Chng nhiu

1. Khi nim

Nhiu (noise) trong thit k mch in t l cc loi tn hiu tp cht c sinh ra mt

cch ngu nhin gy nh hng xu ti tn hiu thng tin . C hai loi nhiu chnh c

ch l nhiu bc x ( Radiation noise) v nhiu thu nhn ( reception noise). Nhiu bc

x l loi nhiu do chnh thit b hoc mch in gy ra cn nhiu thu nhn l loi

nhiu do thit b hoc mch in nhn c khi hot ng gn cc ngun nhiu.

Hnh: Hai loi nhiu c quan tm trong mch PCB

Khi nim nhiu thng i chung vi khi nim EMC(Electromagnetic compatibility)

kh nng tng thch in t m c hiu l gm 2 loi: nhn in t -

EMI(Electromagnetic interference) v min nhim in t -EMS(Electromagnetic

Susceptibility). Vic gim nhiu l vic phi kt hp gii quyt 2 vn l gim EMI v

tng EMS.

Hnh: Kt hp ng thi kh Radiation noise v Reception noise

Trong vic thit k mch in t, nht l mch in t tc cao th vic chng nhiu l

vic cn phi lu v phi lm t khu khi to d n. Chng ta khng th i khi

thit k mch xong ri mi bt u kim tra nhiu v mun ci to bt c g th s phi

thay i li ton b thit k lm tn thi gian thit k, lm mch v chi ph t mch.

Nhiu khng th m phng trc c v n mang tnh ngu nhin do thit k nn vic

tnh ton kh nhiu l tng i kh khn i vi ngi khng c kinh nghim nn i

hi ngi lm mch phi c nhng kin thc nht nh trong thit k mch. Mt iu

ng lu tm na l khi kt hp qu nhiu phng php kh nhiu trn mt mch in

PCB c th khin bn mt mt chi ph kh ln vt qua chi ph d tr ban u to ra

thit b nn vic cn lm l kh nhiu mt cch c hiu qu va thit b hot

ng n nh v tit kim chi ph.

Hnh: Mch in PCB

Cc cun cm n v t in n l cc cm khng o tn ti trong mch in PCB ( printed

circuit board) l mt trong cc nguyn nhn gy nhiu. Cc thnh phn cm khng ny

tn ti gia hai ng mch hoc l gia hai lp cht liu i vi mch nhiu lp.

Hnh: S tn ti ca cc t n v cun cm n trong mt mch PCB

Do qu trnh thit k PCB, ngi thit k tin hnh i hai hay nhiu dy tn hiu song

song qu gn vi nhau hoc do c trng ca board mch in PCB m c th to ra cc

gi tr khc nhau ca t in n v cun cm n, nhng thnh phn ny gp phn vo gy

nh hng qua li tn hiu t ng mch in ny ln ng mch in khc:

Hnh: nh hng ca tn hiu A ln tn hiu B

Bi vy, ti bc u tin ca lm mch in PCB, ngi thit k phi ch tm ti khong

cch gia cc dy tn hiu v dy, cht liu ca board mch in. i vi cc cng ty

lm mch in PCB, c mt khuyn co l thit b CNC ca h c th i c cc ng

dy c khong cch ti thiu l bao nhiu, vi dy l bao nhiu. Ngi lm mch cn

nghin cu k iu ny trc khi t mch ti cc cng ty lm mch trnh phi v li

thit k do nh lm mch khng th p ng c yu cu, mi ln v li s rt mt thi

gian. V d: cng ty KimSonpcb l mt cng ty lm mch ln ti HCM c quy cch mch

in khuyn co l ng mch ti thiu 8mil, khong cch hai ng mch ti thiu 8mil.

Tuy nhin chng ta nn i dy khong cch v kch thc ln hn 10mil hn ch li

mch, nh th s tt hn.

Kch thc ng mch cng nh th cng t nhiu nhng phi m bo ng mch chu

c dng in theo thit k, khong cch gia cc ng mch cng rng cng tt.

Cng c trn trang: http://www.skottanselektronik.com/ cho php ta tnh ton cc ch s cn

thit khi thit k mch nh: tnh h s nhiu gia hai ng mch c b rng W hoc tnh

b rng ti thiu ng mch chu c dng A (Ampere).

L xuyn via cng l mt trong cc nguyn nhn gy nhiu v li trong mch PCB.

Trong thit k chng ta nn hn ch to l via t nht c th. iu ny l ph thuc vo

trnh ca ngi thit k.

2. Cc phng php kh nhiu

A. Cc loi nhiu thng gp

Hnh: Nhiu do nh hng gia hai ng tn hiu

Nhiu do nh hng qua li ca tn hiu b gy ra do s thay i t ngt

mc in p ca tn hiu A lm tn hiu B cng b nh hng theo. Loi

nhiu ny pht sinh do cc thnh phn cm khng n nh trnh by

trn.

Hnh: Nhiu lm bin dng tn hiu

Mt tn hiu A c to thnh bi mt linh kin, nhng trn ng truyn,

tr khng u ra ca linh kin v u vo ca linh kin khc c s

khc nhau s gy ra hin tng phn x sng ti linh kin nhn. Sng phn

x s gy chng lp ln tn hiu chnh v gy bin dng tn hiu. y cng

chnh l nguyn nhn chng ta phi phi hp tr khng m bo tn hiu

thng tin c ton vn nht.

Hnh: Nhiu do ngun khng n nh

Ngun in cung cp cho mch in i khi khng n nh cng gy nh

hng xu ti mch in. Lc ny cc t lc l thc s cn thit n nh

ngun. i vi tn hiu tn s cao, t in ng vai tr nh mt in tr v

gi tr in tr ph thuc v in dung t v tn s tn hiu. Cng thc

c m t:

Zc=

V d i vi tn hiu c tn s100Mhz, t c in dung 10pF c tr khng

l Zc=170(Ohm)

Hnh: p ng tr khng ca t in 10pF

Bng sau cho ta bit mc p ng tr khng ca cc gi tr t khc nhau

i vi cc tn s khc nhau ca h thng.

Hnh: p ng tr khng ca t in vi cc gi tr tn s khc nhau.

T chng ta c th chn cc loi t khc nhau lc nhiu cho cc ng

dng khc nhau,i vi ngun cp gn IC hoc cc ngoi vi th dng t c

0.01-0.1uF ni gia VCC v GND cho ta kt qu tt nht. V n s lm

gim tr khng ng dy ngun IC khong tn s c MHZ. Cn i vi

ngun chnh th nn dng t lc c gi tr ln hn khong tm 1-100uF

kh cc thnh phn rng ca thng thy ng ngun. Nhng khng

nn dng t c gi tr qu ln v dng khi ng c th gy nguy hi cho

mch PCB khi cp ngun.

Hnh: Cc cng dng thc t ca tng gi tr t in trong mch PCB

B. Cc phng php khc phc nhiu

a. iu chnh gi tr ca t v tr lc nhiu cho b to dao ng thch

anh

Trc tin l nu nh c th th hy chn thch anh c tn s nh

nht m p ng c yu cu thit k bi v tn s cng cao th

ch s EMI cng ln, nhiu cng d xy ra. Sau chng ta c th

tham kho s kh nhiu nh trn, iu chnh cc gi tr t C1,

C2 v R2 c kt qu tn hiu tt nht. Thng thng thch anh

c gi tr tm Mhz th t c gi tr tm pF.Mt iu na l thch

anh nn t cng gn IC cng tt.

b. Chn IC dn QFP thay v IC ni DIP

C mt iu c khuyn co l IC dn cho kt qu chng nhiu

tt hn l IC DIP v vy nu c IC tng ng dng QFP, hy s

dng chng

c. Chn IC tch hp single-chip thay v chn mt IC v mt ngoi vi

m rng khc

Nguyn nhn ca vic chn ny rt n gin, v cc dy tn hiu

ni gia hai linh kin l mt thnh phn gy nhiu khng h nh

khi thi cng. Nn nu c th th hy chn loi IC tch hp

(single-chip)

d. Chn ngun cung cp c gi tr nh nu IC c th chy c

y l mt cch tng i n gin, bi v cng sut nhin t l

vi bnh phng bin in p :P=VI=V2Z. Bi vy thay v dng

ngun 5V v dng ngun 3.3V th s gim mt lng nhiu ng

k( thng thng vic lm gim c 57% nhiu)

e. Lc nhiu bng b lc LC

y l mt cch ph bin c dng hin nay, cun Ferrite bead

c chc nng chn cao tn v t th c chc nng lm mt tn

hiu. Tuy nhin cn ch l khng t ferrite bead ti ng tn

hiu c tn s cao hoc l ng clock v cun ferrite c th gy

mt tn hiu trn ng truyn. Thng thng ngi ta t ferrite

bead ti ng ngun ca thit b. Vic ny cng gp phn lm

cho IC v cc linh kin trong mch bt nng hn.

f. Hn ch nhiu bng chnh quy trnh thit k PCB

Cch ny ph thuc vo trnh ca ngi thit k, c th k ra

mt vi kinh nghim m ngi vit bit:

-i dy c t l xuyn Via nht,

-i dy c b rng ln hn vi ng ngun v nh hn vi

ng tn hiu, v d i vi ngun 5V th ng ngun 40mil v

ng tn hiu 10mil l n.

-Hn ch ng tn hiu b gc 90 , nu c th hy lm mt n

-Gia t bypass ngun ca IC khng nn i xuyn via m hy i

via sau t

-Khng c i dy di thch anh dao ng

-Sau khi thit k, ph mass GND ton mch l vic lm cn thit

3. Cc phng php p dng vo mch lun vn