1

CHNG 1

GII THIU CHUNG V K THUT O LNG IN T

1.1 CC KHI NIM V O LNG IN T

o lng hc (Metrology) l lnh vc khoa hc ng dng lin ngnh nghin cu v

cc i tng o, cc php o, cc phng php thc hin v cc cng c m bo

cho chng, k thut o, cc phng php t c chnh xc mong mun.

- o lng (Measurement) l g? o lng l qu trnh thc nghim vt l nhm

nh gi c tham s, cng nh c tnh ca i tng cha bit. Thng thng

o lng l qu trnh so snh i tng cha bit vi mt i tng lm chun (i

tng chun ny thng l n v o), v c kt qu bng s so vi n v o.

+ V d o in p: in p ca mt ngun o c l 5V ngha l in p ca

ngun gp 5 ln in p ca mt ngun chun 1V.

- o lng in t (Electronic Measurement) : l o lng m trong i

lng cn o c chuyn i sang dng tn hiu in mang thng tin o v tn

hiu in c x l v o lng bng cc dng c v mch in t.

+ Nu kt hp o lng in t v cc b bin i phi in - in (sensor - cc b

cm bin) cho php o lng c hu ht cc i lng vt l trong thc t.

- i lng o (Measurand): l cc i lng vt l cha bit cn xc nh tham

s v c tnh nh php o.

- Tn hiu o (Measuring Signal: Tn hiu in mang thng tin o.

- Php o (Measurement): L qu trnh xc nh tham s v c tnh ca i

lng vt l cha bit bng cc phng tin k thut c bit - hay cn c gi l

thit b o.

- Thit b o (Instrument): l phng tin k thut thc hin php o c chc

nng bin i tn hiu mang thng o thnh dng ph hp cho vic s dng v nhn

kt qu o, chng c nhng c tnh o lng c bn c qui nh. Trong thc

t Thit b o thng c hiu l my o (v d: My hin sng, Vn mt s, My

m tn ).

- K thut o (Intrumentation): l mt nhnh khoa hc v cc phng php k

thut cng ngh ng dng trong o lng v iu khin.

2

- Phng php o (Measuring method) : L cch thc thc hin qu trnh o

lng xc nh c tham s v c tnh ca cc i lng o. Phng php o

ph thuc vo nhiu yu t: Phng php nhn thng tin o t i lng o,

Phng php x l thng tin o, Phng php nh gi, so snh thng tin o,

Phng php hin th, lu tr kt qu o Mi loi my o c th coi l mt thit

b o hon chnh thc hin theo mt hay mt vi phng php o c th no .

V c bn qu trnh o lng c th c chia thnh cc bc khc nhau v c

minh ha nh hnh v sau:

Hnh 1.1 Qu trnh o lng

1.2 I TNG CA O LNG IN T

o lng in t c phm vi ng dng rt rng ri, i tng o rt rng. Tuy

nhin trong lnh vc in t - vin thng, i tng ca o lng tp ch yu vo

i tng: H thng tham s v c tnh ca tn hiu v ca mch in t.

- H thng tham s v c tnh ca tn hiu in t:

+ Tham s v cng tn hiu in t gm: Cng dng in, Cng in

p, Cng sut tc dng ca tn hiu...

+ Tham s v thi gian gm: Chu k, tn s ca tn hiu, gc lch pha gia 2 tn

hiu cng tn s, rng ph tn hiu, rng xung, rng sn trc, sn sau

...

+ c tnh tn hiu gm: Ph ca tn hiu, mo dng ca tn hiu, h s iu ch

tn hiu...

+ Tn hiu s gm cc tham s: Mc logic, tn s, chu k...

Thu nh n

thng tin

o

Bi n i, x l, nh

gi, so snh, nh

lng thng tin o

Lu tr ,

hi n th

k t qu o

i

l ng

o

3

- H thng tham s v c tnh ca mch in t:

+ Cc tham s v tr khng: Tr khng tng ng, dn np tng ng, in

tr, in dung, in khng tng ng, tr khng sng, h s phn x, h s tn

hao, h s phm cht ca mch...

+ c tnh ca mch: c tuyn Vn-Ampe, c tuyn bin - tn s, c tuyn

Pha - tn s ca mch...

Ch : Ty theo di tn v h thng tham s v c tnh ca tn hiu v ca mch

in t cn o cng khc nhau.

1.3 PHN LOI PHP O

Php o l cng vic thc hin chnh ca o lng, l vic tm ra gi tr vt l

bng cc th nghim vi s tr gip c cc cng c k thut c bit. Gi tr tm

c gi l kt qu ca php o. Hot ng thc hin trong qu trnh o cho ta

kt qu l mt i lng vt l gi l qu trnh ghi nhn kt qu. Ty thuc vo i

tng nghin cu, vo tnh cht ca cng c o v ngi ta cn thc hin php o

ghi nhn mt ln hay nhiu ln. Nu nh c mt loi ghi nhn th kt qu php o

nhn c l kt qu khi x l cc kt qu t cc ghi nhn .

Php o c bn cht l qu trnh so snh i lng vt l cn o vi mt i lng

vt l c dng lm n v. Kt qu ca php o c biu din bng mt s l t

l ca i lng cn o vi mt n v . Nh vy th thc hin php o, ta cn

thit lp n v o, so snh gi tr ca i lng cn o vi n v v ghi nhn kt

qu so snh c. Thng thng ngi ta thng bin i tn hiu n dng thun

tin nht cho vic so snh.

Nh vy, ta c th tm tt li thnh bn bc chnh ca php o l: thit lp n v

vt l, biu din tn hiu o, so snh tn hiu o vi n v c ly lm chun v

ghi nhn kt qu so snh.

C nhiu cch phn loi phng php o, ty thuc vo phng php nhn kt qu

o, phng php x l thng tin o, di trnh o, iu kin o, sai s...

+ o trc tip : L phng php o m kt qu o nhn c trc tip trn thit b

o t mt ln o duy nht. Thng thng dng cc thit b o tng ng cho chnh

i tng cn o o.

- VD: o in p bng vn-mt, o tn s bng tn s-mt, o cng sut bng t-

mt,...

4

c im ca php o trc tip l qu trnh thc hin n gin v bin php k

thut, tin hnh o c nhanh chng v loi tr c cc sai s do tnh tn.

+ o gin tip : L phng php o m kt qu o nhn c t biu thc tnh tn

cc kt qu ca php o trc tip cc i lng vt l khc nhau.

- VD: o cng sut mt chiu: P=U.I - o in p v dng in bng Vn-mt v

Ampe-mt.

- c im: nhiu php o v thng khng nhn bit ngay c kt qu o.

Trong k thut o lng, thng thng ngi ta mun trnh phng php o gin

tip, v trc ht n yu cu tin hnh nhiu php o (t nht l hai php o) v

thng l khng nhn bit ngay c kt qu o. Song trong mt s trng hp th

khng th trnh c phng php ny.

+ o thng k: L phng php thc hin o nhiu ln mt i lng o vi cng

thit b o v trong cng in kin o, kt qu o c tnh l gi tr trung bnh

thng k ca ca cc ln o o.

c im: Phng php ny cho php loi tr cc sai s ngu nhin v thng

dng khi kim chun thit b o.

Hin nay, k thut o lng pht trin nhiu v phng php o tng quan.

N l mt phng php ring, khng nm trong phng php o trc tip hay

phng php o gin tip. Phng php tng quan dng trong nhng trng hp

cn o cc qu trnh phc tp, m y khng th thit lp mt quan h hm s

no gia cc i lng l cc thng s ca mt qu trnh nghin cu. V d: tn hiu

u vo v tn hiu u ra ca mt h thng no .

Khi o mt thng s ca tn hiu no bng phng php o tng quan, th cn t

nht l hai php o m cc thng s t kt qu o ca chng khng ph thuc ln

nhau. Php o ny c thc hin bi cch xc nh khong thi gian v kt qu

ca mt s thut tn c kh nng nh c tr s ca i lng thch hp.

chnh xc ca php o tng quan c xc nh bng di khong thi gian ca

qu trnh xt. Khi o trc tip tht ra l ngi o phi gi thit h s tng quan

gia i lng o v kt qu rt gn 1, mc d c sai s do quy lut ngu nhin ca

qu trnh bin i gy nn.

Ngoi cc php o c bn ni trn, cn mt s cc phng php o khc thng

c thc hin trong qu trnh tin hnh o lng nh sau:

5

Php o thay th: Php o c tin hnh hai ln, mt ln vi i lng cn o v

mt ln vi i lng o mu. iu chnh hai trng hp o c kt qu ch th

nh nhau.

Php o hiu s: Php o c tin hnh bng cch nh gi hiu s tr s ca i

lng cn o v i lng mu.

Php o vi sai, phng php ch th khng, phng php b, cng l nhng trng

hp ring ca phng php hiu s. Chng thng c dng trong cc mch cu

o hay trong cc mch b.

Php o thng: kt qu o c nh lng trc tip trn thanh ca thit b ch

th. Tt nhin s khc ca cc thang ny c ly chun trc vi i

lng mu cng loi vi i lng o.

Php o ri rc ha (ch th s): i lng cn c o c bin i thnh tin tc

l cc xung ri rc. Tr s ca i lng cn o c tnh bng s xung tng ng

ny.

1.4 CHC NNG V PHN LOI THIT B O

Hu ht cc thit b o c chc nng cung cp cho chng ta kt qa o c i

lng ang kho st. Kt qu ny c ch th hoc c ghi li trong sut qu

trnh o, hoc c dng t ng iu khin i lng ang c o.

V d: trong h thng iu khin nhit , my o nhit c nhim v o v ghi

li kt qu o ca h thng ang hot ng v gip cho h thng x l v iu

khin t ng theo thng s nhit .

Ni chung thit b o lng c chc nng quan trng l kim tra s hot ng ca

h thng t iu khin, ngha l o lng qu trnh trong cng nghip (Industrial

process measurements)

- Phn loi thit b o: Gm 2 nhm chnh

Thit b o n gin: mu, thit b so snh, chuyn i o lng.

Thit b o phc tp: my o, thit b o tng hp v h thng thng tin o lng.

+ Thit b chun: Chun l mu c cp chnh xc cao nht. Chun l phng tin o

m bo vic sao v gi n v o tiu chun.

+ Thit b mu: l thit b o dng sao li i lng vt l c gi tr cho trc vi

chnh xc cao.

+ Thit b so snh: thit b o dng so snh 2 i lng cng loi.

6

+ Thit b chuyn i o lng: Thit b o dng bin i tn hiu mang thng

tin o lng v dng thun tin cho vic truyn tip, bin i tip, x l tip v gi

li, nhng ngi quan st cha th nhn bit trc tip c kt qu o (VD: b K

o lng; b bin dng, bin p o lng; sensor, quang in tr, nhit in tr,

ADC ...)

+ My o (Instrument) : Thit b o dng bin i tn hiu mang thng tin o

lng v dng m ngi quan st c th nhn bit trc tip c (VD: vnmt,

ampe mt,...)

+ Thit b o tng hp: l cc thit b o phc tp, a nng dng kim tra, kim

chun o lng, o lng cc tham s phc tp.

+ H thng thng tin o lng: H thng mng kt ni ca nhiu thit b o, cho

php o lng v iu khin t xa, o lng phn tn...

Vi nhiu cch thc o a dng khc nhau cho nhiu i lng c nhng c tnh

ring bit, mt cch tng qut chng ta c th phn bit 2 dng thit b o ph

thuc vo c tnh.

V d: o dn in chng ta dng thit b o dng in thun ty in l

micro ampe k hoc mili ampe k. Nhng nu chng ta dng thit b o c s kt

hp mch in t o dn in th lc by gi phi bin i dng in o

thnh in p o. Sau mch o in t o dng in di dng in p. Nh vy

chng ta c c tnh khc nhau gia thit b o in v thit b o in t. Hoc c

nhng thit b o ch th kt qu bng kim ch th (thit b o dng analog), hin nay

thit b o ch th bng hin s (thit b o dng digital). y cng l mt c tnh

phn bit ca thit b o.

Ngoi ra thit b o lng cn mang c tnh ca mt thit b in t (nu l thit b

o in t) nh: tng tr vo cao, nhy cao, h s khuych i n nh v c

tin cy m bo cho kt qu o. Cn c thm chc nng, truyn v nhn tn hiu o

lng t xa (telemetry). y cng l mn hc quan trng trong lnh vc o lng

iu khin t xa.

Bng phn loi tng quan thit b o nh hnh 1.2:

Hnh 1.2- Bng phn loi tng quan thit b o

Thit b o

Mc t ng ha

Thit b o

khng t ng

Thit b o t ng

Dng ca tn hiu

Thit b o tng t

Thit b o s

Phng php

bin i

Thit b o bin i thng

Thit b o bin i cn bng

Cc i lng

u vo

Thit b

o dng in

Thit b o tn s

..

.

8

1.5 CHUN HA TRONG O LNG

1.5.1 Cp chun ha

Khi s dng thit b o lng, chng ta mong mun thit b c chun ha

(calibzate) khi c xut xng ngha l c chun ha vi thit b o lng

chun (standard). Vic chun ha thit b o lng c xc nh theo bn cp nh

sau:

Cp 1: Chun quc t (International standard) - cc thit b o lng cp

chun quc t c thc hin nh chun ti Trung tm o lng quc t t ti

Paris (Php), cc thit b o lng chun ha cp 1 ny theo nh k c nh gi

v kim tra li theo tr s o tuyt i ca cc n v c bn vt l c hi ngh

quc t v o lng gii thiu v chp nhn.

Cp 2: Chun quc gia - cc thit b o lng ti cc Vin nh chun quc

gia cc quc gia khc nhau trn th gii c chun ha theo chun quc t v

chng cng c chun ha ti cc vin nh chun quc gia.

Cp 3: Chun khu vc - trong mt quc gia c th c nhiu trung tm nh

chun cho tng khu vc (standard zone center). Cc thit b o lng ti cc trung

tm ny ng nhin phi mang chun quc gia (National standard). Nhng thit

b o lng c nh chun ti cc trung tm nh chun ny s mang chun khu

vc (zone standard).

Cp 4: Chun phng th nghim - trong tng khu vc s c nhng phng th

nghim c cng nhn chun ha cc thit b c dng trong sn xut cng

nghip. Nh vy cc thit b c chun ha ti cc phng th nghim ny s c

chun ha ca phng th nghim. Do cc thit b o lng khi c sn xut ra

c chun ha ti cp no th s mang cht lng tiu chun o lng ca cp .

Cn cc thit b o lng ti cc trung tm o lng, vin nh chun quc

gia phi c chun ha v mang tiu chun cp cao hn. V d phng th nghim

phi trang b cc thit b o lng c tiu chun ca chun vng hoc chun quc

gia, cn cc thit b o lng ti vin nh chun quc gia th phi c chun quc

t. Ngoi ra theo nh k c t ra phi c kim tra v chun ha li cc thit

b o lng.

1.5.2 Cp chnh xc ca thit b o

9

Sau khi c xut xng ch to, thit b o lng s c kim nghim

cht lng, c chun ha theo cp tng ng nh cp trn v s c

phng kim nghim nh cho cp chnh xc sau khi c xc nh sai s (nh nh

ngha di y) cho tng tm o ca thit b. Do khi s dng thit b o lng,

chng ta nn quan tm n cp chnh xc ca thit b o c ghi trn my o hoc

trong s tay k thut ca thit b o. t cp chnh xc ny chng ta s nh gi

c sai s ca kt qu o.

V d: Mt vn-k c ghi cp chnh xc l 1, ngha l gii hn sai s ca n

cho tm o l 1%.

1.6 SAI S TRONG O LNG

o lng l s so snh i lng cha bit (i lng o) vi i lng c

chun ha (i lng mu hoc i lng chun). Qu trnh so snh nh vy bao

gi cng c sai lch. Trong thc t kh xc nh tr s thc cc i lng o. V

vy tr s c o cho bi thit b o c gi l tr s tin cy c (expected

value). Bt k i lng no cng b nh hng bi nhiu thng s. Do kt qu

t khi phn nh ng tr s tin cy c. Cho nn c nhiu h s (factor) nh

hng trong o lng lin quan n thit b o. Ngoi ra c nhng h s khc lin

quan n con ngi s dng thit b o. Nh vy chnh xc ca thit b o c

din t di hnh thc sai s

Khi nim sai s: l chnh lch gia kt qu o v gi tr thc ca i lng o.

N ph thuc vo nhiu yu t nh: thit b o, phng thc o, ngi o Sai s

cng c ngha quan trng khng km g kt qu o, cho php nh gi c

tin cy ca kt qu o.

1.6.1 Nguyn nhn gy sai s

Cc nguyn nhn gy sai s gm:

- Nguyn nhn khch quan: do dng c o khng hon ho, hay l c mt phn

khuyt im hoc h hng, i lng o b can nhiu nn khng hon ton c n

nh, iu kin mi trng khng tiu chun tc ng ln thit b, ln i tng o

hay ngi o ...

- Nguyn nhn ch quan: l sai lm ca ngi o, nh c kt qu o sai, do thiu

thnh tho trong thao tc, phng php tin hnh o khng hp l,...

* Cc ngun sai s: Thit b o khng o c tr s chnh xc v nhng l do sau:

10

- Khng nm vng nhng thng s o v iu kin thit k.

- Thit k nhiu khuyt im.

- Thit b o khng n nh s hot ng.

- Bo tr thit b o km.

- Do ngi vn hnh thit b o khng ng.

- Do nhng gii hn ca thit k.

1.6.2 Phn loi sai s

C nhiu cch phn loi sai s khc nhau: phn loi theo ngun gc,nguyn nhn...

1.6.2.1. Phn loi sai s theo ngun gc gy ra sai s

+ Sai s th: Cc sai s th c th quy cho gii hn ca cc thit b o hoc l cc

sai s do ngi o:

Sai lm (Gross error): mt cch tng qut sai s ny do li lm ca

ngi s dng thit b o nh vic c sai kt qu, hoc ghi sai, hoc s

dng sai khng ng theo qui trnh hot ng...

Sai s gii hn ca thit b o. V d nh nh hng qu ti gy ra bi

mt voltmeter c nhy km. Voltmeter nh vy s r dng ng k t

mch cn o v v vy s t lm gim mc in p chnh xc...

+ Sai s h thng (Systematic error): Sai s do nhng yu t thng xuyn hay cc

yu t c quy lut tc ng, n lm cho sai s ca ln o no cng ging nhau hoc

thay i theo quy lut, Nguyn nhn thng do tnh khng hon ho ca thit b, do

iu kin mi trng tc ng...

Sai s do thit b o: Cc phn t ca thit b o c sai s do cng ngh ch to, s

lo ha do s dng... lm gim sai s ny bng cch bo tr nh k cho thit b

o.

Sai s do nh hng iu kin mi trng: c th nh nhit tng cao, p sut

tng, m tng, cng in trng hoc t trng ngoi tng u nh hng

n sai s ca thit b o lng. Gim sai s ny bng cch gi sao cho iu kin

mi trng t thay i hoc b chnh (compensation) i vi nhit v m. V

dng bin php bo v chng nh hng tnh in v t trng nhiu.

Sai s h thng u c nh hng khc nhau. trng thi tnh v trng thi ng:

trng thi tnh sai s h thng ph thuc vo gii hn ca thit b o hoc do qui

lut vt l chi phi s hot ng ca n.

11

trng thi ng sai s h thng do s khng p ng theo tc thay i nhanh

theo i lng o.

i vi sai s h thng: x l bng cch cng i s gi tr ca sai s h thng vo

kt qu o, hoc hiu chnh li my mc, thit b o vi my mu

+ Sai s ngu nhin (Random error): L sai s do cc yu t bt thng khng

tun theo quy lut tc ng no. Tuy thc hin o trong cng iu khin v tnh

cn thn nh nhau nhng do nhiu yu t bt thng m sinh ra cc kt qu o

khc nhau khi thc hin php o nhiu ln cng mt i lng o. S ny sinh sai

s ngu nhin do nhiu nguyn nhn khch quan tc ng ln i tng o, thit b

o, ngi o ...

V d: gi s in p c o bi mt vn k c c cch khong 1 pht. Mc d

vn k hot ng trong iu kin mi trng khng thay i v c chun ha

trc khi o v i lng in p xem nh khng thay i. Khi tr s c ca

vn k c thay i cht t. S thay i ny khng c hiu chnh bi bt k

phng php nh chun no khc, v do sai s ngu nhin gy ra.

+ Sai s gii hn (Limiting Error) l sai s tng i khi kt qu o v tr lch

ton thang:

[%]100.max

max

X

XM

, Xmax: gi tr ton thang.

Khi kt qu o v tr thang o nh hn v tr lch ton thang th sai s tng i

tng ln. Nh vy mt yu t quan trng khi o lng l kt qu o cng gn v tr

ton thang cng tt.

V d: Dng vn mt thang o 300V, cp chnh xc 2%, tnh sai s tng i khi

o in p 120V?

1.6.2.2 Phn loi theo s ph thuc ca sai s vo i lng o

- Sai s im 0 (sai s cng) l sai s khng ph thuc vo gi tr i lng o.

- Sai s nhy (sai s nhn) l sai s ph thuc vo gi tr i lng o

1.6.2.3 Phn loi theo v tr sinh ra sai s

- Sai s phng php l sai s do phng php o khng hon ho

- Sai s phng tin o l sai s do phng tin o khng hon ho. Gm: sai s h

thng, sai s ngu nhin, sai s im 0, sai s nhy, sai s c bn, sai s ph, sai

s ng, sai s tnh.

12

- Sai s c bn ca phng tin o l sai s ca phng tin o khi s dng trong

iu kin tiu chun

- Sai s ph ca phng tin o l sai s sinh ra khi s dng phng tin o iu

kin khng tiu chun

- Sai s tnh l sai s ca phng tin o khi i lng o khng bin i theo thi

gian

- Sai s ng l sai s ca phng tin o khi i lng o bin i theo thi gian

1.6.3 BIU THC BIU DIN SAI S

- Sai s tuyt i: l hiu gia kt qu o c vi gi tr thc ca i lng o

to XXX

- Sai s tng i chn thc: l gi tr tuyt i ca t s gia sai s tuyt i v

gi tr thc ca i lng o

[%]100.tX

X

- Sai s tng i danh nh:

[%]100.

dX

X

- Sai s tng i qui i: l gi tr tuyt i ca t s gia sai s tuyt i v gi

tr nh mc ca thang o.

[%]100.m

qX

X

Trong : Xdm= Xmax -Xmin : gi tr nh mc ca thang o.

Nu gi tr thang o: 0Xmax th Xm=Xmax (gi tr ton thang - full-scale)

+ chnh xc (Accurate) : Mc gn gi tr thc ca i lng o v gi tr o

c:

t

ot

X

XX1A

.100 [%]

+ chnh xc tng i:

100100.t

ot

X

XX100a

V d: in p 2 u in tr c tr s tin cy c l 50V. Dng vn k o c 49

V.

Nh vy sai s tuyt i: 1VU

13

Sai s tng i: 2%100%50V

1Vct

chnh xc: 2%100%98%a 0,98;0,021A

+ r (Precision): nh gi mc ging nhau ca mi kt qu o vi nhiu kt

qu o khc ca mt i lng o duy nht vi cng my o v iu kin o:

n

n

X

X i

X1iP

nX : tr s trung bnh ca n ln o, Xi - kt qu ca ln o th i.

V d: X5= 97 Kt qu o ca ln o th 5

nX =101,1 - tr s trung bnh ca 10 ln o

Tnh chnh xc ca kt qu o Xi l : %9696,01,101

1,101971

.

1.7 H THNG O LNG

1.7.1 H thng o lng dng tng t

a. H thng o lng 1 knh

Hnh 1.3 H thng o lng analog 1 knh

14

Tn hiu o c to ra t cm bin o lng (transducer) do i lng o

tc ng vo. Tn hiu ny i qua mch ch bin tn hiu (signal conditioner). Sau

i vo b phn trnh by kt qu (display) v thit b ghi (record) cho b

phn c kt qu s dng ngay kt qu o ny. Ngoi ra h thng o lng cn lin

kt vi h thng iu khin t ng bng cch ly tn hiu o ng ra ca mch

ch bin tn hiu a qua mch so snh vi tn hiu chun iu khin i tng

(i lng) ang c o. V d: i lng o l nhit th i tng iu khin

cng l nhit .

b. He thong o lng nhieu kenh:

Trng hp can o nhieu ai lng th moi ai lng o mot

kenh. Nh vay sau moi tn hieu o c lay ra t mach che bien tn hieu

moi kenh se a qua mach phan kenh (multiplexer) e c sap xep

tuan t truyen i tren cung mot he thong dan truyen (day dan hay vo

tuyen). e co s phan biet cac ai lng o, trc khi a vao mach

phan kenh can phai ma hoa hoac ieu che (Modulation MOD) theo

tan so khac nhau (v du nh f10,f20) cho moi tn hieu cua ai lng o.

Tai ni nhan tn hieu lai phai giai ma hoac giai ieu che (demodulation

DEMOD) e lay lai tng tn hieu o. ay cung la hnh thc o lng

t xa (telemety) cho nhieu ai lng o.

15

Hnh 1.4 H thng o lng analog nhiu knh

1.7.2 H thng o lng dng s

Thiet b vi x ly (Microprocessor - P) tham gia vao he thong o lng

nham muc ch x ly nhanh tn hieu o, chong nhieu tot hn so vi tn

hieu o dang Analog khi truyen i xa. Cach ly tot hn va de thc hien

hn neu dung phng phap quang hoc (dung cach thc ghep bang tn

hieu quang (opto coupler). ay cung la hnh thc thng dung hien nay.

Vi s phat trien cua may tnh ca nhan (PC), he thong o lng dung

ky thuat so dung PC e t ong hoa he thong o lng mc o

16

cao hn va thuan li hn khi s dung. Do o, chung ta bc sang mot giai

oan mi May tnh hoa thiet b o lng (computerized instrumentation).

Trong he thong o lng dung ky thuat so, tn hieu dang Analog

c chuyen oi sang tn hieu dang so (digital) bang cac mach ADC

(analog digital s dung, chung ta dung mach DAC (digital analog converter)

e chuyen oi lai.

Ngoai ra he thong o lng dang so con co u iem la s hoat

ong thong minh nh vao chng trnh phan mem (software) cai at vao

may tnh e x ly tn hieu o lng va ieu khien he thong t ong

hoa

Hnh 1.5 H thng o lng digital

1.7.3 H thng o lng t xa

He thong o lng dang so nh s ket noi vi may tnh, a ieu khien t

xa (remote) cac chc nang cua he thong o lng bang cach s dung cac ng

truyen so lieu (BUS) cua bo vi x ly (P). He thong c trnh bay hnh 1.6

Nh may tnh PC ieu khien thiet b o lng thong qua bo giao tiep chuan

(interface bus standard) thong dung la IE 488 hoac RS232C. Phan giao tiep

truyen so a nang (GPIB - general purpose interface bus) c thiet ke e thc

hien s ieu khien.

17

Hnh 1.6 H thng o lng t xa

18

CHNG 2

CC C CU CH TH TRONG MY O

C cu ch th ng mt vai tr quan trng trong thit b o, n c nhim v

hin th chnh xc kt qu o di dng ph hp vi kh nng nhn bit ca con

ngi. C cu ch th cng nh hng n chnh xc, tc ca my o

Trong thc t c nhiu dng c cu ch th khc nhau dng cho o lng, mi loi

c cu o c nhng u nhiu im khc nhau v k thut o, gi thnh, v cng

ngh ch to Cc loi c cu ch th ph bin nh sau:

+ Cc c cu ch th kim.

+ ng tia in t CRT

+ C cu ch th s (dng LED 7 on hay LCD 7 on).

+ Mn hnh ma trn (LED, LCD, Flasma, OLED).

2.1 C CU CH TH KIM (C cu o in c bn - CC)

C cu ch th kim hay cn gi l c cu o in c bn (CC) dng nhiu

trong cc thit b o in (nh o dng in, o in p, o cng sut, o in tr,

) tn s thp. y l nhng dng c o bin i thng. i lng in cn o

X (dng in mang thng tin ca i tng o) c bin i thnh gc quay ca

phn ng (phn c gn kim ch th) so vi phn tnh =f(X)

Mi dng c cu o c cch to ra nng lng in t v cch bin i thnh c

nng to ra momen quay khc nhau. Da vo cc bin i ngi ta phn chia

CC thnh cc loi khc nhau nh sau:

Loi c cu o K hiu

C cu o t in

C cu o in t

C cu o in ng

C cu o tnh in

C cu o cm ng

Log mt in ng

Log mt in t

Log mt t in

19

a. C cu o t in

C cu o t in hot ng theo nguyn l bin i in nng thnh c nng to ra

momen quay nh s tng tc gia t trng ca 1 nam chm vnh cu v t

trng ca dng in I qua khung dy ng.

Hnh 2.1 Cu to ca c cu o t in

Cu to

Cu to ca c cu o t in nh hnh 2.1, gm 2 phn c bn:

Phn tnh: Gm nam chm vnh cu hnh ch U c ch to bng thp c

bit nh hp kim Vnfram, hp kim Crm, 2 m cc t, li st t hnh tr. Gia 2

m cc t, li st t hnh tr to thnh khe hp hnh vnh khuyn cho php khung

dy quay xung quanh v c t trng ng hng tm, khe hp ny c t cm B

ng u.

20

Phn ng: Gm :

- Khung quay: khung ch nht bng nhm, trn khung c cun dy ng cch

in (c 0,03 0,2mm) cho php dng in I chy qua. Ton b khi lng khung

quay phi cng nh cng tt sao cho momen qun tnh rt nh. Khung quay c

t trn trc quay hoc bi dy treo. Dng in I c a vo khung dy thng

qua trc ca khung dy.

a. Khung quay loi trc quay b. Loi dy treo

Hnh 2.2 Khung quay

- Kim ch th c gn cht trn trc quay hoc dy treo. Pha sau kim ch th

c mang i trng sao cho trng tm ca kim ch nm trn trc quay hoc dy

treo v ngoi ra cn c vt iu chnh lch khng (iu khin zero).

- L xo phn khng mt u gn vo trc quay u kia c gi c nh c

nhim v ko kim ch th v v tr ban u hoc to ra lo xo phn khng gi kim ch

th ti v tr cn bng.

Nguyn l hot ng

Khi c dng in i vo cun dy, trn khung dy s xut hin lc in t F:

F = N.B.l.I (2.1)

trong : N - s vng dy qun ca cun dy

B - mt t thng xuyn qua cun dy

l - chiu cao ca khung;

I - cng dng in.

Mmen quay Mq ca lc in t F:

Mq = F.W = N.B.l.W.I (2.2)

trong W l b rng ca khung quay

v i Kq = N.B.l.W - h s t l vi s cu to ca c cu l hng s

Ta c mmen quay Mq ca lc in t F:

Mq = KqI

21

Hnh 2.3: Nguyn l hot ng

ng thi khi l xo (hoc dy treo) to ra mmen cn Tc khi kim ch th

quay do mmen quay Mq lm xon l xo kim sot hoc dy treo:

Mc = Kc (2.6)

Kc - hng s xon ca l xo kim sot hoc dy treo

- gc quay ca kim ch th. Ti gc quay i ca kim ch th ng yn:

Mq = Mc; KqI = KcI

(2.7)

Gc quay I t l tuyn tnh vi dng in I

S m (cn du) cho kim ch th

Khi kim ch th di chuyn di tc ng ca Mq cng xut hin mmen m

Md do dng in ng pht sinh trong cun dy do t thng xuyn qua khung quay

thay i tc thi (H.2.4):

trong : ed:- sc in ng ng; Ri - in tr ca khung quay

22

RD: - in tr m ni hai cun dy.

Hnh 2.4: S m cho kim ch th

- Trng hp RD, khng c mmen m, kim ch th d b dao ng

quanh im s dng li ca kim, v cun dy b h mch khng c dng id trong

khi vn c ed.

- Trng hp RD0, mmen m ln nht c s m cht lm cho s di

chuyn ca kim rt chm v kh khn hn khi b dao ng c hc do di chuyn c

cu o.

- Trng hp RDRDC, in tr m ng mc, kim ch th di chuyn nhanh

khi c dng in vo v khng b dao ng quanh v tr dng ca kim.

Kt lun: lch gc quay ca kim ch th t l tuyn tnh vi cng

dng in qua khung dy. Nh vy, c th khc thang o ca dng in I tuyn

tnh theo gc quay ca kim ch th.

c tnh ca c cu o t in:

- Thang o tuyn tnh.

- Ch lm vic vi dng 1 chiu qua khung dy.

- nhy dng in ca c cu o t in:

0SK

K

dI

dS

C

II

23

Ngha l nhy dng in c tng ng vi s bin thin ca gc quay khi c

s bin thin ca dng in. Trong thc t ngi ta thng dng Imax (dng in ti

a) ca c cu ch th xc nh nhy ngha l nhay cng ln khi Imax cng

nh v max (gc quay ln nht) ca c cu ch th ging nhau (vo khong 1050) .

- Dng ton thang (Itt) rt nh (vi A )

- nhy in p ca c cu: dU

dSV

. Nu ni tr ca khung dy l Ri th:

I

ii

V SRdIR

dS

1

u im: CC t in c u im so vi nhng CC khc nh nhng im sau

y:

- chnh xc cao, c th to ra cc thang o c cp chnh xc ti 0,5%,

- T trng ca nam chm vnh cu mnh nn nhy t b nh hng ca t

trng bn ngoi.

- Cng sut tiu th nh, ty theo dng Imax m cng sut tiu th khong t

25W n 200W.

- V gc quay tuyn tnh theo dng in cho nn thang o c khong chia u

n.

Nhc im

- Cun dy ca khung quay thng chu ng qu ti nh nn thng d b h

hng nu c dng in qu ln i qua.

- Ch s dng vi dng mt chiu.

- Cu to phc tp, d b h hng khi c va p mnh.

ng dng:

C cu o t in c dng rt nhiu lm c cu ch th cho cc thit b o in

nh Vn mt, Ampe mt, dng c o in vng nng, c cu ch th trong php o

cu cn bng

24

Hnh 2.5 Mt s thit b o in s dng CC t in

in k gng quay (H.2.5): Khung quay mang gng phn chiu v h

thng quang hc chiu tia sng vo gng v m sng trn ghi kt qu dng in

i qua. Kt qu c ghi trn thc chia hoc trn giy nhy quang

Hnh 2.6 in k gng quay

b. C cu o in t

C cu o in t hot ng theo nguyn l: nng lng in t c bin i

lin tc thnh c nng nh s tng tc gia t trng ca cun dy tnh khi c

dng in i qua vi phn ng ca c cu l cc l st t.

CC in t c 2 loi:

Loi lc ht (loi cun dy hnh dt)c cu to nh hnh 2.7

Loi lc y (loi cun dy hnh trn) c cu to nh hnh 2.8

Cu to

+ Loi cun dy hnh trn:

25

Hnh 2.7 - C cu in t loi lc y

- Phn tnh: l 1 cun dy hnh tr, pha trong thnh ng c gn l st t mm un

quanh.

- Phn ng: gm 1 l st t cng c un cong v gn vo trc quay nm

i din. Trn trc quay c gn kim ch th v l xo phn khng.

+ Loi cun dy dt:

Hnh 2.8 C cu in t loi lc ht

- Phn tnh: gm 1 cun dy dt, gia c 1 khe hp.

- Phn ng: Gm 1 a st t c gn lch tm, ch 1 phn nm trong khe

hp v c th quay xung quanh trc. Trn trc ca a st t c gn kim ch th v

l xo phn khng.

Nguyn l hot ng chung:

Cun dy tnh khi c dng in I (mt chiu hoc xoay chiu) i qua s to

ra 1 nng lng t:

2

2

1LIWdt

Trong : L l in cm cun dy, tu thuc vo v tr tng i ca l st t ng

v tnh. L=f(x).

Momen quay l: d

dWM dtq

Khi kim ch th quay, mmen phn khng tng: Mpk=-D

Ti v tr cn bng:

26

2

0

0

2

2

2

1,

2

1

2

1

IS

d

dL

DSI

d

dL

D

d

dLI

d

dWD

MM

dt

qpk

Gc quay ca kim ch th t l vi bnh phng ca dng in qua cun dy.

Hnh 2.9 ng h o in p cao s dng CC in t

c im ca CC in t:

- Tiu th nng lng nhiu hn c cu o t in.

- Lm vic c vi c dng in mt chiu v xoay chiu.

- Thang o phi tuyn.

- Cng ngh ch to d dng hn, c cu vng chc, kh nng chu ti tt.

- nhy km do t trng phn tnh yu.

- C hin tng t d trong l st non nn km chnh xc hn c cu o t in.

- Do t trng to ra bi cun dy nh nn d b nh hng bi t trng bn

ngoi, cn phi bo v bng cch chn t cho c cu.

- chnh xc thp do d b nh hng ca t trng bn ngoi v do tn hao ca

st t ln. Tuy nhin vn c dng nhiu trong cc loi ng h o in p cao.

c. C cu o in ng

L c cu c s phi hp gia c cu t in v c cu in t. Hot ng

theo nguyn l bin i lin tc in nng thnh c nng nh s tng tc gia t

trng ca cun dy tnh v cun dy ng khi c dng in i qua.

Cu to

27

C cu o in ng cng c 2 loi l C cu in ng (a) v c cu st in

ng (b), cu to nh hnh 2.10

Cu to c cu in ng gm c cun dy tnh v cun dy ng (khung

quay). Thng thng cun dy ng khng c li st non trnh c hin tng t

tr v dng in xy. Cun ng nm trong vng t trng c to ra bi cun

tnh. Nu cun tnh c cun trn mt li st t th l c cu st in ng.

(a) (b)

Hnh 2.10 C cu o in ng

Nguyn l hot ng

Khi c dng in I1, I2 (mt chiu hoc xoay chiu) i vo cun dy ng v

cun tnh s to ra momen quay:

Mq=KqI1I2=NBSI1I2 (dng in DC)

Hoc

dtiiT

KM qq 211

(dng in AC)

Vy gc quay 21II

D

Kq hoc

dtiiTD

Kq21

1

Trong D l h s ca l xo phn khng hoc ca dy treo.

thang o tuyn tnh theo I1I2 th Kq/D l hng s.

c im ca c cu o in ng

C cu o in ng c u im l nhc im ca c cu t in v c cu

in t

Thng dng lm b ch th cho Vnmt hoc Ampemt hay Watt mt cng

sut ti 1 pha hay 3 pha.

28

Ngoi ra ngi ta cn s dng ch to t s k in ng dng o h s

cng sut cos.

Chiu quay ca c cu in ng v st in ng c xc nh trc khi

hot ng vi dng xoay chiu (nh hnh 2.11). Nh vy khi kim ch th ca c cu

b lch ngc th phi i cc tnh ca cun dy kim ch th quay thun

C cu in ng hay c s dng cho thit b o cng sut ca in p cao.

Hnh 2.11 Chiu quay ca kim ch th ph thuc vo chiu dng in

Hnh 2.12 ng h o cng sut in p cao dng CC in ng.

2.2 NG TIA IN T(CRT)

ng tia in t - CRT (Cathode Ray Tube), hay cn c s dng kh ph

bin trong gi l ng tia m cc c my o lng in t (nh Oscillocope, my

phn tch ph, my v c tnh bin - tn s), cho php hin th dng tn hiu

trn mn hnh.

Nguyn l chung ca CRT:

Cho chm tia e- bay trong chn khng i qua ln lt 2 bn kim loi nm ngang

nhn tn hiu in p y v 2 bn thng ng nhn tn hiu in p x v p ti mn

29

hunh quang (nh hnh 2.13). Do e- chu tc dng ca in trng to bi 2 cp bn

kim loi nn e- s phi bay lch theo phng x v y, lch theo phng y t l

vi tn hiu y, lch theo phng x t l vi tn hiu x. Kt qu vt sng trn mn

hunh quang s nm ti to (x,y). Khi tn hiu x v y thay i vt sng v mt

ng dao ng no trn mn hnh.

y

x

y

x

e-

Mn hunh quangCp l i ngCp l i ngang

Hnh 2.13 Nguyn tc chung ca CRT

Nh vy, CRT l mt loi dng c in t m trong c chm in t e- c

bc x t Katt b nung nng, chng c gia tc, hi t bng in trng hay t

trng, to thnh mt chm in t nh gn bn ti mn hunh quang (hp cht

ca Phosphor), mn hnh pht sng ti im c in t bn ti. Chm in t c

lm lch theo chiu ng v chiu ngang trn mn hnh theo quy lut in p t

vo cc tm lm lch, to ra dng hnh nh (dng dao ng ) trn mn hnh. Dao

ng c th l dng tn hiu (waveforms), hay l cc hnh nh (pictures)

C nhiu loi CRT: Loi CRT khng ch bng t trng (mn hnh ti vi v mn

hnh my vi tnh); Loi CRT khng ch bng in trng (dng nhiu trong cc

thit b o).

2.3 THIT B CH TH DNG LED

a. LED n

LED l mt tip xc p-n, vt liu ch to u l cc lin kt ca nguyn t

nhm 3 v nhm 5 ca bng h thng tun hon Mendeleev nh GaAs (LED c

mu ), GaP (LED c mu hoc mu lc), GaAsP (LED c mu hoc vng).

30

Hnh 2.14 LED n

c nh sng c mu khc nhau th s dng loi bn dn khc nhau hoc

dng nha bc c mu khc nhau.Thng thng LED pht ra tia hong ngoi hn

ngi ta thng bao quanh LED mt lp Phosphor v vy do bc x ca Phosphor

nhn thy ta nhn ra c nh sng pht ra.

Nhc im ca LED l cn dng tng i ln, nhng u im ca n l ngun

in p mt chiu thp, kh nng chuyn mch nhanh, bn, kch thc nh.

-ng dng ca LED.

Ngy nay, LED c coi l mt gii php tit kim nng lng mi. Vi cc u

im ni bt nh tiu hao nhit rt t, LED hu nh khng nung nng mi trng

xung quanh; nh sng n LED n nh, khng gy chi, mi mt, khng pht ra tia

cc tm; n LED c tui th ln n 80.000 100.000 gi. V vy, n LED ngy

cng c ng dng nhiu trong thc t. LED c dng lm b phn hin th

trong cc thit b in, in t, bin qung co, n trang tr, n giao thng n

chiu sng bng LED c u im bn, gn nh, tit kim nng lng.

+ LED c dng lm b phn hin th trong cc thit b in t

b. LED 7 on

Cc dng c o hin th s thng dng b ch th 7 on sng LED ghp li vi

nhau theo hnh s 8. Khi cho dng in chy qua nhng on thch hp c th hin

hnh bt k s no t 0-9.

31

Hnh 2.15- C cu LED 7 on

Cc cch mc LED thng dng: LED 7 on sng Ant chung, LED 7 on sng

Katt chung.

c. Mn hnh ma trn LED

Cc im LED c sp xp thnh ma trn im sng. V d ma trn

LED8x8 nh hnh 2.16, trong cc tn hiu iu khin hng Ri c ni vi

Anode ca tt c cc LED trn cng mt hng, cn cc tn hiu iu khin ct Ci

cng c ni vi Cathode ca tt c cc LED trn cng mt ct. Khi c mt tn

hiu iu khin ct v hng, cc chn Anode ca cc led trn hng tng ng

c cp in p cao, ng thi cc chn Cathode ca cc led trn ct tng ng

c c cp in p thp .Tuy nhin lc ch c mt LED sng, v n c ng

thi in th cao trn Anode v in th thp trn Cathode. Nh vy khi c mt tn

hiu iu khin hng v ct, th ti mt thi im ch c duy nht mt led ti ch

gp nhau ca hng v ct l sng. Cc bng quang bo vi s lng led ln hn

cng c kt ni theo cu trc nh vy.

32

Hnh 2.16 Ma trn LED

2.4 THIT B CH TH DNG LCD - Liquid Crystal Display

a. Cu to ca Mn hnh LCD

Kt hp c hai b lc phn cc v s xoay ca tinh th lng to ln mt mn hnh

tinh th lng.

Hnh 2.17- Nguyn l cu to mn hnh LCD

Polarizing Filters: B lc phn cc.

Alighnment layers: Sp xp lp.

Voltage: in p.

Light: nh sng.

Khi hai b lc phn cc sp xp dc sut theo hng vung gc vi trc

in cc, nh sng i vo t pha trn, i hng 90o dc theo hng ng

hnh son c ca cc phn t tinh th lng, v vy nh sng xuyn qua b lc

di.

Khi c in p t vo, cc phn t tinh th lng nn thng trn ng ra t

hnh ng son c v dng, i hng r ca nh sng, do vy ngn cn

nh sng xuyn qua b lc di (b lc thp)

33

Hnh v miu t nguyn l in hnh ca s xoay mn hnh tinh th lng

trong LCD, cc tinh th lng ni m cc phn t xoay hnh ng son c l

an xen gia hai b lc in cc (phn cc). Khi c in p t vo nh sng

b chn v mn hnh xut hin en.

b. Cc h thng hin th.

C hai kiu cu to mn hnh tinh th lng chnh, khc nhau thit k ngun sng.

Trong kiu th nht, nh sng c pht ra t mt n nn, c v s phng phn

cc nh cc nh sng t nhin. nh sng ny c cho lt qua lp knh lc phn

cc th nht, tr thnh nh sng phn cc phng ch c phng thng ng. nh

sng phn cc phng ny c tip tc cho truyn qua tm thy tinh v lp in

cc trong sut n lp tinh th lng. Sau , chng tip tc i ti knh lc phn

cc th hai; c phng phn cc vung gc vi knh lc th nht, ri i ti mt

ngi quan st. Kiu mn hnh ny thng p dng cho mn hnh mu my tnh

hay TV. to ra mu sc, lp ngoi cng, trc khi nh sng i ra n mt ngi,

c knh lc mu.

loi mn hnh tinh th lng th hai, chng s dng nh sng t nhin i vo t

mt trn v c gng phn x nm sau, di nh sng ny li cho ngi xem. y l

cu to thng gp cc loi mn hnh tinh th lng en trng trong cc thit b b

ti. Do khng cn ngun sng nn chng tit kim nng lng.

+ Mn hnh k t LCD 4x16

34

+ Mn hnh ha LCD

Hnh 2.18 Mn hnh ma trn LCD

35

CHNG 3

O DNG IN V IN P

o in p, o cng dng in l nhng php o c bn c s dng nhiu

khng ch trong k thut m trong c cuc sng hng ngy. Cc tham s ny c th

c o trc tip, gin tip v so snh.

3.1 O DNG IN

Php o dng in c phm vi o rng (t vi pA n v vi MA), di tn rng (t

o dng 1 chiu n o dng xoay chiu tn s ti hng GHz). Ty phm vi o v

di tn o li s dng cc phng php o khc nhau. Tuy nhin trong thc t php

o dng in thng ch c thc hin di tn ti hng trm MHz, cn di tn

s siu cao ngi ta thng o cng sut.

Dng c o dng in c gi l Ampe mt (Ampe k), vi ng h vn nng khi

chc nng o dng th cng c gi l Ampe mt. K hiu ca Ampe mt

trong s l mt vng trn c ch A gia v c th thm k hiu cc cc dng

v m hai bn cho dng in mt chiu:

a. o dng in dng c cu o

a.1 o dng in mt chiu DC

Nguyn l o: c ba c cu ch th ni chng 2 u hot ng vi dng

DC cho nn c dng lm b ch th ampe-k DC. Nhng cn phi m rng tm

o (Range) cho thch hp.

M rng tm o: cho ampe-k c nhiu tm o thch hp, mch o phi c

s m rng tm o cho tng loi c cu ch th.

A

+ -

36

Hnh Mch o dng

M rng tm o cho c cu t in. Dng in tr shunt (H.2.12).

Dng in o: I = Im + IS (2.14)

trong : Im - dng in qua c cu ch th.

IS - dng in i qua in tr shunt.

in tr shunt RS c xc nh:

max

max

mS

t

I RR

I I

trong : Rm - in tr ni ca c cu ch th.

I max - dng in ti a ca c cu ch th.

I t - dng in ti a ca tm o.

V d

50 x 1

(1 50 )S

A KR

mA A

vi Imax = 50 A ; Rm = 1k ; It = 1mA

6 3

6

(50 x 10 x 10 )52,6

(950 x 10 )S

VR

A

i vi ampe-k c nhiu tm o th dng nhiu in tr shunt, mi tm o c

mt in tr shunt, khi chuyn tm o l chuyn in tr shunt (H.2.13). c nhiu

tm o

37

Hoc dng cch chuyn tm theo kiu shunt Ayrton (H.2.14).

Mch o kiu shunt Ayrton c 3 tm o B, C, D. Khi kha A v tr B (tm o

nh nht).

in tr shunt: RSB = R1 + R2 + R3

v tr C: : RSC = R1 + R2 Cn R3 ni tip vi c cu ch th.

v tr D: RSC = R1 Cn R2 + R3 ni tip vi c cu ch th.

V d 2.2: Rm = 1k ; Imax ca c cu 50A. Xc nh ba tm o:

B (1mA); C (10mA); D(100mA) cho R1 , R2 , R3

Gii: tm B (1mA):

(1mA 50A)( R1 + R2 + R3) = 1k50A

3

1 2 3 6

50.1052,6

950.10R R R

tm C (10mA):

6

3 31 2 6

(1 ).50.10 (1 )

9950.10 199

k R A k RR R

A

tm D (100mA):

6

2 32 21 6

(1 )(1 ).50.10

99950.10 1999

k R Rk R R AR

A

38

Thay vo ta c:

31 2 3

(1 )52,6

199

k RR R R

Suy ra

3

(10467,4 1000)47,337

200R

11

1

(1000 52,6 )

1999

0,526

RR

R

Cn R2 = 52,6 (47,337+ 0,526) = 47,37

M rng tm o cho c cu in t: thay i s vng dy qun cho cun

dy c nh vi lc t ng F khng i:

F = n1I1 = n2I2 = n3I3 = ...

V d: F = 300 Ampe vng cho ba tm o; I1 = 1A ;I2 = 5A ; I3 = 10A .

Khi n1 = 300 vng cho tm o I1 = 1A

n2 = 60 vng cho tm o I2 = 5A

n3 = 30 vng cho tm o I3 = 10A .

M rng tm o cho c cu in ng: Trong trng hp ampe-k dng c cu

ch th in ng (st in ng) c mc nh hnh 2.15 th s m rng tm o

bng cch mc in tr shunt song song vi cun dy di ng (nh c cu t in)

trong khi cun c nh c mc ni tip vi cun dy di ng. Cch tnh ton in

tr shunt cng ging nh ampe-k c cu t in.

39

b. o dng AC

Nguyn l o: C cu in t v c cu in ng u hot ng c

vi dng AC. Do c th dng c cu ny trc tip v m rng tm o

dng nh ni trn. Ring c cu in t khi dng phi bin i dng

AC thnh dng DC. Ngoi ra, do tnh chnh xc ca c cu in t nn c

cu ny dng rt nhiu (thng dng) trong phn ln ampe-k (trong my o

vn nng Multimeter cn gi V.O.M.).

Dng c cu t in o dng AC

Dng phng php chnh lu bng diod

Dng in qua diod ni tip vi c cu t in l dng in xoay chiu

chnh lu thnh dng DC. Tr trung bnh ca dng in chnh lu:

*

max

0

1T

cltb dI i dt IT

V d: Dng in AC: IAC = Im sint

Khi :

sin (0 t )2

0 ( t )2

m

cl

TI t

iT

T

Vy 0,318 0,318 2cltb m hdI I I (i vi tn hiu sin). Trng

hp dng in AC c dng bt k th Icltb c tr s ph thuc vo dng v tn

s ca tn hiu. C th dng: iAC =2sin100t(mA) th dng: Icltb =

0,3182(mA) = 0,636(mA)

Dng phng php chnh lu bng cu diod

Dng in xoay chiu c chnh lu hai bn k, khi tr chnh lu trung

bnh:

2cltb clI i dt

T

V d: IAC = Im sint; Icltb = 2 x 0,318 Im = 0,636Im

40

3.2. O IN P

Php o in p c phm vi o rng (t vi V n v vi kV), di tn rng (t o

dng 1 chiu n o dng xoay chiu tn s ti hng GHz). Ty phm vi o v di

tn o li s dng cc phng php o khc nhau. Tuy nhin trong thc t php o

in p thng ch c thc hin di tn ti hng trm MHz, cn di tn s

siu cao ngi ta thng o cng sut.

Dng c o dng in c gi l Vn mt (Vn k), vi ng h vn nng khi

chc nng o in p th cng c gi l Vn mt. K hiu ca Vn Ampe mt

trong s l mt vng trn c ch V gia v c th thm k hiu cc cc dng

v m hai bn cho dng in mt chiu:

Khi o in p phi c mc song song Vn met vi on mch cn o in p.

gim nh hng n mch in cn o, dng in trong mch ca Vn mt phi

cng nh cng tt. iu ny ngha l tr khng tng ng ca Vn mt ZV trong

mch in phi ln hn rt nhiu tr khng tng ng ca on mch cn o

in p.

Khi mc Vn mt vo mch in mt chiu, ch ni cc cc in theo ng chiu

in p. Lun chn thang o ph hp trc khi o: chn thang ln nht trc, ri

h dn cho n khi thu c kt qu nm trong thang o.

a. o in p mt chiu DC

Nguyn l o: in p o c chuyn thnh dng in o i qua c cu ch th

o.

C cu mt thang o:

V

+ -

41

in p o:

V= URs + Um

Nu c cu ch th c Imax v in tr ni tip th:

mS RR

VoI

vi Rm l ni tr ca c cu

RS: in tr ph

Tng tr ca vn-k : ZV = Rs + Rm

M rng tm o

C cu o t in lm vic vi in p mt chiu, nhng in p ton thang kh

nh, nn do phi m rng thang o in p cho ph hp bng cch mc CC

ni tip vi in tr ph Rp.

Hnh: Mch o in p DC nhiu tm o

b. o in p xoay chiu AC dng c cu t in

Nguyn l o: i vi c cu in ng v in t hot ng vi tr hiu dng ca

dng xoay chiu, vn-k AC dng c cu ny phi mc in tr ni tip vi c cu

42

nh trong vn k DC. Cn c cu o t in ch lm vic vi in p mt chiu, do

o khi o in p xoay chiu AC phi bin i in p AC thnh in p DC t

vo CC theo cc cch khc nhau nh: dng Diode chnh lu hoc b bin i

nhit in.

Dng phng php chnh lu bng Diode

Hnh: Mch o in p AC

Mch o c mc nh hnh , diode D1 chnh lu dng AC bn k dng, diode

D2 cho dng in m i qua ( khng qua c cu ch th) cho in p nghch ca

bn k m ca in p AC khng ri trn D1 v c cu ch th. Trnh in c

in p nghch qu ln khi o in p AC c gi tr ln.

Ta c:

)()1()( RMSDVhdImRRRMSACV

M: hdI

hdIIcltbI 2318,0

2

max

Ihd dng in ca in p o VAC ( tnh theo tr hiu dng) qua c cu ch th v

qua R1 tng ng.

hdI

DVRMS

ACV

mRR

)(

1

Dng phng php chnh lu bng Diode cu

43

Ta c:

hdI

DVRMS

ACV

mRR

2)(

Nh vy vi cng mt c cu ( c cu t in) tng tr vo ca vn k AC s nh

hn tng tr ca vn k DC.

i vi vn k in t, tng tr vo ca vn k khng thay i theo tm o i vi

in p AC hoc DC.

Nhc im ca vn k AC dng Diode chnh lu ph thuc vo dng tn hiu v

tn s cao c nh hng ca tng tr v in dung k sinh ca diode.

vn k AC khng ph thuc vo dng v tn s ca tn hiu AC th dng vn

k c b bin i nhit in, cc in tr thay i tm o c ni tip vi in tr

cung cp nhit lng cho cp nhit in.

Hnh: Mch o in p AC dng b nhit in

44

3.3 nh hng ca vn k trn mch o in p

Khi vn k c mc vo phn t cn o in p th c th xem nh tng tr

vo ca vn k mc song song vi phn t .

V d: Vn k o in p hai u in tr R2

Hnh: Mch tng ng khi mc vn k

Dng in i qua R2 khi khng c vn k: I = V/(R1+R2)

Dng in i qua R2 khi c Vn k:

)//2

(1

'

VRRR

VI

Nu R2 rt nh so vi Rv dn n (R2//RV) R2. Khi nh hng ca Vn k

khng ng k i vi mch.

V d: V=+10V; R2 = R1 = 10K; RV = 200K

Ta c:

VKx

K

VV 510

20

10

2

Khi vn k:

VVxK

KV

K

V

KKK

VI

88,4105,19

5,9'

2

5,19

10

)200//10(10

10'

Sai s do nh hng ca vn k: [1-(4,88V/5V)]x100% = 2,4%

45

CHNG 4

O IN TR IN DUNG , IN CM

4.1 O IN TR

4.1.1 O IN TR BNG VN K V AMPKE K

Hnh 4.1 Mch o Rx bng vn k v ampe k

y l phng php xc nh phn t in tr ang hot ng (o nng) theo

yu cu. C hai cch mc o in tr:

Hnh 4.1a: Vn-k mc trc, ampe-k mc sau (li mc r di). Khi in

tr cn o RX c xc nh bi:

I

V

xR

Trong V- cho bi Vn k; Ia - cho bi Ampke k

Theo mch o: V = Va + Vx

Vi Va l in p ri trn Vn k; Vx l in p ri trn Ampe k

Ta thy c sai s trong vic xc nh RX do nh hng ni tr ca ampek. Nu

Ra (ni tr ca ampe-k) rt nh so vi RX th VX>Va. Sai s do nh hng ca

ampe-k khng ng k.

Hnh 4.1b: Ampe-k mc trc, vn-k mc sau (li mc r ngn). in tr RX

vn c xc nh bi:

I

V

xR

trong : I = IX+ IV - cho bi ampe-k vi IV dng in i qua vn-k.

46

Nu tng tr vo ca vn-k rt ln so vi RX th sai s do nh hng ca vn-

k khng ng k.

V d 4.1: o in tr r ca t in (RX) khi hot ng in p qui nh. Mch

o c mc theo hnh 4.2. Vn-k c tm o 50V v nhy 20k/VDC c mc

ni tip vi t in C cn o. Kim ch th in p 10 vn.

Hnh 4.2: o in tr r Rx

Khi o in p ri trn t in.

VC = VS V = 300V 10V = 290V

Dng in ti a Imaxca c cu ch th bng 50 A (kim ch 10V)

Vy in tr r ca t in

MA

V

xR 29

10

290

V d 4.2: Trong mch hnh 4.1a, vn-k c nhy 10k/V ch 500 vn v ampe-

k ch 0,5A c RA = 10. Vn-k t tm o 1000V. Xc nh in tr RX.

Gii:

Theo vn k v ampe k: 1000

5.0

500

A

V

I

V

xR

Nu phn tch ta c: V = Vx + Va = (Ra + Rx).I; Ra + Rx = 1000

Suy ra gi tr thc ca Rx = 1000 - 10 =990

Vy sai s so nh hng ca ampe k v vn k:

%1%1001000

10

V d 4.3 Nu vn-k v ampe-k c mc theo hnh 4.1b th vn-k v ampe-k

c bao nhiu? Khi R = 990

Gii: in tr tng ng gia vn-k v RX.

RV // RX = (10k/V1000V) // 990 = 989,9

47

Vn-k ch th:

VV

xR

vR

aR

xR

vR

Vx

V 4959,999

9,989500

)//(

)//(

500

Ampe k ch th:

AV

xR

vR

vV

xI

vII 5,0

9,989

495

//

Do hai v d 2 v 3 l nu o RX bng cch ly tr s c ca vn-k chia

cho tr s c ca ampe-k th tr s c ca v d 3 chnh xc hn v:

RX = V/I=495V/0,5A=990.

Trong khi tr s o c v d 2: RX = 500V/05, A =1000

4.1.2 O IN TR DUNG OHM K

Trong my o vn nng (multimeter V.O.M) c phn o in tr (ohmk).

Trong trng hp dng ohm-k o in tr th trng thi o l phn t in tr

o (RX) khng c nng lng (o ngui), mch o s l ngun nng lng ring

(ngun pin).

a. Mch nguyn l o in tr

Mch o c mc theo hnh 4.3

Hnh 4.3 a. Mch ohm k b. Thang o khng tuyn tnh ca ohm k

48

y l mch ohm k kiu mc ni tip, dng in qua c cu ch th R1:

mRR

xR

bE

mI

1

Vi R1: in tr chun ca tm o; Rm : in tr ni ca c cu.

Khi Rx 0 ; Im Imax (dng cc i ca c cu o)

Khi Rx ; Im 0 (khng c dng qua c cu o)

V d: Eb = 1,5V; Imax = 100A; R1 + Rm = 15k.

Xc nh ch th ca kim khi RX = 0 v s ch th tr s in tr khi Im = 1/2

thang o; 1/4 thang o; 3/4 thang o.

Gii: T phng trnh trn khi RX 0:

Ak

IV

100150

5,1

Ti tr s 1/2 thang o: I =100A/2 = 50A

km

RRx

RA

V15)

1(

50

5,1

Khi dng Im = 1/4 thang o: Im= 25A Rx = 45k

Ti dng Im = 3/4 thang o: Im = 75A Rx = 5k

Nh vy gi tr thang o in tr khng tuyn tnh theo dng in I (H.4.3b).

b. Mch o in tr thc t

Trong thc t ngun pin Eb c th thay i. Khi RX 0, Im qua c cu khng

bng Imax, do mch o c th mc thm R2 (H.3.5) bin tr ny dng chnh

im 0 cho mch o khi Eb thay i. Nh vy trc khi o phi ngn mch hai

u AB, iu chnh R2 sao cho ohm-k ch 0.

49

Theo mch trn ta c:

mRRR

xR

bE

bI

//21

Nu R2 // Rm nt R1 th

1R

xR

bE

bI

Nh vy in p : Vm = Ib (R2 //Rm)

S c dng Im qua c cu ch th:

mR

mRR

bI

mR

mV

mI

)//2

(

Do mi ln o Rx 0, iu chnh R2 c:

max.1

)//2

(

I

mRR

mRR

bE

mI

Sao cho khi Eb c s thay i th s ch th Rx s khng thay i.

4.2 O IN DUNG

Mch o c mc theo hnh 4.1. Tng tr ca in dung CX c xc nh bi

vn-k v ampe-k (nu s hao mt do in mi ca t in khng ng k).

Hnh 4.5: Mch o Cx dng Vn k v ampe k

50

V

I

xC

xCI

V

xC

Z 1

Ngun tn hiu cung cp cho mch o phi l ngun tn hiu hnh sin, c

mo dng nh (ha tn c xem khng ng k). Bin v tn s ca tn hiu

phi n nh (khng thay i). Nu tn hiu c sng hi (ha tn) bc cao th s to

ra sai s ng k cho kt qu o.

4.3 O IN CM

Hnh 4.6: Mch Lx, Rx dng vn k v ampe k

Mch o in cm Lx c mc nh hnh 4.6. Tng tr ca in cm Lx c xc

nh:

22222

xRZ

I

xL

xL

xR

I

V

xL

Z

51

CHNG 5

MY HIN SNG (OSCILLOCPOE)

5.1 GII THIU CHUNG

Trong lnh vc in, in t, v Vin thng c nhiu dng tn hiu khc nhau, mi

dng tn hiu c mt s tham s c trng no . Trong o lng in t, mt

trong nhng yu cu c bn xc nh tn hiu l quan st dng ca tn hiu.

Hnh 5.1- Hnh nh My hin sng (oscillocope)

Thit b trc tip dng nghin cu dng ca tn hiu gi l my hin sng

(oscillocope). Oscillocope thc hin v dao ng ca tn hiu trn mn hnh.

5.1.1 Cc u im v kh nng ng dng ca Osicllocope

Oscillocope l loi thit b o a nng: ngoi vic cho php quan st dng tn hiu,

cn c th o c hu ht cc thng s ca cc loi tn hiu in. Ngoi ra cn c

52

th ghi li c trn phim nh cc gi tr tc thi ca cc tn hiu in bin i c

chu k hay phi chu k.

Oscillocope l loi my o c nhiu tnh nng tt nh : tr khng vo ln; nhy

cao (o c in p t vi V ti hng trc kV); qun tnh t, di tn rng (t 0 Hz

ti vi trc GHz), mn ch th (c th dng ng tia in t) kh sc nt v mn hnh

rng (t 70-150mm) my cng ln cht lng cng cao th mn sng hin th cng

ln....

Oscillocope c s v dng rt rng ri v l mt trong nhng dng c o quan

trng nht trong qu trnh kim tra mch v thit b in t, ch yu c dng

quan st dng tn hiu thay i theo thi gian u vo/ra, hay cc v tr khc nhau

trong mch, bn cnh n cn cho php o cc tham s ca tn hiu nh: Cc tr

s in p, chu k, tn s, gc lch pha, mo dng, h s iu ch,... Ngoi ra

khi kt hp vi mt s thit b chuyn i dng nng lng th Oscillocope c th

o lng c nhiu dng i lng vt l bin i khc nhau nh trong c hc,

trong sinh hc, trong y hc...

Bn cnh khi kt hp vi mt s thit b ph tr khc th Oscillocope c th tr

thnh my o cc thng s ca mch in t. (V d v c tuyn bin tn s ca

mch...).

Tm li Oscillocope l mt thit b o vn nng khng nhng c dng kh rng

ri trong lnh vc in t - Vin thng m cn c dng nhiu trong cc ngnh

cng nghip khc na.

5.1.2 Phn loi OSCILLOCOPE

C nhiu cch phn loi oscillocope khc nhau tu theo ng dng v cu to...

Phn loi theo ch ng b:

Oscillocope khng ng b dng quan st nhng tn hiu phi chu

k.

Oscillocope ng b dng quan st tn hiu c chu k.

Phn loi theo di tn lm vic:

Oscillocope tn s thp.

Oscillocope tn s cao,

Oscillocope tn s siu cao.

Phn loi theo cu to:

53

Oscillocope 1 knh.

Oscillocope 2 knh.

Oscillocope hn hp (2 knh tng t +16 knh tn hiu s)).

Oscillocope c nh kiu tng t hay kiu s.

Oscillocope xung quan st tn hiu c khong thi gian ton ti ngn.

5.2 OSCILLOCOPE TNG T

5.2.1 S khi v nguyn l lm vic ca oscillocope tng t 1 knh.

Nh xt trn, c rt nhiu loi oscillocope khc nhau: oscillocope s,

oscillocope tng t, oscillocope 1 tia hay 2 tia ... nhng sau y ta ch xt chi tit

cu to v nguyn l hot ng ca oscillocope tng t 1 tia.

a. Cu trc chung ca Oscillocope tng t dng CRT

Probe: Dy o

Vertical System: Knh lch ng Y

Attenuator: B phn p

Vertical Amplifier: Khuch i lch

ng Y

Trigger System: Khi kch khi (ng

b)

Horizontal System: Knh lch ngang X

Sweep Generator: B to in p qut

Horizontal Amplifier: Khuch i lch

ngang

Display System: Knh iu chnh sng

Z

54

Hnh 4.2- Cu trc chung ca Oscillocope tng t 1 knh

Cu trc chung ca Oscillocope tng t 1 knh dng CRT gm:

+ CRT: Mn ch th ng tia in t khng ch bng in trng. C nhim v hin

th dng sng trn mn hnh. y b phn trung tm ca Oscillocope. V l i

tng iu khin chnh trong oscillocope.

V cu to ng tia in t l mt ng chn khng v thu tinh, bn trong c cha

cc in cc. u ng hnh tr c cha sng in t v 2 cp phin lm lch. u

cui ng loe to hnh nn ct, y ng l mn hunh quang c tc dng pht sng

khi c tia e- p vo. ng in t c nhim v to ra tia e- xut pht t Catot n

mn hnh to ra vt sng c dng ph thuc vo quy lut ca tn hiu a n cc

phin lm lch Y1Y2 v X1X2 ca ng tia. Cc khi khc ca Oscillocope in p

iu khin cp li ng UY1Y2 v cp li ngang UX1X2.

+ Knh lch ng Y (Vertical System): C nhim v nhn tn hiu cn quan st

Uth c a vo t dy o (Probe) thc hin cc chc nng bin i tn hiu v

to ra tn hiu ph hp (dng in p i xng) a ti cp li ng Y1Y2 ca CRT.

+ Khi ng b (Triger System): hay cn c gi l Khi kch khi, c nhim

v nhn tn hiu ng b (tn hiu kch khi) Ub to ra xung ng b Uxb iu

khin knh lch ngang X.

+ Knh lch ngang X (Horizontal System): To ra in p qut rng ca hay nhn

tn hiu qut t bn ngoi (qua u vo Ext) to ra in p qut ngang a ti

cp li ngang X1X2 ca CRT.

Ty theo dng in p qut m hnh nh sng trn CRT c dng khc nhau:

+ Nu Uq l in p rng ca tuyn tnh th dao ng l dng tn hiu theo thi

gian.

+ Uq ging dng tn hiu vo th dao ng l cc hnh nh phc tp dng cnh

hoa, ... c gi l dao ng Lissajous.

b. Nguyn l v cc phng php qut

a in p ca tn hiu cn nghin cu ln cp phin lch Y, v in p qut rng

ca ln cp phin lch X. Do tc dng ng thi ca c hai in trng ln 2 cp

phin m tia in t dch chuyn c theo phng trc X v Y. Qu o ca tia in

t dch chuyn trn mn s vch nn hnh dng ca in p nghin cu bin thin

55

theo thi gian. Nu in p qut l hm lin tc theo thi gian th c gi l qut

lin tc, nu in p qut l hm gin on theo thi gian th c gi l qut i.

b.1. Nguyn l qut tuyn tnh lin tc

in p qut tuyn tnh lin tc c tc dng li tia in t dch chuyn lp i lp li

1 cch lin tc theo phng ngang t l bc nht vi thi gian. qut tuyn tnh

lin tc cn phi dng in p bin i tuyn tnh lin tc (tng tuyn tnh hay gim

tuyn tnh).

Gi s:

+ tmUthU .sin a vo knh Y v a ti cp li ng Y1Y2

+ taqU . a ti cp li ngang X1X2 -> in p trn cc cp li tia nh sau:

ySthUyyUyU 21

xSqUxxUxU 21

Trong : + oySyKyS : nhy ca knh Y

+ oxSxKxS : nhy ca knh X

+ KX v KY l h s khuych i tng cng ca knh Y v X.

Nh vy lch tia trn mn hnh theo chiu chiu ng v chiu ngang

)1(sin

.sin.sin

..

.sin.

mxmYy

aoxSxK

xmYtmYy

taoxSxK

oxSxKqUoxSxUx

tmUoySyK

oySyKthUoySyUy

Trong :

aoxSxKm

mUoySyKmY

.

.

Biu thc (1) chnh l th ca dao ng trn mn hnh, n c dng ging dng

thU cn quan st . Nh vy khi in p qut c a vo cp li ngang X1X2 l

in p tuyn tnh th dng dao ng trn mn hnh chnh l dng tn hiu cn

nghin cu theo thi gian. Minh ha nguyn l qut tuyn tnh nh hnh 4.3

56

Hnh 4.3Minh ha nguyn l qut tuyn tnh

Nu t th tia in t vt qu gii hn mn hnh in p qut c s dng phi

l dng in p qut rng ca tuyn tnh. in p qut rng ca l tng thi gian

quyt ngc ng=0 trng hp ny s khng c tia qut ngc. Tuy nhin trong

thc t ng#0. ngthqT Do ton ti thi gian qut nguc nn im sng trn

mn hnh s chuyn ngc t tri qua phi to nn 1 ng qut ngc khng

mong mun, loi tr th chn Tth >>ng. loi tr hon ton, trong thi gian

qut ngc ngi ta to ra 1 xung m a ti cc iu ch G ca CRT x tia

qut ngc .

Nu tn s qut cao, mn hunh quang c d huy mc cn thit th khi

mi ch c Uq t vo cp phin X1X2 c mt ng sng theo phng ngang.

Khi c c Uth t vo cp phin Y v nu Tq = nTth th trn mn xut hin dao

ng ca mt hay vi chu k ca in p nghin cu (Uth).

8

2

3

5 7

6

0

1

0

X

Y Y

t

X

1 4 8

Y1 Y7

1

t

th

Uq

ng

Tq

t

UG t

Xung xa tia qut

ng c

57

Hnh 5.4Minh ha nguyn l to nh trn mn hnh

c nh quan st vi cht lng cao cn chn:

ng

58

Gi s tn hiu xung Uth c h s lp y nh

59

(b) thq TT5

6 : ng vi 5 chu k qut lin tip dao ng xut hin cc v tr

khc nhau I, II, III, IV, V, nh vy dao ng lp li sau khong thi gian rt ln,

bng 6 chu k in p qut, do khi quan st dao ng s chuyn ng trn

mn hnh -> Oscillocope mt ng b.

(c) thq TT : Qua mi chu k qut dao ng dao ng xut hin trn mn hnh

trn mt ng duy nht, nh vy dao ng quan st c n nh, r nt, nh

vy Oscillocope t iu kin ng b.

Tm li iu kin ng b i vi ch qut tuyn tnh lin tc nh sau:

Tq = n.Tth (n: nguyn dng)

Nh vy tha mn in kin ng b, chu k in p qut tuyn tnh lin tc

phi bng s nguyn ln chu k tn hiu cn quan st.

Qu trnh thit lp v duy tr iu kin ny l qu trnh ng b ca Oscillocope.

Qu trnh ny c thc hin theo s ng b.

+ Cc ch ng b:

- ng b trong: tn hiu ng b ly t knh Y ca OSCILLOCOPE

- ng b ngoi (EXT)

- ng b in li (LINE)

d. S khi chi tit ca oscillocope tng t.

S khi in hnh ca mt Oscillocope tng t (c ng tia in t khng ch

bng in trng) nh sau:

60

Hnh 5.7: S khi ca Oscillocope tng t

+ Chc nng cc khi trong s cu to ca oscillocope tng t.

Mn hnh ng tia CRT:

Knh lch ng y (Knh tn hiu):

C tc dng bin i in p tn hiu cn nghin cu ph hp vi lch tia theo

chiu ng. Knh lch ng y bao gm:

M ch

vo v

phn p Y

Ti n

khu ch

i Dy

tr

Khu ch

i Y

i

x ng

T o

xung

chu n

K/ i

ng

b v

t o

d ng

T o

xung

ng

b

T o

i n p

qut

i lin

t c

M ch

vo v K

X

K/ i X

i

x ng

Ch n

c c tnh

K/ i

Z

Knh l ch ng Y

Knh l ch ngang X v ng b

Knh Z

T i G c a CRT

UZ

Ux

Uqut

S3

S2

S1

AC

DC

GND

Uth

Vpp

CH

EXT

LINE

AC

50Hz

Ub

Ux

Uxb

1

2

3

CRT

X1

X2

Y1

Y2

61

+ Khi suy gim hay b phn p vo thng l mch in dung-in tr v c h s

phn p khng i trong di tn rng a in p c gi tr bin thch hp vo

mch vo v tng khuch i knh y c th m rng lng trnh in p cn o.

Chuyn mch ca b phn p c ghi ra ngoi mt my Volts/Div (Div chia

dc).

V d s tng tng khu suy gim R-C nh hnh 4.8a:

(a)

R1

C1R

2

C2

U1 U2

(b)

Hnh 5.8 Chuyn mch phn p

H s chia p ca khu phn p RC l:

2

21

2

1

Z

ZZ

U

UH

Trong Z1, Z2 l tr khng tng ng ca mi khu phn p.

11

11

1 CRj

RZ

v

22

22

1 CRj

RZ

h s phn p khng ph thuc tn s, chn R1C1=R2C2, khi :

2

21

R

RRH

- khng ph thuc vo tn s .

+ Khi mch vo v tin khuch i y: Tng Zv ca knh y (hay chnh ca

oscillocope) v phi hp tr khng vo ca knh y vi tr khng sng ca cp

dn tn hiu n. Tin K y lm tng nhy knh y: (Ktng=Ktin Ky +KK i

xng) Mch ny cn tham gia vo di thng tn ca knh y. (Mch vo thng l

tng K dng dalinhtor mc CC, JFET, MOS-FET, sau l tin K y l KTT

mc vi sai). Trit p iu chnh a ra mt my bin i Ktin K dao ng

bin thin theo chiu y.

+ Khi dy tr : thng l chui cc phn t LC dng khi tn hiu Uy l dng xung,

to tr gia xung vo a n phin lm lch ng vi in p a n phin

62

lch ngang khi quan st tn hiu khng b mt xn trc ca xung, v s dng

trong trng hp qut i.

+ B khuch i y i xng : lm tng nhy chung ca knh y, thc hin o pha

tn hiu cung cp i xng cho cp phin lm lch tiu t chm tia ti mi

tt, nhy mi i xng nhau i vi trc x, v khng lm mo dng th dao

ng do cch cung cp tn hiu khng i xng gy nn.

+ Khi to dao ng c bin chun Upp c nh kim chun thang khc ca

chuyn mch phn p y trc khi thc hin php o bin in p. Thng l l

b dao ng a hi i xng t dao ng.

Knh lch ngang x.

ng to nn in p qut v truyn t in p cp phin lm lch ngang.

V khuch i tn hiu ng b vi in p lch ngang. Trong knh lch ngang

gm c:

+ Trng hp qut trong th c : B to in p qut rng ca (qut ni ) tng hay

gim tuyn tnh (c th l in p rng ca qut lin tc hay qut i) .

+ Trng hp qut ngoi th cn c :

- Mch vo v tin khuch i thng l b phi hp tr khng v suy gim

gim nh bin in p qut ngoi n mc cn thit.

+ B khuch i i xng knh X khuch i in p qut trong hay qut ngoi

n mc cn thit.

Khi ng b

Gii quyt vn pha ca tn hiu cn quan st Uq vi tn hiu qut ngang, c

dao ng c nh v trung thc, r rng. C 3 loi tn hiu ng b a n

chuyn mch S1:

- ng b ngoi : Tn hiu ng b ngoi cho qua b o cc tnh, sau ti

khuch i v to dng xung ng b, tip qua b to xung kch pht qut , x,

63

chiu sng, v tip ti b to qut (lin tc hay i) v cui cng a ti b

khuch i X i xng. Trng hp ny dng khi quan st tn hiu xung c rng

hp, tn s xung ln.

- ng b trong : Ly mt phn tn hiu Uy cn quan st t khi tin Khuch i Y

a xung ng b, trng hp ny dng quan st tn hiu Uy l sin hoc xung

trong di tn s thp, cao.

- ng b 50 Hz xoay chiu : Ly mt phn tn hiu in p xoay chiu ngui nui

50Hz a vo chuyn mch ng b. Dng quan st tn hiu phm vi tn s

thp, tn s cng nghip dng sin...

Knh khuch i z :

Bao gm mch vo, i cc tnh, khuch i z v vo cc iu ch G thay i

sng trn mn. S dng trong trng hp c tn hiu iu ch sng vo.

Khi ngun nui :

m bo cp ngun cho ton b my hin sng.

Mt s ch lm vic:

- Qut lin tc ng b trong (ngoi): Dng quan st nh ca tn hiu lin tc

theo thi gian v o cc tham s ca chng. S2 v tr CH (hoc EXT nu l ng

b ngoi), S3 v tr 2. Tn hiu t li vo knh Y, qua Mch vo v b phn p Y

c khuch i ti mt mc nht nh, sau c gi chm li ri a qua B

K Y i xng to 2 tn hiu c bin ln, o pha nhau a ti 2 phin

ng

- Qut i ng b trong: Dng quan st v o tham s ca dy xung khng tun

hon hoc dy xung tun hon c hng ln. S2 v tr CH, S3 v tr 1. Qu

trnh hot ng: ging ch 1

- Ch khuch i (ch qut lissajous): Dng o tn s, gc lch pha,

su iu ch, v c tnh Vn-Ampe ca it hoc dng lm thit b so snh. Hnh

nhn c trn mn OSCILLOCOPE gi l hnh Lissajous. S3 v tr 3. B to

qut trong c ngt ra khi qu trnh hot ng. OSCILLOCOPE lm vic theo 2

knh c lp X,Y v u vo X cng l u vo tn hiu.

5.2.2 Oscillocope nhiu knh.

64

Trong nhng trng hp cn so snh nhiu tn hiu cn o, ta phi kho st hai hay

nhiu qu trnh trn mt Oscillocope. Vn ny c gii quyt bng cc bin

php:

- Mi qu trnh nghin cu c dng mt tia in t ring bit.

- Ch dng mt tia in t ghi c hai qu trnh nhng lm cho tia in t thay i

c chu k ghi t qu trnh ny sang qa trnh khc.

Phng php th nht phi dng nhiu Oscillocope khc nhau, mi Oscillocope

nghin cu mt qu trnh ring bit. Cch thc hin nh vy th rt tn km, v phi

dng nhiu Oscillocope. Hn na, v nhy ca cc ng tia in t khc nhau, t

l xch v thi gian khng ging nhau, nn phng php ny t dng.

Trn thc t, ngi ta dng Oscillocope nhiu tia, m ph bin l loi hai tia. Trong

cc loi Oscillocope ny, ng tia in t c cu to theo hai cch.

- Loi ng tia c ngn i (hoc nhiu hn), h thng sng in t. Nhng h thng

ny to nn hai tia in t (hay nhiu tia) tc dng ln cng mt mn hnh.

- Loi ng c tia in t pht ra t cng mt catt ra mt s tia.

C hai loi ng trn u c kh khn trong ch to l lm sao kh b c tc

dng nh hng ln nhau ca cc tia in t. Kh khn ny cng ln khi s tia in

t cng nhiu. V vy, thng thng ch c loi ng c hai tia. Trong mt s qu

trnh c cng tn s, c th kho st ng thi trn mn ca mt Oscillocope c ng

tia in t c mt tia. Cch ny c thc hin theo bin php th hai ni trn;

n c km theo mt b phn ph ca Oscillocope na l chuyn mch in t.

Chuyn mch in t l thit b dng n in t hoc n bn dn, u vo c

a ti c hai qu trnh in p cn nghin cu. u ra ca n a ti cp phin

lch Y (hay b khuych i y) ca Oscillocope.

Tc dng ca chuyn mch in t l lm cho tia in t chuyn i thi gian qut

ghi qu trnh cn nghin cu ny sang qu trnh cn nghin cu khc. S chuyn

mch trn c thc hin do s khng ch dao ng xung vung i xng c to

ra t mt b a hi. in p chuyn mch cn yu cu dng xung ca n gn vung

gc, c nh vy th s chuyn trng thi mi tc thi, khng gy m ri dao ng

cn quan st. Xung in p ny cn phi i xng, tc thi gian hai khong chu

k dng v m phi bng nhau, c nh vy th sng ca hai dao ng mi

bng nhau.

65

a. Oscillocope 2 knh dng CRT 2 tia

Cu to ca Oscillocope hai tia c minh ha nh hnh 4.9

Hnh 5.9- CRT 2 tia

Cu to c bn ca Oscillocope in t hai tia ging nh Oscillocope mt tia,

nhng Oscillocope hai tia cn ch rng trong mt ng tia in t c hai sng

phng tia in t ring bit, tc l ngn i h thng sng in t, ta c hai sng

phng tia in t ring bit. Mi chm tia in t cho mt vt dng sng. Mi tia

in t c sng in t to ra t catt qua cc in cc n mn hunh quang

c qua cc cp phin lm lch ring ca n (Y11; Y12 v Y21; Y22) li tia

in t (1) v (2) theo chiu ng. Dng sng qut rng ca t b to gc thi gian

a vo cp phin lch ngang v c hai chm tia in t ny c lm lch ngang

mn hnh mt cch ng thi. Oscillocope s dng CRT 2 tia c li vo cp phin

lch ng tch bit hon ton, knh A v knh B. Mi knh u c cc mch

khuch i lm lch ring bit ca n ti mt cp phin lm lch ng. B to

gc thi gian iu khin mt b duy nht cc tm li tia ngang.

66

b. Oscillocope 2 knh dng CRT 1 tia kt hp chuyn mch in t

Hnh 5.10- Hnh nh ca Oscillocope tng t 2 knh dng CRT 1 tia

S nguyn l ca Oscillocope 2 knh dng CRT 1 tia kt hp chuyn mch in

t nh hnh 4.1167. Trong hai tn hiu cn quan st (U1(t), U2(t)) s c a

vo 2 knh lch ng ring bit ging nhau ri a ti chuyn mch in t.

Chuyn mch in t s c iu khin ti 1 thi im ch c 1 tn hiu qua n

v a ti khuych i Y i xng a ti cp li ng Y1Y2 v tn hiu s

c hin th trn mn hnh.Hai tn hiu s c ln lt hin th trn mn hnh

theo 2 ch qut: qut tun t v qut xen k.

+Ch qut tun t (Alt Mode): Nguyn l: ln lt qua mi chu k qut, cc tn

hiu U1(t) (gi s l dng in p hnh sin) v U2(t) (gi s l dng in p tam

gic) c a qua chuyn mch in t v a qua khuch i Y i xng ln

lt hin th trn mn hnh. Gi s trong cc chu k qut l U1 ng hin th cn

trong cc chu k qut chn U2 c hin th. Minh ha ch qut tun t nh

u im ca ch qut tun t l tc chuyn mch khng cn ln, rt ph hp

quan st nhng tn hiu c tn s cao, nhc im im l khi tn s tn hiu

quan st nh, Tq ln dao ng quan st khng n nh.

67

Hnh 5.11- S nguyn l oscillocope 2 knh

+ Ch qut xen k (Chop Mode): Trong 1 chu k Tq, ngi ta chia lm nhiu

khong thi gian bng nhau . Ln lt qua mi khong thi gian ny cc tn hiu

U1, U2 c a qua chuyn mch in t v a ti k/i Y i xng hin th

ln mn hnh. Gi s trong khong thi gian l U1 c hin th, trong khong thi

gian chn U2 c hin th. Nh vy, trong 1 chu k Tq, c 2 tn hiu u c

hin th trn mn hnh i dng cc on sng t nt xut hin xen k nhau. Tuy

nhin nhng ch t nt dng sng to ra ngn ti mc khng th nhn ra

chng khi tn s chuyn mch l cao. Khi tn hiu nghin cu tn s thp th tn

hiu hin hnh trn mn my xem sng gn nh lin tc. Khi tn hiu nghin cu

tn s cao, thc hin khng ng b (chn Tq ) th on ngt b lp do d huy

ca ng v lu nh ca mt.

khc phc nhc im ny ca kiu lun phin i vi tn s thp ta s dng

chuyn mch ngt qung. Bi v nhng ch t qung trong tng vt ngn ti mc

khng th nhn thy c, khin c hai dng sng tn hiu u c hin hnh mt

cch lin tc, d dng cho vic quan st so snh.

Knh lch ng CH1

U1

DC

AC

GND

Volts/div .

VAR

Y_Pos

Phn p

Dch mc

Tin K

Knh lch ng CH2

U2

DC

AC

GND

Volts/Div .

VAR

Y_Pos

Phn p

Dch mc

Tin K

Chuyn mch

in t

CRT

1 tia Khi ng b

UExt

AC 50Hz

Source

Vertical Mode

Chop /Alt

Trig. Level

X-Y

1 2

To in p qut

Uxb

Uk

Var

Sweep

Time/div

Slope+/-

68

u im ca ch qut xen k l dao ng n nh khi quan st nhng tn hiu

c tn s nh, nhng nhc im l tc chuyn mch lm vic phi ln, do

khng ph hp khi quan st nhng tn hiu c tn s ln.

Hnh 5.12- Minh ha cc ch qut

5.3 DY O DNG CHO OSCILLOCOPE

Dy o (Probe) c s dng a cc tn hiu vo my o ni chung hay

Oscillocope ni ring. Dy o thng c cu to gm u d (thng l u mc

vo im o trong mch), cc phn tin x l (phn p, hoc khuch i,), cp

dn ng trc, v connector chun BNC. Dy o cng c vai tr quan trng v nh

hng ln n sai s ca php .

C 2 dng dy o chnh:

+ Dy o th ng.

+ Dy o tch cc.

5.3.1 Dy o th ng tr khng cao

Hnh 5.13- Hnh nh ca dy o th ng

Dy o th ng tr khng cao thng c 2 ch lm vic: Khng suy gim (v tr

x1) v c suy gim (v tr x10 lm suy gim tn hiu vo 10 ln). S tng

ng ca dy o loi ny trong cc trng hp nh hnh 4.14.

U1

t

U2

t

Uq

t

U1

U2

U1

U2

Ch qut tun t (Alt) Ch qut xen k (Chop)

69

(a) Dy o khng suy gim (v tr x1)

(b)- Dy o c suy gim (v tr x10)

Hnh 5.14- S tng tng dy o th ng c khu suy gim (x1|x10)

5.3.2 Dy o tch cc

Dy o tch cc thng c tch hp thm cc b khuch i tn hiu, hay

b bin dng vo u o in p. Cc dng dy o dng nh hnh 5.15 v hnh

5.16.

Hnh 5.15 Dy o dng dng in tr Shun

Mt b chuyn i to ra in th u vo t l vi dng trong mch in s

dng th c gi l "u d dng in ". Mt in tr v mt u d in th 1:1

nh hnh 4.15 v chng l phng thc s dng trng thi chc chn. Tuy th

vic xen in tr vo mch in s dng c mt vi bt li. S to ra st p ln

Rin

Vin

u vo my hin sng

R

Rin

50

iu

70

trc in tr tng ng trong Oscillocope s c bt li nh hng n s vn hnh

mch in v phi kt ni u ra t ca my d ti mch in im dng in

c gim st. S dng u d vi sai s cho php o dng vo v ra ca nt m n

khng c ni t.

u d dng in thc hin c nu s dng bin p c hiu qu, cho in

tr nh vo mch in s dng hnh 4.16, t ng vo u d mt bin p c cun

dy th cp ns vng cp iu khin u d 50 m u cui v d in tr 50 ni

ti u vo -xi-l. Dng in c o trong mch s dng c dn qua cun dy

s cp 1 vng, khi n ph thuc vo bin p, phng php ny khng c o dng

in DC, i, e... n b ghp AC. Phng trnh cho bin p c trnh by r rng nh

sau:

(1) is = iu/ns

(2) nhy ca u d l Rin/ns V/A.

(3) in tr ca cun s cp d nhin l Rin/ns

Khi bin p u d dng in s dng chiu mch in trn dy ni trong

mch in s dng, in tr Rin/ns2 c biu din thm vo dy m nh hng ti

ca dng in o khng th l cui cng.

ng tic l u d ch to thng xuyn gp tnh trng s cc vng qun

ca cun th cp my bin p l khng r rng, nhng c th a li in tr u

cui v nhy, s vng cun th cp c th tnh tn d dng.

u d dng in thc hin vi vic s dng "hiu ng ln" hoc mt "b t dao

ng ln", ly tc thi cng qua nam chm trong li bin p pht sinh ra mt

tn hiu in th, c khuch i v a ti u vo -xi-l. Phng php ny

o dng DC nhng n c hn ch khi o tn hiu tn s tng i thp.

u d dng in ghp phi hp vi mt bin p v mt b to ng ln a

vo mt khi tch phn v phi hp cc u ra cung cp nhng c trng nht

ca hai kiu . rng bng o c c gi tr xp x 50MHz. Trong kiu ny d

dng in bin p c mt on dy ngn qua li bin p v dy ny c xen vo

nhnh ca mch in s dng cn o. C kiu sp t li bin p a vo hai phn

di chuyn c song bin p c th c phm vi xung quanh cc vng dy, n c

tch ri u kia. T s vng dy bin p v nhy u d c th thay i bi hai

vng hoc nhiu vng ca dy mang dng in qua bin p. Cng hoc tr dng

71

in trong cc nhnh khc nhau ca mch in s dng c th thc hin bng s

lin kt cc dng in ng thi qua u d dng in, nhng s c vi php o cp

cho gia cc nhnh.

Hnh 5.16- u d bin i dng

5.4 OSCILLOCOPE S

5.4.1 Kh nng ca xil s

xil in t s c cc u im l:

- Duy tr hnh nh dng ca tn hiu trn mn hnh vi khong thi gian khng

hn ch.

- Tc c c th thay i trong gii hn rng.

- Cc on hnh nh lu gi c th xem li c tc thp hn nhiu, tc

qut c th ti 1cm/1h.

- To c hnh nh dao ng tt hn, tng phn hn loi xil tng t.

- n gin hn trong s dng, vn hnh.

- C th truyn trc tip s liu ca tn hiu cn quan st di dng s, ghp

trc tip vi my tnh hay c x l trong xil.

Rin

Vin

u vo my hin sng

Rin

50

is

iu

ns

72

5.4.2 Cu trc oscillocope s

S khi cu to ca xil (c nh) s, c v c bn nh 05.

Hnh 5.17- S khi nguyn l cu to Oscillocope s

Khi chuyn mch S (ng trc) c v tr 1 th xil lm vic nh mt xil a nng

thng thng.

Khi chuyn mch S t v tr 2 th xil lm vic l mt xil c nh s. in p

tn hiu cn quan st c a vo u vo Y, ti b bin i tng t - s ADC.

Ti thi im (t1), khi iu khin gi mt lnh ti u vo iu khin ca b

ADC v khi ng qu trnh bin i. Kt qu l in p tn hiu c s ha, c

ngha l b bin i ly mu dng tn hu nhiu im v bin i gi tr tc thi

ca bin ti mi im thnh gi tr m nh phn t l vi bin . Ti thi

im kt thc qu trnh bin i, b ADC gi tn hiu kt thc ti b iu khin.

Mi s nh phn c chuyn ti b nh v c nh v tr nh ring bit. Bi

v y l b nh khng linh hot (c nh nonvolatile memory) nn n c th lu

tr lng lu tr ln cc s nh phn vi bt k di thi gian no. Khi cn thit,

mt lnh t khi iu khin c th lm cho cc s nh phn ny c sp xp theo

chui li theo th t xc nh v c a ti b bin i DAC. B bin i s -

tng t s bin cc gi tr nh phn thnh in p tng t, v in p ny c

a qua b khuch i Y v ti cp phin lm lch Y ca ng tia in t.

Do b nh c lin tip qut nhiu ln trong mt giy nn mn hnh c sng

lin tc v hin ln dng sng l hnh v cc im sng, biu th dng sng cn

quan st.

t c mt ng sng lin tc, cn c th c thm mt mch ni suy (lm

mt) gia b DAC v b khuch i Y.

73

Mt im hn ch ca xil c nh s va m t trn l di tn b hn ch, do tc

ca b bin i ADC thp (thng thng hin nay, xil c nh s c di tn 1-

10 MHz).

Gn y, cc xil c nh s c di tn rng c pht trin nh c ci t

microprocessor, cc b bin i ADC c tc bin i nhanh hn, k thut s ha

mi hn, cch ni suy v phng php th hin tn hiu.

Mt loi xil c nh khc c trnh by nh 08.

Hnh 5.18- - S khi nguyn l cu to Oscillocope s s dng Vi x l

S 5.18 khc vi s 5.17 ch b dao ng qut thc s l b bin i

DAC knh X, c iu khin t s liu ca Vi x l. u ra b DAC to ra in

p nhy bc, sao cho s nhy bc thang khng khc bit qa nhiu so vi in p

bc thang c to t b dao ng qut tng t.

Vi ADC loi 10 bit, s bc nhy l 210=1024. Ton b on in p ra c chia

thnh 1023 bc ring bit, v s lch ngang ca tia in t thc t l t l theo

thi gian. Tc bin i DAC v b iu khin qut quyt nh tc qut cc

i. Tc qut c th iu chnh c bng vic thay i s n u vo s ca

b DAC.

Cn t hp cc b phn pha trn, gm: ADC, b nh, v DAC ca knh Y cho

php kh nng thay i tr ca tn hiu vo h thng lm lch Y trong mt gii hn

rng, ng thi n c th kt hp c vi DAC ca knh X, nh vy m bo s

ng b chnh xc.

5.5 NG DNG O LNG DNG OSCILLOCOPE

5.5.1 o tham s tn hiu in p

74

(a)

U(t)Ch1+ - Ch2+ -

-xi-l

(b)

Hnh 5.19- o tham s ca tn hiu in p U(t)

Gi s s dng Oscillocope tng t 2 knh o tham s ca tn hiu in p.

a tn hiu in p cn o U(t) (gi s l dng in p hnh sin) vo u vo knh

CH1 hoc CH2 ca Oscillocope. iu chnh Oscillocope ch qut trong tuyn

tnh lin tc v ng b sao cho c t nht mt chu k tn hiu hin th trn mn

hnh vi bin ln v nm trong gii hn mn hnh (v d hnh nh dao ng

nh hnh 5.19-b). Gi s cc h s lch ng v ngang ca Oscillocope l:

Volts/div = 2mV/div, Time/div=1ms.

+ o in p nh nh Upp: Da vo dao ng , xc nh lch theo chiu

ng gia nh v nh di H(div) nh hnh v.

Upp=H(div) x [Volts/div]

=> Upp= 7 div x (2mV/div) = 14 mV

Vi U(t) l dng in p hnh sin: =>Bin Um=Upp/2; gi tr hiu dng

22URMS

ppU .

+ o chu k T: Da vo dao ng , xc nh lch theo chiu ngang ca 1 chu

k tn hiu L (div) (v d lch gia 2 nh lin tip.

T=L(div)x[Time/div]

=> f=1/T

+ o thnh phn 1 chiu UDC: c th thc hin theo 2 cch sau y:

- Xc nh v tr ng in p 0V (GND) bng cch a chuyn mch (AC-GND-

DC)->GND, sau a li chuyn mch v v tr DC, thnh phn mt chiu chnh

l khong in p gia ng 0V v ng trung bnh ca dng in p hin th

trn mn hnh ch DC.

L

H

75

- a chuyn mch (AC-GND-DC)->AC, nh du mt nh bt k ca dao ng

, sau a chuyn mch v v tr DC, xc nh dch chuyn ca nh

H(div).

divVoltsdivH

divVoltsdivH

/).(

/).(UDC

- Nu nh dch chuyn ln trn

- Nu nh dch chuyn xung di

5.5.2 o tn s bng phng php Lissajous

o chu k hay tn s bng phng php qut tuyn tnh c chnh xc khng cao

nht l khi tn hiu c tn s ln. Mt khc phng php b hn ch bi gii hn

ca tn s qut ca Oscillocope, o vi nhng yu cu php o tn s ln vi

yu cu chnh xc cao ngi ta phi chuyn sang o bng phng php

Lissajous (phng php qut X-Y, hay phng php khuch i).

Bn cht ca o tn s bng phng php l phng php so snh tn s ca tn

hiu cha bit vi mt tn s chun bit c chnh xc cao thng qua hnh nh

ca dao ng Lissajous. S o ca phng php ny nh 4.20.

Ch1+ - Ch2+ -

-xi-l

+

Ufx

+

Uch fch=100 MHz

(a)- S o (b)- Kt qu o

Hnh 5.20- o tn s bng phng php Lissajous

Gi s ta o bng Lissajous Oscillocope 2 knh, ta phi iu chnh:

+ Tn hiu cn o tn s:Ufx Knh CH1 Knh Y

+ in p chun Ufch Knh CH2 Knh X.

+ iu chnh Oxilo lm vic ch qut Lissajous (Ufx Y1-Y2; Ufch X1-X2).

Chn chuyn mch X-Y

Vert.Mode CH1 UCH1Knh Y

Source CH2 UCH2Knh X

+iu chnh cc chuyn mch Volts/div (CH1 v CH2);POS-Y (CH1);POS-X

nhn c dao ng Lissajous nm chnh gia v trong gii hn mn hnh.

76

+Thay i tn s chun fch nhn c dao ng Lissajous n nh trn mn

hnh.

Xc nh fx:

Xc nh s im ct dao ng ca mt ct tuyn nm ngang

(phng X) bt k : nX

Xc nh s im ct dao ng ca mt ct tuyn thng ng

(phng Y) bt k : nY

T s gia tn s ca tn hiu da vo knh X v tn s ca tn hiu

a vo knh Y s l nghch vi t s ca s im ct dao ng ca

ct tuyn theo phng X v phng Y tng ng:

X

Y

Y

X

n

n

f

f

Gi s kt qu o nh hnh 4.180-b, ta c:

Y

X

X

Y

CH

CH

ch

x

n

n

f

f

f

f

f

f

2

1

=> MHzn

nff

Y

Xchx 200

2

4.100

Php o tn s bng phng php Lissajous c chnh xc bng vi chnh xc

ca tn s fch, v gii hn tn s o c ln, bng gii hn tn s ca knh lch

ng.

xc nh s im ct d dng, thng iu chnh fch sao dao ng khng qu

phc tp v s im ct dao ng khng qu ln.

5.5.3 o gc lch pha

Gi thit o di pha ca tn hiu qua 1 mng 4 cc (M4C) s dng Oscillocope 2

knh.

a. S dng phng php qut tuyn tnh

S o nh hnh 4.21: in p vo hnh sin U1(t) c a vo knh CH1, in

p ra U2(t) c a vo knh CH2.

+ iu chnh Oscillocope ch hin th 2 knh, qut trong tuyn tnh lin tc.

+ iu chnh cc h s lch tia v v tr sao cho nhn c t nht mt chu k ca

cc tn hiu, bin ln v nm trong gii hn mn hnh. Gi s kt qu hin th

nh hnh 4.22-a.

77

Hnh 5.21- S o di pha ca mng 4 cc

(a)- o bng phng php qut tuyn tnh (b) o bng phng php Lissajous

Hnh 5.22- Kt qu o gc lch pha

b. S dng phng php qut lissajous

S o nh hnh 5.21: in p vo hnh sin U1(t) c a vo knh CH1, in

p ra U2(t) c a vo knh CH2. iu chnh Oscillocope ch qut

Lissajous sao cho:

U1(t) knh CH1 knh Y

U2(t) knh CH2 knh X

iu chnh cc chuyn mch nh sau:

Chn chuyn mch X-Y (chuyn sang ch qut lissajous)

Vert.Mode CH1 = UCH1Knh Y

Source CH2 = UCH2Knh X

+ iu chnh cc h s Volts/div (CH1 v CH2), POS-Y (CH1), POS-X nhn

c dao ng Lissajous nm chnh gia v trong gii hn mn hnh. Dao ng

s c ng thng hoc ng Elip hay ng trn.

+ Xc nh gc trung tm ca dao ng : a cc chuyn mch kt ni u vo

ca c 2 knh v v tr GND, trn mn hnh s l 1 im sng, dch chuyn im

sng v chnh gia mn hnh (im O).

78

+ a cc chuyn mch kt ni u vo v v tr AC, khi s nhn c dao ng

c dng ng thng hoc Elip. Gi s kt qu l ng Elip nh hnh 4.22-b.

+Xc nh gc lch pha: Xc nh cc khong lch khng v cc i (Y0 v Ym)

hoc (X0 v Xm).

Xm

X

Ym

Y 00sin

=>

Xm

X

Ym

Y 0arcsin

0arcsin

Tu theo tng dng dao ng m cch nh gi tr khc nhau. Phng php

ny khng xc nh c du ca gc lch pha. Mun xc nh c du ca

hay mun bit tn hiu no sm pha hay chm pha hn, ta s chuyn sang quan st

rt nhanh ch qut tuyt tnh.

5.5.4. V c tuyn Vn-Ampe ca it

D 1

N1183

R1 1

00

+ Ch1+ - Ch2+ -

-xi-l

(a) S o (b)- Kt qu o

Hnh 5.23- V c tuyn Vn-Ampe ca it

+ Chn R1=100 hoc 1k.

+ iu chnh my to sng pht ra xung tam gic, iu chnh bin xung (khong

10V) v mc in p mt chiu ca xung (phm OFFSET) bng 0V, tn s ca

xung khong 200-300 Hz.

+ Thc hin cc bc iu chnh v c tuyn V-A ca it theo l thuyt

hc.

+ Chuyn xil sang ch o x-y (ch qut Lissajous).

+ o cc tnh knh 2 : Pull (POS_Y ca knh CH2).

+ Xc nh gc to bng cch n cc phm GND ca c hai knh CH1, CH2 sau

chuyn sang ch DC

79

+ Bt my pht xung v iu chnh bin hoc mc mt chiu ca xung nhn

c dng c tuyn V-A ca it. V dng c tuyn o c.

+ Xc nh in p thng ca it.

5.5.5. V c tuyn ra ca BJT

Mc mch o nh 5.24-a.

Rc 1

00

+

Ch1+ - Ch2+ -

-xi-l

2N2222

E 5

VR 100k R1 1k

A+

(a)- S o (b)- Kt qu o

Hnh 54.24- V c tuyn ra ca BJT

+ iu chnh my to sng pht ra xung tam gic, iu chnh bin xung (khong

10V) v mc in p mt chiu ca xung (phm OFFSET) bng 0V, tn s ca

xung khong 200-300 Hz.

+ Thc hin cc bc iu chnh v c tuyn ra ca BJT theo l thuyt hc.

+ Chuyn xil sang ch o x-y (ch qut Lissajous).

+ o cc tnh knh 2 : Pull (POS_Y ca knh CH2).

+ Xc nh gc to bng cch n cc phm GND ca c hai knh CH1, CH2 sau

chuyn sang ch DC

+ Bt my pht xung v iu chnh bin hoc mc mt chiu ca xung nhn

c dng c tuyn ra ca BJT. V dng c tuyn o c.

+ Thay i bin tr VR v c tuyn ra ng vi cc gi tr dng IB khc nhau.

80

CHNG 6 - O TN S, KHONG THI GIAN V GC LCH PHA

Tn s, chu k, cc khong thi gian, gc pha l cc tham s quan trng ca tn

hiu. Trong k thut in t, thng hay dng cc tn hiu c ph tn s rt rng.

Di ph tn s ny bt u t cc tn s bng mt vi phn trm Hz n hng trm

GHz. Ton b tn ph ny c th chia lm nhiu di tn s c tnh cht khc nhau:

Di tn thp: < 16Hz

Di tn s m thanh: 16 Hz < f < 20 KHz

Di tn s siu m: 20 KHz < f < 200 KHz

Di tn s cao: 200 KHz < f < 30 MHz

Di tn s siu cao: 30 MHz < f < 3000 MHz

Di tn s quang hc: > 3GHz

Cc di tn s khc nhau c cc phng php o tn s khc nhau. Gii hn dng

v k thut o lng cc tn s cao tn tng ln cng vi s pht trin ca k thut

in t v ngy nay xc nh c cc tn s hng trm GHz.

Cc tham s v tn s:

Xt tn hiu xoay chiu iu ha bin thin