng, cng sut ngun pht

Cc bc sng hot ng780, 850, 1310, 1550 hay 1625nm

Cng sut phtTnh bng dBm

Loi ngun quangLED hay Laser

nhy thuTnh bng dBm

B tch sngPIN, APD hay IDP

Loi iu chAM, FM, PCM hay s

T s li bit (cho h thng s)10-9 hay 10-12

T s tn hiu/tp m S/NTnh bng dB

S connector hay mi hn ni s dng trong h thngTn hiu s suy hao tng theo s connector hay mi hn ni

Cc yu cu v mi trng hay hn ch khc m, nhit , nh sng

Cc yu cu c hcChng chy, dng trong nh hay ngoi tri, cp treo hay cp chn

Trong khong cch truyn dn tn hiu l vn cn ch nht. N lin quan n vic chn la cng sut cng nh loi ngun pht quang, loi si quang (si n mode cho khong cch truyn i xa hn), t chn loi v nhy my thu quang ph hp. Ngoi ra cng chn loi iu ch cho thch hp vi khong truyn xa.

Khi thit k 1 h thng mng quang phc tp cn tnh ton suy hao tng, c lng cng sut quang cn dng, bc sng hot ng, suy hao si quang v nhy thu quang.

Phn tch nhy thu quang xc nh mc quang ti thiu cn c thu c. nhy thu chu nh hng do tp m mt ngun pht quang, tp m si quang, tp m ca b tch sng, nhiu k t, t s li bit v trt thi gian (jitter).

Cng cn xem xt v vn mi trng. Bi nhit nh hng ti hiu sut ca LED, laser v si quang. Khi thit k a ra cc yu cu chung v an ton chy n, pht x EMI v cc thng s khc. Chc chn mi trng hin ti s c nhiu nh hng n mng cp quang nn phi la chn loi cp cn thn.

Cn cn nhc v chi ph cho 1 h thng mng cp quang. Cc thnh phn nh loi my pht quang, bc sng pht, loi connector, loi si cp quang, loi my tch sng quang s nh hng n ton b chi ph cng nh hiu sut ca mng. Ni chung cn thit k 1 h thng mng cp quang c chi ph hiu qu nht ph hp vi nhu cu s dng.

Mt s ni dung, khi nim cn quan tm khi thit k h thng mng cp quang:

Suy hao vt liu trong h thng: Lin quan n kt qu bng tn v khong cch, c th hin bng suy hao quang trn mt si quang no . Nu l h thng c xy lp t trc v ch cn nng cp th thng tin ny c xc nh d dng. Nu l xy dng 1 mng mi th phi bit c khong cch truyn (v d khong cch t my pht n my thu) tnh ton suy hao vt liu si. T xc nh cng sut my pht quang v/ hoc my pht trung gian.

Loi tn hiu pht i: bao gm tn hiu hnh, m thanh, s liu v ch ra l tn hiu s hay tng t.

La chn loi si quang (nh bng trn): loi n mode hay a mode. Khong cch truyn, loi tn hiu pht v ng dng s xc nh loi si no dng l ph hp nht. truyn i xa, tc cao, nhiu knh th chn si n mode, ngc li nu truyn vi khong cch ngn, tc thp, 1 knh ring l th chn si a mode c gi thnh thp hn.

La chn loi connector quang: cng nh si quang, tu vo h thng khc nhau s c cc yu cu khc nhau. Phi chn loi connector gim phn x, d lp t, khi lng ph hp v c giao din thch hp vi cc h thng c sn.

1. Cu trc Topo ca mng cp quang:

Khi thit k mng quang cho mt khu vc no , cn tham kho cu trc Topo ca mng quang tng th c tnh quy hoch cho hp l, khoa hc, khng gy chng cho, lng ph vt t. Nu l thit k mi th cng phi xy dng c cu trc mng quang mang tnh tng th. (Xem cu trc Topo mng cp quang - Chng II ).

2. Ngun cng sut quang:

Nh ta bit, tn hiu quang c pht ra t Diode Laser trong my pht quang. Cng sut quang pht ra c o bng dBmW.

Cng thc i mW sang dBm: 10log (mW) = dBmNh vy, 1mW tng ng vi 0dBm. Nu cng sut cng ra ca my pht quang l 3dBm cho gi tr bng 2 ln mc ti 0dB, tc l 3dBm = 2mW. Tng t nh vy, 6dBm = 4mW; 9dBm = 8mW; 12dBm = 16mW.

Ngy nay, hu ht cc my pht quang 1310nm u cho cng sut quang t 4-20mW. Khi cng ngh mi pht trin ngi ta c xu hng s dng thit b c bc sng quang 1550nm mang li nhiu u im, tuy nhin cng cn phi cn nhc n chi ph.

3. Suy hao cp:

Cp quang c suy hao 0.35dB/km ti 1310nm, 0.2dB/km ti 1550nm. Tuy nhin khi tnh ton thc t th cn tnh thm vo suy hao cp theo l thuyt mt s cc h s s cp di y.

4. Suy hao vng cp:

Khi cp quang c ko treo trn ct, ngay c khi c dy gia cng th chiu di cp thc t gia 2 im ct vn di hn chiu di o l thuyt. Khong chnh lch ny c gi l vng ca cp. Khi tnh ton thit k thng cng thm 2-5% b li vng ny.

Thm ch khi thi cng ko cp ngm th n cng khng chy theo 1 ng thng tp m n chy theo ng zic-zc. Khi thit k cp i ngm thng cng thm 2% chiu di cp.

5. D phng cp:

Nu cp quang t hng th cn phi thc hin hn ni li. Qu trnh hn ni yu cu cn phi dn 1 on cp d. phng mi bt trc xy ra nn d on cp quang. V d c 2km cp quang th cun d nh nht t 10m n ln nht l 100m.

6. Suy hao ca cc mi hn gia nhit v hn c hc:

Ni chung cc mi hn (mi ni) u thch hp cho vic trin khai v vn hnh mng cp quang. C th thc hin hn bng c hc hoc hn t ng nh cc my hn gia nhit. Hn c hc yu cu cc k s phi c tay ngh cao. Mt mi hn c hc b hng c th cho mc suy hao ti 1dB, nu tt th l 0.1dB. Khi so snh vi suy hao ca 1km cp quang l 0.35dB th mi thy rng y l mc suy hao rt ln. Nu hn bng my hn gia nhit s cho cht lng tt hn vi mc suy hao l 0.05dB/ mi hn.

7. S cc mi hn ni:

Cp quang thng c chiu di ch to l 2km/cun. V th mi on cp 2km s yu cu ti thiu 2 mi hn ti 2 u cui. Trung bnh 1 mi hn c mc suy hao 0.05dB, nu l hn gia nhit. Nh vy, ngoi mc suy hao l thuyt l 0.35dB/km th cn phi cng thm phn suy hao mi hn ny, tc l 0.4dB/km.

8. Suy hao quang:

T u ra ca my pht quang, cng sut quang s b suy hao do h thng mng phn phi trc khi a n my thu. Do ngi thit k phi tnh ton sao cho tng cng sut quang pht ra s b li c mc suy hao quang trong mng phn phi, l suy hao qua h thng cp, b chia quang, connector, mi hn ni m bo mc vo u thu vn t c yu cu.

Thng s suy hao ca mt s thit b tnh ti ca s 1310nm nh sau:

STTLoi thit b quangMc suy hao Ghi ch

1Cp quang n mode, 9/125(m0.35dB/kmti bc sng 1310nm

2Mi hn0.05 0.1dB /mi hn0.05 dB/mi: hn gia nhit

0.1 dB/mi: hn c hc

3Connector SC hoc FC0.2-0.3dBThng thng tnh =0.3dB

4Adapter SC hoc FC0.15-0.3dBThng thng tnh =0.2dB

5ui cp (Pigtail)0.3dB1 u c connector

6Dy nhy (Jumper)0.6dB2 u c connector

7B chia quang0.3-2dB (1x2) : 0.3dB

(1x3) : 0.6dB

(1x4) : 0.6dB

(1x5) : 0.9dB

(1x6) : 0.9dB

(1x7) : 0.9dB

(1x8) : 0.9dB

(1x9) : 1.2dB

(1x10): 1.2dB (1x11) : 1.2dB

(1x12) : 1.2dB

(1x13) : 1.2dB

(1x14) : 1.2dB

(1x15) : 1.2dB

(1x16) : 1.2dB

(1x17) : 1.5dB

(1x18) : 1.5dB

(1x19) : 1.5dB

(1x20) : 1.5dB

Ghi ch: Mc suy hao qua b chia quang tnh thm trn bao gm suy hao ca 1 connector + suy hao ca cc mi hn bn trong b chia quang.

V d: Suy hao in hnh ca cng 20% ca b chia quang (1x2) l:

= Suy hao 20% quang (7.14dB) + Suy hao tnh thm (0.3dB) = 7.44dB

Trong khi suy hao in hnh ca cng 20% ca b chia quang (1x4) l:

= Suy hao 20% quang (7.14dB) + Suy hao tnh thm (0.6dB) = 7.74dB

m bo cht lng tn hiu, khuyn co mc tn hiu quang vo my thu quang/ node quang nm trong di (-2 n 0dBm).

9. Suy hao d phng:

Thng thng khi thit k mc suy hao d phng l 0.4dB. Mc d phng ny c a ra nhm b li lch tnh ton nh chiu di cp, cc mi hn ni m bo h thng vn hot ng tt. Mc d phng ny cng cn b li s suy gim cht lng ca cc thit b theo thi gian nh c tnh si cp quang, cng sut pht quang ca my pht laser thp hn, cc mi hn ni

Nh vy ta thit lp c cng thc tnh nh sau:

Cng sut my pht quang yu cu = Suy hao b chia quang + suy hao cp + suy hao cc mi hn + phn d phng 0.4dB + suy hao connector.

10. Loi cp quang:

Trong h thng mng quang rt t khi ngi ta s dng cp c 1-2 si quang. Ti thiu phi l 6-8 si. Thng thng dng loi 12 v 24 si. Cng c th dng loi cp c s si ln hn tu thuc vo tng h thng mng thit k.

Khi thit k cng phi tnh s si d phng cho ng ngc chiu.

11. V d thit k h thng mng quang:

Thit k mng phn phi t Headend phn phi tn hiu n 4 khu vc, ln lt cch Headend 3Km, 6Km, 9Km, 12Km (Xem hnh VI.2) v t cc b chia quang ti Headend. Nh vy s c 4 u chia quang cp n 4 khu vc trn, v vy s c 3 b chia quang 1x2 c cc t l chia khc nhau (xem hnh VI.3).

12. Thit b chia quang:

Tapoff v Slitter l cc thit b s dng trong mng ng trc dng phn phi tn hiu in. Ni chung cc b chia Splitter chia tn hiu u vo thnh cc u ra tng ng nhau, cn cc b Tapoff th phn ln tn hiu u vo c chuyn ti u ra, cc nhnh tapoff u ra c nh suy hao thng theo mc -10dB n - 20dB.

Trong mng quang tn hiu cng c phn nhnh bi cc b chia quang, ch yu dng cc b chia 1x2, tuy nhin 2 u ra thng khng ging nhau, m bin i theo t l khc nhau. Do thut ng chia quang u (optical splitter) hay khng u (optical tapoff, optical coupler) u ch b chia quang. Cc b chia quang ny c th c hoc khng c cc connector (trung bnh suy hao 0.35dB/connector).

Cn ch khi c t s b chia quang do nh cung cp a ra. Mt s nh sn xut quy nh theo mc pht quang, v d b chia 40:60 c ngha l cng 40% c cng sut (mW) truyn qua bng 40% cng sut quang u vo. Mt s nh sn xut khc li quy nh theo mc suy hao, v d b chia 40:60, ngha l cng th nht suy hao 40%, cng th 2 suy hao 60% so vi mc cng sut quang u vo. C hai cch trn u ng, quan trng l ngi thit k phi nm c s liu khi s dng. Trong ti liu ny, s dng quy nh theo mc pht quang ph hp vi thit b v phng php tnh ton thit k ca cng ty (xem hnh VI.3).

C hai phng php tnh suy hao qua 1 b chia quang, gi s c b chia quang (1x3) c t s (a%/ b%/ c%) v cn tnh cng sut quang ti cc u ra:

Phng php 1: tnh ton khi xt mc quang vo Lin c n v l (mW).

Mc tn hiu ti cc cng ra ca b chia quang s l:

Vi phng php ny, ta c mc tn hiu ti cc u ra b chia quang c n v l (mW), sau phi i n v (mW) thnh dBm tin tnh ton trong h thng.

Phng php 2: tnh ton khi xt mc quang vo Lin c n v l (dBm).

Mc cng sut tn hiu ti cc cng ra ca b chia quang s l:

Ta thy rng gi tr log (%) y chnh l phn suy hao khi tn hiu i qua u vo ti cng chia tng ng. Vi phng php ny, ta c mc tn hiu ti cc u ra b chia quang c n v l (dBm) v tin tnh ton trong h thng.

Hai phng php trn ch a ra phng php tnh mc chia l tng khi tn hiu quang i qua b chia. Trong thc t, ta cn tnh thm suy hao do mi hn trong b chia, nh vy, sau khi tnh mc cng sut ti cc cng ra ta cn tr i gi tr suy hao (0.3-2dB). Sau tnh tip suy hao khi tn hiu i qua connector ca cc cng b chia.

Di y l bng tham kho suy hao cng chia quang theo t l phn trm:

Cng quang

%Suy hao

513.5

1010.25

158.42

207.14

256.16

305.36

354.68

404.09

453.63

503.15

552.7

602.3

651.98

701.62

751.32

801.07

850.75

900.53

950.29

13. Suy hao connector:

Khi tnh ton thit k, mt connector thng c suy hao 0.35dB( tng ng vi suy hao ca 7 mi hn), do vy nn hn ch dng connector. iu ny c ngha l nu thc s cn thit mi thc hin u ni ODF ngoi ra ta s dng hn ni mng sng nhm tng tnh n nh v gim mc suy hao xung mc ti thiu.

14. Tnh ton suy hao tng th mng:

Theo hnh VI.3 c th tnh c tng suy hao:

Gi s my pht quang c cng sut 12dBm, cng sut ny c chia bi b chia quang T-1 thnh 2 ng: ng 30% (suy hao 5.66dB), 70% (suy hao 1.92dB).

Cng suy hao 5.66dB ca b chia T-1 c a vo b chia T-2 c t l chia 45:55 (suy hao 3.93dB/3dB), khi u ra c suy hao cao hn (45%) c cng sut quang l:

12 5.66 3.93 = 2.41dBm.

Tn hiu quang ny i qua 3Km cp quang n im u cui A (3 Km cp quang c suy hao l: 3x0.4=1.2dB).

Do tn hiu quang cn li l 2.41-1.2=1.21dBm, vi tng suy hao ca connector l 0.7dB, nn tn hiu ti A l 1.21-0.7=0.51dBm. Tn hiu d phng tnh l 0.4dB khi tn hiu vo my thu quang l 0.11dBm.

Tng t s tnh c tn hiu quang ti B l: -0.16dBm, ti C: 1.45dBm, ti D: 1.18dBm.

Ch : Bi ton trn ch mang tnh cht v d tnh ton. Trong thc t i vi mng trn ta ch cn dng 1 b chia quang 4 ng theo t l cp n cc im A, B, C, D.

Trn y m t phng php thit k mng quang cho hng xui chiu, phng php thit k quang hng ngc chiu cng c nhng tnh ton tng t nh trn, tuy nhin, c mt s im cn lu sau:

V c tnh ca si quang, suy hao quang, loi si quang s dng, loi ngun quang c nu trn. Thit k mng quang ngc chiu n gin hn so vi mng quang xui chiu v mng quang hng xui chiu mang tnh cht phn chia tn hiu, mng quang hng ngc chiu l chy thng hoc tp trung (nu s dng phng php ghp quang hin i). Chnh v vy, khi thit k quang ngc chiu cn tnh ton khong cch c ly t node quang v CMTS t tnh c suy hao trn ng truyn t v s dng my pht quang ngc chiu v cng sut pht sao cho hp l.

Cng sut pht quang ngc chiu thng c cc mc: -4dBm, 0dBm, +3 dBm. Tuy nhin, my thu quang ngc dng cng thng c nhy rt cao, mc thp nht:-16dBm. Khuyn co nn mc cng sut quang u vo my thu l -6dBm.

V vy, khi thit k mng quang hng xui chiu cn tnh ton khong cch, cc im t trm lp, ODF hp l mng c th hot ng c c hng ngc chiu trn cng mt tuyn cp, trnh xy ra tnh trng phi ko thm mt tuyn cp mi cho hng ngc chiu.II. Kho st thit k mng ng trc xui chiu:Trc khi thit k mt h thng, phi thc hin phn tch k k thut h thng. Kt qu nh gi k thut chnh xc v y s gip xc nh c cn phi nng cp hay thit k mi h thng no .

Cc ni dung cn phn tch khi nng cp hoc xy dng li mng CATV:

1. S h dn /100m?

2. Tng s h dn lp t truyn hnh cp/node quang?

3. Tnh trung bnh s ct (ct in v ct bu in)/100m?

4. Tng s cp treo/cp ngm ? mt

5. Loi chn ct b tng?

6. Loi np cng yu cu khi ngm ho? ------ C / Khng ------ Bao nhiu?

7. Tn s h thng hin ti s dng ------ MHz

8. C k hoch nng cp t trc khng? Bng tn s dng trc l ---- MHz

9. D nh thay i tn s hin ti ----- MHz

10. Loi cp ng trc s dng?

11. Loi ngun cung cp ang dng?

12. Loi thit b th ng? tch cc?

13. Loi connector ang dng?

14. Cu trc h thng c cha?

15. S tng khuch i di nht trn ng trc chnh? ng trc nhnh?

16. S tng khuch i di th 2 trn ng trc chnh? ng trc nhnh?

17. S tng khuch i di th 3 trn ng trc chnh? ng trc nhnh?

18. S si quang dng / node quang?

19. C h s bn v mng hin ti hay khng?

20. Tim nng ca h thng mng? Thp / Trung bnh / Cao?

21. % cp ng trc cn thay th?

22. % cp chn trc tip? cp i ngm?

23. Cc khu vc c bit c s c no khng?

24. Thay th cc thit b mi vo v tr thit b c c ph hp khng? (ch v connector)

25. % cc thit b c c tiu chun, cht lng ph hp vi mng thit k mi?

26. S khuch i hin s dng c kh nng thay th modul mi?

27. Loi connector thu bao? Loi 1: -----%

Loi 2: -----%

28. % cp thu bao RG6, RG11?

29. Bng tn cho cc b chia thu bao s dng ? -----MHz

30. Lit k cc loi hp cha thit b s dng?

31. Lit k cc loi hp c th nng cp/ci to?

Trc khi tnh ton thit k cng cn thc hin kim tra li ton b k hoch ca d n: nng cp, xy dng mi hay xy dng li. Cng vic thm tra ny rt quan trng i vi tin trnh xy dng mng CATV. Nu khng nh gi ng v chi tit th vic thit k cng nh thi cng sau ny s b sai, mng s c cht lng km. Ngc li, nu lm tt s thun li cho thit k ng tin , m bo cht lng, chnh xc.

Ngoi ra cng cn phi c 1 bn khu vc thit k. Khi kim tra, kho st cng phi rt ch tm ra nhng im bt thng. V d khi c mng dn nc chy ngm qua khu nh no th phi ch thch ln bn trnh trng hp cp ngm c thit k chy ct ngang qua .

Mt iu na cng cn ch khi th hin trn bn v l h thng cc k hiu phi thng nht m bo cho mi ngi u c v hiu c.

1. Thit k node quang:

xc nh phm vi bao ph ca node quang th da vo cc yu t sau:

Mt nh dn trong khu vc

Cc tuyn cp c sn

Cc tuyn ng (ch ng ngm, b sng)

Kh nng pht trin, m rng trong tng lai.

Vng bao ph cu 1 node quang khng nn ct ngang 1 khu nh( m c chung ngh nghip, tn gio, s hu) hoc cc khu c ranh gii c lp v c th hin ti hoc tng lai cc khu vc ny s c chng trnh ring ca h.

V tr t node quang: t node quang ti ni cho vng bao ph l ln nht v cp c cho khong 500 h dn vi s lng cp ko l nh nht. Nu t node quang ti v tr khng thun li th thng thng phi dng thm 1 node quang na ph vng cn li, v do lm tng gi thnh ca mng.

Mc RF u ra ca node quang: in hnh l +46dBmV@862MHz. Mc tn hiu knh u, knh cui c th bng nhau hoc theo dc dng c lch nm trong khong iu chnh EQ ca node quang.2. Quy nh thit k mng ng trc:

Bng tn xui dng: 85 862MHz, bng tn ngc dng: 5-65 MHz. Cc khuch i c Gain: in hnh G = 30dB, in p hot ng 24-60Vac ti 50Hz.

S tng khuch i ti a: 5 (T node quang n u cui). Cc khuch i c thit k, thit lp mc tn hiu vo/ra tun th nguyn tc gain n v v nguyn tc cn bng nhiu-mo hi. Mc RF u ra ca cc khuch i: 46dBmV@862MHz Mc tn hiu knh u, knh cui c th bng nhau hoc theo dc dng c lch nm trong khong iu chnh ca khi EQ trung gian gia hai moun khuch i(iEQ) ca khuch i. Trong , tun th iu kin mc tn hiu vo moun khuch i th nht phi bng phng. Mc RF u ra ca cc b chia ng nhnh cui cng cp tn hiu cho h thu bao c gi tr: 80 ( 4 dBuV, trong chnh lch mc tn hiu gia 2 tn s 110 MHz v 862 MHz bng 4 dB khng phn bit dc dng hay m( khng p dng vi khu chung c). Mc tn hiu ti thu bao, cht lng ng truyn: p ng yu cu trong bng thng s tiu chun ng truyn(chng I/phn II/mc 1).3. Tnh ton suy hao:

Tnh suy hao ca cp ng trc, b chia ng trc, b chia ng nhnh, b cn bng ng dy ti 2 tn s 110 MHz v 862 MHz. (cn c theo thng s tiu chun nu trong chng VIII). i vi cp ng trc:

Chn loi cp ng trc vic thit k v trin khai thi cng c hiu qu nht. Khng dng loi cp ng trc trc chnh c ng knh li ln hn 0.875 (2.2225mm) hoc cp nhnh c ng knh li ln hn 0.625 (1.5875mm). Tnh ton suy hao cp gi tr ln nht do nh sn xut a ra.

Bng thng s suy hao cp thng c o trong iu kin nhit phng 200C. tng/ gim suy hao t l thun vi tng/ gim nhit , h s ny l 0.181818%/ 0C( tham kho hng Commscope). Khi p dng tnh ton cn tnh mc suy hao do nhit thc t trung bnh ti 300C:Suy hao@30oC = suyhao@20oC+suyhao@20oC*0.181818%(30 - 20) i vi cc thit b th ng( b chia nh hng, b chia, b chn ngun, b cn bng ng truyn.) s dng trong bng tn t 5-1000MHz.

i vi cc b chia:

S dng cc b chia c bng tn t 5-1000MHz.

Khi thit k, tnh ton suy hao ca b chia theo 2 cch:

C1: suy hao danh nh do nh sn xut a ra +10%

C2: ly theo suy hao ln nht (Nu n nh hn suy hao danh nh th +10%)

T ct in ngoi va h s dng:

B chia thu bao 2 cng ra cho phm vi nh dn t 1-2 h dn.

B chia thu bao 4 cng ra cho phm vi nh dn t 3-4 h dn.

B chia thu bao 6 cng ra cho phm vi nh dn t 5-6 h dn.

B chia thu bao 8 cng ra cho phm vi nh dn t 7-8 h dn.

4. Mt s lu khi thit k khu chung c, nng cp, ci to mng: Thit k khu chung c:

S dng node quang dnh ring phc v cho cc khu nh, khng vt qu 1000 h dn. Mi cng cp tn hiu RF khng vt qu 300 h.Khi kho st cn cung cp cc thng tin v khu chung c cho ngi thit k nh sau:

Ct cp tn hiu

Chiu di ca to nh

S n nguyn (cu thang)

S tng, s h ca to nh

Khi tnh ton thit k, dng cp RG11 c suy hao thch hp dn n to nh. t cc thit b tch cc trn ct in ngoi to nh. Ngun in c cp t xa. Nu cn thit phi t thit b tch cc trong to nh th phi thit k cp ngun in cho chng. Khng s dng ngun AC cho u ra ca ngun cung cp PS tr khi ngun PS c chc nng gim p.

Dng b chia cp tn hiu n mi n nguyn. i vi khch sn th cng thit k coi nh 1 to nh. Chia tng n nguyn theo s cc tng xc nh s cng chia cn thit cp cho mi tng. Nu khng bit s tng th phi cung cp s h dn mi tng ngi thit k bit tnh ton.

Khi cn nng cp, ci to mt khu vc no th cn ch mt s im sau:

Cung cp bn , bn v khu vc cp nht, chnh xc cho ngi thit k.

Chn v tr ti u cho node quang

Tn dng ti a cp ng trc ang s dng. c bit i vi khu vc ngm ho v cc khu vc c chi ph nhn cng cao.

Ti thiu vic thay cp mi (t hn 15% cp c sn)

Tng thm cp m rng t im cp trc c sn n cc v tr phn nhnh khc. M rng ti a vng bao ph v gim chiu di cc tng thit b th ng.

Gi nguyn v tr cc khuch i trc c sn bng cch s dng loi khuch i c 3 cng ra (1 cng chnh v 2 cng phn nhnh) c Gain 30dB.

Cc khu vc xy dng li th cn ch mt s im sau:

Thit k c cu trc ng trc (khng t cc b chia thu bao gia cc thit b tch cc ngoi tr l tng thit b tch cc cui cng).

Cung cp bn , bn v khu vc cp nht, chnh xc cho ngi thit k.

Chn v tr ti u cho node quang

Gim thiu cc v tr ko cp chy song song (t hn 36% cp trong bn thit k)

Gim thiu s cc thit b tch cc.

Cc tng khuch i cao tn:

S tng khuch i tu thuc vo loi thit k(xy dng li hoc nng cp), mt dn c trong khu vc, tuyn cp c sn, tim nng m rng trong tng lai. Ni chung, gii hn s tng khuch i c xc nh theo nhng tiu ch di y: (khng vt qu 5 tng khuch i- v phi tun th theo nguyn tc gain n v v cn bng nhiu-hi)

Thit k xy dng li khu vc c mt cao v trung bnh (>35h/100m):

Tt nht l dng 3 khuch i tnh t sau v tr node quang.

Thit k xy dng li khu vc c mt thp, t hoc khng c tim nng pht trin ( 4 ng

Ni vo im gia / Ct vng Ring

16. Cc k hiu quang khc:

Cp quang

Du # = s si quang

Du * = do ngi s dng nh ngha

Vng lp quang

Du * = do ngi s dng nh ngha

Connector

Du * = loi connector

B chia quang

Du # = % hoc suy hao quang hoc

17. Cc khi d liu:

Khi d liu i km cc khuch i

Gi a ra mt s thng tin sau:

S th t khuch i

M s ngun cung cp

Tng khuch i

Mc u vo/ra ca khuch i ti cc tn s thit k xui dng/ngc dng

Gi tr cn bng (EQ) xui dng/ngc dng

Gi tr nh suy hao (PAD) xui dng/ngc dng

Khong cch t khuch i trc

hoc

V d:

Block K ngc chiu:

Khi d liu cui ng dy

Gi a ra mt s thng tin sau:

Tn hiu s mc cao

Tn hiu tng t mc cao

- Tn hiu tng t mc thp hoc hoc

V d:

CHNG VIIITHNG S K THUT TIU CHUN MT S THIT B, VT T

Vic xy dng h thng thng s k thut tiu chun cho cc thit b, vt t mang ngha v cng quan trng. y chnh l c s c s ng nht gia khu thit k v thi cng, mang li tnh ng nht v n nh cho mng.

Cc thng s k thut ca mt s vt t c nu trong chng ny khng ch nh theo mt hng cung cp nht nh m c xy dng p ng nhng yu cu k thut tiu chun c nu trong cc chng sau, to s thng nht trong qu trnh mua sm, sn xut vt t, nghim thu cht lng thit b phc v thi cng.

I. Cp:1. a.Cp ng trc treo QR540:

Cu to:

Li dn in c cu to bng ng nguyn cht hoc nhm ph ng. Bao ph li dn in l lp cch in polyethylene vi cht dnh bo v bn ngoi chng thm nc v gi cho connector c kt ni chc chn. Tip theo l lp ng nhm c tnh nng chng nhiu xm nhp cao vo cng nh gi cho tn hiu khng b lt ra (y l im khc bit ln nht gia cp trc v cp phn phi, thu bao). Ngoi cng l lp v polyethylene (PE) v dy treo c cu to bng thp.

Kch thc vt l: quyt nh cht lng cp, cht lng tn hiu.

ng knh li dn in: 3,15 mm

ng knh ngoi lp cch in: 13,03 mm

ng knh ngoi lp v nhm: 13,93 mm

dy lp v nhm: 0,45 mm

ng knh ngoi v cp PE: 15,5 mm

dy v cp PE: 0,89 mm

ng knh dy treo: 2,77 mm

c tnh c kh: Bn knh un cong b nht: 10,2 cm

Lc cng chu ng ln nht: 100 kgf

Cc thng s in:

Tr khng sng: 75 + 2 Ohms

in dung k sinh: 50(3 nF/km

Vn tc truyn dn: 88%

in tr vng vi dng D/C ln nht@20oC:

+ Li ph ng: 5,28 Ohms/km

+ Li ng:

4,14 Ohms/km

b.cp ng trc ngm QR540

Cu to:

Cc thng s vt l:

-ng knh li dn in : 3,15mm.

-ng knh lp cch in : 13,03mm.-ng knh ngoi lp v nhm: 13,72- dy lp v nhm: 0,343mm.

-C lp v bc chu lc,chng gp cp, chng thm nc (cho chc nng ngm).-ng knh ngoi lp v cp PE: 15,49.

- dy lp v cp PE: 0,74mm

Bng thng s Suy hao @ 20oC

Tn s (MHz)Suy hao (dB/100 m)Tn s (MHz)Suy hao (dB/100 m)

50,463003,71

301,123253,87

451,353504,03

501,443754,17

551,544004,33

831,94254,49

1082,174504,59

1502,595004,89

1812,895505,12

1932,956005,38

2113,127506,07

2203,228656,56

2503,3810007,12

2703,51

2. Cp phn phi RG11: a.Cp treo RG11

Cu to:

Li dn in c cu to bng thp ph ng. Bao ph li dn in l lp cch in polyethylene vi cht dnh bo v bn ngoi chng thm nc, gm nhm v gi cho connector c kt ni chc chn. Tip theo l lp nhm l v si nhm bn bao ph 60% cp c tnh nng chng nhiu xm nhp vo cng nh gi cho tn hiu khng b lt ra. Ngoi cng l lp v PVC v dy treo c cu to bng thp.

Kch thc vt l: quyt nh cht lng cp, cht lng tn hiu.

ng knh li dn in: 1,63 mm

ng knh ngoi lp cch in: 7,11 mm

ng knh ngoi lp l nhm u tin: 7,29 mm

ng knh ngoi v cp: 10,03 mm

dy v cp: 1,07 mm

ng knh dy treo: 1,83mm

Cc thng s in:

Tr khng sng: 75 Ohms

b.Cp ngm RG11.

Cu

Cc thng s vt l:

-ng knh li dn in : 1,63mm.

-ng knh ngoi lp cch in: 7,11mm.-ng knh ngoi lp l nhm u tin: 7,29mm.

-C lp v bc cho chc nng ngm (chng thm nc,chu lc,chng gp cp).

-ng knh lp v ngoi cng : 10,16mm. dy lp v ngoi 1,07mm.

Bng thng s Suy hao @ 20oC

Tn s (MHz)Suy hao (dB/100 m)Tn s (MHz)Suy hao (dB/100 m)

51,254008,53

553,154509,02

833,875009,51

1875,745509,97

2116,2360010,43

2506,7275011,97

3007,3886513,05

3507,94100014,27

3. Cp thu bao RG6:

a.cp treo RG6.

Cu to:

Li dn in c cu to bng thp ph ng. Bao ph li dn in l lp cch in polyethylene vi cht dnh bo v bn ngoi chng thm nc, gm nhm v gi cho connector c kt ni chc chn. Tip theo l lp nhm l v si nhm bn bao ph 60% cp c tnh nng chng nhiu xm nhp vo cng nh gi cho tn hiu khng b lt ra. Ngoi cng l lp v PVC.

Kch thc vt l: quyt nh cht lng cp, cht lng tn hiu.

ng knh li dn in: 1,02 mm

ng knh ngoi lp cch in: 4,57 mm

ng knh ngoi lp l nhm u tin: 4,75 mm

ng knh ngoi v cp: 6,91 mm

dy v cp: 0.76 mm

Cc thng s in:

Tr khng sng: 75 Ohms

b.Cp ngm RG6.

Cu to:

Cc thng s vt l:

-ng knh li dn in: 1,02mm.

-ng knh ngoi lp cach in: 4,57mm.

-ng knh ngoi lp l nhm u tin: 4,75mm.-C lp v bc cho chc nng ngm (chng thm nc,chng gp cp,chu lc).

-ng knh ngoi cnglp v cp :6,91mm.

- dy lp v cp :0,76mm.Bng thng s Suy hao @ 20oC

Tn s (MHz)Suy hao (dB/100 m) Tn s (MHz)Suy hao (dB/100 m)

51,940013,61

555,2545014,43

836,450015,09

1879,3555016,08

2111060016,73

25010,8275018,54

30011,6486520,01

35012,63100021,49

II. Khuch i:

1. Khuch i trc:

c im:

1 cng vo, 1 hoc 2 cng ra.

S dng cng ngh khuch i GaAs hoc cng ngh lai GaAs FET.

Ch s nhiu (Noise Figure) thp.

iu khin mc gain gia cc tng khuch i chiu xung.

iu chnh suy hao, dc linh hot.

C kh nng tiu hao in t.

Thng s k thut :

Thng sGi trGhi ch

Chiu tn hiu xung

Di tn s hot ng85 ... 862MHz

Suy hao phn hi18dB

Gain 36dB

Ch s nhiu (Noise figure) 8dB

Di iu chnh suy hao u vo

(iu khin bng nm iu chnh hoc pad)0-18dB

Di iu chnh dc u vo

(iu khin bng nm iu chnh hoc pad)0-18dB

iu chnh dc gia hai tng khuch i8/0dB

im kim tra mc tn hiu (test point)20dB

phng(0.8dB

CTB (mc ra phng/mc ra c dc 8dB)-67dBc/-73dBc1

CSO (mc ra phng/mc ra c dc 8dB)-64dBc/-68dBc1

XMOD (mc ra phng/mc ra c dc 8dB)-67dBc/-70dBc1

(1) iu kin o: 77 knh NTSC, mc tn hiu ra bng 48dBmV, dc dng 8dB (65-862dB)

Tn hiu chiu ln

Di tn s5 ... 65 MHz

Suy hao phn hi18dB

Gain22dB

im test u vo-20dB

Di iu chnh gain

(iu khin bng nm iu chnh hoc pad)0-18 dB

Di iu chnh dc

(iu khin bng nm iu chnh hoc pad)0-8dB

phng(1dB

Ch s nhiu (Noise figure)6 dB

Thng s khc:

in p cung cp25...65 VAC

Cng sut tiu th (bao gm c modun khuch i ngc dng)16W

Connector kt ni u vo, u ra5/8

u kt ni im testF-female

Nhit hot ng-40 ( +55(C

Khuyn co: Nn s dng loi khuch i c chnh suy hao v dc bng pad cm mang li s n nh cho vic thit lp mc tn hiu.

2. Khuch i nhnh:

c im:

1 cng vo, 1 hoc 2 cng ra.

S dng cng ngh khuch i GaAs hoc cng ngh lai GaAs FET

Ch s nhiu (Noise Figure) thp.

iu khin mc gain gia cc tng khuch i chiu xung

iu chnh suy hao, dc linh hot.

C kh nng tiu hao in t.

Thng s k thut :

Thng sGi trGhi ch

Chiu tn hiu xung

Di tn s hot ng85 ... 862MHz

Suy hao phn hi18dB

Gain33/39dB

Ch s nhiu (Noise figure) 8dB

Di iu chnh suy hao u vo

(iu khin bng nm iu chnh hoc pad)0-18dB

Di iu chnh dc u vo

(iu khin bng nm iu chnh hoc pad)0-18dB

iu chnh dc gia hai tng khuch i8/0dB

im kim tra mc tn hiu (test point)20dB

phng(0.8dB

CTB (mc ra phng/mc ra c dc 8dB)-62dBc/-68dBc1

CSO (mc ra phng/mc ra c dc 8dB)-62dBc/-66dBc1

XMOD (mc ra phng/mc ra c dc 8dB)-62dBc/-65dBc1

(1) iu kin o: 77 knh NTSC, mc tn hiu ra bng 48dBmV, dc dng 8dB (65-862dB)

Tn hiu chiu ln

Di tn s5 ... 65 MHz

Suy hao phn hi18dB

Gain22dB

im test u vo-20dB

Di iu chnh gain

(iu khin bng nm iu chnh hoc pad)0-18 dB

Di iu chnh dc

(iu khin bng nm iu chnh hoc pad)0-8dB

phng(1dB

Ch s nhiu (Noise figure)6 dB

Thng s khc:

in p cung cp25...65 VAC

Cng sut tiu th (bao gm c modun khuch i ngc dng)16W

Connector kt ni u vo, u ra5/8

u kt ni im testF-female

Nhit hot ng-40 ( +55(C

Khuyn co: Nn s dng loi khuch i c chnh suy hao v dc bng pad cm mang li s n nh cho vic thit lp mc tn hiu.

III. Thit b quang:

1. Node quang:

Node quang hot ng 2 chiu theo c ch module linh hot. C th nng cp cho chiu xung thm module thu tn hiu quang v chiu ln thm module pht tn hiu quang phc v qu trnh nng cp mng trong tng lai. Tng khuch i hot ng hiu sut cao mng c di tn hiu ra rng. Cn c kh nng m rng ln 3,4 cng ra khi lp thm 1 b chia tn hiu trong.

c im:

Node quang c th iu khin bng laptop hoc cc thit b cm tay.

Tn hiu quang chiu ln s dng cng ngh laser.

Cc tng khuch i s dng cng ngh GaAs hoc GaAs FET.

03 hoc 04 cng ra.

Thng s k thut :

Thng sGi trGhi ch

Thng s quang chiu xung

Bc sng1290 ( 1600 nm

Ngun quang vo (mc thu)-7 ( +2dBm

Optical Return loss 45dB

Di tn s85 ( 862 MHz

Mt nhiu dng7pA/Hz

p ng 0.85mA/mW @ 1310nm

u niSC/APC hoc FC/APC

Thng s quang chiu ln

Cng sut quang phtLaser DFB: 2mW

Laser FP: 1 mW

Bc sng1310 ( 40 nm

u niSC/APC hoc FC/APC

Thng s RF chiu xung

Mc ra 108 dBuV

Di iu chnh gain

(iu khin bng nm iu chnh hoc pad)0 ( 20 dB

Di iu chnh dc

(iu khin bng nm iu chnh hoc pad)-1 ( 20 dB

Suy hao phn hi20dB

Ch s nhiu Noise Figure 6.5dB

iu khin gain v dcBng tay hoc iu khin bng thit b cm tay

im test-20dB

CTB- 78 dBc1

CSO- 76 dBc1

XMOD- 72 dBc1

(1) iu kin o: 77 knh NTSC, mc tn hiu ra bng 48dBmV, dc dng 8dB (65-862dB)

Thng s RF chiu ln

Di tn s RF5 ( 65 MHz

Suy hao phn hi20 dB

im test-20dB

Mc tn hiu RF voOptimal: 2 ( 8 dBmV

Cc thng s khc

in p cung cp27 ( 65 VAC

Cng sut tiu th 70W

S iu bin Hum70dB

u kt ni cng racho 5/8 hoc F11 connector

u kt ni im testF-female

Nhit hot ng-40 ( +55(C

Khuyn co: Nn s dng loi node quang c chnh suy hao v dc bng pad cm mang li s n nh cho vic thit lp mc tn hiu.

2. My pht quang xui dng:

Thng sGi trGhi ch

Thng s quang

Bc sng, loi si:1310 ( 10 nm,

n mode 9/125(m

Laser DFB-7 ( +2dBm

Suy hao phn hi 55dB

Cng sutLa chn: 3dBm(13dBm

u niSC/APC hoc FC/APC

Thng s RF

Tn s45 ( 870 Mhz

Mc voCn c theo cng sut pht:

(15 ( 20 dBmV)

Suy hao phn hi18dB

C/N 52.5 dB

CTB- 71 dBc1

CSO- 65 dBc1

XMOD- 65 dBc1

Tr khng u vo, u ni75Ohms, F

(1) iu kin o: 60 knh PAL

3. My thu quang xui dng:

Thng sGi trGhi ch

Thng s quang

Bc sng, loi si1290 ( 1600 nm,n mode 9/125(m

Mc quang vo-3 ( +5dBm

Suy hao phn hi 45dB

Mt nhiu dng= 60 dB

Thng s li ra chung:

n nh tn s :

50ppm

Mc thit lp suy hao li ra:

0-10dB

Mc tuyn tnh@8Mhz:

0.5dB

Tr nhm :