B GIO DC V O TO

TRNG I HC BCH KHOA H NI

VIN IN ------- -------

BO CO TIU LUN [1]

ti:

MN CHN IN T

Ging vin ging dy : TS. Nguyn Vit Sn

Hc vin : Nguyn An Ton

SHHV : CA140027

Chuyn ngnh : iu khin v T ng ha

Lp : 14AKTH.KH

H Ni, thng 02/2015

MN CHN IN T

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MC LC

Chng 1:TNG QUAN V MN CHN ................................................................................ 3

Chng 2:HIU SUT CHE CHN .......................................................................................... 9

Chng 3:HIU SUT CHE CHN VI NGUN TRNG XA .......................................... 12

3.1. Tnh chnh xc ................................................................................................................ 12

3.2. Tnh gn ng ................................................................................................................ 13

Chng 4:HIU SUT CHE CHN VI NGUN TRNG GN ....................................... 17

4.1. Khc nhau gia trng gn vi trng xa: ...................................................................... 17

4.2. Ngun in trng: ........................................................................................................ 21

4.3. Ngun t trng: ............................................................................................................ 22

Chng 5:CHE CHN T TRNG TN S THP ............................................................ 23

Chng 6:HIU NG KHE H ............................................................................................... 28

TI LIU THAM KHO ......................................................................................................... 31

MN CHN IN T

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Chng 1

TNG QUAN V MN CHN

Thut ng shield thng cp n mt v1 kim loi m bao quanh hon ton mt

thit b in t hoc mt phn ca thit b .

Mt mn chn c 2 mc ch, nh minh ha trong hnh 1.1. Th nht, nh th hin

trong hnh 1.1a, mn chn l ngn chn nhng bc x t cc thit b in t hoc mt

phn ca cc thit b in t, bc x ra bn ngoi thit b. Mc ch y l ngn chn

1 enclosure

(a) (b)

(c)

Hnh 1.1 Minh ha vic s dng v bc (mn chn)

(a) ngn bc xa ra ngoi v (b) ngn bc x bn ngoi vo; (c) phng bc mn chn.

MN CHN IN T

Trang4

nhng bc x gy ra bi thit b m khng tun th cc gii hn v bc x hoc ngn

chn cc thit b gy nhiu cc thit b in t khc. Mc ch th hai ca mn chn, nh

th hin trong hnh 1.1b l ngn cc bc x t bn ngoi n thit b t cc thit b

in t khc, ci m c th gy nhiu cho thit b. Mn chn c th c s dng gim

nhy vi cc tn hiu bn ngoi nh cc raa cng sut cao, i radio v TV. Bc nh

v mt phng bc mn chn c s dng th nghim v EMC1 c th hin trong

hnh 1.1c.

Nh vy, mn chn l mt ro cn i vi vic lan truyn trng in t. Ta c th

nh gi hiu sut ca mt mn chn qua t l gia ln cng in trng (t

trng)2 n3 mn chn vi ln cng in trng (t trng) truyn qua mn

chn. Ngoi ra, chng ta c th xem y l t s gia cng in trng (t trng)

truyn n thit b in t vi cng in trng (t trng) b mn chn chn li4.

Theo ngha th hai ny, chng ta c th lng t ha hiu sut ca mn chn nh mt gi

tr suy gim trung gian5. Cc khi nim trn mang tnh nh lng v ngha ca thut

ng mn chn v s c nh lng chnh xc hn nhng mc sau.

Vic tnh ton gi tr l tng ca hiu sut mn chn s c trnh by trong cc

mc sau v s c c mt vi gi tr tng i ln ca hiu sut mn chn n hng

trm dB. Hiu sut mn chn 100dB ngha l cng in tng (t trng ) n b

gim 100.000 ln khi n ra khi mn chn! c c hiu sut mn chn l tng v

cc ln nh trn, th mn chn phi bao quanh hon ton thit b in t v phi khng

b xuyn qua6 bi l trng, ng ni, khe h hoc cp. Bt k mt s xuyn qua no

khng ng cch, u c th lm gim mnh m hiu qu ca mn chn.V d, xt mt

hp kim loi kn (mn chn) c mt dy dn xuyn qua n nh th hin trn hnh 1.2a.

Gi s mt ngun bc x bn cnh nh mt ng-ten bc x mt trng in t. Trng

in t ny s mc vng qua7 dy dn, to ra mt dng in trong dy dn. Dng in

ny s chy t do qua v thit b v mc vng qua cc linh kin in t bn trong. V

ngc li, nhiu bn trong mn chn s mc vng qua dy dn, chy trn dy dn ra

ngoi v thit b, ri bc x. Mt mn chn b xuyn qua kiu ny s loi b hon ton 1electromagnetic compatibility 2 electric (magnetic) field 3 incident 4shield removed to that with the shield in place 5 insertion loss 6 penetration 7 coupled

MN CHN IN T

Trang5

hiu qu ca mn chn. Vic xuyn cp nh vy phi c x l ng cch m bo

hiu qu ca mn chn. Mt s phng php ph bin nh dng b lc ti im vo / ra

hoc s dng cp c v bc vi mn chn bc bn ngoi v c ni vi mn chn ca

thit b nh th hin trong hnh 1.2b [2]. Ta thy, cp s c dng in cm ng bi trng

bn ngoi1. n gin ch cn ni mn chn ca cp vi mn chn ca thit b bng mt

dy dn nh trong hnh 1.2b th dng in trn mn chn cp s c dn vo b

1external field

Hnh 1.2 Nhng nguyn nhn nh hng n hiu sut mn chn trong thc t

MN CHN IN T

Trang6

mt ca thit b, v tr m chng c th, mt ln na bc x n cc linh kin bn dn bn

trong thit b, nh vy gim bt hiu qu ca mn chn thit b.

Mt ln na, iu ngc li cng ng; nu mn chn cp khng c ni bn

ngoi vi v thit b (mn chn), dng in nhiu bn trong v bc c th chy qua v ra

bn ngoi mn chn, ni m chng c th bc x. Loi b mt mn chn ton phn1 t

cp ngoi vi (bn ngoi) thc s c th lm gim s bc x ca cp! Mt dn chng cho

nhn nh ny l cp my in Centronics m ni gia PC vi my in ny. Nh mt quy

tc chung, mt mn chn bc xung quanh si cp cha hn l gim bc x ca si cp.

Gii thch hin trng ny c minh ha trong hnh1.2c. c hiu qu che chn ca

mn chn si cp trong vic gim bc x ca si cp, th mn chn si cp phi c

ni vi mt im in p khng (mt im t l tng)2. Nu in p ca im cui3 ca

mn chn v im t logic ca PCB4 l khc nhau, nh minh ha trong hnh 1.2c, th

chng ta s to ra mt ng-ten n cc (mn chn cp). Nu chiu i ca mn chn cp

hn 01

4 , mn chn tr thnh mt b bc x hiu qu. Cp ngoi vi nh cp my in cho

PC thng c chiu di yu cu 1,5m, khong mt phn t bc sng 50MHz. Cng

hng trong cc bc x ca mt thit b do dng common-mode5 trn cc loi cp ngoi

vi thng xuyn c quan st trong phm vi tng s 50-100MHz. Ngt kt ni cp

ngoi vi vi thit b thng loi b nhng cng hng ny v lm ni bt6 bc x ca

chng.

S thm nhp ph bin khc l t mt l trng hay khe h trn tng chn.

Trng bn trong hoc bn ngoi mn chn7 s bc x qua khe h ny, nh vy lm gim

hiu qu ca mn chn. iu ny c minh ha bi mt nh l quan trng bit l

nguyn l Babinet [3]. Trong phn minh ha nguyn l ny, xt mt khe m c ct

trn mt mn dn in l tng nh th hin trn hnh 1.3a. Mt dy dn c ni qua

vo hai im ca khe h, v kch thch n. Trng xa8 m c bc x bi ng-ten khe

1 overall shield 2 a zero-potential point (an ideal ground) 3 pigtail 4Printed Circuit Board 5 common-mode: Chokes (mc 5.9) (nhiu tc ng vo) 6accentuated 7 fields interior to the shield or exterior 8near field: l trng bc x (ca ng-ten) trong vng bn knh 1 bc sng tnh t ng-ten pht. far field: l vng xa hn trng gn ca mt ng-ten.

MN CHN IN T

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h c k hiu l sE , sE , sH v sH . By gi th xt cu trc b th hin trn hnh

1.3b, n bao gm vic thay th mn kim loi bng khng gian t do v thay th khe h

bng mt vt dn1 l tng c hnh dng ging nh khe h. ng-ten c kch thch mt

ln na, dy dn c ni n 2 na ca cu trc b. Trng xa bc x bi cu trc

b ny c k hiu l cE , cE , cH and cH .

Nguyn l Babinet: Trng xa bc x bi mn ban u vi khe h v trng xa

bc x bi cu trc b c quan h vi nhau nh sau [3]:

s cE H (1.1)

s cE H (1.2)

20

cs

EH

(1.3)

1 conductor

Hnh 1.3 Minh ha v hiu qu ca mt khe h trong che chn

(minh ha nguyn l Babinet)

MN CHN IN T

Trang8

20

c

s

EH

(1.4)

iu ny minh ha rng khe h c th c bc x hiu dng1 bng ng-ten c kch

thc dn l kch thc ca khe h. iu ny cung cp ci nhn su sc2 v khe h hiu

dng trong mn chn lm gim hiu qu ca mn chn. V d, gi s rng khe h trong

hnh 1.3a c kch thc bng mt na bc sng, 0 / 2L . Nguyn l Babinet quy nh

rng cc tnh cht bc x ca khe h ny s ging nh (s hon i gia E v H) mt

ng-ten lng cc cng chiu di khe h nh minh ha trong hnh 1.3b. Ta c bit

rng na bc sng di ca ng-ten lng cc nh trong hnh 1.3b vi 0 / 2L l cu

trc bc x rt hiu qu. Do , mt khe h c kch thc nh th cng bc x rt hiu

qu. Ta thy rng nu chiu rng w khe h nh hn, th khe h (v lng cc b3) vn

bc x hiu qu. Do , ta khng nn c suy ngh rng4 Nu ta khng th nhn thy nh

sng qua khe h, th n s khng bc x tt. iu ny tt nhin khng ng. Khe h

xut hin nhiu ni trn v bc (mn chn)5. Np to ng dn vo bn trong v bc

(mn chn) v chng c khe h xung quanh. Do , khe h ca chng c gia cng

bng cch t min lt dn in6 ti khe h hoc beryllium finger stock.

Tm li, mn chn thc t khng th c tin cy hon ton loi b cc bc x

ca thit b. C nhiu v d ca cc thit b in t m khng dng v bc (mn chn),

nhng chng vn c th tun th cc quy nh gii hn v bc x v dn bc x7. Cc

my nh ch in t v my in c bc trong v nha. Chi ph v tnh khng thc t

ca mt v chn kim loi bc b vic s dng chng trong cc thit b. Mt khc, my

tnh c nhn v cc siu my tnh ln s dng v bc kim loi. X l mt cch hiu qu

tt c cc thm nhp cung cp hiu qu s dng ca cc mn chn cho cc loi thit b.

Nh mt nguyn tc c bn, nh thit k EMC s kt hp cc nguyn tc thit k EMC

cng nhau trong mt thit b d cho thit b c c che chn hay khng.

1 effective radiators 2 considerable insight 3 complementary dipole 4 we should not be caught in the trap of thinking that 5shielded enclosure 6 placing conductive gasketing material 7 conducted emissions

MN CHN IN T

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Chng 2

HIU SUT CHE CHN

Trong mc ny s cp n hiu sut che chn ca mt mn chn kim loi. Cc

khi nim chung v hiu qu che chn c cp trn. Vi cc khi nim ny, ta

xt cc vn chung ca ro chn kim loi c dy t, in dn sut , hng s in

mi tng i 1r , v t thm tng i r , th hin trn hnh 2.1. Khi c sng

in t ti ro chn ny, th sng phn x c to ra, v mt phn sng ti truyn qua

ro chn. Hiu sut che chn ca ro chn c nh ngha theo cng in trng,

n vi decibel (dB) l:

10

20log

i

t

ESE

E (2.1)

Ch rng, cng thc ny s cho mt kt qu dng, v cng in trng ti

lun ln hn cng in trng ra khi ro chn. V d, hiu sut che chn l 120dB

ngha l ln ca cng in trng truyn qua b gim so vi ln ca cng

in trng ti l 106. Mt s nh ngha v hiu sut che chn l t s gia cng

in trng truyn qua trn cng in trng ti. nh ngha ny s cho kt qu m

Hnh 2.1 Minh ha hiu qu che chn ca ro chn dn in c sng phng ng nht n b mt

MN CHN IN T

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v n v l dB, n l m ca (2.1). V cng t trng, hiu sut che chn, c th

c nh ngha l:

10

20log

i

t

HSE

H (2.2)

Nu cng t trng ti l sng phng ng nht v mi trng mi bn ro

chn l ng nht, th hai nh ngha l ging nhau, nn cng in trng v t

trng c lin quan bi tr khng c trng1 ca mi trng cho sng phng ng nht.

i vi trng gn hoc l mi trng khc nhau hai bn bin th chng khng bng

nhau. Tuy nhin, nh ngha v hiu sut che chn trong quan h cng in trng

(2.1) thng c ly lm tiu chun cho c hai tnh hung.

C mt vi hin tng lm gim cng in trng ti khi n i qua ro chn.

Hy xt s trn hnh 2.2, th hin kt qu ny.

Kt qu u tin l phn x trn mt bn tri ca ro chn. Mt phn ca cng

in trng ti m b phn x l do h s phn x b mt. Mt phn sng m vt qua b

mt ny th tip tc i qua tng chn. Khi n i qua mi trng dn ny, th bin ca

n gim dn theo hm ze , trong l hng s suy gim ca vt liu. iu ny c

gi l tn tht hp th. i vi vt liu mn chn m dn in tt, hng s suy gim

lin quan n dy lp mt ca vt liu , l 1 / v 1 / f , trong

1 intrinsic impedance

Hnh 2.2 Minh ha v phn x nhiu ln trong mt mn chn

MN CHN IN T

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l t thm ca ro chn v l in dn sut ca n. V vy, cng ca in

trng b suy gim l /ze . Nu dy t ca ro chn ln hn nhiu so vi dy lp

mt1 ca vt liu ro chn tn s sng ti, th sng m c truyn qua mt u tin

th b suy gim rt ln khi n p vo mt bn phi. Mt phn ca sng ti ny b phn

x, v mt phn c truyn qua ro chn vo mi trng bn phi ca ro chn. Phn

phn x ca sng ny b truyn tr li ro chn v p vo mt u tin, c th ti t kim

loi. Mt ln na mt phn ca sng ny c truyn qua mt bn tri v thm vo tng

cng in trng phn x bn tri mi trng, v mt phn b phn x v tip tc

n bn phi. Phn ny li tip tc suy yu khi n truyn qua ro chn. Mt ln na, n i

qua ro chn v p vo mt bn phi, mt phn b phn x v mt phn c truyn qua

mt bn phi. Phn truyn qua mt bn phi thm vo tng cng in trng m

c truyn qua mn chn. Qu trnh c tip tc nh vy, nhng cng t trng

phn x v truyn qua b sung b suy yu dn bi s di chuyn ca chng trong ro chn

dn in. Nu mn chn c thit k c dy m n ln hn nhiu so vi dy

lp mt ca vt liu tn s ca cng in trng ti cho trc, th c mt vi h

qu n s phn x lin tc b mt bn trong ca ro chn. S phn x v truyn qua

nhiu ln ny c c th b qua do dy mn chn l ln hn nhiu so vi dy lp

mt, v ch c phn x v truyn qua ban u bn tri v phi b mt l cn c xem

xt. Tuy nhin, hiu sut che chn a ra (2.1) c th c chia thnh ba thnh phn

tiu biu l tn tht phn x, tn tht hp th v phn x nhiu ln ( n v dB):

dB dB dB dBSE R A M (2.3)

Trong : R c trng cho tn tht phn x gy ra bi phn x b mt bn tri v

phi, A c trng cho tn tht hp th ca sng khi n i qua ro chn, v M c trng

cho hiu ng ph do phn x v truyn qua nhiu ln. Ta thy, cc phn x li s to ra

cng in trng truyn qua b mt bn tri v s thm vo cng in trng

ban u. V vy, vic phn x nhiu ln M s l mt s m v s lm gim hiu sut che

chn (v R v A s dng). By gi, ta bt tay vo vic nh lng nhng yu t gp phn

vo hiu sut che chn ca mt ro chn.

1 skin depth

MN CHN IN T

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Chng 3

HIU SUT CHE CHN VI NGUN TRNG XA

Trong mc ny, ta s gi s ngun trng in t m ti ro chn l xa so vi

mn chn th trng in t ti xem nh sng phng ng nht1. Trc tin, ta s xc

nh cch tnh chnh xc hiu sut che chn, ri s xc nh mt cch gn ng m vn

mang li kt qu tng t cho mn chn m c to t vt dn tt, vi dy t rt ln

so vi dy lp mt tn s sng ti.

3.1. Tnh chnh xc

tnh chnh xc hiu sut che chn ca ro chn kim loi, ta xem li hnh 2.1.

Mn chn dn in c dy t, in dn sut , hng s in mi 0 , t thm .

Mn chn dn in c sng phng ng nht ti mt bn tri ca n. Gi s mi trng

hai bn ca mn chn l khng gian t do2. Chn h ta vung gc nh hnh 2.1, vi

b mt bn tri cha hai trc xy c z = 0 v b mt bn phi c z = t.

Vi gi thit rng ro chn c to t vt liu dn in tt, nn tr khng c

trng ca vt dn l rt nh so vi khng gian t do: 0 . V chng ta cng gi thit

rng dy lp mt nh hn nhiu so vi dy t ca ro chn. Khi , hiu sut che

chn (n v dB) theo (2.1) l:

/010 1020log 20log4dB

dB

tdB dB

A

R

SE e M

(3.1)

Trong : 0 l tr khng c trng ca vng khng gian t do:

000

(3.2)

l tr khng c trng ca mn chn:

0

j

j

(3.3)

1uniform plane wave 2free-space

MN CHN IN T

Trang13

l dy lp mt vt liu ro chn tn s sng ti:

1

f

(3.4)

MdB l tn tht phn x nhiu ln:

2 / 21020log 1t j t

dBM e e (3.5)

Trong cng thc (3.1) th cc thnh phn do phn x, do hp th v do phn x

nhiu ln c tch bit vi nhau nh cng thc (2.3).

3.2. Tnh gn ng

By gi, ta xt cc kt qu trc (3.1) vn vi gi thuyt rng ro chn c to

t vt liu dn in tt 0 , v b dy ca ro chn l rt ln so vi dy lp mt

tn s ca sng ti t ,

T cng thc (3.1), ta c tn tht phn x:

01020log 4dBR

(3.6)

Ta tnh xp x tr khng c trng ca vt liu dn in tt nh sau:

1

1 ( / )

j

j

j

j

j

(3.7)

45

(ro chn l vt liu dn in tt / 1 )

V: 000

(3.8)

Thay vo (3.6), ta c:

100

120log

4dB

r

R

(3.9)

MN CHN IN T

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Trong , ta gi s 0 r v 0 . Vi in dn sut ca kim loi, n thng

c tham chiu vi ng, cht m c in dn sut 75,8.10Cu S/m. V vy, in dn

sut ca cc kim loi khc c vit l Cu r , trong , r l in dn sut tng

i so vi ng. Thay vo (3.9), ta c:

10168 10logr

dB

r

Rf

(3.10)

Ta thy rng, tn tht phn x rt ln tn s thp v in dn sut ca kim loi

ln. Tng t, vt liu t tnh 1r , lm gim tn tht phn x. Tn tht phn x gim

vi tc -10dB/decade vi tn s. V d, xt mt mn chn c lm t ng ( 1r ).

Tn tht phn x 1kHz l 138dB. 10MHz, tn tht phn x l 98dB. Mt khc, l

thp c 1000r v 0,1r . 1kHz, tn tht phn x l 98dB, v 10MHz th n b

gim xung 58dB.

T cng thc (3.1), ta c tn tht hp th:

/1020logt

dBA e (3.11)

Ta cng c th c n gin ha (3.11). Ta c dy lp mt:

1

f

0,06609

( )r r

mf

(3.12)

Thay (3.12) vo (3.11), ta c:

1020

logdBt

A e

8,686t

(3.13)

131,4 r rt f

Cng thc (3.13) cho thy rng tn tht hp th tng theo f thang o dB khi

tn s tng. V vy, tn tht hp th tng kh nhanh khi tn s tng. Vt liu st t c

1r nn tn tht hp th ln hn ng (gi thit rng 1r r ). Tn tht hp th c

th c hiu l t l gia b dy ca mn chn v dy lp mt, t (3.13) ta c:

MN CHN IN T

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8,686

dB

tA

8,7 dB (khi 1t

)

17,4dB (khi 2t

) (3.14)

iu ny cho thy tm quan trng ca dy lp mt trong tn tht hp th.

Ta thy rng, tn tht phn x l mt hm t l vi /r r , trong khi tn tht hp

th li l mt hm t l ca r r . Bng 3.1 th hin cc h s ca chng khi vt liu thay

i.

Hnh 3.1 th hin cc thnh phn trong hiu sut che chn ca mt tm ng c b

dy 20mil khi tn s thay i t 10Hz n 10MHz. Ta thy rng, tn tht hp th tri hn

hn tn s trn 2MHz. Hnh 3.2 th hin cc thnh phn trong hiu sut che chn ca

mt tm thp c b dy 20mil. D liu ny c v t 10Hz n ch 1MHz. Ch rng

tn tht phn x ca kim loi ny tri hn hn tn s di 20kHz. Nhng s liu ny

cho thy rng tn tht phn x l thnh phn ng gp ch yu vo hiu sut che chn

tn s thp cho c mn chn bng vt liu hp kim ca st hoc kim loi mu. cc tn

s cao hn th vt liu hp kim ca st tng tn tht hp th v hiu sut che chn cng

tng. Tm li, cng in trng truyn qua ch yu qua mt th hai, trong khi

cng t trng truyn qua ch yu mt th nht, v vy hp th l quan trng nht

Bng 3.1

MN CHN IN T

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gim cng t trng.

Hnh 3.2 Hiu sut che chn ca mt tm thp c dy 20 mil

Hnh 3.1 Hiu sut che chn ca mt tm ng c b dy 20mil

MN CHN IN T

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Chng 4

HIU SUT CHE CHN VI NGUN TRNG GN

cc phn tch trc v hiu sut che chn gi thit l sng phng ng nht

n b mt ca mn chn. iu ny l do gi thit rng mn chn t vng trng xa

ca ngun trng in t ti1. Trong mc ny, ta s xt ngun trng gn2. Ta s kho

st cc k thut da vo mn chn vi loi ngun ny; d l ngun cng t trng

hay ngun cng in trng. Phi nhn mnh rng, trng gn th cu trc phc tp

hn nhiu so vi trng xa (trng m n gin v sng phng ng nht ging nhau).

Do , phn tch hiu ng ca tm phng, ro chn dn in trng gn l mt qu

trnh rt phc tp.

Tht khng hp l khi mong i c mt cng thc n gin v chnh xc cao cho

che chn trng gn nh t c (chnh xc) cho che chn trng xa m cp

mc trc. Kt qu sau l kt qu xp x gn ng (kt qu rt phc tp). Trung tm ca

phng php gn ng ny l thay th tr khng c trng ca khng gian t do

0 0 0/ , bng tr khng sng3 wZ , da vo lng cc Hertz (lng cc in)

4 v

vng t trng nh (lng cc t)5. Mt d y l mt xp x hi th, nhng n c

nghin cu v cho kt qu kh chnh xc nu chng ta lm mt s thay i n gin.

4.1. Khc nhau gia trng gn vi trng xa6:

iu quan trng l phi phn bit gia cc loi ngun khi chng gn mn chn.

Ta thy rng, im cch ngun ny xa, trng in t ging nh sng phng trong

(1) cc thnh phn ca vng trng xa E v H l vung gc vi nhau v (2) t s

gia hai vector trng in t l tr khng c trng ca mi trng 0/E H . Trong

vng trng gn ca ngun ny th khng cn tha mn nhng iu kin trn na. c

bit, cn phi v tr hn 03 th mi gi c hai tnh cht trn. Ni chung, vng trng

gn c nhiu thnh phn trng in t hn vng trng xa. Hn na, cc thnh phn

1incident field 2 near-field sources 3 wave impedence 4 Hertzian (electric) dipole 5 small magnetic loop (dipole) 6 near field versus far field

MN CHN IN T

Trang18

trng in t khng bin i n gin nh nghch o khong cch 1/ r , nhng cng

ph thuc vo 21 / r v 31 / r .Mt ch tiu hp l xc nh bin trng gn / trng xa

s l ni m t s E trn H xp x bng 0 . T s ny c gi l tr khng sng:

wE

ZH

(4.1)

Tr khng sng c tnh theo cng thc:

2 3

0 0 00

20 0

1 1

( ) ( )1

( )

w

j

r r rZ

j

r r

(4.2)

Trong hng s bc sng 0 0 0 v v tr ca mt im trong h trc ta

cu c nh ngha l khong cch radial ti im r.

ln ca tr khng sng theo khong cch ti ngun c th hin trn hnh

4.1a. Ti trng xa th 1 / r tri hn nn 0

wZ . Cng thc 4.2 c rt gn trong

trng gn l:

00

1wZ j

r

(trng gn, 0 1r )

0

0

90r

(4.3)

Trong vng trng gn th cng in trng t l vi 31 / r , trong khi cng

t trng th t l vi 21 / r :

3

2

1

1

Er

Hr

(Ngun in trng, trng gn) (4.4)

Ngoi ra, trong vng trng xa ca tr khng sng lng cc in1 l rt ln so

vi tr khng tng ng ca mi trng. Cho nn lng cc in c xem l ngun

tr khng cao2. ln tr khng sng ca mt ngun in trng l:

1 electric dipole 2 high-impedance source

MN CHN IN T

Trang19

0

12

we

Zf r

060r

(4.5)

Trong , ch s di e th hin mt ngun in trng (lng cc Hertz1). Cng

thc (4.3) c vit li theo bc sng trong khng gian t do 0 0 /v f vi 0 120

v 0 02 / .

Lng cc t nguyn t (vng t trng nguyn t)2 l i ngu ca lng cc

in nguyn t m ta c th chuyn i cng in trng v cng t trng s

nhn kt qu tng ng. Cc thnh phn trng xa ca lng cc t nguyn t l E v

1 Hertzian dipole 2 elemental magnetic dipole (loop)

Hnh 4.1 Tr khng sng ca lng cc Hertzian (a) v lng cc t (b)

MN CHN IN T

Trang20

H . Tr khng sng ca ngun ny c nh ngha l:

wE

ZH

(4.6)

Hay: 2

0 00

2 30 0 0

1

( )1 1

( ) ( )

w

j

r rZ

j

r r r

(4.7)

ln ca tr khng sng theo khong cch ti ngun c th hin trn hnh

4.1b. Ti trng xa th 1 / r tri hn nn 0

wZ . Cng thc 4.7 c rt gn trong

trng gn l:

0 0

wZ j r (trng gn, 0 1r )

0 0 90r (4.8)

Cng t trng trng gn t l vi 31 / r trong khi cng in trng

li t l vi 21 / r :

3

2

1

1

Hr

Er

(Ngun t trng, trng gn) (4.9)

Ngoi ra, trong trng gn ca lng cc t th tr khng sng rt nh so vi tr

khng c trng ca mi trng. Cho nn lng cc t c xem l ngun tr khng

thp. ln tr khng sng ca mt ngun t trng l:

0

2wm

Z f r

0

2369r

(4.10)

Trong , ch s di m th hin mt ngun t trng (vng t trng1) v thay

0 0 0/ , 0 0 0 v 0 02 / vo (4.8).

S phn bit gia ngun in trng v t trng s cho php chng ta chuyn i

nhiu kt qu c trong trng xa cho trng hp ngun trng gn. C nhiu v d

cho cc ngun nh vy.

1 magnetic loop

MN CHN IN T

Trang21

V d nh mt my bin p c to ra vi nhiu vng dy qun quanh li t.

Trng in t ln cn ngun ny phn ln l t trng. Nh vy, my bin p ging

vi vng t trng1. Vi ngun ny th trng gn c tnh cht l tr khng sng nh hn

rt nhiu so vi 0 , v cng in trng t l vi 21 / r trong khi cng t trng

t l vi 31 / r .

V d nh ngun in trng l b phng in v im cn xt l im cui ca

h quang, nh trong chi than ca ng c in mt chiu. i vi ngun ny th trng

gn c tnh cht l tr khng sng rt ln so vi 0 , v cng t trng t l vi 21 / r

trong khi cng in trng t l vi 31 / r .

4.2. Ngun in trng:

K thut c bn ca mn chn m c xt vi ngun trng xa th thng thy

vi ngun trng gn, nhng loi ngun ny th gii hn phng php xc nh hiu qu

che chn. Mt gii php chnh xc cho vn ny l kh khn hn nhiu so vi ngun

sng phng ng nht. Nn mt cch gn ng l ta vit hiu sut mn chn di dng

cc khi nim phn x, hp th v phn x nhiu ln, v s dng cc kt qu trc nhng

thay tr khng c trng 0 bng tr khng sng

wZ trong cc cng thc. Khi nim tn

tht hp th khng b nh hng bi loi ngun ny.

Tn tht phn x thu c bng cch thay tr khng c trng bng tr khng

sng ca khng gian t do trong (3.6):

10

20log4w

dB

ZR

(4.11)

Thay tr khng sng i vi ngun in trng t (4.5) v xp x vi vt dn tt

t (3.7) ta c:

, 10 3 2322 10logr

e dB

r

Rf r

(4.12)

Tn tht phn x i vi mt ngun in trng c v trn hnh 4.2 vi mn

chn ng v thay i khong cch t ngun n mn chn. Tn tht phn x i vi

ngun sng phng ng nht c th hin ng hi t tim cn khi khong cch t

ngun n mn chn tng. Ta thy rng trn th th tn tht phn x i vi trng gn

MN CHN IN T

Trang22

ca ngun in t cao hn ng k so vi mt ngun sng phng ng nht. N cng

tng khi m gim khong cch gia ngun v mn chn.

4.3. Ngun t trng:

Tn tht hp th ging vi ngun sng phng ng nht. Tn tht phn x i vi

trng gn ngun t trng thu c bng cch thay tr khng sng t (4.10) vo (4.11),

ta c:

2

, 1014,57 10logr

m dB

r

frR

(4.13)

Hnh 4.2 cng th hin tn tht phn x i vi trng gn, ngun t trng khi

thay i khong cch gia ngun ti mn chn v thay i tn s vi mn chn bng

ng. Ta thy rng tn tht phn x gim khi gim tn s v rt nh so vi tn tht phn

x ca sng phng. Do , tn tht phn x thng c b qua tn s thp. Ta cng c

tn tht hp th cng nh tn s thp, nn cc k thut khc phi c s dng i vi

mn chn i ph vi tn s thp, trng gn ca ngun t trng.

Hnh 4.2 Tn tht phn x trng gn ca ngun in trng v t trng

MN CHN IN T

Trang23

Chng 5

CHE CHN T TRNG TN S THP

Trong cc phn trc, ta thy tn tht phn x vi ngun trng xa (sng

phng ng nht) th mn chn chim u th nhng tn s thp, trong khi tn tht hp

th th mn chn chim u th nhng tn s cao. i vi trng gn, ngun in

trng, th ln ca chng cng khng thay i vi trng hp sng phng ng nht:

Tn tht phn x vn chim u th nhng tn s thp, v tn tht hp th cng chim

u th nhng tn s cao. Vi trng gn, ngun t trng, th chng khc nhau nhiu

nhng tn s thp. Tn tht hp th trn mn chn l ch yu trng gn, ngun t

trng ti tt c cc tn s. Tuy nhin, c tn tht phn x v tn tht hp th l kh nh

trng gn, ngun t trng ti nhng tn s thp, v vy cn phi c phng php

chng li ngun t trng tn s thp.

C hai phng php c bn mn chn chng li ngun t trng tn s thp:

to ng trnh1 cho ng sc t vi vt liu c t thm cao v to ng sc t i

lp2 theo nh lut Faraday, m ta bit nh l phng php vng ngn mch3. ng

trnh ca ng sc t i vi ng t tr thp4 ( t thm cao) c minh ha trn

hnh 5.1a. Gi s mi trng bn ngoi l khng gian t do vi 0 v mn chn

c to t vt liu st t s c 0r vi 1r , t trng s c xu hng tp trung

vo ng st t c t tr thp v nh vy s b lm lch hng khi nh hng n khu

vc bn trong ca mn chn. Trong phng php vng ngn mch minh ha trn hnh

5.1b vng dn in nh mt dy dn c t sao cho t trng i ti v xuyn qua b

mt th b chn bi vng ngn mch, do theo nh lut Faraday, trong vng s xut

hin mt dng in cm ng indI , v to ra ng sc t ind . ng sc t cm ng

ny c cc hoc hng chng li t trng ti ban u, vy nn t trng tng ln cn

ca vng ngn mch b trit tiu. C v s ng dng ca hai phng php trn. Nht l

trong nhng trng hp mn chn b gim hiu sut do t trng tn s thp.

1 diversion 2 opposing flux 3 shorted-turn 4 low-reluctance

MN CHN IN T

Trang24

C hai yu t c th lm gim hiu qu ca k thut ng trnh t thng m ta

phi nh l:

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

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

Nh sn xut vt liu st t thng ghi r t thm tng i ca vt liu tn

s thp nh l 1kHz nn gi tr ny r c th rt ln. V d nh, hp kim Mui1 c t

thm tng i hn 10,000 khi tn s bng 0 (DC) n 1kHz, nh th hin trn hnh 5.2.

Trn 1kHz, t thm tng i ca hp kim Mui gim t ngt, v trn 20kHz th n

khng ln hn thp cn ngui2. V vy, vt liu c t thm cao nh hp kim Mui th

hiu qu hn vi t trng so vi vt liu r hn nh thp khi tn s di vi chc kHz.

mn chn chng li c t trng trn 20kHz, th thp c hiu sut bng vt liu c

t thm cao. y l l do ti sao ro chn cho b cp ngun chuyn mch3 c to

t thp nhiu hn l t hp kim Mui. Thp th r hn hp kim Mui v cng hiu qu tn

s chuyn mch c s (20 100kHz) v sng hi chuyn mch. Mn chn ca ngun

in ngn chn tn s thp, t trng bc cao ca bin p cho b chuyn mch4 khi bc

x n cc mch in hay dy dn khc ca thit b, ni m chng c th gy ra nhiu vn

hoc c dn ra ngoi qua dy ngun, ri gy ra bc x.

1 Mumetal 2 cold-rolled steel 3 switching power supplies 4 switching transformer

Hnh 5.1 Hai phng php quan trng ca mn chn chng li t trng tn s thp:

(a) S dng vt liu st t c t thm cao lm lch hng t trng;

(b) S dng "vng ngn mch" dn in to t trng i lp.

MN CHN IN T

Trang25

Mt khc, mn chn chng li nhiu 60Hz hiu qu hn vi hp kim Mui nu

cng t trng khng qu ln chng lm bo ha vt liu. Hin tng bo ha

ny ca vt liu st t bi t trng bc cao c minh ha trn hnh 5.3a. dc ca

ng cong B-H t l vi t thm tng i ca vt liu. Cng t trng t l vi

ampere vng1 nu nh ni tip cc vng dy m mang dng in I qun quanh vt liu.

Nh vy, dng in ln thng to t trng bc cao ni m dc ca ng cong

phng hn, kt qu l r gim i vi t trng bc cao. V vy, c v nh hp kim Mui

c th l vt liu che chn hiu qu vi t tng 60Hz v t thm tng i ca n cao

tn s thp ny, nhng tht ra khng ng nh vt, v rng dng in ln thng c tn

s 60Hz. Nh sn xut vt liu st t thng ch ra trng thi c trng ca t thm

tng i ban u ca vt liu ca h ti (1) tn s thp, c trng 1kHz; v ti (2) t

trng bc thp2. Mt phng php tiu biu gim thiu hiu ng bo ha l dng hai

mn chn nh th hin trn hnh 5.3b. Mn chn u tin c r thp v cm ng t

thp bo ha t. Mc ch ca mn chn ny l gim t trng ti n khng bo

1 ampere turns 2 low field levels

Hnh 5.2 Minh ha s ph thuc vo tn s ca cc vt liu khc nhau

MN CHN IN T

Trang26

ha mn chn th hai, mn chn m c r cao v c kh nng cao i vi bo ha t. Tiu

biu l lp th nht gy ra tn tht phn x cho t trng.

Hnh 5.4 S dng hiu ng vng ngn mch kh bc x t trng ca t thng r ca my bin p:

(a) hnh chiu cnh; (b) hnh chiu bng.

Hnh 5.3 Minh ha hin tng bo ha t trong vt liu st t:

(a) ng cong B-H;

(b) dng mn chn t nhiu lp gim hiu ng bo ha.

MN CHN IN T

Trang27

ng dng thng thng ca vng ngn mch gim bt t trng ca my bin

p cp ngun cho b chuyn mch. Vng dn in bao gm di bng ng c qun

quanh my bin p nh th hin trn hnh 5.4a v b. Mc ch ca vng ngn mch hay

di bng1 ny l kh t trng bc x ca t thng r2 ca my bin p.

Hnh nh ca my bin p cp ngun cho b chuyn mch vi vng ngn mch

qun quanh chng c th hin trn hnh 5.5.

1 band 2leakage flux

Hnh 5.5 Hnh nh v vng ngn mch ca my bin p chuyn mch dng kh t trng bc x

MN CHN IN T

Trang28

Chng 6

HIU NG KHE H

Nh ch ra trc , c nhiu trng hp l trng trn mn chn l khng th

trnh khi bi nhng l do thc t. Mt trong nhng trng hp l vic thng gi.

Qut c s ng chuyn hi nng bn trong mn chn ra ngoi. Ta thy rng cc

loi l trng th thng s lng ln ca loi l nh th ln hn loi l ln. Nguyn nhn

quan trng ca vn ny c minh ha trong hnh 6.1.

Xt mn chn rn th hin trong hnh 6.1a. Dng in cm ng chy trong mn

chn ny v t thng lin i1 ca chng m to ra t thng tn2, t thng m chng li

hay kh t thng ti. T thng ti to ra dng in cm ng trn b mt, dng in m

c th xem l to ra t thng phn x. T thng phn x c cc tnh sao cho kh c

t thng ti p ng iu kin bin l tng in trng tip tuyn vi vt dn in tt

phi bng khng. m mn chn c th kh c nh trn, th dng in cm ng phi

c php chy qua m khng b cn tr. Gi s ta to mt khe h trn mn chn v n

vung gc vi hng ca dng in cm ng. Khe h s lm gin on dng in chy

qua v s lm gim hiu sut che chn. rng ca khe h khng nh hng ng k

n iu ny, th hin trn hnh 6.1b, c. Mt khc, nu ta hng ca khe h song song

vi hng ca dng in cm ng, th khe h s nh hng t hn n vic che chn, nh

th hin trn hnh 6.1d. Tht l khng kh thi xc nh hng ca dng in cm ng

v t hng ca khe h cho thch hp, v vy s lng ln ca cc l nh c thay th

1 associated fields 2 scattered fields

Hnh 6.1 Minh ha hiu ng khe h ln dng in cm ng ca mn chn.

MN CHN IN T

Trang29

cho khe h di nh minh ha trn hnh 6.1e. Ni cch khc l nhiu l nh to ra nhiu l

thng gi bng khe h di, nhng lm nhiu lon dng in cm ng th b hn nhiu, v

bng cch cng gim h.

Cng tht s cn thit khi to np y trn v thit b (mn chn). Ti im ng

np th mt khe h c to ra quanh np y m c th xem nh mt ng-tenna khe1,

nh cp trc . Mc d khong h c th kh nh, nhng kh nng bc x ca

khe h c th kh ln, nh minh ha bng nguyn l Babinet, ni cp trc.

V d, gi s rng c mt np y pha trn v thit b (mn chn) nh minh ha trn

hnh 6.2a.

Nguyn l Babinet th hin rng khi chuyn i khe h bng vt dn rn cng kch

thc vi khe h s to ra trng bc x ging vi t khe h. Trong minh ha ny th

chiu di ca khe h quan trng hn b dy ca n khi xc nh bc x ca khe h. Nu

chiu di khe h bng mt na bc sng, th bc x tng t vi ng-ten lng cc mt

na bc sng. iu ny gii thch ti sao cn thit dng nhiu c vt quanh np l

ph v cc ng-ten khe h tim nng ny, minh ha trong hnh 6.2b, v rng cc ng-ten

1 slot antenna

Hnh 6.2 Minh ha hiu ng khe h ti np y:

(a) bc x khe h theo nguyn l Babinet;

(b) nhiu c vt nm gn nhau ph v chiu di khe h;

(c) minh ha v tr thch hp v khng thch hp ca ming m nhm gim gim pht x t c vt.

MN CHN IN T

Trang30

tuyn tnh ngn hn thng hiu qu bc x km hn so vi ng-ten di. Ming m kim

loi thng c s dng lm kn khe h nh minh ha trn hnh 6.2c. N thng c

dng li thp hoc ng beryllium finger stock. Ming m ny s c t pha bn

trong so vi cc c vt, v rng nu chng c t pha bn ngoi cc c vt th bc x t

cc l c vt s khng b ngn chn.

Cc ro chn h cng c th c bo v bng cch s dng ng dn sng above

cutoff1. Mt ng sng vung vi cnh bng d th suy gim ca ng dn sng theo

chiu di l th t l vi l

de

. V vy, suy gim hay hiu sut che chn nh vo ng dn

sng l:

10

10

20log

.20log

ld

dBSE e

l ed

27,3l

d (6.1)

Ta thy hiu sut che chn t l thun vi chiu di ng dn sng. iu ny l c

s s dng ng dn sng below cutoff2 cho php lung khng kh i vo trong v

thit b (mn chn) v ng thi ngn chn s lan truyn tn s thp hn tn s cutoff vo

trong mn chn.

[4][5]

1 waveguide above cutoff 2 waveguide below cutoff

MN CHN IN T

Trang31

TI LIU THAM KHO

[1] C. R. Paul, Introduction to Electromagnetic Compatibility. John Wiley & Sons,

2006.

[2] E. F. Vance, Electromagnetic-interference control, IEEE Trans. Electromagn.

Compat, vol. EMC-22, pp. 319 328, 1980.

[3] C. A. Balanis, Antenna Theory. Wiley, 2005.

[4] P. A. Laplante, Electrical Engineering Dictionary. CRC Press LLC, 2000.

[5] S. Gibilisco, The Illustrated Dictionary of Electronics. The McGraw Hill, 2001.