[123doc.vn] Luanvan s

B GIO DC V O TO

I HC NNG

TRN NGUYN

NGHIN CU CI THIN HIU QU BC CHN

M BO TNH TNG THCH IN T

LUN VN THC S K THUT

Nng Nm 2014

B GIO DC V O TO

I HC NNG

TRN NGUYN

NGHIN CU CI THIN HIU QU BC CHN


Chuyn ngnh : K thut in t

M s : 60.52.70

LUN VN THC S K THUT

Ngi hng dn khoa hc: PGS.TS TNG TN CHIN

Nng Nm 2014

LI CAM OAN

Ti xin cam oan y l cng trnh nghin cu ca ring ti.

Cc s liu, kt qu nu trong lun vn l trung thc v cha tng

c ai cng b trong bt k cng trnh no khc.

Tc gi lun vn

TRN NGUYN

MC LC

M U ................................................................................................... 1

1. Tnh cp thit ca ti ................................................................... 1

2. Mc tiu nghin cu ........................................................................ 1

3. i tng v phm vi nghin cu .................................................... 2

4. Phng php nghin cu ................................................................. 2

5. B cc ti .................................................................................. 2

CHNG 1: TNG QUAN V TNG THCH IN T V CC

NGUN NHIU ....................................................................................... 4

1.1 GII THIU CHNG ....................................................................... 4

1.2 KHI NIM TNG THCH IN T ............................................. 4

1.3 KIM TRA TNG THCH IN T ................................................ 5

1.3.1 Cc quy tc v TTT.................................................................. 5

1.3.2 Thit b ...................................................................................... 7

1.3.3 Quy trnh kim tra ...................................................................... 9

1.4 PHN TCH CC NGUN NHIU .................................................. 10

1.4.1 Cc ngun nhiu t nhin.......................................................... 10

1.4.2 Cc ngun nhiu cng nghip.................................................... 11

1.5 S BC X ...................................................................................... 13

1.5.1 Bc x pht x .......................................................................... 13

1.5.2 Bc x t dy dn ..................................................................... 15

1.6 KT LUN CHNG ....................................................................... 16

CHNG 2: HIU QU BC CHN CA VT LIU ...................... 17

2.1 GII THIU CHNG ..................................................................... 17

2.2 TR KHNG CA VT LIU .......................................................... 17

2.2.1. Tr khng ca khng kh.......................................................... 18

2.2.2. Tr khng ca kim loi .......................................................... 19

2.2.3. Chiu su lp da ...................................................................... 19

2.3 HIU QU BC CHN CA SUPERALLOY, ALUMINUM V

MUMETAN ............................................................................................. 20

2.3.1 Gii thiu chung ....................................................................... 20

2.3.2 Tnh ton hiu qu bc chn ...................................................... 21

2.3.3 Tn hao hp th ........................................................................ 22

2.3.4 Tn hao phn x ...................................................................... 24

2.3.5 Hiu qu bc chn khi tn hao hp th trn 10 dB ................ 30

2.3.6 H s hiu chnh phn x li ..................................................... 32

2.3.7 Hiu qu bc chn ................................................................... 37

2.4 KT LUN CHNG ....................................................................... 40

CHNG 3: CI THIN HIU QU BC CHN M BO

TNH TNG THCH IN T.......................................................... 42

3.1 GII THIU CHNG ..................................................................... 42

3.2 TRNG GN V TRNG XA .................................................... 42

3.3 TNG HP TN HAO PHN X V HP TH ............................. 44

3.4 PHNG PHP BC CHN TRONG LNH VC TTT ................. 46

3.5 CC PHNG PHP CI THIN HIU QU BC CHN ............. 47

3.5.1 Phng php bc chn nhiu lp ............................................... 47

3.5.2 Phng php bc chn i ........................................................ 49

3.5.3 Phng php bc chn chng t trng tn s thp ................. 50

3.5.4 Phng php t khe h v chia nh khe h .............................. 52

3.5.5 Ci thin hiu qu bc chn ti khp ni .................................... 54

3.5.6 Phng php s dng cc ng dn sng ..................................... 56

3.6 TNH TON HIU QU BC CHN TRONG TRNG HP BC

CHN KHNG KN................................................................................ 58

3.6.1 Hiu qu bc chn ca tm chn c khe h ............................... 59

3.6.2 Hiu qu bc chn ca tm chn khi khe h chia nh ................. 61

3.6.3 Hiu qu bc chn ca mt li bng vt liu dn in ............... 62

3.6.4 Hiu qu bc chn ca khe h dng ng dn sng ...................... 63

3.7 KIM TRA HP BC CHN TRONG THC T.............................. 67

3.8 KT LUN CHNG ...................................................................... 68

CHNG 4: M PHNG CI THIN HIU QU BC CHN

M BO TNH TNG THCH IN T ....................................... 69

4.1 GII THIU CHNG ..................................................................... 69

4.2 GII THIU S B V PHN MM CST (Computer Simulation

Technology) ............................................................................................. 69

4.3 XY DNG TIN TRNH M PHNG CI THIN HIU QU BC

CHN...................................................................................................... 71

4.4 THIT LP CC THNG S HP BC CHN ............................... 72

4.5 M PHNG BC X IN T HP BC CHN KN HON TON. 73

4.6 KT QU M PHNG BC X IN T HP BC CHN C KHE

H ........................................................................................................... 74

4.6.1 M phng bc x in t khi hp bc chn tn ti khe h 4x3cm 74

4.6.2 M phng bc x in t khi hp bc chn c khe h 4x3, 12x0.2,

5x0.2, 0.8x0.2cm. ..................................................................................... 75

4.6.3 M phng bc x in t ca hp bc chn khi chia khe 4x3cm

thnh 15 khe ............................................................................................. 76

4.6.4 M phng bc x in t ca hp bc chn khi chia khe 4x3cm

thnh 20 khe ............................................................................................. 78

4.6.5 M phng bc x in t khi hp bc chn c cc khe h cng

din tch ................................................................................................... 80

4.7 KT QU M PHNG BC X IN T CA HP BC CHN

BNG CC VT LIU KHC NHAU .................................................... 81

4.8 KT QU M PHNG BC X IN T HP BC CHN C KHE

H DNG NG DN SNG .................................................................. 82

4.8.1 M phng bc x in t hp bc chn c khe h dng ng dn

sng c chiu di v rng ln lc thay i ................................................ 82

4.8.2 M phng bc x in t hp bc chn c mt ng lc gic v 28

ng dng t ong c cng thit din l 184mm2 ........................................... 84

4.8.3 M phng bc x in t hp bc chn c khe h dng t ong vi

s lng ng khc nhau ............................................................................ 85


S DNG TM CHN I ................................................................... 87

4.10 SO SNH HIU QU BC CHN ................................................. 88

4.11 KT LUN CHNG .................................................................... 92

KT LUN V HNG PHT TRIN TI ................................. 93

DANH MC TI LIU THAM KHO ................................................. 94

QUYT NH GIAO TI LUN VN (Bn sao)

PH LC

DANH MC CC K HIU, CH VIT TT

K HIU

A Absorption Loss (dB) Tn hao hp th

C Re-Reflection Correction Factor H s hiu chnh phn x li

E Electric field strength (V/m) Cng in trng

H Magnetic field strength (A/m) Cng t trng

R Reflection Loss Tn hao phn x

t Thickness (mils, m, or mm) dy

Zm Intrinsic impedance of Tr khng ca kim loi

metal (ohms)

Zb Intrinsic impedance of Tr khng ca kim loi mng

thin metal (ohms)

Zair Intrinsic impedance of air (ohms)Tr khng ca khng kh

Zs Source impedance (ohms) Tr khng ngun

Zw Wave impedance Tr khng sng

Skin depth (cm or m) m thu

Permittivity (farads/m) Hng s in mi

0 Permittivity of air or space Hng s in mi ca khng kh

(8,84.10-12

farads/m)

Permeability (henries/m) H s t thm

0 Permeability of air H s t thm ca khng kh

(4.10-7

henries/m)

Conductivity (mhos/m) in dn sut

cu Conductivity of copper (mhos/m)in dn sut ca ng

DANH MC CC CH VIT TT

CST Computer Simulation Cng ngh m phng

Technology my tnh

DAC Digital to Analog Converter Chuyn i s ra tng t

DFT Discrete Fourier transform Bin i Fourier ri rc

EMC Electromagnetic Compatibility Tng thch in t

EME Electromagnetic Emission Pht x in t

EMI Electromagnetic Interference Giao thoa in t

EMS Electromagnetic Susceptibility nhy in t

EUT Equipment Under Test Thit b gim st, kim tra

FFT Fast Fourier Transform Bin i Fourier nhanh

RE Radiated Emission Bc x pht x

RF Radio Frequency Tn s v tuyn

SE Shielding Effectiveness Hiu qu bc chn

SEE Shielding effectiveness Hiu qu bc chn i vi

electric field thnh phn in trng

SEH Shielding effectiveness Hiu qu bc chn i vi

magnetic field thnh phn t trng

TLM Transmission Line Matrix Ma trn ng truyn

TE Transverse Electric in trng ngang

TM Transverse Magnetic T trng ngang

DANH MC CC HNH

S hiu Tn hnh Trang

1.1 Ba yu t trong tin trnh EMC 4

1.2 Cc nguyn tc TTT 6

1.3 S m t cc ng ghp EMI 10

1.4 Bc x t bo mch in 14

1.5 Bc x t cp 15

2.1 ng sng bc x truyn qua mt tm chn 22

2.2 Tn hao hp th cho t trng, in trng v sng

phng 24

2.3 Tn hao phn x trong t trng 27

2.4 Tn hao phn x trong in trng 28

2.5 Tn hao phn x trong sng phng 29

2.6 Tng tn hao trong t trng 31

2.7 Tng tn hao trong in trng 31

2.8 Tng tn hao trong sng phng 32

2.9 H s hiu chnh phn x li trong t trng 35

2.10 H s hiu chnh phn x li trong in trng 36

2.11 H s hiu chnh phn x li trong sng phng 36

2.12a Hiu qu bc chn trong t trng 37

2.12b Hiu qu bc chn trong t trng n 200dB 38

2.13a Hiu qu bc chn trong in trng 38

2.13b Hiu qu bc chn trong in trng n 200dB 39

2.14a Hiu qu bc chn trong sng phng 39

2.14b Hiu qu bc chn trong sng phng n 200dB 40

3.1 S chuyn i t trng gn n trng xa xy ra ti

/2 43

3.2 Tr khng sng ph thuc vo khong cch t ngun

v trng l in trng hay t trng 43

3.3 Hiu qu che chn ca l chn ng dy 0.02inch

(20mil) trong trng xa 45

3.4 S dng vt liu t tnh nh mt l chn 46

3.5 M t bc chn 47

3.6 Bc chn nhiu lp 48

3.7 Tm chn i 49

3.8a Phng php dng vt liu c t thm cao lm

lch hng t trng 51

3.8b Phng php to ra t trng i lp vi t trng

ti 51

3.9 S dng nhiu lp chn chng li hin tng

bo ha t ca vt liu 52

3.10a Dng in cm ng chy trn tm chn kim loi khi

khng c cc khe h 53

3.10b Dng in cm ng chy trn tm chn kim loi khi

c khe h 53

3.11 Ci thin bc chn bng vic t khe h song song

vi hng dng in cm ng 53

3.12 Ci thin bc chn bng phng php chia khe h

ln thnh nhiu khe h nh 54

3.13 K thut m ni 55

3.14 nh hng ca cc np y hp bc chn 56

3.15 S dng ng dn sng cung cp h thng thng

gi v lm gim bc x in t qua n 57

3.16 ng dn sng dng t ong 57

3.17 Kch thc khe h 59

3.18 Chia nh khe h 61

3.19 Mn li 62

3.20 ng dn sng hnh tr trn vi ng knh d v

chiu di z 65

3.21 ng dn sng hnh lc gic 66

3.22 Qu trnh kim tra bc x hp bc chn trong thc t 67

4.1 Giao din phn mm m phng CST 70

4.2 S tin trnh m phng 71

4.3 Thng s hp bc chn 72

4.4 Hp bc chn m phng bng phn mm CST 72

4.5 th bc x in t ca hp kn ti v tr cch hp

3m 73

4.6 th bc x in t ti v tr cch hp 3m 74

4.7 th bc x in t ti v tr cch hp 3m

khi hp bc chn c khe h 4x3cm v 12x0.2cm 75

4.8

th bc x in t ti v tr cch hp 3m

khi hp bc chn c khe h 12x0.2cm, 5x0.2,

0.8x0.2

76

4.9 th bc x in t ti v tr cch hp 3m

khi hp bc chn c khe h 4x3cm v 15 khe 0.2x4 77

4.10 Hp bc chn vi 15 khe v 20 khe h c tng din

tch 12cm2

78

4.11

th bc x in t ti v tr cch hp 3m khi hp

bc chn c 15 khe v 20 khe c tng din tch

12cm2

78

4.12 Hp bc chn vi 20 khe h c tng din tch 12cm

2

vi cch b tr khe v kch thc khe khc nhau 79

4.13


bc chn tn ti 20 khe h c tng din tch 12cm2

vi cch b tr khe v kch thc khe khc nhau

79

4.14 th bc x in t ti v tr cch hp 3m khi hp

bc chn c khe h 9x0.2, 3x0.6, 0.6x3, 0.2x9cm 80

4.15


bc chn c khe h 0.2x9cm vi cc vt liu khc

nhau

81

4.16 Hp bc chn c khe h dng ng lc gic 82

4.17


bc chn c khe h dng ng lc gic c g = 6.36mm

v chiu di ng thay i

82

4.18


bc chn c khe h dng ng lc gic c d = 6.35mm

v chiu rng ng thay i

83

4.19 Hp bc chn c khe h dng t ong 84


bc chn c khe h dng t ong 84

4.21


bc chn c khe h dng t ong vi s lng ng

khc nhau

85

4.22


bc chn c khe h dng t ong vi s lng ng

khc nhau

86

4.23 Mt ct hp bc chn s dng tm chn i ti v tr

c khe h 87


bc chn s dng tm chn i 87

4.25 Hiu qu bc chn ca hp kn v hp c khe h 88

4.26 Hiu qu bc chn ca hp c khe h vi chiu di

khc nhau 89

4.27 Hiu qu bc chn ca hp khi khe h c t l thay

i nhng cng din tch 89

4.28 Hiu qu bc chn ca hp khi khe h dng ng lc

gic c chiu di D= 6.35 v chiu rng thay i 90

4.29 Hiu qu bc chn ca hp khi khe h dng ng lc

gic c chiu rng g=6.36 v chiu di thay i 90

4.30 Hiu qu bc chn khi hp bc chn c khe h dng

t ong vi s lng ng khc nhau 91

4.31 Hiu qu bc chn khi hp bc chn s dng tm

chn i 92

1

M U

1. Tnh cp thit ca ti

S pht trin khng ngng ca khoa hc k thut, cc thit b in, in

t, tin hc ngy cng khng th thiu trong i sng. Cc thit b cn thit

cho nhu cu con ngi ngy mt tng ng k, trong mt khng gian gii hn

li c nhiu thit b cng hot ng. V vy, s nh hng gia cc thit b do

bc x in t ngy mt ng k.

Chnh v vy, tng thch in t ngy cng c quan tm hn. Tng

thch in t nhm gii quyt vn khi cc thit b vn hnh trong mt mi

trng c s hin din ca cc thit b khc hoc c tn hiu nhiu t mi

trng chung quanh tc ng vo m chng vn hot ng tt. thc hin

c iu ny, chng ta phi dng mt k thut nh l mt phng tin

trnh nhng hiu ng khng mong mun m s nhiu lon trn c th gy ra.

Mt trong nhng bin php hiu qu lm gim s bc x gy nhiu ca

thit b cng nh cc ngun gy nhiu t cc thit b, mi trng chung quanh

l dng bc chn. Trong thc t bc chn lun tn ti cc khe h do nhu

cu lm mt, giao tip, cp ngun. Cc khe h ny lm gim i hiu qu

bc chn do ta phi nghin cu tm cch khc phc, ci thin hiu qu ca

bc chn. V vy ti tin hnh thc hin ti NGHIN CU CI THIN

HIU QU BC CHN M BO TNH TNG THCH IN T

nhm m bo tnh tng thch in t ca cc thit b.

2. Mc tiu nghin cu

ti tp trung nghin cu cc vn sau:

- Nghin cu nh hng ca cc ngun nhiu trong lnh vc tng

thch in t.

2

- Nghin cu ci thin tnh tng thch in t bng k thut bc chn.

3. i tng v phm vi nghin cu

- Nghin cu cc ngun nhiu ca nhng thit b in, in t, tin hc ...

- Nghin cu cc vt liu bc chn ci thin tnh tng thch in t

- ng dng phn mm CST thit lp m phng ci thin hiu qu bc

chn trong lnh vc tng thch in t.

4. Phng php nghin cu

- Thu thp, phn tch cc ti liu v thng tin lin quan n ti

- Phn tch cc ngun nhiu trong lnh vc tng thch in t

- Nghin cu m phng ci thin tnh tng thch in t bng k thut

bc chn.

5. B cc ti

Lun vn c chia lm 4 chng nh sau:

Chng 1: Tng Quan v tng thch in t v cc ngun nhiu

Chng ny s trnh by cc khi nim tng thch in t, kim tra

tng thch in t, phn tch cc ngun nhiu, cc loi nhiu, s bc x

pht x, hiu ng nhiu cng nh mc nh hng ca n n cc thit b

in t.

Chng 2: Hiu qu bc chn ca vt liu

Chng ny trnh by tp trung v c tnh ca vt liu v hiu qu bc

chn ca cc loi vt liu c trng trong vic gii quyt vn TTT .

Chng 3: Ci thin hiu qu bc chn m bo tnh tng

thch in t

Chng ny trnh by tp trung v cc c s trong vic gii quyt vn

TTT cng nh nghin cu cc phng php ci thin hiu qu bc

chn nhm m bo TTT.

3

Chng 4: M phng ci thin hiu qu bc chn m bo tnh

tng thch in t

Chng ny s dng phn mm CST (Computer Simulation

Technology) m phng kim chng s ci thin hiu qu bc chn, so snh

hiu qu bc chn trong cc trng hp c th.

6. Tng quan ti liu nghin cu

Ti liu nghin cu c s dng trong qu trnh lm lun vn l cc

sch chuyn ngnh, bi bo trn cc tp ch chuyn ngnh, bo co ca cc

hi tho chuyn ngnh.

4

CHNG 1

TNG QUAN V TNG THCH IN T

V CC NGUN NHIU

1.1 GII THIU CHNG

Chng ny cp v khi nim tng thch in t, cc quy tc, quy

trnh v thic b kim tra tnh tng thch in t. Trong chng ny cng

phn tch cc ngun nhiu bao gm ngun nhiu t nhin v nhiu cng

nghip, cc vn tc ng qua li gia cc h thng in t thng qua cc

kiu ghp ni, vn bc x, pht x

1.2 KHI NIM TNG THCH IN T

Nhiu giao thoa in t (EMI: Electromagnetic Interference) c

nh ngha nh mt tn hiu in khng mong mun, n em li nhng

kt qu khng mong i trong h thng. Chng hn, EMI gy ra nhiu m

chng ta c th nghe c t cc thit b thu sng radio, gy ra s trc

trc cho h thng thm ch c th dn n nhng thm ha nghim trng.

Thut ng EMC (Electromagnetic Compatibility) lin quan n mt h

thng in t c kh nng thc hin chc nng tng thch vi cc h

thng in t khc v khng to ra, khng nhy vi nhiu.

Nu mt h thng l EMC th phi tha mn ba tiu chun sau:

Khng gy ra nhiu vi cc h thng khc.

Khng nhy vi s pht x t cc h thng khc.

Khng gy ra nhiu cho chnh n.

Hnh 1.1: Ba yu t trong tin trnh EMC

5

Cc vn ca EMC lin quan n s pht sinh, s truyn v s thu

nhn nng lng in t. Hnh 1.1 minh ha ba yu t ca mt vn

EMC: ngun to ra s pht x v ng ghp ni mang nng lng pht x

chuyn t ngun n b thu v v vy nng lng in t khng mong

mun c chuyn i thnh mt s tc ng khng mong i. Bng cch

chia ng ghp ni thnh hai loi, c hai nhm nh cho vn EMC,

l: bc x v dn.

T quan im ca b thu v b pht, cc vn EMC c th c chia

thnh pht x in t (EME: Electromagnetic Emission) v nhy in t

(EMS: Electromagnetic Susceptibility).

C ba cch c p dng lm gim nhiu bc x v nhiu dn.

Trit s pht x ti ngun pht.

Lm v hiu ha ng ghp ni cng nhiu cng tt.

Lm cho b thu min dch vi ngun pht.

Tm li, thit b in t tng thch in t (ElectroMagnetic

Compatibility EMC) l h thng, thit b c kh nng hot ng tng

thch vi nhng h thng, thit b khc v khng gy nhiu cho mi trng,

khng chu nh hng ca nhiu t mi trng v khng gy ra nhiu cho

chnh n.

1.3 KIM TRA TNG THCH IN T

1.3.1 Cc quy tc v TTT

Cc vn TTT thng thng c th gm s kt hp gia cc quy tc

nh s kt hp ca cc tn s, kch thc ca cc thnh phn, vic lp rp cc

b phn li vi nhau

TTT c chia thnh 3 nguyn tc da trn k thut kt ni:

- ng dn bc x

6

- ng dn thit b dn

- S kt hp ca 2 k thut trn

Trong mi k thut bc x hay thit b dn th c chia thnh 2 quy

tc nh na l s pht x v s min nhim. Ta c s sau:

Hnh 1.2 Cc nguyn tc TTT

Nh vy hin tng bc x chia lm hai loi:

- Bc x pht x (RE): lin quan n c ch sinh ra cc nng lng bc

x in t khng mong mun vo mi trng chung quanh gy nh hng

n cc thit b in t khc.

- Bc x min nhim (RI): lin quan n c ch chng li cc bc x

in t t cc thit b khc vo cc b phn ang hot ng ca h thng.

V hin tng dn cng c chia lm hai loi:

- Dn pht x (CE): lin quan n c ch cc nng lng in t c

to ra t cc mch in t nh hng n cc b phn khc trong mch (c

bit l ngun cung cp AC) thng qua cc dy cp truyn dn tn hiu gia

cc thit b.

7

- Dn min nhim (CI): lin quan n kh nng chng li cc nhiu

in t sinh ra t b ngun AC, cc mch in t n thit b hot ng ca

h thng.

Cn ESD bao gm s kt hp gia hin tng bc x v hin tng

dn.

1.3.2 Thit b

+ My phn tch ph

+ B phn thm d

+ Anten pht / anten thu

+ Khuch i in t

+ B phn khuy

M t mt s cc thit b nh sau:

a. My phn tch ph

My phn tch ph ch yu c s dng sao chp thnh phn ph

hoc ph cng sut ca mt s tn hiu kt ni.

My phn tch ph tng t s dng mt b lc thng di c t

ng iu chnh trn mt phm vi tn s tng ng vi ph o. Mt my

thu i tn cng c th c s dng. Tn hiu dao ng ni c trn ln

vi tn hiu u vo, chuyn i cc tn hiu iu ch nhiu tn s c

thun tin.

My phn tch ph k thut s, x l dng sng ly mu, phn tch

ph tn s ca n vi php bin i Fourier ri rc (DFT). Thm ch mt

s c th kt hp cc k thut c m t trn, v d l phn tch ph

thi gian thc. Cc tn hiu x l c r xung chuyn i s dng tn

s phch v sau c phn tch vi b phn tch bin i Fourier

nhanh (FFT).

8

b. My pht tn hiu

My pht tn hiu sn xut tn hiu c dng sng in t c chu k hoc

khng c chu k trong c hai hnh thc k thut s hoc tng t. My pht

sn xut dng sng vi b x l tn hiu k thut s v s dng k thut s

chuyn i ra tng t (DAC) to ra mt u ra tng t. My pht tn

hiu c kh nng sn xut tn hiu sng sin trong khong t tn s thp ln

n nhiu GHz.

c. B khuch i in t

B khuch i in t ly tn hiu t ngun cung cp v to ra mt tn

hiu mi c cng sut cao cng vi nhit thi ra. Cc nh sn xut b khuch

i lun lun phn u gim tr khng ca cc phn t mch khuch i.

iu ny lm tng hiu qu v gim thiu cng sut tiu tn, lm cho cc b

phn khuch i hot ng vi tin cy cao.

d. Anten pht / anten thu

Anten c s dng truyn hoc nhn sng in t trong bung.

Cc c tnh quan trng nht i vi anten pht, anten thu trong bung trit

nhiu l bng thng. Bng thng l phm vi tn s anten c th thu hoc nhn

nng lng ng. Cc c tnh khc t c nh nh hng, li v s

phn cc b mt trong bung do hin tng cng hng.

e. B phn thm d

B phn thm d bao gm ba anten bng thng rng c lp nh

hng trc giao m mi my o mt thnh phn ca in trng. Cc tn hiu

t u d c truyn k thut s thng qua mt lin kt si quang n n v

c hoc my tnh. RMS (root c ngha l vung, gi tr hiu dng) gi tr cho

ba thnh phn ca trng biu din cho trng tng hp.

9

So vi cc anten kt ni vi mt my phn tch ph phn tch s khc

bit gia nhng tn s, u d trng p ng cho tt c cc thnh phn tn

s hot ng. Do tn hiu sch trong mt thit lp th nghim min nhim

l rt quan trng, c bit l trng sng iu ha c pht ra bi mt b

khuch i cng sut.

f. H thng khuy

H thng khuy l kim loi phn x tt mc ch chnh l thay i

iu kin bin trong bung trit nhiu. Vic thit k b cng hng cng c

th lm thay i tn s thp nht c th s dng cho bung. Trong vng quay

ca m hnh h thng khuy, sng ng trong bung s c thay i v tt

c cc im trong khi phi lm vic s chu cng mt mc ti thiu, ti a v

trng in t trung bnh. Bung hon thnh iu kin ny c gi l khuy

tt, do cp mt trng thng k ng hng cho th nghim EMC / EMI.

1.3.3 Quy trnh kim tra

Cc bc chnh trong qu trnh kim tra nh sau:

- M t k lng cc hot ng ca EUT vi s , biu v s

b tr.

- EUT, dng c dy dn vi cc kt ni v m phng nu cn thit.

- Thit b gim st, cp si quang, chng phi khng nh hng n

s min nhim ca EUT khi thm vo.

- Kim tra cc tn s (cc mc min nhim) da trn cc nguyn nhn

lm cho EUT sai s t cc thit b gy nhiu hoc cc dch v v tuyn

- Cc ch hot ng trong phng kim tra EUT da trn vic phn

tch cc ch .

- Phn mm kim tra sn phm, ch hot ng.

10

1.4 PHN TCH CC NGUN NHIU

Do s pht trin mnh m ca cng ngh in t nn cc loi ngun

giao thoa in t (EMI) cng tng ln, nh hng ca EMI ngy cng a dng

v phc tp. Nhng hiu ng ny e da n sc khe v s an ton ca con

ngi, hoc gy ra s trc trc h thng in in t khc, lm cho cc thit

b hot ng khng ng theo yu cu v d b hng hc. Nhiu EMI c th

lan truyn theo kiu dn hoc bc x c minh ha nh hnh v:

Hnh 1.3 S m t cc ng ghp EMI

Cc ngun nhiu EMI trong lnh vc TTT c chia lm 2 phn

chnh: Cc ngun nhiu t nhin v cc ngun nhiu cng nghip.

1.4.1 Cc ngun nhiu t nhin

Cc ngun nhiu t nhin c th l :

- T trng mt t v cc cn dng t.

- in trng trong iu kin thi tit tt.

- Hot ng ca cc cn dng v nhiu kh quyn.

- Dng v trng t.

- Nhiu v tr.

11

Trng in t c ngun gc t nhin xut hin khp ni gia mt

t v tng in ly (t 0 n 100km), trong phn ln l hot ng ca cc

cn dng v chng c xem nh mt ngun bc x.

1.4.2 Cc ngun nhiu cng nghip

a. Cc ngun nhiu v tuyn in

Tt c cc thit b in hoc in t tn ti cc cp ngun nhiu v

tuyn in khc nhau. Cc thit b ny c th c phn loi ty theo bn cht

ca cc tn hiu v tuyn in m thit b to ra v ty theo kiu nhiu m n

sinh ra.

Cc kiu nhiu ny c th sinh ra bi:

- Cc thng tin pht i ca my pht v tuyn in.

- Cc s bc x hi ca cc thng tin pht ny.

- Cc tn hiu nhiu dn hoc bc x ca thit b in hoc in t.

a1. My pht v tuyn in

Hin nay trn th gii c hng chc triu my pht v tuyn in

c thng k. i khi cc my pht ny gy nhiu lon cc my thu ca cc

dch v v tuyn in khc hoc ngay c vic lm nhiu lon s vn hnh ca

cc thit b in t nhy cm hoc cc thit b bo v km i vi bc x

trng in t mc cao.

a2. Cc ngun bc x khng c

- Cc my mc cng nghip, khoa hc v y t.

- Thit b x l thng tin.

- Cc h thng nh la ca cc xe ng c n.

- Cc thit b in gia dng, cc thit b xch tay c ng c in.

- Mng in cao p v h p.

- Hiu ng vng trn cc ng truyn nng lng.

12

- n hunh quang v cc bng hiu pht sng.

- Cc h thng hin th v cc my thu hnh vi in p rt cao.

- n chp in t.

- H thng chuyn mch c in

b. Cc hiu ng ca tn hiu nhiu

b1. S phn b thng k ca mc nhiu

Tnh cht ngu nhin ca nhiu v khi nim v xc sut ca nhiu l

cc yu t chnh d on cht lng vn hnh v s ti u ha ca h

thng v tuyn in. S bin ng ca nhiu v tuyn in thng c m

t bi khi nim v phn b xc sut bin ca nhiu. S phn b ny ch ra

s phn trm thi gian trong hnh bao ca nhiu vt qua khi cc mc d

liu, cc mc ny c o ti cc tn s c xc nh r v mt kiu tch

sng xc nh.

Tri vi nhiu c ngun gc t nhin, nhiu cng nghip c nhng c

tnh rt khc nhau v s phn tch thng k ca n cng phc tp hn.

b2. Nhiu cc h thng thu v tuyn in

Cc h thng thu v tuyn in, c bit l cc h thng t trong cc

vng cng nghip, chng c nhy cm cao i vi tt c cc loi ngun

nhiu. nghin cu nhng nguy c b nhiu lon ca mt h thng thu, cn

thit phi bit cc c tnh v tuyn in ca ngun nhiu, cc c tnh ca h

thng thu v bn cht ghp gia ngun nhiu vi h thng thu.

b3. Nhiu do bc x ca mt thit b khng phi l v tuyn in

Ngng nhy cm v bc x in trng l mc trng in t ti a

thit b c th chu c m khng b sai st trong vn hnh cng nh khng

b st gim v cht lng. m bo mt s nhy cm ti thiu cho cc

thit b in t, cn phi m bo mt ngng nhy cm ln hn hoc bng 1

V/m mi tn s. Tuy nhin, ty theo mc ch cng nh an ton ca s

13

vn hnh thit b m y ban K thut in quc t a ra cc ngng nhy

cm cho tng loi thit b khc nhau. V d: hng 1 (1V/m), hng 2 (3V/m),

hng 3 (10V/m)

Cc ngun giao thoa in t EMI in hnh trong bnh vin bao gm

ng c, n hunh quang, thit b gn vi ngun in Phu thut in, dao

t in hoc m in l ngun EMI ln trong bnh vin.

Cc ngun EMI khc l in thoi di ng v cc dch v v tuyn cu

thng. Nhng ngun ny to ra cng trng cao khong cch ngn,

n thay i t l nghch vi khong cch t anten.

1.5 S BC X

Khi thit k mt sn phm theo cc c im k thut th cn phi bit

c h thng hay mi trng m n c lp t, bnh thng chng ta s

phi tch ring hai mt ca s bc x v tnh nhy cm ca thit b, thit k

p ng vi cc yu cu ti thiu ca mi mt. S bc x c chia nh thnh

cc bc x pht x (tn s cao) t h thng v cc bc x dy dn (tn s

thp) hin din trn cc dy cp giao tip v cp ngun. Theo quy c th

im ngn cch gia hai dng bc x l ti tn s 30MHz. Bc x pht x c

th chia ra thnh cc bc x m bt ngun t cc dng in sai lch trn cc

bo mch, cc dy dn khc v cc bc x t cc dng in kiu chung

(common-mode) trn cc bo mch hoc cu trc dy dn, hoc trn cc dy

cp ni cc thit b.

1.5.1 Bc x pht x

a. Bc x t bo mch

Hu ht thit b, ngun gy nhiu ch yu l cc dng in chy trong

mch (nh tn hiu clock, video, iu khin v cc dao ng khc), tt c hu

nh u nm trn cc bo mch. Mt lng nng lng no s bc x trc

14

tip t bo mch mang theo dng in gy nhiu (hnh 1.4). Cng in

trng khong cch 10m (theo tiu chun Chu u) c xc nh theo

cng thc [1]:

E = 263.10-12

. f2. A. Is (V/m) (1.2)

Trong :

f : tn s (MHz)

A: din tch vng dng in (cm2)

Is: cng dng in trong vng (mA)

Hnh 1.4 Bc x t bo mch in

b. Bc x t cp

i vi cp, khi c dng in chy qua vi mt tn s no th cng

bc x mt in trng tng ng. Cng in trng bc x ca cp

khong cch 10m c xc nh [1]:

15

E = 1.26.10-4

. f. L. I (V/m) (1.3)

Trong :

f: tn s dng in chy trong cp (MHz)

L: chiu di dy cp (m)

ICM : cng dng in chy trong cp (mA)

Hnh 1.5 Bc x t cp

1.5.2 Bc x t dy dn

Cc ngun gy nhiu t bn trong cc mch in hoc t ngun

cung cp ca thit b c mc vng sang cp ngun v dn n thit b

khc. Ngun nhiu ny cng c th mc vng dng in cm hoc in

dung t thit b khc sang cp ngun. Kt qu l nhiu c th xut hin

nh ch sai lch (gia masse ngun v mas se my hoc gia cc dy

dn tn hiu) hoc ch chung (gia masse ngun, masse my, tn hiu

v t) hoc s kt hp c hai ch trn. Bc x ch sai lch c th

kt hp vi nhiu chuyn mch tn s thp t ngun cung cp, trong khi

bc x ch chung li kt hp vi cc thnh phn nhiu tn s cao.

16

1.6 KT LUN CHNG

C th ni rng tng thch in t l mt lnh vc cn tng i mi

m nc ta. V vy vic tm hiu, nghin cu n p dng vo thc t l

ht sc cn thit. Cc ngun nhiu trong lnh vc tng thch in t kh a

dng v phc tp, hai phn t ng vai tr chnh trong h thng l ngun gy

nhiu v thnh phn b nhiu. Trong phn ny trnh by c cc loi

nhiu v nguyn nhn gy nhiu. Cc nguyn tc tng thch in t, cc thit

b cn thit cho qu trnh kim tra tnh tng thch in t.

17

CHNG 2

HIU QU BC CHN CA VT LIU

2.1 GII THIU CHNG

Chng ny trnh by cc vn v m thu, tr khng ca vt liu

kim loi, khng kh. ng thi trong chng cng tp trung nghin cu hiu

qu che chn ca ba loi vt liu Superalloy, Nhm (Al) v Mumetal c

dy 0,35*10-3

inch, c t khong cch mt mt t ngun in t. Cc tn

s c kho st t 10Hz n 1GHz.

2.2 TR KHNG CA VT LIU

Tt c cc vt liu c tr khng ph thuc vo dn in, h s t

thm v hng s in mi ca vt liu. Sng in t truyn qua vt liu, tr

khng ca sng gn vi tr khng ca vt liu.

Phng trnh tng qut cho tr khng ni l [7]:

(2.1)

Trong :

= 2f , n v radians

f : tn s, n v Hz

: h s t thm ca vt liu, =0 r

0 :h s t thm ca khng kh hoc chn khng , 0= 4*10-7

H/m

r : h s t thm ca vt liu i vi khng kh

: in dn ca vt liu, =cur

cu: in dn ca ng cu=5,8*107 mhos/meter

18

r : in dn ca vt liu i vi ng.

: hng s in mi ca vt liu, =0r

0: hng s in mi ca khng kh hoc chn khng, 0=8,84*10-12

F/m

r: hng s in mi ca vt liu i vi khng kh

2.2.1. Tr khng ca khng kh

Trong vic xc nh tr khng ca khng kh, mc d j l nh, in

dn nh hn nhiu so vi j, gn bng khng. V vy, i vi tr khng

sng ca khng kh, phng trnh (2.1) s tr thnh:

Zair =377ohms (2.2)

Sng in t truyn qua khng kh mt khong cch (r) t ngun, ni

r , gi l trng xa c tr khng bng Zair.

Trong trng gn, ni r , tr khng sng ph thuc vo tr khng

ngun v khong cch t ngun.

Gi s khong cch t ngun l nh so vi bc sng (), tr khng

sng tr thnh[7]:

Zw = =k.377 ohms (2.3)

Trong :

E: cng in trng (V/m)

H: cng t trng (A/m)

k=1, nu r

19

k= , nu ngun c tr khng cao v r

Nhng ZW khng th vt qu tr khng ngun.

k= , nu ngun c tr khng thp v r

Nhng ZW khng th nh hn tr khng ngun.

2.2.2. Tr khng ca kim loi

Trong vic xc nh tr khng ca mt kim loi, c in dn cao >>

. Gi nh dy ca kim loi (t) ln hn ba ln so vi m thu (t >>

3), tr khng ca kim loi (Zm) ca phng trnh (2.1) tr thnh:

Zm= ohms/square (2.4)

hoc trong quan h tng i so vi ng:

Zm=369 micro-ohms/square (2.5)

Zm cng c th c biu din theo m thu () cho bt k kim loi

no:

Zm= ohms/square (2.6)

Trong

= meters (2.7)

2.2.3. Chiu su lp da

m thu l dy ca kim loi m bin ca dng in ti bt k

tn s no u b suy gim n 1/e ln (37%) ca cng dng in ti b

mt. m thu 2, dng in b suy gim n 1/e2 ln (14%). V vy,

63% (1-1/e ) dng in chy qua gia b mt kim loi v m thu 1, 86%

20

(1-1/e2) gia b mt v m thu 2 v ln n 99% ti m thu 5.

Nu dy ca kim loi nh hn dy trn, tr khng ca n r rng l cao

hn so vi Zm c tnh ton.

i vi kim loi mng tr khng (ZB) tr thnh [7]:

ZB= ohms/square cho bt k gi tr ca t/ (2.8)

i vi t/ 3 (2.11)

v

K = cho t < 3 (2.12)

2.3 HIU QU BC CHN CA SUPERALLOY, ALUMINUM V

MUMETAN

2.3.1 Gii thiu chung

- V t trng, nguyn tc Faraday khng p dng. Tuy nhin, vt

liu t tnh c t thm cao ( >> 1) v dy ln c th to ra s suy

gim t trng.

21

Mt khc, vt liu dn in mng c h s t thm thp cng c kh

nng cung cp che chn hiu qu i vi t trng. L chn lm bng vt liu

dn in, do t trng thay i m to ra dng xoy trn l chn to hiu qu

che chn. Dng in xoy ny chng li s bin thin ca t trng nh

hng bn trong l chn. Kt qu l, khi tn s tng, hiu qu che chn s

tng ln tng ng.

- V in trng, theo nguyn tc Faraday, in trng bn trong vt

dn in hnh cu gn nh bng khng.

dy ca l chn ng mt vai tr khng ng k khi cc electron t

do di chuyn trong vt liu dn in.

-i vi trng xa, tr khng sng c xc nh l

Zw = =377 (2.13)

t c iu kin ny, khong cch n ngun bc x cn phi

xa, hay ni cch khc, khu vc trng xa. C t trng v in trng u

gim bin l 20dB nu khong cch c tng ln gp mi ln.

Trong khu vc trng gn, hiu qu che chn phi c xem xt ring

i vi t trng v in trng. T l gia cc trng ph thuc vo khong

cch t ngun bc x. T trng iu khin trng gn khi ngun c tr

khng thp, ngc li in trng iu khin khi cc ngun c tr khng cao.

Khi khong cch ti ngun l /2, tr khng sng l 377 .

2.3.2 Tnh ton hiu qu bc chn

Hiu qu bc chn cho thy kh nng ca vt liu kim loi bo v,

chng li cc trng in t bn ngoi v l ro cn ngn chn cc trng bn

trong lm h hi cc thit b khc. Cc thnh phn ca n bao gm tn hao

hp th, tn hao phn x v h s hiu chnh phn x li.

22

Hnh 2.1. ng sng bc x truyn qua mt tm chn

SEMagnetic = A + RMagetic + CMagnetic (2.14)

SEElectric= A + RElectric + CElectric (2.15)

SEPlaneWave= A + RPlaneWave + CPlaneWave (2.16)

trong :

SE: Hiu qu bc chn

A: Tn hao hp th

R: Tn hao phn x

C: H s hiu chnh phn x li

Cui cng, hiu qu bc chn hon ton ca mt tm kim loi l tng

hp ca ba yu t, tn hao hp th, tn hao phn x v h s hiu chnh phn

x li. Vic tnh ton phi c p dng cho tt c ba lnh vc, in trng, t

trng v sng phng .

2.3.3 Tn hao hp th

23

a. Phng trnh tn hao

Tn hao hp th c tnh ton trc v cc thnh phn in trng, t

trng, sng phng c tn hao hp th ging ht nhau. Phng trnh tn hao

hp th l mt hm c trng bc chn EMI ca kim loi c s dng (nh

trong Ph lc B) v tn hao hp th ca tm chn l [3]:

(dB) (2.17)

trong :

K1=131,4 nu l n v l mt

K1=3.34 nu l n v l inch

l : dy ca bc chn

r: h s t thm

gr: in dn

Cc kt qu tnh ton p dng cho t trng, in trng v sng

phng. xc nh vt liu che chn thch hp c th c la chn theo h

s t thm v h s in dn cho tn hao hp thu thch hp.

Bng 1 ti ph lc B m t c im che chn EMI tng i, trong

c t thm v dn in tng i ca cc kim loi.

b. Kt qu tn hao hp th

Tn hao hp th c tnh bi Matlab sau khi nhp cc iu kin ban

u. th ny p dng cho tt c ba lnh vc l t trng, in trng v

sng phng.

24

Hnh 2.2 Tn hao hp th i vi t trng, in trng v sng phng

T kt qu hnh 2.2 ta thy rng tn hao hp th ca Nhm (Al) l nh

nht, lm cho n d b tn thng nht i vi nng lng phn x li. Nh

vy, c ba vt liu phi s dng h s hiu chnh phn x li, c bit l

Nhm s da vo h s ny cho phm vi rng nht ca tn s.

2.3.4 Tn hao phn x

Tn hao phn x ca mt tm chn lp ghp, tn hao phn x ca mt

ng truyn, n t n nh khi tr khng ca in t trng cao hn nhiu

hoc thp hn so vi tr khng ca tm chn. Trong trng hp tn hao phn

101

102

103

104

105

106

107

108

109

10-3

10-2

10-1

100

101

102

103

104

TON HAO HAP THU

TAN SO(Hz)

TO

N H

AO

HA

P T

HU

(d

B)

Superalloy

Aluminum

Mumetal

25

x l thp, ta s dng kim loi c t thm cao hn v tng dy tng

cng hiu qu che chn.

Trong t trng, tr khng ca tm chn v tr khng ca trng l

gn cn bng tn s thp. iu ny to ra s tn hao phn x mc ti

thiu. Khi tng tn s, lm tng tn hao phn x trong t trng. Do , tn

hao phn x gn nh t l thun vi tn s.

Trong in trng, tn s cao tr khng ca tm chn v tr khng

ca trng l cn bng v tn hao phn x tr nn nh hn. Do , tn hao

phn x gn nh t l nghch vi tn s trong in trng.

a. Phng trnh tn hao

Mi trng c phng trnh tn hao phn x ring bit. i vi t

trng, phng trnh s l [3]:

(2.18)

Trong :

C1= 0,0117 nu r tnh theo n v mt

C1= 0,462 nu r tnh theo n v inches

C2= 5,35 nu r tnh theo n v mt

C2= 0,136 nu r tnh theo n v inches

r: l khong cch t ngun in t n bc chn

f: tn s

r: h s t thm

26

gr: in dn

Phng trnh tn hao phn x s dng y l dnh cho t trng tr

khng thp. y c xem l trng gn, trong r l khong cch t ngun

in t, nh hn bc sng ca t trng chia 2 (r

27

gr: in dn

b. Kt qu tn hao phn x

Kt qu t chng trnh Matlab cho thy tn hao phn x i vi t

trng, in trng v sng phng.

T trng:

Hnh 2.3 Tn hao phn x ca thnh phn t trng

101

102

103

104

105

106

107

108

109

0

20

40

60

80

100

120TON HAO PHAN XA CUA THANH PHAN TU TRUONG

TAN SO(Hz)

TO

N

HA

O

PH

AN

X

A

(

dB

)

Superalloy

Aluminum

Mumetal

28

Hnh 2.3 cho thy Nhm (Al) c tn hao phn x nhiu nht trong t

trng trong khi Superalloy v Mumetal c mc tng t nhau mc d gi tr

t thm tng i ca n l rt khc nhau.

in trng:

Hnh 2.4 Tn hao phn x ca thnh phn in trng

Tng t, hnh 2.4 cho thy Nhm (Al) c tn hao phn x nhiu nht

trong in trng trong khi Superalloy v Mumetal c mc tng t nhau.

101

102

103

104

105

106

107

108

109

-50

0

50

100

150

200

250

300TON HAO PHAN XA CUA THANH PHAN DIEN TRUONG

TAN SO(Hz)

TO

N

HA

O

PH

AN

X

A(

dB

)

Superalloy

Aluminum

Mumetal

29

Sng phng

Hnh 2.5 Tn hao phn x i vi sng phng

Mt ln na, hnh 2.5 cho thy Nhm (Al) c tn hao phn x

nhiu nht trong sng phng trong khi Superalloy v Mumetal c mc

tng t nhau.

101

102

103

104

105

106

107

108

109

0

20

40

60

80

100

120

140

160TON HAO PHAN XA DOI VOI SONG PHANG

TAN SO (Hz)

TO

N H

AO

PH

AN

XA

(d

B)

Superalloy

Aluminum

Mumetal

30

2.3.5 Hiu qu bc chn khi tn hao hp th trn 10 dB

a. Phng trnh tn hao

Khi tn hao hp th ln hn 10dB, nng lng phn x khng th xm

nhp qua bc chn, ta thy rng vic tnh ton cc yu t phn x li l khng

cn thit. Do , hiu qu bc chn trong c ba trng hp nu tn hao hp

thu ln hn 10dB c th c tnh ton bng cch tng hp cc tn hao hp

th v tn hao phn x.

SEMagnetic = A + RMagetic (2.21)

SEElectric= A + RElectric (2.22)

SEPlaneWave= A + RPlaneWave (2.23)

Trong :

A: Tn hao hp th

R: Tn hao phn x

Tuy nhin, t kt qu ca cc tnh ton nh trong phn kt qu, tn hao

hp th ca tng kim loi vt qu 10dB trong khong tn s nht nh, th

hin kh nng ca nng lng phn x xuyn qua che chn. iu ny i hi

s tnh ton cc h s hiu chnh phn x li.

b. Kt qu hiu qu bc chn khi tn hao hp th trn 10 dB

y l cc th cho thy hiu qu bc chn c tnh bi Matlab cho

trng hp tn hao hp thu ln hn 10dB. y l tng ca tn hao hp thu v

tn hao phn x. H s hiu chnh phn x li l khng cn thit v vic b

sung s khng nh hng ng k n hiu qu bc chn.

31

Hnh 2.6: Tn hao ca thnh phn t trng

Hnh 2.7: Tn hao ca thnh phn in trng

101

102

103

104

105

106

107

108

109

101

102

103

104

TON HAO CUA THANH PHAN TU TRUONG

TAN SO(Hz)

TO

NG

TO

N H

AO

(dB

)

Superalloy

Aluminum

Mumetal

101

102

103

104

105

106

107

108

109

101

102

103

104

TON HAO CUA THANH PHAN DIEN TRUONG

TAN SO(Hz)

TO

NG

TO

N H

AO

(dB

)

Superalloy

Aluminum

Mumetal

32

Hnh 2.8: Tn hao i vi sng phng

Trong c ba lnh vc, Nhm (Al) c hiu qu bc chn trong phm vi

hp nht, xp x t 20dB n 250dB, trong phm vi tn s 10 Hz n 1 GHz.

Trong phm vi t 10Hz n 1MHz, Nhm (Al) c hiu qu bc chn tt hn

Superalloy v Mumetal v tn hao phn x ln hn .

Tuy nhin, i vi tn s hn 1MHz, mc tn hao hp thu ca

Superalloy v Mumetal vt qua Nhm (Al), nn hiu qu bc chn tt

hn Nhm.

2.3.6 H s hiu chnh phn x li

101

102

103

104

105

106

107

108

109

101

102

103

104

TON HAO DOI VOI SONG PHANG

TAN SO (Hz)

TO

N H

AO

(dB

)

Superalloy

Aluminum

Mumetal

33

a. Phng trnh h s hiu chnh phn x li

Phng trnh cho h s hiu chnh phn x li C [3]

(2.24)

Trong :

A: Tn hao hp th

: H s phn x hai bin

Trong ba lnh vc c h s phn x ti hai bin c a ra trong cc

iu kin ca cc tham s tnh ton ban u m.

Phng trnh cho t trng [3]:

(2.25)

(2.26)

Trong :

r: Khong cch t ngun in t n bc chn

f: tn s

r: h s t thm

gr: in dn

Phng trnh cho in trng [3]:

34

(2.27)

(2.28)

Trong :

r: Khong cch t ngun in t n bc chn

f: tn s

r: h s t thm

gr: in dn

Phng trnh cho sng phng [3]:

(2.29)

(2.30)

Trong :

f: tn s

r: h s t thm

gr: in dn

Bng cch s dng h s phn x li thch hp ti hai bin v

tham s tnh ton ban u m, cc yu t iu chnh phn x li ph hp

35

tng ng cho mi trng hp c tnh ton iu chnh hiu qu che

chn ng n.

Cc yu t iu chnh phn x li l cn thit cho tn hao hp th t hn

10 dB ngn chn nng lng phn x xm nhp qua bc chn.

b. th h s hiu chnh phn x li

Cc th di cho thy cc h s hiu chnh phn x cho mt trong ba

trng hp.

T trng:

Hnh 2.9: H s hiu chnh phn x li ca thnh phn t trng

101

102

103

104

105

106

107

108

109

0

10

20

30

40

50

60HE SO HIEU CHINH PHAN XA LAI,C, DOI VOI TU TRUONG

TAN SO(Hz)

HE

SO

HIE

U C

HIN

H P

HA

N X

A L

AI,

C (

dB

)

Superalloy

Aluminum

Mumetal

36

in trng:

Hnh 2.10 : H s hiu chnh phn x li ca thnh phn in trng

Sng phng

Hnh 2.11. H s hiu chnh phn x li i vi sng phng

101

102

103

104

105

106

107

108

109

0

50

100

150HE SO HIEU CHINH PHAN XA LAI ,C, DOI VOI DIEN TRUONG

TAN SO (Hz)

HE S

O H

IEU C

HIN

H P

HAN X

A L

AI, C

(dB

)

Superalloy

Aluminum

Mumetal

101

102

103

104

105

106

107

108

109

0

50

100

150HE SO HIEU CHINH PHAN XA LAI ,C,DOI VOI SONG PHANG

TAN SO (Hz)

HE

SO

HIE

U C

HIN

H P

HA

N X

A L

AI,

C (

dB

)

Superalloy

Aluminum

Mumetal

37

Khi kim tra hnh 2.9, 2.10, v 2.11 cng vi hnh 2.2, r rng h s

hiu chnh phn x l cn thit khi tn hao hp th nh hn 10dB. Khi xem

hnh 2.2, tn hao hp th ca Nhm khng vt qu 10dB cho n 100

MHz, Superalloy v Mumetal l 100 KHz.

Trong c ba hnh, h s tin n khng khi tn s t 100MHz cho

Nhm (Al) v 100KHz cho Superalloy v Mumetal. V vy, h s hiu chnh

phn x li khng cn thit khi tn hao hp thu ln hn 10dB.

2.3.7. Hiu qu bc chn

Cc th miu t hiu qu bc chn c tnh khi c s dng h s

hiu chnh phn x li cho mi trng hp.

T trng

Hnh 2.12a: Hiu qu bc chn ca thnh phn t trng

101

102

103

104

105

106

107

108

109

-500

0

500

1000

1500

2000HIEU QUA BOC CHAN CUA THANH PHAN TU TRUONG

TAN SO(Hz)

HIE

U Q

UA

BO

C C

HA

N (

dB

)

Superalloy

Aluminum

Mumetal

38

Hnh 2.12b. Hiu qu bc chn ca thnh phn t trng n 200dB

in trng

Hnh 2.13a. Hiu qu bc chn ca thnh phn in trng

102

103

104

105

106

107

108

109

0

50

100

150

200

250

HIEU QUA BOC CHAN CUA THANH PHAN TU TRUONG

TAN SO(Hz)

HIE

U Q

UA

BO

C C

HA

N (

dB

)

Superalloy

Aluminum

Mumetal

101

102

103

104

105

106

107

108

109

0

200

400

600

800

1000

1200

1400

1600

1800HIEU QUA BOC CHAN DOI VOI DIEN TRUONG

TAN SO(Hz)

HIE

U Q

UA

BO

C C

HA

N(d

B)

Superalloy

Aluminum

Mumetal

39

Hnh 2.13b: Hiu qu bc chn ca thnh phn in trng n 200dB

Sng phng

Hnh 2.14a. Hiu qu bc chn i vi sng phng

101

102

103

104

105

106

107

108

109

20

40

60

80

100

120

140

160

180

200

HIEU QUA BOC CHAN DOI VOI DIEN TRUONG

TAN SO(Hz)

HIE

U Q

UA B

OC C

HAN(d

B)

Superalloy

Aluminum

Mumetal

101

102

103

104

105

106

107

108

109

0

200

400

600

800

1000

1200

1400

1600

1800HIEU QUA BOC CHAN DOI VOI SONG PHANG

TAN SO(Hz)

HIE

U Q

UA

BO

C C

HA

N(d

B)

Superalloy

Aluminum

Mumetal

40

Hnh 2.14b Hiu qu bc chn i vi sng phng n 200dB

T kt qu th hin hnh 2.12a n 2.14b cho thy hiu qu che chn

cho c ba vt liu trong c ba trng hp, Nhm (Al) l km hiu qu nht v

Superalloy c hiu qu nht.

2.4 KT LUN CHNG

Trong chng trnh by c cc vn v tr khng, m thu

v tnh ton c hiu qu bc chn ca ba loi vt liu Nhm (Al),

Superalloy, Mumetal. T kt qu trn cho thy rng, Nhm (Al) vn c kh

nng bc chn hiu qu 40dB cho cc tn s ln hn hoc bng 1MHz trong

t trng, nh hn hoc bng 1MHz trong in trng v ln hn hoc bng

5 KHz trong sng phng.

101

102

103

104

105

106

107

108

109

0

50

100

150

200

HIEU QUA BOC CHAN DOI VOI SONG PHANG

TAN SO(Hz)

HIE

U Q

UA

BO

C C

HA

N(d

B)

Superalloy

Aluminum

Mumetal

41

Tn s ti thiu Superalloy c kh nng bc chn trong t trng l 5

KHz. Ngc li, Superalloy c hiu qu bc chn trong ton b ph tn s i

vi in trng v t 50Hz trong sng phng.

Mumetal c kt qu tng t nh Superalloy. Tn s ti thiu cho hiu

qu bc chn l 1MHz trong t trng, ton b ph tn s trong in trng

v 500 Hz trong sng phng.

42

CHNG 3

CI THIN HIU QU BC CHN


3.1 GII THIU CHNG

Trong chng ny, trnh by cc vn c bn v bc chn, tc dng

ca vic bc chn, cc yu t nh hng n hiu qu bc chn. ng thi

trong chng ny cng trnh by cc phng php ci thin hiu qu bc chn

khi bc chn i hi phi c khe h hoc cc khe h tn ti do li cc khp

ni cng nh hiu qu bc chn ca hp c khe h dng ng dn sng hay

dng t ong.

3.2 TRNG GN V TRNG XA

Cc c tnh ca trng c xc nh bi ngun, cc phng tin

truyn thng xung quanh ngun v khong cch t ngun n im quan st.

Ti mt im gn vi ngun, cc c tnh ca trng c xc nh ch yu

bi cc c tnh ngun. Xa ngun, cc tnh cht ca trng ph thuc ch yu

vo mi trng m trng truyn qua. Do , khng gian xung quanh mt

ngun bc x c th c chia thnh hai khu vc, nh th hin trong hnh 3.2.

mt khong cch ln hn /2 l trng xa. khong cch nh hn

/2 l trng gn. Khu vc xung quanh /2 l khu vc chuyn tip gia cc

trng gn v trng xa.

T s gia in trng (E) vi t trng (H) l tr khng sng. i vi

trng xa t s E/H bng tr khng c tnh ca mi trng.

i vi trng gn t s trn c xc nh bi cc c tnh ca ngun

v khong cch t ngun n ni trng c kho st. Nu ngun c dng

43

cao v in p thp (E/H 377), trng gn ch yu l

in trng.

Hnh 3.1 S chuyn i t trng gn n trng xa xy ra ti /2

Hnh 3.2 Tr khng sng ph thuc vo khong cch t ngun

v trng l in trng hay t trng

44

i vi mt anten dy thng c tr khng ngun cao. Tr khng sng

gn anten ch yu l in trng cng cao. Khi khong cch c tng ln,

in trng suy gim v cng v n to ra mt t trng b sung. i vi

trng gn, in trng suy gim vi tc (1/r)3 trong khi t trng suy

gim vi tc (1/r)2. Do , tr khng sng t mt anten dy thng gim

dn theo khong cch v gn vi tr khng ca khng gian t do trong trng

xa, th hin trong hnh 3.2.

Mt t trng ch yu c sinh ra bi mt anten tr khng sng gn

anten l thp. Khi khong cch t ngun tng, t trng suy gim vi tc

(1/ r) 3

v in trng suy gim vi tc (1/r)2. Do tr khng sng tng

theo khong cch v tim cn vi tr khng ca khng gian t do khong

cch /2.

i vi trng xa c hai in trng v t trng u suy gim t l

vi 1/r. Trong trng gn, in trng v t trng phi c xem xt mt

cch ring bit, khi t l ca hai trng khng l hng s. Trong trng xa,

chng kt hp to thnh sng phng c tr khng 377. V vy, khi sng

phng c cp, chng c gi nh l trong trng xa. Khi tng in

trng v t trng c cp chng c gi nh l trong trng gn.

3.3 TNG HP TN HAO PHN X V HP TH

Sng phng:

Tng tn hao ca sng phng trong trng xa l s kt hp ca tn hao

hp th v tn hao phn x, c trnh by phn trn. H s hiu chnh C

trong nhiu tn hao phn x thng b b qua trong sng phng, khi tn hao

phn x kh cao v h s hiu chnh nh.

Hnh 3.3 cho thy s tn hao hoc hiu qu che chn ca mt l chn

ng dy 0.02 inch. C th thy, tn hao phn x suy gim khi tn s ngy

45

cng tng, bi v tr khng l chn tng theo tn s. Tn hao hp th, tng

theo tn s, do m thu gim. Hiu qu che chn thp nht xy ra ti mt

s tn s trung gian, trong trng hp ny l 10 kHz. Hnh 3.3 cho thy sng

phng tn s thp, tn hao phn x l ch yu, trong khi hu ht s tn hao

tn s cao l do tn hao hp th.

Hnh 3.3 Hiu qu che chn ca l chn ng dy 0.02 inch (20mil) trong

trng xa

in trng:

Tng tn hao trong in trng l s kt hp tn hao hp th v tn

hao phn x, c trnh by phn trc. H s hiu chnh C trong nhiu

tn hao phn x thng b b qua trong trng hp in trng, khi tn hao

phn x l rt ln v h s hiu chnh nh. tn s thp, tn hao phn x l

46

c ch chnh ca hiu qu che chn trong in trng. tn s cao, tn hao

hp th l c ch chnh cho hiu qu che chn.

T trng:

Tng tn hao trong t trng l s kt hp tn hao hp th (Eq. 2.17)

v tn hao phn x (Eq. 2.18), c th hin trong phng trnh 2.14. Nu l

chn dy (tn hao hp th >10dB), h s hiu chnh C c th c b qua.

Nu l chn mng, h s hiu chnh t biu thc 2.24 phi c xem xt.

Tn hao phn x trong trng gn vi t trng tn s thp l nh. S

tn hao chnh cho t trng l tn hao hp th. V c hai tn hao hp th v

tn hao phn x u nh tn s thp nn tng hiu qu che chn l thp. Do

rt kh che chn t trng tn s thp. Bo v b sung i vi t

trng tn s thp c th t c ch bng cch cung cp mt ng r t tr

thp, chuyn hng t trng xung quanh mch c bo v. iu ny

c th hin trong hnh 3.4

Hnh 3.4 S dng vt liu t tnh nh mt l chn

3.4 PHNG PHP BC CHN TRONG LNH VC TTT

Ta s dng cc tm, l chn bng kim loi bao bc xung quanh mt sn

phm hay mt thnh phn ca sn phm in t gi l bc chn. Chng c hai

tc dng, th nht hnh 3.5a l ngn chn s pht x bc x in t t cc

sn phm hay mt thnh phn thit b in t khc bn trong hp che chn

47

sang mi trng bn ngoi. Th hai nh hnh 3.5b l ngn chn cc bc x

bn ngoi n cc thit b in t bn trong lp v che chn. Mc ch

chung ca lp v che chn chnh l vic ngn chn cc bc x ln nhau gia

cc thit b in t, bc x ca sn phm ny khng c tc ng n sn

phm in t kia v ngc li.

Hnh 3.5 M t bc chn: (a) Vic gi bc x nm trong hp bc chn, (b)

Vic ngn chn bc x bn ngoi vo trong hp

Nh vy, bc chn c tc dng ngn chn s lan truyn ca bc x

in t v hiu qu bc chn (SE) c tnh bng t s gia ln cng

in trng (t trng) n tm l chn v ln cng in trng

(t trng) sau khi i xuyn qua l chn kim loi. Thng thng, yu cu

hiu qu bc chn phi ln hn 100dB. Tuy nhin t c mt hiu

qu bc chn ln n nh vy th hp bc chn cc thit b in t phi kn

hon ton. Mt vi khe h hoc ch vt nt trong hp s lm gim ng k

hiu qu bc chn.

3.5 CC PHNG PHP CI THIN HIU QU BC CHN

3.5.1 Phng php bc chn nhiu lp

Hnh 3.6 cho thy che chn bng nhiu bn mng, c n s l chn vi

tr khng Zm1, Zm2, ... Zmn bao gm c kim loi v khe h khng kh. Tng tn

hao phn x c th c th hin nh l tng ca phn x.

48

Hnh 3.6 Bc chn nhiu lp

Tn hao ti mi b mt chuyn tip [8]

R = 20log10

= 20log10 + 20log10 + + 20log10 (3.1)

S suy gim qua cc tm chn n gin l tng ca n tm chn

A = 8.686(1t1 + 2t2 + + ntn ) dB (3.2)

n v tn l hng s suy gim v dy ca tm chn mng th n tng

ng. Cn lu rng Zm1, Zm2 khc nhau vi cn bc hai ca tn s. Do ,

tn hao phn x ti b mt tip gip kim loi vi kim loi l c lp vi tn

s. N l mt hm ca tn s cho cc b mt kim loi-khng kh. N c

xem l hiu qu che chn ca cc tm chn c th c tng ln bng cch

iu khin tr khng ca vt liu v dy.

H s iu chnh t s phn x ni lin tip l [8]

49

IR = 20log10

= 20log10 + 20log10 +

+ (3.3)

Trong :

vn = (3.4)

Kn = (1 + j) (3.5)

Zmtn l tr khng nhn t bn phi ca mi phn.

3.5.2 Phng php bc chn i

i vi mt bc chn ln, che chn rt cao thng c cung cp bi

hai tm chn kim loi c lp c cch nhau mt thnh vn p kh (Hnh

3.7) c th c coi nh mt cht in mi cch in.

Hnh 3.7 Tm chn i

Cc thnh phn ca l chn lc ny l [8]:

50

R = 20log10 + 20log10 + 20log10 + 20log10

(3.6)

A = 8,686(1t1 - 2t2 + 3t3)

= 8,686(1t1 + 3t3) 2 0 (3.7)

IR = 20log10 + 20log10 +

20log10 (3.8)

i vi trng hp c bit trong c hai tm kim loi l cng mt

vt liu, dy v gi nh s tn hao do g l khng ng k, tn hao hp

th v tn hao phn x u l ca mt tm chn n. Ti khng gian cng

hng che chn, hiu qu che chn ca l chn i c th ln hn ln n 6

dB so vi tng ca hai l chn ring bit c cng tng s dy kim loi.

3.5.3 Phng php bc chn chng t trng tn s thp

T trng tn s thp c tn hao phn x v tn hao hp th khi gp

bc chn u rt nh nn vic che chn chng li cc ngun t tn s thp

l rt cn thit. C hai phng php chnh trong vic chng li cc bc x ca

t trng tn s thp:

- S dng cc vt liu c t thm cao lm lch hng ca t

trng.

- S dng phng php to ra mt t trng c chiu chng li bc x

ca t trng ti theo nh lut Faraday.

Gi s mi trng bn ngoi c t thm l 0 cn tm che chn

c lm t vt liu c t thm l 0 r

(vi r>>1 ), th t trng s

c xu hng tp trung vo ni c t thm cao v nh vy n s lm lch

hng t trng khng nh hng n cc thit b bn trong cc tm chn.

51

Hnh 3.8 a) Phng php dng vt liu c t thm cao lm lch hng

t trng, b) Phng php to ra t trng i lp vi

t trng ti

Phng php th hai ngi ta s dng mt vng dy dn c t sao

cho t trng ti hng vo vng dy. Theo nh lut Faraday, s xut hin

mt dng in cm ng indI trn vng dy v t thng ind nh hnh 3.8b. T

thng ny c hng chng li t trng gc gy ra n. Do , n lm gim

bc x ca t trng khi n gp vng dy ny.

Tuy nhin, vic to ra mt vt liu c t thm cao l khng h d

dng. Hai nguyn nhn chnh l:

- t thm ca vt liu gim khi tn s tng.

- t thm ca vt liu gim khi cng t trng tng.

Do vy, rt kh ch to c vt liu c t thm cao p ng

c yu cu chng li cc bc x t trng tn s thp trong iu kin

cc bc x lun thay i lin tc c v tn s ln ln. gii quyt

vn trn, ngi ta s dng mt lc hai lp che chn c minh ha nh

sau:

52

Hnh 3.9 S dng nhiu lp chn chng li hin tng bo ha t ca

vt liu

Trong , lp chn u tin c t thm thp, mc bo ha cao v

lp chn th hai c t thm cao, mc bo ha thp. Lp chn u tin l

lm gim cng t trng ti m bo n khng gy ra hin tng bo

ha lp chn th hai. Do , t thm tng i ca lp chn th hai vn

cn cao gip ngn chn cc bc x t trng tt hn. ng thi vic s dng

nhiu lp che chn l lm tng s suy hao do phn x khi t trng ti gp s

lng cc b mt chn nhiu hn.

3.5.4 Phng php t khe h v chia nh khe h

Khi trng ti gp tm chn, chng cm ng sinh ra cc dng in

chy trn b mt tm chn nh hnh 3.10a, n c tc dng nh mt trng

phn x.

Trng phn x ny c xu hng chng li bc x ca trng ti

p ng ng cc iu kin bin, ti l tng cng in trng tip xc

vi mt mt phng vt liu dn in phi bng 0. Tuy nhin, tm chn c

th loi b c bc x ca trng ti theo nguyn tc trn th cc dng in

cm ng sinh ra chy trn phi c lu thng hon ton m khng gp

bt k mt s cn tr no. Nhng do s tn ti ca cc khe h trn b mt tm

53

chn lm cn tr v gy ra s gin on i vi cc dng in ny nh

hnh 3.10b. V vy, hiu qu bc chn s b gim i.

Hnh 3.10 a)Dng in cm ng chy trn tm chn kim loi

khi khng c cc khe h, b)Dng in cm ng chy

trn tm chn kim loi khi c khe h

Do , ci thin hiu qu bc chn, ta phi thit k cc khe h t

cc v tr nh th no, ln ra sao... m bo cc dng in cm ng chy

trn tm chn c lu thng mt cch tt nht c th. Phng php tt nht

chnh l t khe h sao cho chiu di khe h tip xc vi hng dng in

cm ng cng nh cng tt. Trong trng hp ny, ta s t khe h theo

hng song song vi hng ca dng in cm ng. Lc ny dng in cm

ng c lu thng tt hn, v vy, khe h t lm nh hng n hiu qu ca

vic bc chn.

Hnh 3.11 Ci thin bc chn bng vic t khe h

song song vi hng dng in cm ng

Tuy nhin, xc nh chnh xc c hng ca dng in cm ng

cng nh vic t cc khe h ng v tr thch hp l iu rt kh khn. Do

, mt phng php na ci thin hiu qu bc chn chnh l chia khe h

54

ln thnh nhiu khe h nh. V vy, dng in cm ng c lu thng rt tt

lm tng hiu qu bc chn mt cch ng k.

Hnh 3.12 Ci thin bc chn bng phng php

chia khe h ln thnh nhiu khe h nh

Vic chia khe h ra thnh nhiu khe h nh th chiu di ca mi khe

h phi c chn ph hp vi dy tn s ca bc x in t. tng

cng hiu qu ca vic bc chn th chiu di ca mi khe c chn

thng nh hn nhiu so vi na bc sng (/2).

3.5.5 Ci thin hiu qu bc chn ti khp ni

Tng hiu qu che chn ca bc chn b hn ch bi li ca khp ni

lm cho dng in chy trong l chn. Hiu qu che chn ca khp ni ph

thuc ch yu vo kh nng chng to ra mt in tr tip xc thp qua khp.

in tr tip xc l mt hm ca vt liu, dn in ca lp bn b mt v

p lc tip xc.

Ba yu t sau s lm tng hiu qu che chn ng k:

1. Tip xc dn in: Tt c cc b mt khp ni phi c khp nhau

v dn in.

2. Khp ni gi ln nhau: b mt khp ni nn gi ln nhau mc

ln, n cung cp in dung ni cho khp ni c chc nng nh mt ngn

mch in tn s cao. S gi ln nhau ca khp ni mc ti thiu t l

gia b mt v k h 5:1 l mt la chn tt.

55

3. Ming m, im tip xc ca khp ni: Tip xc tt gia cc b

mt tip xc c th t c bng cch s dng ming m dn. Cc tnh cht

v in ca cc ming m phi ging vi l chn duy tr mt mc dn

in cao ti b mt chuyn tip v trnh nhng khong trng khng kh

hoc in tr cao. Dng in cm ng trong l chn, c bn cng mt hng

nh in trng ti. Mt ming m t ngang vi dng in ny l km hiu

qu hn so vi t song song vi dng in. Sng phn cc trn cha cc

thnh phn dc v ngang bng nhau. Do , cc ming m phi c hiu qu

nh nhau trong c hai hng. Mt s cc ming m c hiu sut ph thuc

vo hnh dng khp ni, in tr tip xc v lc t ti cc khp.

Hnh 3.13 cho thy hai k thut in hnh ca ming m khp che

chn.

Hnh 3.13 K thut m ni

Ngoi ra, bt k mt hp bc chn no cng cn c cc np ng m.

Do xut hin rt nhiu cc khong h ti cc v tr ny v bc x thng qua

n tng ng nh bc x ca mt anten. Mc d cc khong h ny c th

rt nh nhng nng lng bc x ca n c th l rt ln theo nguyn l

Babinet. V d, yu cu cn c mt np y hp bc chn minh ha hnh

3.14a.

56

Hnh 3.14 nh hng ca cc np y hp bc chn

Theo nguyn l Babinet, nu thay th cc khong h ny bng mt dy

dn rn c kch thc cng vi kch thc ca khong h th bc x in

t ca chng l tng ng nhau. iu ny cho thy rng chiu di ca

khong h quan trng hn b dy ca n trong vic xc nh bc x in t

qua cc khong h ny. Do , nu chiu di ca cc khong h ng bng

na bc sng th bc x ca n tng ng nh bc x ca mt anten

dipole. ph v kh nng hnh thnh nng lng bc x nh mt anten, ta

thng t cc c vt xung quanh np y do cc anten ngn c bc x in t

nh hn so vi cc anten di. iu ny c minh ha hnh 3.14b, khi chn

cc c vt xung quanh np y che chn th lm gim bc x in t t cc

khong h.

3.5.6 Phng php s dng cc ng dn sng

Khi hp bc chn i hi c hiu qu bc chn cao hoc hp bc chn

cn c khong h ln nh h thng thng gi hnh 3.15, chng ta c th

dng bc chn dng ng dn sng.

57

Hnh 3.15 S dng ng dn sng cung cp h thng thng gi v lm

gim bc x in t qua n: (a) nhn trc, (b) nhn bn cnh

Vi ng dn sng hnh tr trn, nu chiu di ca n gp 3 ln ng

knh th hiu qu bc chn s t mc rt cao 100dB.

i vi ng dn sng dng t ong:

Che chn ton vn ca v chn RF c duy tr ti cc im c

ng thng gi . Tm vt liu kim loi dng t ong c s dng cho mc

ch ny nh trong hnh 3.16. Tm vt liu kim loi dng t ong tn

dng li th ca nguyn l ng dn sng khi p dng cho cc t bo t

ong ring l.

Hnh 3.16 ng dn sng dng t ong

58

Nu cc t bo lc gic c xp x bng ng dn sng trn, che chn

khong 100dB c th t c ln n tn s cao nht tha cc mi quan h

[8]:

d (3.9)

t 3d (3.10)

Trong d l ng knh ca ng dn sng trn, t l chiu di ca ng

dn sng v l bc sng tng ng vi tn s cao nht.

3.6 TNH TON HIU QU BC CHN TRONG TRNG HP

BC CHN KHNG KN

Trng hp thit b bc chn c khe h, h thng thng gi, ng ni

v khp ni lm gim SE ca hp bc chn. Trong trng hp ny, mt

phng php c bit tnh SE ca cu trc l cn thit.

u tin, tnh SE cho mt tm chn ca vt liu dn in ti mi tn s

quan tm.

Th hai, tnh ton SE khi c khe ti mi tn s. Sau s dng gi tr

SE thp hn ti mi tn s. Thng thng c mt s loi khe h, phng

php kt hp cc hiu ng ca n cng tng t nh cch tnh tng s tr

khng ca cc in tr song song [7]:

(3.11)

Tuy nhin, khi SE c tnh theo dB, mi SE phi c chuyn i

tr li ra t l trc khi cng. Tng SE c chuyn i tr li ra dB.

Tng SE nhiu khe s nh hn so vi SE ring l thp nht. Phn di s

trnh by cc phng php xc nh SE ring l cho nhiu loi khe h.

59

3.6.1 Hiu qu bc chn ca tm chn c khe h

Hnh 3.17: Kch thc khe h

i vi mt khe hnh ch nht nh trong hnh 3.17 [7]:

SEdB = 97 20log(LfMHz) + 20log + SEshad + 30

(3.12)

trong :

L, S : chiu di v chiu cao ca khe (mm)

d: chiu sau ca khe, thng cng l b dy ca vt liu (mm)

SEshad: hiu ng che, xem trong bng 3 hoc mc nh l 3dB

ln : log t nhin

i vi mt l trn nh trong hnh 3.17 [7]:

SEdB = 99 20log(LfMHz) + SEshad + 30 (3.13)

trong :

L : ng knh (mm)

60

Trong c hai trng hp, nu tm chn mng (d

61

W = Chiu rng ca hp

L = Chiu di ca khe

H = Chiu cao ca hp

S = Chiu rng ca khe

D = Chiu su ca hp

3.6.2 Hiu qu bc chn ca tm chn khi khe h chia nh

tnh ton cc SE ca mt tm chn vi mt khe h c chia nh,

u tin tnh ton SE tm chn vi mt khe h ln c cc cnh l ng nt

t trong hnh 3.18 vi phng php tnh nh ni trn. Sau , s dng

phng trnh 3.18, tnh ton nng cao ( SE) do chia nh khe h. Cng thm

s ci thin cho tnh ton ban u c c tng SE. [7]

Hnh 3.18 Chia nh khe h

62

= 20log 20log (3.14)

Trong :

LB v SB : Chiu di v chiu rng khe trc khi chia nh.

LA v SA : Chiu di v chiu rng khe sau khi chia nh.

Khi khe ban u c chia thnh cc khe nh vi cng t l L/S nh

khe ban u, s hng th hai bin mt.

3.6.3 Hiu qu bc chn ca mt li bng vt liu dn in

Sng phng [7]:

Ti ni r

SE = 20log dB, khi g (3.15)

SE =0, khi g (3.16)

trong :

r : Khong cch t ngun n li (mt)

: Chiu di bc sng (mt)

g : khong cch li (mt) , nh hnh v

Hnh 3.19 Mn li

63

Trng gn, r

T trng:

SEH = 20log = 20log (3.17)

in trng [7]:

SEE = 20log = 20log (3.18)

Hoc, SE c th c tnh cho sng phng bng cch tnh theo biu

thc 3.15

SE = 20log

sau cng thm : 20log cho t trng gn

hoc cng thm : 20log cho in trng gn

SE lc ny khng th cao hn so vi SE ca mt tm chn kim loi

mng tng ng. Nhng phng trnh ny l hp l khi g 10-6

. Khi g l

mt phn rt nh ca bc sng, chng hn g 10-6

, mn li trng ging

nh mt tm kim loi mng. V vy, cc phng trnh vt liu dn in nn

c s dng tnh ton cho c hai trng gn v trng xa.

3.6.4 Hiu qu bc chn ca khe h dng ng dn sng

Cc khong h trn cc tm chn c th lm suy gim cc bc x in

t nu chng c hnh dng ging vi ng dn sng. Ti cc mode lan truyn

sng, tn s cng hng (tc l tn s m ti sng c th lan truyn m t

b hp th) l mt hm ph thuc vo hai s nguyn (k hiu m m v n) v

cc mode lan truyn ng vi tn s ny (k hiu l TEmn, TMmn). Trong ng

dn sng c thit din l hnh ch nht ngi ta dng mode TE10, ng c thit

64

din hnh tr trn th dng mode TE11. Vi ng dn sng hnh ch nht, tn s

ct c tnh nh sau [2]:

2 20,

2c mn

vf m n

l (3.19)

Vi mode TE10 trong ng hnh ch nht nn [2] :

0,10

8

9

2

1.5 10( )

5.9 10( )

c

vf

l

ml

inchesl

(3.20)

Trong fc l tn s ct, l l chiu di ca ng dn sng ch nht.

i vi ng dn sng hnh tr trn th [5]:

96.9 10( )cf inches

d (3.21)

d: ng knh ca ng dn sng.

Suy hao qua ng dn sng ch nht c xc nh theo cng thc [2]:

2

,

0 0 1c mn

mn

f

f (3.22)

Gi s rng tn s sng ti f nh hn nhiu so vi tn s ct fc, ta

c [2]:

(3.23)

Vi mode TE10 suy ra [2]:

65

10l

(3.24)

Suy hao lan truyn ca sng in t qua ng dn sng vi chiu di z

(chiu di mt ct ngang) t l vi ze . V vy, hiu qu bc chn lin quan

n mt ng dn sng hnh ch nht l [2]:

10

10

10 10

20log

20log

27.3

z

dBSE e

z e

z

l

(3.25)

Tng t, vi ng dn sng hnh tr trn ta c [5]:

32dBz

SEd

(3.26)

Hnh 3.20 ng dn sng hnh tr trn vi ng knh d v chiu di z

Cc kt qu trn cho thy sng in t s b suy hao nhanh chng khi

n i qua cc ng dn sng v t l thun vi chiu di ng dn sng. Cc ng

dn sng ny khng ch thc hin nhim v thng gi m cn c kh nng

ngn chn s lan truyn ca bc x in t. Nhiu ng dn sng nh c

ghp li vi nhau theo cu trc t ong (hnh 3.21) l mt trong nhng

phng php hu hiu thc hin ngn chn cc bc x ny.

66

L thuyt v sng nh hng cho cc biu thc xp x cho s suy hao

ca cc ng dn sng dng t ong [9]

(3.27)

fc l tn s ct ca ng dn sng.

Phng trnh SE cho ng dn sng hnh lc gic[9]

(3.28)

g = chiu ngang ca ng dn sng trong milimet

d = chiu di ca ng dn sng trong milimet

f = tn s hot ng trong MHz

fc = ct tn s

R = t l g (tiu chun l g = 3,18 mm)

Hiu qu bc chn SE ca hp bc chn c khe h dng t ong ph

thuc ch yu vo chiu rng v chiu di ca t bo t ong.

Hnh 3.21 ng dn sng hnh lc gic

67

3.7 KIM TRA HP BC CHN TRONG THC T

Trc khi a cc sn phm n ngi tiu dng th qu trnh kim tra

xem hp bc chn c m bo c cc vn TTT hay khng. S

kim tra bc x hp bc chn trong thc t c m t nh sau:

Hnh 3.22 Qu trnh kim tra bc x hp bc chn trong thc t

Qu trnh ny c thc hin trong mt phng kn v kt qu c xc

nh ti mt my phn tch. My phn tch ny c hai cng: cng 1 c kt

ni vi ngun bn trong hp bc chn cn kim tra v cng 2 c kt ni

n mt anten thu. My phn tch c t bn ngoi phng kn o h s

phn x S11 v h s truyn S21. Bc x EMI t hp bc chn s c mt

68

anten nhn c (cch 3m) a v my phn tch tnh ton. Bc x ny

c xc nh theo cng thc [6]:

21 1AF SE V (3.29)

Trong :

AF l h s anten thu

V1 l in p vo ti cng 1

Nu bc x ny nh hn ngng bc x quy nh theo mt tiu chun

yu cu th hp bc chn t yu cu.

3.8 KT LUN CHNG

Bc chn c tc dng ngn chn s pht x bc x in t t cc sn

phm hay mt thnh phn thit b in t khc bn trong hp che chn sang

mi trng bn ngoi hoc ngn chn cc bc x bn ngoi n cc thit b

in t bn trong lp v che chn. Tuy nhin khi bc chn c khe h v mt

l do no , lm gim hiu qu bc chn ca hp. V vy trong chng ny

trnh by cc phng php chia nh khe h, t khe h sao cho t cn tr

dng in cm ng nht, chiu di khe l

69

CHNG 4

M PHNG CI THIN HIU QU

BC CHN M BO TNH TNG THCH IN T

4.1 GII THIU CHNG

Trong chng ny, ti s dng dng phn mm CST (Computer

Simulation Technology) m phng bc x in t trong cc trng hp

khc nhau kim chng li cc vn nu trn. Hp bc chn c kch

thc trong 30cm x14cm x22cm c cc khe h lm bc x in t ra mi

trng xung quanh vt mc cho php, do vy cn c bin php ci thin

hiu qu bc chn ca hp. Trong chng ny, thc hin m phng kim

chng cc phng php ci thin hiu qu bc chn trong cc trng hp c

th nhm lm cho sn phm m bo tnh tng thch in t.

4.2 GII THIU S B V PHN MM CST (Computer Simulation

Technology)

Phn mm m phng CST c ng dng ch yu trong cc lnh vc

sng cc ngn, RF, EMC/EMI, gia tc ht v cc ng dng tn s thp khc

nhau. Chng c s dng trong cc ngnh cng nghip nh hng khng v

tr, t ng ha, t, in t, k thut in, y t...

N cho php m hnh ha cc hin tng in t v nh gi thit k

cho hiu sut ti u. CST MICROWAVE STUDIO ( CST MWS ) l cng

c tin tin m phng 3D nhanh v chnh xc cho cc thit b tn s cao.

N cho php phn tch nhanh v chnh xc ca anten, b lc v EMC vv.

CST EM STUDIO ( CST EMS) l mt cng c s dng cho vic thit k v

phn tch cc tn s tnh v thp ng dng cho ng c, cm bin, thit b

70

truyn ng, my bin p v v bc che chn vv. CST PARTICAL STUDIO (

CST PS) cho php m phng cc ht mang in t do di chuyn, ng dng

cho ng tia cathode vv. CST CABLE STUDIO ( CST CS ) l cng c m

phng tnh ton vn tn hiu v EMC/EMI, ng dng phn tch khai thc

truyn hnh cp vv. CST PCB STUDIO ( CST PCBs ) l cng c m

phng tnh ton vn tn hiu v EMC/EMI trn bo mch in. CST MPHYSICS

STUDIO ( CST MPS ) l cng c dng phn tch ng sut nhit v c

khTrong ti ny phn mm CST c ng dng m phng ci thin

hiu qu bc chn.

Hnh 4.1 Giao din phn mm m phng CST

71

4.3 XY DNG TIN TRNH M PHNG CI THIN HIU QU

BC CHN

Hnh 4.2 S tin trnh m phng

Bt u

Thit lp cc thng s cho hp bc chn

M phng bc x in t hp bc chn kn hon ton

M phng bc x in

t khi hp bc chn c

khe h 4x3cm

M phng bc x in

t khi hp bc chn c

khe h 4x3, 12x0.2,

5x0.2, 0.8x0.2cm

M phng bc x in

t ca hp bc chn

khi chia khe 4x3cm

thnh 15 khe

M phng bc x in

t ca hp bc chn

khi chia khe 4x3cm

thnh 20 khe

M phng bc x in t

khi hp bc chn tn ti

cc khe h cng din tch .

So snh hiu qu bc chn gia cc

trng hp vi nhau.

Nhn xt, nh gi

M phng bc x

in t hp bc chn

c khe h dng ng

dn sng c chiu di

v rng ln lt thay

i

M phng bc x

in t hp bc chn

c mt ng lc gic

v 28 ng dng t ong

c cng thit din l

184mm2

M phng bc x

in t hp bc chn

c khe h dng t ong

vi s lng ng khc

nhau

M phng bc x in t khi hp bc chn

c khe h 0.2x9cm

M phng bc x

in t khi hp bc

chn c khe h

0.2x9cm v c

thm tm chn

cng loi dy

0,5mm

M phng bc x

in t khi hp bc

chn c khe h

0.2x9cm vi tm

chn i.

M phng bc

x in t ca

hp bc chn vi

cc vt liu khc

nhau.

72

4.4 THIT LP CC THNG S HP BC CHN

Hp bc chn c cc thng s c thit lp nh hnh v:

Hnh 4.3 Thng s hp bc chn

Hnh 4.4 Hp bc chn m phng bng phn mm CST

Bc chn c s dng m bo tnh tng thch in t c dng

hnh hp ch nht v c cu to t 6 mt lm t vt liu bng Nhm v cc

vt liu khc c trnh by c th phn sau. Bc chn c 5 mt dy l

0.635 cm v mt mt c b dy l 0.05 cm. Cc khe h c to ra trn mt

Nhm c b dy 0.05cm. Kch thc ca mt bn trong hnh hp ch nht l

22 14 30 cm.

73

Khoan l pha trn hp bc chn nh hnh 4.3 cp ngun vo hp

thng qua cp ng trc c tr khng 50 v bn knh ca li dn in trong

l 0.085 inch. T cp ng trc ny, kt ni vi li ca n bng mt si dy

dn bng ng c ng knh l 0,16cm c ko xung mt y ca hp

m bo bc x in t t dy dn lan truyn u khp ni bn trong hp.

Cui si dy dn ni vi mt in tr 47. in tr ny tng ng nh

qu trnh tn tht nng lng do nhit nhm ph hp vi thc t kt qu

m phng c khch quan hn.

Di tn s m phng s c chn t 700 MHz n 1700 MHz. Khe h

trong m phng ny s dng vi kch thc 4x3 cm, 9x0.2, ... ng thi hp

bc chn cng m phng khi tn ti khe h dng ng dn sng. Cc trng

hp c th s c trnh by cc phn sau.

4.5 M PHNG BC X IN T HP BC CHN KN HON TON

Bc chn c s dng ci thin hiu qu bc chn c dng hnh hp

ch nht vi kch thc v cc thng s nh trnh by phn trn. Trong

trng hp ny bc chn kn hon ton v bc x in t t hp ra mi

trng xung quanh ti v tr cch 3 mt c m phng bng phn mm CST

nh sau:

Hnh 4.5 th bc x in t ca hp kn ti v tr cch hp 3m

74

T th ta thy, bc x in t o ti v tr cch hp 3 mt l -80 dB

[microV/m] y l ngng phn tch ca phn mm. V vy cc gi tr bc x

in t rt nh ny th hu nh n khng lm nh hng n hot ng ca

cc thit b in t khc xung quanh. iu ny cho thy hp che chn kn

hon ton th hiu qu bc chn s rt tt, m bo vn tng thch in t.

4.6 KT QU M PHNG BC X IN T HP BC CHN C

KHE H

4.6.1 M phng bc x in t khi hp bc chn tn ti khe h 4x3cm

Trong trng hp yu cu bc chn phi c khe h phc v cho mt

mc ch no , i vi trng hp ny bc chn cha khe h 4x3cm c

din tch 12cm2.

th bc x in t o ti v tr cch hp 3m nh sau:

Hnh 4.6 th bc x in t ti v tr cch hp 3m

T kt qu ta thy, khi bc chn c khe h th bc x in t ra mi

trng xung quanh s tng cao, hiu qu bc chn gim i rt ng k. Trn

th ta thy, cng in trng u trn 70 dB[microV/m], c di tn

75

trn 110dB[microV/m], cng trng ln nh vy s gy nhiu cho cc

thit b ln cn. Lc ny, thit b khng m bo tnh tng thch in t. V

vy ta cn c gii php nng cao hiu qu bc chn ca hp khi c khe h

vi din tch l 12cm2. Bc x in t o ti v tr cch hp 3m lc ny phi

nh hn mc ngng cho php, theo chun A l 57dB[microV/m] thit b

m bo tnh tng thch in t. Gii php s c trnh by phn sau.

4.6.2 M phng bc x in t khi hp bc chn c khe h 4x3,

12x0.2, 5x0.2, 0.8x0.2cm.

Trong mc ny, ta tin hnh m phng o cng trng bc x ti

v tr cch hp bc chn ba mt khi hp bc chn c cc khe h ln lc l

4x3, 12x0.2, 5x0.2, 0.8x0.2.


khi hp bc chn c khe h 4x3cm v 12x0.2cm

T hnh 4.7 ta thy, hp bc chn c khe h 4x3cm c din tch khe h

ln hn nhiu so vi din tch khe 12x0.2cm nhng bc x in t khi c khe

12x0.2cm li ln hn trng hp c khe 4x3cm. iu ny chng t rng bc

x in t t hp ra mi trng xung quanh ph thuc ch yu vo chiu di

hn l chiu rng ca khe nh cp chng ba.

76


khi hp bc chn c khe h 12x0.2cm, 5x0.2, 0.8x0.2

Theo hnh 4.8 cho ta thy, khi cc khe h c cng chiu rng v chiu

di khe thay i th cng bc x in t ca hp ra mi trng xung

quanh cng thay i theo. Trong trng hp ny bc x in t tng theo

chiu di khe h. Bi v khi chiu di khe h tng ln th chng lm cho dng

in cm ng chy trn mt cc tm chn s cng b cn tr. Kt qu lm cho

kh nng chng li cc bc x in t ca cc tm chn cng thp nn hiu

qu bc chn ca hp cng gim theo. Trn th hnh 4.8 cho thy, ch

trng hp bc chn c khe 0.8x0.2 gy bc x in t ra mi trng xung

quanh l di mc cho php theo chun A, m bo tnh tng thch in t.

Cc trng hp cn li bc x in t u ln hn mc cho php nn ta phi

c bin php lm gim bc x in t m tnh tng thch in t. iu

ny s c trnh by phn sau.

4.6.3 M phng bc x in t ca hp bc chn khi chia khe

4x3cm thnh 15 khe

Trong mc ny, ta ci thin hiu qu bc chn ca hp bc chn khi

hp c khe h vi din tch 12cm2. Khi hp bc chn yu cu phi c din

tch khe h l 12cm2 nhm phc v cho mt mc ch no , hnh 4.5 cho ta

77

thy khi hp bc chn c khe 4x3cm s bc x in t ra mi trng xung

quanh vt qu mc cho php. V vy ta s nng cao hiu qu bc chn bng

cch chia nh khe 4x3cm thnh 15 khe 0.2x4. Lc ny tng din tch khe h

l nh nhau u 12cm2. Cng in trng o ti v tr cch hp 3m trong

hai trng hp nh hnh 4.9.


khi hp bc chn c khe h 4x3cm v 15 khe 0.2x4

T hnh 4.9 cho thy cng trng trong trng hp 15 khe 0.2x4

nh hn nhiu so vi trng hp 1 khe 4x3cm. Cng in trng nhn

chung t mc trn 80dB[microV/m] gim cn di 57dB[microV/m] tr di

hp cn di 1400Mhz. V vy ta thy hiu qu bc chn c ci thin

ng k. Bi v khi khe ln c chia thnh nhiu khe nh lm cho cc dng

in cm ng chy trn cc tm chn t b cn tr hn so vi trng hp khe

h ln. Tuy vy hiu qu bc chn ca hp trong trng hp ny vn cha t

yu cu do bc x in t trong vng ln cn ti tn s 1350MHz u trn

57dB[microV/m]. Do vy ta cn tip tc ci thin hiu qu bc chn ca hp

c trnh by trong phn tip theo.

78

4.6.4 M phng bc x in t ca hp bc chn khi chia khe

4x3cm thnh 20 khe

Trong mc 4.8 ta thy, Hiu qu bc chn ca hp khi c khe h 12cm2

c chia thnh 15 khe c ci thin ng k nhng bc x in t t

hp ra mi trng xung quanh vn cn mnh, khng m bo tnh tng

thch in t. V vy ta cn tip tc ci thin hiu qu bc chn bng cch

chia khe ln thnh 20 khe b tr nh hnh 4.10. Hng di 10 khe c kch

thc 0.2x3 v hng trn 10 khe 0.3x2cm. Kt qu m phng nh hnh 4.11

Hnh 4.10 Hp bc chn vi 15 khe v 20 khe h c tng din tch 12cm2


khi hp bc chn c 15 khe v 20 khe c tng din tch 12cm2

T kt qu m phng ta thy, khi hp bc chn c khe h 12cm2 c

chia thnh 20 khe nh hnh 4.10 th cng trng o c ti khong cch

79

3m t hp thp hn so vi trng hp 15 khe v di 57dB[microV/m]. Bc

chn t yu cu, m bo tnh tng thch in t. Hiu qu bc chn ca

hp c nng cao bi v chiu di khe lc ny c chia ngn li nn bc

x in t hp ra mi trng xung quanh cng b yu i. Tuy nhin nu ta

chia cc khe c cng kch thc 0.2x3, t chng song song v thng hng

vi nhau nh hnh 4.12 th hiu qu bc chn ca hp c nng cao hn.

Kt qu m phng thu c nh hnh 4.13.

Hnh 4.12 Hp bc chn vi 20 khe h c tng din tch 12cm2

vi cch b tr khe v kch thc khe khc nhau

Hnh 4.13 th bc x in t ti v tr cch hp 3m khi hp bc chn tn

ti 20 khe h c tng din tch 12cm2 vi cch b tr khe v kch thc khe

khc nhau

th hnh 4.13 chng t, khi hp bc chn c 20 khe cng kch thc

0.2x3cm v t chng song song v thng hng th hiu qu bc chn ca hp

80

tt nht so vi cc trng hp cn li. Bi v khi b tr nh vy, s cn tr

dng in cm ng ca cc khe l thp nht.

4.6.5 M phng bc x in t khi hp bc chn c cc khe h

cng din tch

Trong phn ny, ta m phng bc chn c cc khe h c t l di rng

khc nhau nhng c cng din tch. C th bc chn c m phng khi c

cc khe vi kch thc 9x0.2, 3x0.6, 0.6x3, 0.2x9 chng u c cng din tch

l 1.8cm2. Kt qu m phng nh hnh 4.14.


khi hp bc chn c khe h 9x0.2, 3x0.6, 0.6x3, 0.2x9cm

T th ta thy bc x in t thay i theo t l di rng ca khe.

Cng in trng tng dn theo chiu di phng ngang ca khe. iu

ny cho thy, bc x in t khng ch ph thuc vo kch thc ca khe h

m cn ph thuc vo t l ca khe h. Mc d khe h c cng din tch

nhng nu khe h no c s cn tr vi dng in cm ng chy trn cc tm

chn cng nh th hiu qu hp bc chn s cng cao. Trong trng hp hp

bc chn i hi phi c khe h vi din tch 1.8cm2 th ta to khe h c kch

thc 0.2x9cm, lc ny cng trng o ti v tr cch hp 3m u nh

81

hn 47dB[microV/m] l t yu cu theo chun B, m bo tnh tng thch

in t.


BNG CC VT LIU KHC NHAU

Trong phn ny, hp bc chn c khe h 0.2x9 c m phng vi cc

vt liu khc nhau . Kt qu m phng thu c nh hnh 4.15.


khi hp bc chn c khe h 0.2x9cm vi cc vt liu khc nhau

T th hnh 4.15 cho thy, khi hp bc chn thay i vt liu

th hiu qu bc chn ca hp cng thay i theo. Hp bc chn lm bng

kim loi hoc hp kim th cng in trng o c ti v tr cch

hp 3m u nh hn 50dB[microV/m] thp hn nhiu so vi trng hp

hp bc chn lm bng vt liu cch in nh g, mica. Cng in

trng o c ti v tr cch hp 3m khi hp bc chn lm vt liu cch

in nh g hoc mica u trn 120 dB[microV/m]. th cho thy ch

c s khc bit ng k gia vic s dng vt liu dn in hay cch in.

Cn gia cc vt liu dn in hoc vt liu cch in vi nhau th t c

s khc bit. Hiu qu bc chn ca hp lm bng vt liu dn in r

82

rng cao hn nhiu so vi hp lm bng vt liu cch in. Bi v bc x

in t lan truyn qua mi trng cch in, trong khi n b hp th v

phn x li khi gp thnh hp bng vt liu dn in.

4.8 KT QU M PHNG BC X IN T HP BC CHN C

KHE H DNG NG DN SNG

4.8.1 M phng bc x in t hp bc chn c khe h dng ng

dn sng c chiu di v rng ln lc thay i

Trong phn ny, ta s xem xt hiu qu bc chn ca hp khi c khe h

dng ng lc gic. C th trong mc ny ta m phng trng hp ng c

g=6.36mm cn chiu di ng ln lc l 3.145, 6.35, 9.54, 25.44mm. Kt qu

m phng nh hnh 4.16.

Hnh 4.16 Hp bc chn c khe h dng ng lc gic

Hnh 4.17 th bc x in t ti v tr cch hp 3m khi hp bc chn

c khe h dng ng lc gic c g = 6.36mm v chiu di ng thay i

83

T th hnh 4.17 cho thy khi hp bc chn c khe h dng ng dn

sng th bc x in t t hp gim ng k so vi hp c khe h cng din

tch. Bi v khi sng truyn qua ng lc gic s b suy hao t bit khi sng c

tn s di tn s ct. ng thi kt qu m phng cng cho thy, cc ng c

cng g=6.36mm tc c cng din tch thit din nhng ng cng di th bc

x in t cng thp.

Khi hp bc chn c khe hnh lc gic vi cnh g=6.36mm th bc x

in t t hp ra mi trng xung quanh mnh hn mc cho php (trn

57dB[microV/m] ti khong cch 3m), khng m bo tnh tng thch in

t. Khi khe hnh lc gic trn c dng ng vi chiu di d= 6.35mm th hiu

qu bc chn ca hp c ci thin ng k. Cng trng o ti v tr

cch hp 3m di 57dB[microV/m] t chun A. Khi chiu di ng bng

9.54mm th hiu qu bc chn ca hp cng c ci thin. Lc ny, cng

trng o ti v tr cch hp 3m di 47dB[microV/m] t chun B. Cc

thit b c m bo tnh tng thch in t.

Khi di ng lc gic d=25.44mm, t l di rng l 4:1 th hiu qu

bc chn ca hp c ci thin ng k. Cng trn

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