Nghiên cứu xử lý tín hiệu nhỏ phục vụ đo lường

Sii' dung phuong phap nay tin hieu qua dp dien dung a m6i nhiet dp duge ghi nhan rat nhieu giii tri N-512.. Tinh toan dp bu dien dung — tu dong trong khi neu sii' dung phuang phap khac

DAI HOC QUOC GIA HA NOI

TRITCING DAI HOC KHOA HOC TU^NHIEN

DE TAI :

NGHIEN Ciru Xir LY TIN HIEU NHO

PHUC Vy DO LUCING

MA SO: QT-06-45

CHU TRl DE TAI : PGS TS PHAM QLOC TRIEL

CAC CAN B O T H A M GIA: ThS Nguyen Anh Due

HVCH N"uv6n Huu Lam

JA^I H O C U U O C G l A r ^ N O ; n^ j N G TAM THONG TIN THUVIEN

^VML

HA NOI - 2006

BAO CAO TOM TAT

De tai;

NGHIEN CUtf XL^ LY TIN HIEU NHO PHUC VU DO LUONG

Ma so: QT-06-45

CHU TRI DE TAI PGS TS PHAM QUOC TRIEU

CAN B O THAM GIA: ThS Nguyen Anh Due

HVCH Nguyen Huu Lam

Muc tieu nghicn ciiu

- Nghien cuu cac phuong phap xu ly tin hieu nho, lay vf du tin hieu DLTS ling dung vao do luang cac dai lugng vat ly, nhan manh phuang phap Fourier, Tach song dong bo Boxcar va Lock-in

- Nghien cuu van di tang cuang tin hieu dong ihoi giiim thieu tap nhieu

- Tdng hap khai quat ly thuyet va ap dung thuc te

Noi dung nghien ciiu

- Tong quan ly thuyet ve mot so phirang phap xu ly tin hieu DLTS

- Nghien cuu cac guii phap nang cao chat luang do tin hieu nho

- Nghien cuii cac hieu li'ng lien quan den xu ly tin hieu

- Dinh huong xay dung phep do

- Hudng dSn Luan van cao hoc va viet bai bao khoa hoc

Cac ket qua dat diroc

- Nghien cuu tinh chat ciia tin hieu nho tren ca sa moi tuang quan giua tin hieu

va tap nhi^u tac dong vao he do vat ly

- Nghien cuu tap hop va khai quat boa mot so phuang phap xu ly tin hieu nho nhu phuang phap Fourier, phuang phap Tach song dong bo, phuang phap Boxcar, phuang phap Lock-in Tin hieu nho duac dan chung la tin hieu DLTS Tuy nhien, hoan toan c6 the su dung cac phuang phap noi tren dd do dac, xu ly cac tin hieu nho khac trong he do

- Huang dSn 01 Luan van cao hoc theo huang d6 tai Ket qua ciia Luan van: Thiet ke xay dung thanh cong bo khuech dai lock-in so hai pha bang phan mem Labview (two-phase lock-in amplifier) ung dung trong nghien cuu va dao tao tai Khoa Vat ly

- Viet 01 bai bao khoa hoc ve cac phuong phap xu ly tin hieu nho ung dung trong do luang DLTS

Tinh hinh sii dung kinh phi

- Kinh phi duac cap: 20.000.000 d

- Kinh phi da quyet toan: 20.000.000 d

KHOA QUAN LY (Kv va "hi rd ho ten

CHU TRI DE TAI (Ky va ghi ro ho ten

- Study on problems of increase signals and decreace the noises

- Summarise the theories in order to apply into measued systems

- Study of the effects concerning the signal proccessing

- Orientate to build measured systems of MSc Thesis

- Full fill the contains of the scientific article

Project results

- 01 research paper

- 01 MSc Thesis of N^uven Anh Due (2006)

MUC LUC

• •

Trang

Madau 2

CFIUDNG L TIN HIEU DLTS TRONG By\N DAN ^

LI Tin hieu nho DLTS 3

1.2 Xac dinh mot so thong so \\i tin hieu nho DLTS 4

1.2T Xac dinh nong do muc Np 4

1.2.2 Xac dinh tiet dicn bat 5

1.2.3 Xac dinh miic sau 5

CHUDNG 2- CAC PHUONG PHy\P XU' LY TIN HIEU DLTS 6

2.1 Phuang phap Fourier 6

2.1.1 Co soTy thuyet 6

2.1.2 Tinh Uu viel ciia phucmg phap 7

2.1.3 So do khoi va nguyen tac boat dong 8

2.2 Phuo'ng phap dich cu'a 9

2.2.1 C a s a l y thuyet 9

2.2.2 Uu nhuoc diem ciia phuong phap dich cu'a 9

2.3 Phuong phap tach song dong bo A - B 10

2.4 Phuo'ng phap Boxcar j I

2.4.1 C a s a l y thuyet I j

2.4.2 Sa do khoi chii'c nang , r

CHUONG 3- PFIUONG PHy\P KHUECH DAI LOCK-IN 17

3.1 Gioi thieu chung 17

3.2 Cac khai niem co" ban 17

3.3 Bo phat hien nhay pha (Phase-sensitive deicclion) 22

3.3.1 Gioi thieu 22

3.3.2 Nguyen ly boat dong 22

3.3.3 Cac khai niem lien quan den bp PSD 26

3.4 Khuech dai lock-in hai pha (fwo-phase Lock-in amplifier) 27

3.5 Cac nguon nhieu 28

Ket luan 33

Tuu trung lai, nhiem vu ciia nguai lam thi nghiem do dac cac tin hieu nho la phai loai bo, boc tach toi da anh huang ciia tap nhieu vao tin hieu do, phat hien ro rang nhat tin hieu vat ly c^n quan tam

De tai nay nghien cuu ve mot so phuang phap xii' ly tin hieu nho trong phep do dai luang dien nhu phuo'ng phap Fourier, phuong phap tach song dong bo, phuang phap Boxcar, phuang phap Lock-in Day chi la mot so phuo'ng phap thong dung, mot

so cong doan trong qua trinh het sue da dang, phong phii cua thuc nghiem vat ly Tuy nhien chiing cung gop phan tao ra nhieu cong trinh khoa hoc co gia tri nhieu he do gia tri cao tren the gioi cung nhu o Viet nam Nam vu'ng nguyen ly va giai phap thuc hien cac phuang phap nay co the giup cac nha thuc nghiem trong nghien cuu doi tugng quan tarn cua minh

Bao cao j^om 3 chuo'n

Chuangl trinh bay ve mot loai tin hieu nho dien hinh trong nghien cuu vat ly ban ddn: tin hieu qua do ciia cac tam sau (DLTS) Xu ly tot tin hieu nay ta co the' nhan duac nhieu thong tin v^ cac tam sau trong chat ban d5n, anh huang ciia no tai chat luang linh kien ban dan

Chuong 2 trinh bay ve cac phuo'ng phap xu ly tin hieu nho DLTS Chuong nay chii yeu dua ra cac nguyen ly va each thuc hien de co the xan dung vao cac he do tin hieu nho khac

Chuong 3 trinh bay ve phuang phap Lock-in xii' ly tin hieu nho rat hieu qua trong cac phep do thuc nghiem vdi nguyen ly va each ap dung do tin hieu nho

Hy vong rang, nhieu nguyen ly do, nhi(!u giai phap thuc nghiem nham nang cao chat luang phep do se duac nghien cuu tiep luc irong thoi ^ian tai

CHUONG 1 TIN HIEU DLTS TRONG BAN DAN

1.1 Tin hieu nho DLTS

Khi bi kich thich ( do nhiet, dien hoac quang) tam sau bi lap d^y dien tu va se tra lai trang thai diing neu ngiang kich thich So dien tu tren cac lam sau bi thay doi trong qua trinh chuyen tiep va duac bieu diln bang phuo'ng trinh sau:

Cp, e„ : la toe dp phat hat tai bang nhiet tuong ung vai 16 trong \a dien tii

kp , kn la cac hang so lien quan den ban chat dan dien

Nj la nong dp tam

Co the thay doi dp lap day dien tii' ciia tam sau bang chuyen tiep P - N hoac bang hang rao Schottky

Gia su CO mot lo'p chuyen tiep N'- P hoac diod Schottky loai P co tam sau nam

a niia duoi \'ung cam Hay vo'i chuyen tiep P"^ - N hoac diod Schottky loai N co tam sau nam a nii'a tren vung cam Dien dung tren mot don vi dien tich 16p ngan la;

2(r, ±V) '

Trong do V^^ la the khuech tan

± V tuong ung la the phan cue nguac va thuan

N, la n6niz d6 ion trong lop ngan

N,= (N^+N-,)-n-,- ^ ^ _ _ (1.3)

Tir cong thiic (1.2) va (1.3) ta thay dien dung cua lop chuyen tiep the hien miic

dp lap day dien tu ciia tam sau

Neu e^ » e p do la truang hop tam sau bat dien tu thi cong thuc (1.1) tra thanh

dn

dl

n,( e„ - n C j + Nr.n.Cn, (1.4) Truang hop phan cue thuan cho chuyen tiep Nong dp dien tu trong mii^n chuven

tiep cao n ti le \(i\ N^^ \a e„ « n.Cn thi (1.4) se la:

N, -n-,-(t)- jN, n^(0)) exp( -n.C,.t) Khi t — • x thi np(t)—•Nj Tat ca cac tam deu bat dien tu

Truang hop phan cue nguoc lap chuyen tiep Cac hat tai dien bi diy khoi lap

ngan Khi do c^»x\.C^ , dien tii' duge giai phong khoi tam sau

Vay trong duang dien dung qua dp C(t) mang ddy dii cac thong so ca ban ciia tam sau nhu: n6ng dp muc Nj, tiet dien bat dien tii a,^ , nang luang tam E, , toe dp phat xa nhiet c^

1.2 Xac dinh mot so thong so tu tin hieu nho DLTS

1.2.1 Xac dinh nong dp miic Nj

Trong truang hop dien dung thay doi nho do qua trinh lap d^y hoan toan tam bat N[ trong vung ngheo va dp pha tap la deu theo kh6ng gian vai ban dan loai N ta co:

N,=2(N,-N,) \(

C la dien dung lop ngheo

Neu tap phan bo khong dong nhat trong kh6ng gian thi nong dp tam sau duac tinh theo cong thuc chinh xac hon la:

doi nho the d V ciia xung phan cue V ung vai dp rpng vung ngheo x

N^ la nong dp tam nong

Vav tu do thi AC va V ta co the tinh ra noni^ d6 tam sau

1.2.2 Xac dinh tiet dien bat

Tu su phu thupc ciia dien dung theo thai gian ta tinh tiet dien bdt : e^^cr,, 0', n doi \oi hat tai co' ban Phuong phap na> cho ket qua chinh xac ma don gian

Doi voi qua trinh bai hat tai khong co' ban ihi phiic tap vi no phu thuoc vao co che tiem

De xac dinh tiet dien bat nguoi ta con co each khac la tinh tu toe dp phat xa trong vung ngheo e,^

M V N ^ e x p t - ^ ' ^

Gia tri eiia tiet dien bat xac dinh bang phuong phap nay cao han mot bac so vai phuang phap quang Dieu nay co the do cac qua trinh bat xay ra trong mot dien truang manh ciia chuyen tiep

1.2.3 Xac dinh mure sau

TCr phep do miic dp phat xa nhiet e^, ciia hat tai ta co the xac dinh miic sau

Thiet lap do thi In ( e j phu thupc I/T Dp nghieng ciia duang phu thupc chinh

la nang lugng kich boat phat hat lai

Tuy nhien nang lugng kich boat khong phai liic nao cung triing vai nang lugng

do duge Di^u nay co the do cac nguyen nhan sau:

1/ Trong c6ng thuc (1.12) phan truac ciia ham exp phu thupc nhiet dp

-Tiet dien bat a , phu thuoc vao nhiet do thuanu la khoniz biel truoc

-Thanh phan (r,, A; phu thupc bac 2 ciia nhiet dp (- J-}

2/ Su phu thupc vao nhiet dp la khac nhau do co' che bat hat tai khac nhau De hieu

chinh su anh huong ciia nhiet dp trong tinh toan co may each sau day:

- Hieu chinh theo cac phu thupc manh trong nhu'ng khoang nhiet dp nhat dinh

-Phu thuoc '/r,,A - T" hieu chinh tir 2kT ciia do thi l n ( e J \ a T

CHUONG 2 CAC PHUONG PHAP XLTLY TIN HIEU DLTS

2.1 Phuomg phap Fourier

2.1.1 C a s a l y thuyet

Phuang phap qua dp tam sau Fourier duge xay dung voi su so hoa dien dung,

thai gian qua do va xac dinh he so roi rac Fourier Ket qua do duge dieu khien tu

dong cho dp chinh xac cao Phuang phap nay g6m 4 dac trung co ban sau:

1- Dat N gia tri do lam mSu tu pho dien dung qua dp Khai trien theo chu6i

Fourier de tim ra cac he so roi rac

2- Tinh duge true tiep tren ph6 qua dp hangd so tho gian r va bien dp phd

bang moi lien he giua cac he so Fourier

3- Di^u chinh dp rpng chu ky T, de dang doi voi cac nhiet dp khac nhau Dac

biet han che nhieu tot

4- Loai nhieu tot bang each phan lich qua dp dien dung theo cac he so Fourier

Thuc te cho ket qua chinh xac thuong duac tinh toan doi vai cac he so a nhiJng bac

dau tien

Phuang phap qua dp tam sau Fourier vdn dua tren moi tuang quan giua cac

thong so ban dSn nhu trong c6ng thuc (1.10) va (1.12) Tuy nhien duge phan tich

duoi dang chu6i Fourier

A/ la m^u thai gian nghi, N la so khoang thoi gian nghi

TCr cac he so Fourier ta tinh duge bien dp ph6 suy ra tinh duge nong d6 tam

Sii' dung phuong phap nay tin hieu qua dp dien dung a m6i nhiet dp duge ghi

nhan rat nhieu giii tri ( N-512 1024, )trong khoang thoi gian T,, bang pho ki thuat

so

Vi \ay CO N cap gia Iri r va l nen co the ap dung luy \ cac phuong phap de

tinh cac thong so tam sau Song sii" dung phuo'ng phap nay cac thong so ban dan duge rut ra true tiep tu cac he so Fourier th6ng qua cac dinh ciia chiing

2,1.2 Tinh uu viet ciia phuong phap

1 Cho ket qua chinh xac ma chi can quel nhiet mot hln, con doi vc'^i cac phuang phap khac thi phai quel nhiet hoac do it nhat la nam Ian moi danh gia duge ket qua

2 Toe dp xii ly nhanh, dp nhay cao sai so nho

3 Tinh toan dp bu dien dung — tu dong trong khi neu sii' dung phuang phap khac thi phai thuc hien thii c6ng

4 So hod va tu dong luu iru' cac so lieu

5 Kha nang loai nhieu cao

6 Dt dieu chinh b^ng cac th6ng so nhu bien dp, dp rpng chu ky

7 Van hanh dan gian

Son" CO han che la chi dumz duoc trons he do tu d6n^

2.1.3 Sa do khoi va nguyen tac hoat dong

A

Khoiif

Co

BucVc iihav 2K

• >

Nhom c() \ac

Do dicndung

}.L

Danh gui nhom qua do

Hinh 2.1 S(J do klioi ehde ndn^^ ei'ui phiiwv^ phap do cpid do tam sdii Fourier

• Nguyen tie hoat dong:

Dal mSu do vao trong buong miu, buong m5u duge hiit chan khong thong qua

may bom, sau do dat gioi ban nhiet dp cho phep do Tac dung xung vao mSu do Co the la xung anh sang hoac the xung Cho qua thiet bi do dien dung, diing ph6 ki thuat

so ghi nhan va \ e d6 thi pho

May tu dong khai trien pho dien dung theo chu6i Fourier roi tinh ra cac he so

de tii do danh gia nhom qua dp

Neu nhom qua dp da xac dinh thi buoc nhay nhiet dp la 0.5K con kh6ng xac dinh thi buoc nhay la 2K Qua trinh tiep luc cho den khi nhiet dp dat toi giai ban, con khong phep do lap lai

2.2 Phirang phap dich cura

2.2.1 C a s a l y thuyet

De xac dinh cac thong so' ciia lam sau tu duang qua dp dien dung

C(t) = Qj.exp (-e^t) ta xet tai 2 thai diem tj va l.^r.t, vai r=2.3,

Ta co:

AC = C(/,) - C(i,) = Q[exp(- e,/,) - exp(- ej'l,)] (2.6)

Cho t| tang dan thi AC dat cue dai a thai diem t,,,, nao do Dt xac dinh t ^,, tii

Vay tai mot nhiet dp T nao do dung phuong phap dich tj la co duang qua dp

C(t) ma cue dai ciia no cho ta tinh duge e,^ Biet cap th6ng so (e,, T) thi se xac dinh

duge E]^ va a,^ trong he do tu dong duang C(t) duge ghi tu dong tai m thai diem khac

T

nhau m6t each gian doan theo thai gian t, voi t = CS.At, CS = l,2,3 m va A/ ^ -^,

m

Tgla thai gian do dien dung qua dp

May tinh se tu dong tinh hieu AC - C(t) - C(r.l) a nhu'ng thoi diem t lien tiep

nhau de ve len d6 thi su phu thupc C(t) tai vai gia Iri nhiet dp khac nhau

Gpi phuang phap dich ciia la do viec quet thoi gian duge thuc hien nha may

tinh b^ng each dich dan cac cii'a Sampling

2.2.2 iTu nhugc diem ciia phuong phap dich cua

- Xii ly ket qua tu dong, tinh toan chinh xac nhanh chong, do tin cay cao

- Tranh sai so dpc trong trong vice xac dinh cac gia tri thai gian ciia c6ng Boxcar tai

cac dinh pho (t^.^J

- De xac dinh nang lugng ciia mot tam chi can ghi duang C(t) tai 4 den 5 nhiet dp

khac nhau Yeu cfiu so file kh6ng nhieu

- Khi do nhiet dp thi dp on dinh va tinh chinh xac doi hoi rat cao

- Trong qua trinh do chi thay doi nhiet dp mot Ian

- Chi ap dung tren he do tu dong

- Pho (L6) gian doan theo thdi gian, neu khoang each gian doan nay cang nho (m

Idti) thi ket qua linh toan cang chinh xac trong trudng hgp so lieu khong chiia tap

- Tai mot so nhiet do chua xuat ro bieu hien tam sau

2.3 Phuang phap tach song dong bp A - B

Du6i su tac dong ciia the phan cue mot chieu gay nen mot vung dien tich qua

dp tai mi^n chuyen tiep Su thay doi trang thai tinh dien ciia vung qua dp duge coi la

su thay doi dien dung ciia mot tu dien hoac dong dicn ciia mot mach kin chiia viing

dien tich khong gian phd Ghi nhan su thay doi ciia dong dien la pho dong qua dp

Bieu thiic ciia dong qua dp la ham ciia thai gian

J,^{i) = /,„,, e x p [ - e , , ( / -i,,)\ ( 2 9 )

Vai J,^,, ^ q-c,.Nr va L^ - i.7 ' cxp(-^) (2.10)

KI

Tin hieu dong qua dp J(t,T) tai mot nhiet dp T nao do duoi tac dung ciia xung

phan cue mot chieu co dang J = J(t), T ~ const

Thuc hien vice chia chu ky xung kich mau lam bon phan bang nhau a bp xii

ly Tin hieu J(t) chi duge th6ng 6 hai khoang thoi gian

T, T, 37;

— < I < — va — < I < T,

4 2 4

Gpi tin hieu J(t) a khoang dau la A va khoang sau la B Sii dung bp tach song

so sanh hai dien tich A va B

Neu gpi R - R(l) la ham truytin dac trung cho tin hieu R ty le vai dp Idn ciia

A va B Qua m6i he sii ly no co phuang trinh la:

R{i) - \C[i).F{i).di ( 2 1 1 )

Trong do F(t) gpi la h a m Ipc voi he tach song dong bp A -B, h a m Ipc F(t) co dang:

Vai T, la chu ky lap lai ciia xung kich mSu

Ham truydn co dan^:

TCr (2.13) tim duge cue tri ciia ham truyen la 0.133Q, doi voi dien dung qua dp

va 0,403C^ doi vai dong qua dp

Qua trinh thuc nghiem la dinh truo'c e,,^, ( bang tan so f), do nhiet dp ciia mlu thay doi Dinh cue dai pho DLTS cho ta xac dinh nhiet dp T^^, Thuc hien phep do vai cac e^j khac nhau, do tuang ling T,^, Tu cac cap gia tri do duoc ve do thi su phu thupc

Ln

T: theo KT Duong bieu dien cho biet nang lugng ion boa Ej, tiet dien b4t hat tiii ciia cac mii'c sau Vay vai phuang phap nay phep do da chuyen quan he hai bien so J(t,T) sang quan he mot bien so AJ(T) bang vice lay hieu hai khoang A va B tren duang tin hieu dong qua dp

Cung CO the thuc hien phep do nhu va\' tren he analog dung tu ghi

2.4 Phuong phap Boxcar

Neu khong quan tam den thanh phan khong doi C^ va de y r^ng V„ - T~;

V^ « T~ thi (T,, coi nhu khong doi theo nhiet dp va cong thiic (2.14), (2.15) co the viet

V6i Cy, A la cac hang so va A - a,, va goc toa dp tai day vung d^n E,

- Doi vai ban d2n cau true N^ - P, phuang trinh bieu diem cho tam bat 16 tro'ng hoan toan tuang tu (2.16) va (2.17), khi do e^ duge thay bing Cp

Trong c6ng thiic (2.16) va (2.17) chiia cac thong tin chii yeu can thiet ve tam sau hu nang lugng ion hoa E, toe dp phat xa dien tii e,^, tiei dien bat dien tii a^ ( n^m trong A), nong dp tam sau N-, ( nam trong e^,)

Tii cac phuong trinh (2.16) va (2.17) cho ta thay duong dien dung qua dp phu thupc vao hai bien so la nhiet dp va thoi gian Ta \iei: C = C(t.T) ky thuat xii ly tin hieu qua dp dien dung bang he Boxcar kep nhu sau:

- Tin hieu dien dung qua dp dua vao bp xii' ly va duge ghi lai dua ra chi a hai thai diem la t, va t 0 loi ra cua bp xii' ly co hai gia tri C(t,) C(U} lai duge dua vao bp

khuech dai vi sai Loi ra khuech dai vi sai cho dien ap ling \'6i dang duang qua dp va

vi tri ciia hai c6ng t, va t

Hinh 2.2: Mo td ky thuat Boxear kep

Tin hieu di qua m6i bp xii ly duge dac trung boi ham truyen:

R{t) = \C(t).F{i).dt (2.18)

F(t): La ham loc dac trung cho bp xii ly;

Tf: La chu ky lap lai ciia bp xung kich mSu

Ham ipc F(t) doi vai he Boxcar co dang:

Mat khac theo (2.17), ta co the viet (2.21) nhu sau;

Rif) = (::.[exp(- e,,./, ) - exp( - ',./ )]

(2.21)

(2.22)

Do (2.16) ta thay toe dp phat dien tiie^ lai phu thupc vao nhiet dp: e^ - e,/T)

Khi do R theo su thay doi nhiet dp ciia miu ( co nghla e„ thay doi ) thi R se di qua cue tri thoa man di6u kien:

dR de cj- /, e x p ( - c^,./, ) 4- e x p i - e ,.i )| = 0 ( 2 2 3 )

Vay ghi nhan tai hai thoi diem tj, t-, thong qua he Boxcar kep va thay doi nhiet

dp m3u thi ham truyen R se dat gia tri cue dai Toe dp phat xa dien tii tinh duge tu gia tri cue dai do la e,^ Neu dinh san hai thoi diem t,, t co nghia la an dinh mot gia tri to'c dp phai xa Ghi nhan ham truyen R theo su thay doi nhiet dp ciia mSu thi se

xac dinh duge c\xc dai ciia R tai mot nhiet dp nao do Su phu thupc ciia ham truyen R

vao nhiet dp gpi la ph6 DLTS

- Gia tri e^ duge lua chpn trudc do gpi la su lua chpn ciia s6 toe dp Khi cho nhiet dp thay d6i thi e^ sx thay doi den gia tri e^^ da djnh truac ciia ciia so toe do

Khi do ham truyen R dat cue dai, ky thuat DLTS co tinh uu viet a dac diem nay No

da bien su phu thupc C(t,T) ba chieu thanh R(T) hai chi^u cho ta xii ly di dang ban

- Trong qua trinh thuc nghiem cho nhiet dp ciia m3u thay doi va gia tri e^ dinh trudc ( cac vi tri t,, t2 dinh truac ) Tir pho DLTS xac dinh duge nhiet dp T^^ a cue dai pho Tu cong ihiic (2.16) co:

- N6ng dp cac tam co the' tinh true tiep tu su thay d6i dien dung ling vai su lap day hoan toan cac tam b^ng xung tiem bao hoa doi vai truang hgp hat tai khong ca ban hoac bing xung hat tai co' ban rpng nhat co the co doi voi truang hgp tam hat tai ca ban

- Doi vai cau Iriic N^ - P thi bieu thiic nong do lam bat dien tu la:

AC -V, =2

Trong do:

C: La dien dung cua diod khi phan cue ngugc tinh;

AC: La su thay d6i dien dung tai t = 0 gay boi xung tiem bao hoa;

(N^ - ND): La n6ng dp aceptor tong cong a phia P ciia vung tiep giap

Hinh 2.3: Minh hoa phep do DLTS vdi nam ci'ra so toe do kluic nhau vd edeli xde dinh

ndnq luang ion hod cua tam sau

Bang phucmg phap DLTS cac thong so duge xac dinh co sai so kh6ng vugt qua 10% Uu diem ciia phuang phap nay la chi can xac dinh dinh cua tin hieu de tim T^^^

ma khong cin quan lam bien dp tin hieu la bao nhieu nen anh huang ciia hang so

truyen tren he dien tii den phep do la khong he co

Vay viec dat cac ciia s6 t|, t (co nghla dat truoc e„ ) da chuyen quan he 2 bien

AC{LT) sang thanh quan he mpl bien A ( ( / ) mot each tu dong \ a co the xay dung duge tren he analog tu ghi

2.4.2 S o do khoi chijc nang

Phuong phiip Boxcar dung trong he do DLTS co sa do khoi sau:

Neu chi sir dung cac khoi tii (l)den (9) cho he do dien dung DLTS thi ta co he

do dien dung nho duge chi thi b^ng dong ho mot chieu Hinh tren la sa do khoi chiic nang ciia he do dien dung DLTS dung ky thuat xii ly Boxcar kep

Chiic nang ciia tiing khoi nhu sau:

Khoi (I) la khoi phat tin hieu hinh sin cao tan lam viec a tdn so 5.6 MHz

Mach phat sii dung bp dao dong thach anh vai cac cong NAND ciia vi mach SN7400 Tin hieu hinh sin 5.6 MHz sau khi qua cac tang Ipc lua se duge dua vao cau

dien dung (2), khoi quay pha (4) a do edu (2) duge nuoi b4ng nguon 5.6MHz Tin

hieu sau cau (2) lai duge dua vao khoi khuech dai Ipc lua (5) Khoi nay duge xay dung b^ng vi mach SC2929 va cac khung cong huang lam viec a tdn so' 5.6MHz sao cho dp truyen qua ciia khoi dat 0.5 MHz Khoi co nhiem vu Ipc ra duang dien dung tii song cao tan hay noi each khac Ipc duang bao bien dp tii song mang

Khoi TSNP (7) duge dieu chinh de nhay nhat voi bien doi do thanh phan dien tra Vi ly do nay ma bp quay pha (4) nam truac khoi tin hieu chuain co vai tro quan trpng trong giai doan tit^n xu ly ciia he do

Tin hieu thu duge (duong qua dp) sau khoi TSNP co dp trung thuc phu thupc

dp don sdc ciia tin hieu cao tan hinh sin 5.6 MHz

Khoi khuech dai mot chi^u (8) co loi ra dua vao dong h6 chi thi mot chieu (9)

a day ta co th6 quan sat sU thay doi gia tri tuyel doi cua dien dung mau do theo cac

tac nhan tu ben ngoai

Bp xii ly tin hieu hai cong la khoi Boxcar, khoi co 3 loi A, B, C Trong do loi

A la loi phiit xung phan cue mdu do trong khoi cau dien dung Loi B la loi nhan tin hieu qua dp dien dung sau khoi khuech dai Khoi C la loi ra tu ghi sau qua trinh xii

ly tin hieu tu dong tai 2 thai diem dat s5n

CHl/ONG 3

3.1 Gioi thieu chung

Khuech dai Lock-in (Lock-in amplifier) duoc dung de phat hien va khuech dai nhirng tin hieu AC cue nho, co the dat toi ca vai nanovolt No co the do chinh xac nhirng tin hieu nho ban nhieu nhieu Ian

Khuech dai Lock-in sii dung ky thuat phat hien nhay pha, no chi giii lai nhimg thanh phan tin hieu co ciing tan so voi tin hieu chuan, con nhung thanh phan tan so khac vai tan so tin hieu chuan se bi loai nen khong anh hucrng gi den phep do

Xet vi du sau:

Gia six tin hieu can do co dang sin voi bien dp 10 nV tan so 10 kflz Neu sii dung bp khuech dai tap am thap voi nhieu loi vao khoang 5 nV/VlI/, dai tan ciia bp khuech

dai la 100 kHz va he so khuech dai la 1000 thi a loi ra cua bp khuech dai chiing ta

co: dp Ian ciia tin hieu la 10 |.iV (10 nV x 1000) va dp Ian ciia nhieu la 1.6 mV (5

neu nhu khong tach duge thanh phan tan so quan tam ra khoi nhieu

Neu chiing ta dung mot bp Ipc dai voi dp pham chat Q ^ 100 (day la thong so ciia bp Ipc cue tot), tan so trung tam la 10 kHz thi tin hieu nam trong dai tan 100 Hz (10 kHz/Q) se duge truyen qua Nhieu trong truang hgp na\ se la 50 j.iV (5 nV/vHz x VlOO Hz x 1000) va dp Ion ciia tin hieu van la 10 )_iV Nhu vay nhieu o loi ra van Ian ban tin hieu nhieu Ian nen chiing ta se Ichong thuc hien chinh xac duge phep do Han niia he so kliuech dai cung Idiong lam tang duge ty so tin hieu tren tap (signal to noise)

Bay gia chung ta se khao sat vai tro cua bp phat hien nhay pha PSD (Phase-sensitive detector) Bp phat hien nhay pha co the phat hien tin hieu a tan so 10 kHz voi dai rpng hep co khoang 0.01 Hz! Trong truong hgp nay nhieu o trong dai chi eon khoang 0.5 |.iV (5 nV/VHz x Vo.Ol kHz x 1000) trong khi do do Ian tin bieu van la

10 |aV Ty so tin hieu tren tap bay gia la 10/0.5 = 20 nhu vay chung ta hoan toan c6 the thuc hien duge phep do mot each chinh xac

Mot bp phat hien nhay pha PSD khong nhung co tac dong voi bien dp tin hieu cung tan so vai tin hieu chuan ma con nhay voi su sai khac pha giiia tin hieu va tin hieu chuan Vi the nen mot he thong dira tren hoat dong cua bp PSD co the do duge ca ve bien dp va pha cua tin hieu luan hoan tat nhien la co su hien dien cua nhieu Cac he thong dua tren nguyen ly boat dong cua bp PSD duge gpi la cac he Lock-in Neu trone cac he khuech dai co su dung nauven Iv hoat doim cua bo PSD thi duoc lioi la khuech dai lock-in [ |

3.2 Cac khai niem co ban

So do klioi CO ban cua mot he kbuech dai lock-in duge trinh bay tren hinh 1.1 Viec chia chiing thanh cac khoi rieng biet giiip ehiing ta de hieu va co the tiep can mot each CO hieu qua va nhanh nhat u ^ T U ^ O u o c ^ ^ T T ^ '

RUNGTAV IHONG IJ- ihU ,/ihfN

Mot he khuech dai lock-in bao gom nhimg bp phan nhu: tang tien khuech dai, bo nhan, bo loc tan thap va kenh tin hieu chuan Sau day chiing ta se tim hieu mot so cac khai niem lien quan:

3.2.1 Kenh tin hieu (The signal channel)

Tac dung ciia bp tien khuech dai (preamplifier) la khuech dai tin hieu dat tai gia tri phii hgp vai dai ciia bp nhan (multiplier) hay gia tri tin hieu la vugt trpi so vai nhieu ciia bo nhan, ngoai ra no con cho phep thay doi he so khuech dai ciia he hay la thay doi dp nhay ciia he khuech dai

Kenh tin hieu

Lowjiass Fihcr

He so KD

Ref input

OLllpUl

Kenh Ref

Hinh 3.1 - Bp khuech dai lock-in co ban

Nha tang tien kliuech dai ma chiing ta co the chpn duge he so khuech dai phu hgp Muc dich o day khong chi lam tang he so khuech dai ma con phai tang duge ti so tin hieu tren tap Voi each do thi chung ta se dam bao giam toi thieu nhung thanh phan nhieu khong can thiet ra khoi tin hieu Nhu \'ay doi voi cac he thi nghiem ta phai chpn tang tien khuech dai phii hgp va co tap am thap (lov\-noise)

3.2.2 Bo tach song dong bo

Hinh 1.2 la cau tao cua mot bp tach song dong bp (synchronous detector) No bao gom mot bp loc tan thap (low-pass filter) va mot bp nhan ly tuong (ideal multiplier) Sau day chiing ta se khao sat theo quan diem toan hpc

Hinh 3.2 - Bo tach sone dons bo

Tin hieu va tin hieu chuan a loi vao bp nhan co dang hinh sin, co the hieu dung l4n

j(0 = V2V,cos[6?,/ + ^ J

r ( 0 = V2V^cos[6i>,^ + ^,]

d loi ra ciia bp nhan chiing ta nhan duge tich ciia tin hieu s(t) va tin hieu chuan r(t)

CO dang la tong ciia cac thanh phan tan so khac nhau:

V^(0 = V,K, COs[(a;, + W, )t 4- ^^ + ^ J + Vy^ COS[{CO^ - CO, )t + {(j)^ - (I), )]

Vai thuat toan nay se duge de cap lai a cac phan sau Neu chpn tan so cat cua bp loc tan thap thoa man nho ho'n tan so 03^ thi cac thanh tin hieu co tan so la tong ciia cac thanh phan (ojr "*' cor) se khong xuat hien a loi ra Ngoai ra neu dai tan ciia bp loc tan thap Ion ban nhimg thanh phan tan so Aco ^ |(o, - O),! thi o loi ra cua bp Ipc se chi xuat hien nhiing thanh phan tan so Aoj De tinh bien dp ciia thanh phan tin hieu do thi chiing ta phai biet ham dap ling tan so cua bp Ipc HLO*^^)- Dp Ion bien dp loi ra duge bieu dien bang phuong trinh:

khV,K,./i,(A^)

trong do AI(CO)^\Hi(joj)\

Do AL((O) CO tan so cat nho hon rat nhieu so voi oj,- nen chiing ta thay rang he chi co the chap nhan nhCmg thanh phan tin hieu co tan so gan voi tan so tin hieu chuan Dieu nay co the duge mo ta theo khai niem cua so truyen (transmission window), vai tan so trung tam la tan so tin hieu chuan Dac trung AL(CO) phu thupc vao tan so (co) duge mo la tren hinh 1.3 Tom lai chung ta thay rang voi su ket hgp ciia tin hieu chuan, bp nhan va bp Ipc tan thap thi tin hieu loi ra chi con lai nhQ*ng thanh phan tan

so Ian can tan so tin hieu chuan

-3dB bcindu idlh

\^, -3dB

\ bandwidth

\

(*

Hinh 3.2> - (a) Dap ung bien dp - tan so cua bo loc Ian thap

(b) Cua so truven cua bp loc tan thap \ai lan so trung tam oj^

• Giiii dieu che vdi tin hieu chuan dong bo (Demodulation with a

synchronous reference)

Xet so d6 CO dang nhu hinh 3.4 Bay gio chiing ta se khao sat y nghia thuc tien ciia phuang phap Tin hieu dua vao he thi nghiem va tin hieu chuan duge lay tu mot nguon

Ta CO mot nguon tin hieu dang sin co tan so cOr duge diing lam nguon kich dong thi nghiem va lam tin hieu chuan Tin hieu loi ra ciia he tbi nghiem co cung tan so vai

nguon kich dong va bj dich pha di ^^ Tin hieu chuan duge dua qua bp dich pha sau

do duge dua tai bo nhan cung tin hieu tu tbi nghiem

6 day chung ta thay cOs ~ cOr boi vi chiing duge nhan tu mot nguon Nhu vay a loi ra

chi con lai thanh phan mot chieu co dang:

trongdo: (p = (/>^ - ^ , k^ ^K-AtiO)

Thong thuong thi bien dp tin hieu chuan duge co dinh nen k,- cung la mot hang so Nhu vay bien dp loi ra bay gio ti le thuan voi bien dp tin hieu va dp lech pha ciia tin hieu va tin hieu chuan Sau day chung la se khao sat tac dung cua bp dich pha trong viec do cac thong so ciia tin hieu nhu bien dp va pha

Hinh 3.4 - Detector dong bp vai bp dich pha

• Giai dieu che bien do (amplitude demodulation)

Khi chiing ta dieu chinh sao cho pha cua tin hieu chuan cung pha voi tin hieu thi loi

ra cua bp nhan se la:

v,(0-^.vn,, (0

trong do: /??,(/) ^ ^??(^)®h^{i)

h^(t): dap ling xung cua bp Ipc ni(t): tin hieu dieu che

®; phep nhan chap Trong truong hgp nay pho tin hieu loi ra cua bp tach song dong bp co dang:

V^Xfco) = k,.M(faj)H^{jaj)

^M{jco)H^{fCo)^

d day: M(j(o) la bien doi Fourier ciia tin hieu dieu che,

Hgifoj) = kJT^{jco) la ham dap ling tan so ciia bp tach song

• Giai dieu che pha (phase demodulation)

De su dung bp tach song d6ng bp nhu la bp giai dieu ch§ pha (phase demodulator) chiing ta phai thira nhan rang bien dp tin hieu la hSng s6 Truoc hit chiing ta dieu chinh sao cho tin hieu chuan lech pha pi/2 vai tin hieu:

Xet trong truong hgp long quat thi (t)^ thay doi iheo thai gian (t),^(t) ((f)^(t) « 1) Bay gia chiing ta phai xem xet anh huang cua bp Ipc tan thap len ihanh ph^n tan s6 ciia

(|)m(t) Neu pha dieu che (phase modulation) co bien doi Fourier la ^Jjco), ta co

dac trung pho loi ra:

V,{jco)^k,.V^.<^J_fCo)H,(jco)

Hoac CO the bieu dien theo thoi gian:

VoiO-krKAin®h,u)

Dp nhay cua he doi voi su thay doi pha ti le thuan voi bien dp cua tin hieu Noi each

khac dap ling tan so cua bp tach song tl^jijco) co ciing qu\ luai doi voi bien dp va

pha

• Dieu che tron (Mixed modulations)

Noi chung thi chung ta cho rang tin hieu quan tam xuat hien voi ca bien dp va pha dieu che co dang:

Voi [^^^^(OJ « I radian

Nhu ta da biet thi su dieu che bien dp Idiong phu thupc vao su thay doi pha Nhu vay trong truong hgp ciing pha voi sir thay doi nho ciia pha cung khong anh huong Ion den dp chinh xac khi ta do bien dp Voi khoang bien doi pha ±10" thi dan den su bien thien ±1 !/2% khi chiing ta do bien dp

K^, sin()) '"

Thanh phan vuong pha

i\ COS'

Thanh phan cung pha

Hinh 3.5 - Bieu dien pha ciia tin hieu va tin hieu chuan

Vai nhu'ng he phiic tap ban thi no bao gom hai bp tach song dong bp Chiing cho phep do ca bien dp va pha ma khong can phai gioi ban su dich pha toi da Chia khoa ciia phuang phap nay duge bieu dien tren hinh 1.5 Chiing ta se xet ky ban van de nay a phan khuech dai lock-in hai pha (two-phase lock-in)

3.3 Bo phat hien nhay pha (Phase-sensitive detection)

Su ket hgp cua mang cac Ichoa dien tu va bp ipc tan thap a loi ra tao thanh bp phat hien nhay pha (phase-sensitive detector) Bp phat hien nhay pha con co cac dang

khac la bp trpn (mixer) va bp giai dieu che (demodulator) Vi vay khi xem xet cac

dac trung ciia bp phat hien nhay pha chinh la xet den su ket hop giua Idioa chuyen mach va bp Ipc

Sau day chung ta se di khao sat nguyen tac hoat dong cua bp phat hien nhay pha chii khong di vao mach dien cu the

3.3.2 Nguyen ly hoat dong

Nguyen ly hoat dong cua bp phat hien nhay pha di,ra vao hoat dong ciia khoa hai trang thai duge dieu khien bai nguon tin hieu chuan Khoa thay doi vi tri giira hai diem A va B dua vao sir thay doi the cua tin hieu chuan Dieu nay dan toi loi ra duge n6i vai he khuech dai ^1 ung vai nua duong cua tin hieu chuan, con ting voi nua am ciia tin hieu chuan thi loi ra dupe noi voi he khuech dai -1 Voi thoi gian ciia niia am

va niia duong la bang nhau, chiing ta co dang song duge bieu dien tren hinh 3.7

Hinh 3.7 - Iruong hgp tin hieu ciing pha vai tin hieu chuan

Tren hinh 3.7 thi tin hieu hinh sin la ctling pha voi tin hieu chuan Tin hieu loi ra sau khoa chuyen mach se co dang chinh luu hai nua chu ky Tin hieu nay duge dua tai

bp Ipc tan thap, a loi ra ta duge tin hieu mot chieu DC ty le thuan voi bien dp ciia tin hieu

Trong phan Ion cac ung dicing thi tin hieu va tin hieu chuan khi den bp phat hien nhay pha se bi lech pha Flinh 3.8 bieu dien cac truang hop co dp lech pha khac nhau Hinh 3.8(a) truang hgp ngugc pha, thu duge the mot chieu am Hinh 31.8(b) truang hgp dp lech pha 90'', trong truang hgp nay the loi sau khoa chuyen mach co dang luong ci,fc nen loi ra sau bp Ipc se bang 0 Cuoi cung la hinh 3.8(c) dp lech pha

la hk ky nen the mot chieu loi ra co gia tri nam giu'a gia tri cue dai duong va am

tuang li'ng voi dp lech pha thay doi tu 0" den 180'\

a b c

Hinh 3.8 - (a) lech pha (j) = I8(f: (b) lech pha (j) -^ 90"; (c) pha tu\ >'

De xac dinh chinh xac moi quan he giu'a tin hieu va tin hieu chuan, chiing ta phai

thiia nhan hoat dong cua Idicia chuyen mach duge dieu khien boi tin hieu xung

vuong nhan gia tri +1 va -1 Nhu vay chiing la co the su dung mo hinh bp nhan ly

tuang dupe bieu dien tren hinh 3.9 trong do tin hieu chuan co dang bien doi Fourier

Hinh 3.9(b) bieu dien the loi ra sau khoa chuyen mach trong truong hgp tin hieu va tin hieu chuan khong dong bp Tin hieu chuyen mach co gia tri trung binh bang 0 va dang tin hieu co the thu duge boi bieu thiic sau:

a phan truoc tan so cat cua bp Ipc tan thap nho hon tan so chuan Cac thanh phan tan

so 2a),-, 4o),, 6(0,-, etc se bi loai bo a loi ra chi con lai thanh phan mot chieu DC phu thupc vao dp lech pha:

K - ^ ^ K ^ , ^ , ( 0 ) c o s ( ^ , - ^ J

K

Ai(0): la bien dp cua dap ung bp Ipc tai tan so bang 0

Dua vao cac he so cua bieu thii'c dap ung cua bp phat hien nhay pha ta thay no giong voi dap ling cua bp phat hien nhay pha ly tuong boat dong voi tin hieu chuan dang sin Su khac co ban do la trong truong hgp na\ bo phat hien nha\ pha co the boat dong vai nhimg tin hieu co tan so 3c'),, 5to,- etc Dieu nay dupe minh hpa tren hinh

3.10 doi \ a i tin hieu dang sin co lan so gap 3 lan tan so chuan Bieu thiic loi ra ciia

he truong hgp nay co he so la 1/3, 1/5 tuong ung voi dp Ion cua cac he so phan tich Fourier cua tin hieu chuan