Trong do ENBW (equivalent noise bandwidth ) la nhieu trong loan dai cua bp Ipc tan thap c6 hang so thai gian T. ENBW duge xac dinh bai hang so thoi gian va dp
doc (slope) ciia bp Ipc. Wait time la thai gian de dat toi 99% gia tri cuoi ciia nọ Ta CO bang sau: Slope ENBW 6dB/oct 1/(4T) 6dB/ocl 1/(8T) 7T 6dB/ocl 3/(32T) 9T 6dB/oct 5/(64T) lOT • Shot noise
Dong dien co nhieu va phu thupc vao cac hat tai dien húu han. Do dong chuyen dong CO huong cua cac hat tai dien la khong dong nhat nen sinh ra nhieu dong. Nhieu do duge gpi la Shot noisẹ No co the chuyen thanh the nhieu Idii dong nhieu di qua mot dien trạ Shot noise hay nhieu dong duge xac dinh boi cong thiic:
Jn..eO-ms) = {2qL\f)''
Trong do q la dien tich cua electron (1.6 10" C), 1 la dong AC hieu dung hoac la dong mot chieu DC phu thupc vao mach va Af la dai tan lam viec.
Khi sii dung khuech dai lock-in de do dong tin hieu AC ma co dai tan nho thi Shot
noise co gia tri khong dang kẹ
• Nhieu 1/f
Gia sir cac dien tro co gia trj 10 Q, khong ke la duge lam tir vat lieu gi deu co ciing nhieu Johnson. Them vao do chiing con co nhieu boi^ siá thay doi dien tro do dong chay qua dien tro thay doị Doi vai dien tro la hgp chat carbon, thi gia tri nhieu hieu dung thuong bi6n thien tu 0.1 ).iV den 3 |aV ling voi dien the IV dat tren dien trọ Con vai dien iró la kirn loai thi nhieu nay co gia tri nho hon khoang 10 lan. Nhieu nay co ph6 la 1/f, con gpi la nhieu 1/f, chiing lam cho cac phep do a tan so th§p gap
kho khan. Nhieu 1/f xuat hien trong tat ca cac he kbuech dai hay cac bp detector. Chiing con xuat hien trong cac he chan khong va chat ban dan.
• Cac nguon nhieu ngoai (External noise sources)
Ngoai cac nguon nhieu noi da giai thieu a tren thi con co cac nguon nhieu ngoai xuat hien trong phong thi nghiem. Cac nguon nhieu nay la cac nguon khong dong
bp, chiing khong lien quan den tin hieu chuan cung nhu khong xuat hien a tan so
chuan va cac boa ba ciia nọ Vi du cac nguon nhieu nay dupe gay ra bai bong den dien, motors, radio, may tinh, etc. Cac nguon nhieu anh huang den phep do, bai vi chiing ta phai tang he so Dynamic reserve hay tang hang so thai gian.
Tuy nhien van co cac nguon nhieu lien quan den nguon tin hieu chuan nen co the lam tang cuang hay suy giam tin hieu, phep do tra nen kliong chinh xac. Cac nguon nhieu tren co the duoc ban che bang each bo tri thi nghiem hgp lỵ Sau day chiing ta se khao sat cac each ma nguon nhieu anh huang den he:
• Lien ket dien dung
Co the xem mot nguon the xoay chieu AC dat canh he thi nghiem lien ket vai bp detector thong qua mot tu dien C,ira>, con gpi la dien dung lien ket. Mac dii Cstray co the rat nho nhung nhieu lien ket Ion hon rat nhieu lan tin hieu cua hẹ Va no dac biet anh huoni^ khi nhieu lien ket donu bo voi tan so tin hieu chuan.
Stray Capacitance experiment | ' DetectorT 1 1 Noise Source 1 1
Hinh 1.13 - Lien ket dien duns
Chiing ta co the xem dong nhieu gay ra boi C,t,a> co dang sau
^i:'.^ r F
siren J \7;-i/r irjịs^
dl
Trong do co - 2Ti:f, f la tin so nhieu; V,,^,,, la bien dp nhieụ
Vi du, nSu ngu6n nhilu la mot mach cong suat. nen f = 60 FIz va V^oise = 120 V. Csirav la dien duna tuong duong cua tu dien co cac ban tu song song. Gia sii cac ban tu CO dien tich 1 cm", khoang each giua cac ban la 10 cm thi C,tra> = 0.009 pF. Dong nhilu tinh duge la 400 pA (a 60 Hz), no co the Ion ban rat nhieu so voi dong ciia tin hieụ N^u nsuon nhieu o tdn s6 cao hon thi nhieu lien ket c6 the Ion hon niiạ
N^u tSn so nhieu bSng t4n sc^ chuan thi phep do tro nen khcmg chinh xac vi khuech dai lock-in chi loai duge cac thanh phan nhieu a tan so khac tan so chuan. Nhu vay mu6n phep do dat dp chinh xac cao ta phai thuc hien cac bien phap tranh nhieu lien ket dien dung nhu sau:
• Tranh xa cac nguon nhieu (giam Cstra>)- Khong de day tin hieu gan nguon nhieu
• Thiet ke cac thi nghiem co tra khang nho (sut the do dong nhieu se giam) • Bpc kim he thi nghiem (cho vao hop kim loai)
• Lien ket cam ling
Lien ket cam ung ciia mot dc)ng xoay chieu .AC len he ihi nghiem la thong qua tii* truang bien thien tac dong len he thi nghiem. Mot dong dien thay doi a gan he thi nghiem lam cho tu thong thay doi va tao nen mot suat dien dong (d(j)B/dt) tren day noi giiia detector va hẹ No boat dong lien ket nhu mot bien the, khi do he thi nghiem d6ng vai tro nhu cupn thii cap con nguon nhieu dong vai tro nhu cupn so cap. Ex r oeriment ^ ^ Detector 1 . - — r 1 B(t) 4. r- i 5 1 Noise Source \
Hinh 1.14 - Lien ket cam ling
Cac bien phap de ban che nhieu do lien ket cam i;rng: • Loai bo hoac tat cac nguon nhieu
• Day tin hieu phai dung day cap dong tiaic • Bpc kim he thi nghiem (cho vao hop kim loai)
• Dung he detector co tro khang cao de do dong, khong do the
• Lien ket dien tro
Cac dons dien xu6ng dat tai cac diem dat khac nhau cung tao ra mot dien the nhieụ Hinh 3.15 minh hpa detector do tin hieu so \ a i dat tai mot diem xa vai he thi nghiem. Nhu vay thC' ciia he thi nghiem se bang the cua detector do duge cong vai nhilu do sut the tren dien tro noi giua diem dat ciia he va detector. Boi vi detector va
he thi n<^hiein n6i dk\ tai cac diem khac nhau nen tai cac diem do c6 cac the tuong
Experiment
Detector
^^T^^7
"^WJ
Noise Source
Hinh 3.15 -- Lien ket dien tra
Cac bien phap de ban che nhieu do lien kt§t dien tro:
• Noi chung cac diem dat tai mot \ i tri
• Day noi dat phai lan de giam dien tro
• Dao d o n g co* hoc
Khong phai tat ca cac nguon nhieu deu la nguon dien. Cac nhieu ca hpc cung co the chuyen thanh nhieu dien ha\ cac hieu ling. Nhung iha\ doi co hpc (nhiing dao dong)
ciia he hoac day cap co the chuyen thanh nhieu dien tren loan bp dai tan lam viec ciia khuech dai lock-in.
Vi du: Xet mot day cap dong true noi giúa detector va khuech dai lock-in. Dien dung ciia cap co the xem la mot ham cua su thay doi hinh dang cua nọ Nhúng dao dong CO hpc cua cap chuyen thanh si,r bien thien cua dien dung theo thai gian. Ta c6:
, . 6 / F ,.6/C dO
C — + / — = --- = 1 dl dl dl dl dl dl
Dua vao bit§u thuc tren chiing ta thay nhung dao dong tren day cap gay ra su bien thien dC/dt va lam cho dong trong da\ cap cung bien doị Dong nay se anh huang den detector va tin hieu can dọ
Cac bien phap de ban che nhieu do dao dong ca hpc: • Loai bo cac he ca hpc gan thi nghiem
KET LUAN
Sau mot nam nghien ciiu, de tai da thuc hien mot so cong viec va co mot so ket qua cu the nhu sau:
• Nghien ciiu tinh chat ciia tin hieu nho tren ca sa moi tuang quan giCa tin
hieu va tap nhieu tac dong vao he do vat lỵ
• Nghien ciiu tap hgp va khai quat hoa mot so phuang phap xii ly tin hieu nho nhu phuong phap Fourier, phuang phap Tach scSng d6ng bp. phuang phap Boxcar, phuang phap Lock-in. Tin hieu nho duge đn chiing la tin hieu DLTS. Tuy nhien, hoan toan co the sii dung cac phuang phap noi tren de do dac, xii ly cac tin hieu nho khac trong he dọ
• Huong dan 01 Luan van cao hpc theo huang de taị Ket qua ciia Luan van: Thiet ke xay dung thanh cong bp khuech dai lock-in so hai pha bang phan mem Labview (two-phase lock-in amplil'ier) ling dung trong nghien cuu va dao tao tai Khoa Vat lỵ
• Viet 01 bai bao khoa hpc ve cac phuong phap xu ly tin hieu nho ling dung trong do luong DLTS.
Chiing toi tran trpng cam on Ban KHCN-DHQGIFN, Truang DHKHTN, Khoa Vat ly va cac dong nghiep da quan tam giiip do chting toi trong qua trinh thuc hien de tai naỵ
TAI LIEU THAM KHAO
ị;E
rieng Viet: