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3.3.2Nguyen 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
Relerence
Hinh 3.6 - Phase-sensitive detector
1) r\ r\ { \ / / ' ^~\^ \ \ \ / / ^ '\ t • V ' J '.J '-J ~j ! l ' J ' \ ! ' -1 ' 1 ^\ - .^^
[he loi r.i
I y \Z V NZ ^ ^ _
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 kỵ 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 hieụ
Trong phan Ion cac ung dicing thi tin hieu va tin hieu chuan khi den bp phat hien nhay pha se bi lech phạ Flinh 3.8 bieu dien cac truang hop co dp lech pha khac nhaụ 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 giúa gia tri cue dai duong va am
h kAc/W.' kAc/W.' 0 Oi V 0 giii i n iruim hinh r\ • * — > • v | t r gia Líi tn L/ / gid tri trunii binh 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 giúa 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 nhu sau:
/ - ( / ) -
71
4 cos((o ( + (l),) — cos3(<:?;./ ^(1),) + -có^iicoj -^(j)/) 3 ' 5
o Sl'JIKll Lô\-|)âs n i l c r r < c r . &MV1 XLlllL- \ UOHLl I l u l p i U • • ^ ( 1 ) • M T ) ^ I T . ( I j ' - ' U ) ^ \ ^ \ i ! N / bi a I f V L I '
Hinh 3.9 - (a) bo nhan l\ tuxrng: (b) loi ra sau khoa chuyen mach vai tin hieu dang
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:
' ; ( / ) - r ( / ) - v ( / )
trong do s(t) = \2y^cos(cột -\-(/)^)
Thuc hien phep nhan va phan tach thanh phan tong va hieu cua cac thanh phan tan so ta thu duge: 2V2F r v , , ( / ) = -[COS(OJ^.l±(Oj + tp,. ± ^ J /r - • cos(3('.'/ / ± cof ^- 3(/j^. ± (j).^) 3 -h - COS{5cO,.t X. CO J -r 5(1)^. ± ij)^ )... J
Den day ta thay co sir tuong duong voi bp tach song dong bp ly tuong (ideal synchronous detector) da thao luan a phan truac. Ta dat o, - (O,- va nhu da thira nhan 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 bọ 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 thiíc 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.
V 1 0 1 0 n S 0 h. ;^ \J r\ \J \J r\ r\ \j r\ \j '-J — — • I I X iruiíj tnnli
Hinh 3.10- Phut hien tin hieu lidi hdc cao (adsbygoogle = window.adsbygoogle || []).push({});
3,3.3 Cac khai niem lien quan den bo PSD
• Do nhay Full-scale
Bp phat hien nhay pha mang mot y nghla rpng, vi ly do do ma chiing co the hoat dong voi cac i^mg dung, voi cac loai tin hieu khac nhaụ T\iy nhien, ta thay rang dap ling ciia bien dp loi ra phu thupc vao thanh phan bien doi Fourier cua tin hieu, va chiing khac nhau trong cac truong hgp khac nhaụ
De thuan tien cho viec lua chpn cac thiet ke va tíng dung thi cac he so ty le cua bp phat hien nhay pha phai la khong doi doi voi mot tin hieu dang sin dac trung. Mot he so ty le dan gian do la ty le giiía the mot chieu AC loi ra va the hieu dung loi vao dong bp dang sin trong truang hgp ciing phạ Trong thuc te thi the loi ra mot chieu d.c. duge gioi ban boi gia tri cue dai nao dọ Cac gia tri do duge gpi la the loi ra full- scale, thuong CO gia tri la ±10V. Dp nhay full-scale cua bp phat hien nhay pha duge dinh nghla la the hieu dung cua tin hieu dong bp. cung pha dang sin trong truang hgp the mot chieu d.c. loi ra dat gia tri full-scalẹ No duoc do bang V. mV hay |aV. Neu ta biet dp nhay full-scale la So, the hieu dung tin hieu dang sin la V^, dp lech pha la (j). Ta co phuong trinh the loi ra:
V^=V,{VJS^)cos(<l))
trong do: Vp la the loi ra full-scale
Vai each bieu dien nhu tren ta thay: neu bp phat hien nhay pha co he so Sp nho thi no he CO the de dang dat duge Hill-scale voi nhung tin hieu (VJ bẹ
• Dynamic reserve
Doi voi khuech dai lock-in chung ta hay gap thuat ngiTr Dynamic reservẹ Neu loi
vao bao g6m tin tin hieu full-scale a tan so tin hieu chuan cong voi nhieu a cac tan
so khac thi Dynamic reserve duoc dinh nghla nhu sau:
Dynamic reserve- signal & noise cue dai o loi vao
full scale
Vi du: neu full-scale la 1|J.V, Dynamic reserve la 60dB thi nhieu Ion nhat la ImV
ma loi vao van khong bj vugt thang.
Nhu vay nhieu ma Dynamic reserve gioi ban thi se khong bi vugt thang a bat cii mot
noi nao do trong thiet bi nhu: tang khuech dai, PSD (phase-sensitive detector), bp Ipc tan thap hay khuech dai DC. Dieu nay co the thuc hien nha viec dieu chinh he so
khuech daị De dat duge he so Dynamic reserve cao thi he so khuech dai loi vao phai
dat thap. Dieu nay co nghla la tin hieu loi ra bp PSD co gia tri rat nhọ Bp Ipc tan thap se loai bo nhimg thanh phan nhieu va giu" lai thanh phan mot chieu DC, sau do no duge khuech dai co the den lOV full-scalẹ Tuy nhien, nhu ta da biet thi cac bp
khuech dai lock-in tuong tu co van de khi gia tri Dynamic reserve lan, no se anh
huong den dp tuyen tinh ciia thiet bị Ngoai ra, nhieu lan thi no cung se anh huang den phep do theo mot each nao dọ
Dynamic reserve con phu thupc vao tan so ciia nhieụ Ro rang la nhieu a tan so tin
hieu chuan van khong bi suy bao nen Dynamic reserve a tan so do la OdB. Khi tan so nhieu khac voi tan so chuan thi Dynamic reserve tang. Bai vi tan so loi ra sau
PSD la llnoisc - Ik-ij bp Ipc lan thap sau bp PSD se loai cac thanh phan nhieu dọ
actual reserve 50 dB 40 clB 2GdB OdB 60 dB 5cecified reserve
lov.' pass niter
Dand\Md:;i
iref fnoise
Hinh 3.11 - Moi quan he giua d\ namic reserve voi tan so nhieu
Hinh 3.11 mo ta moi quan he giua he so Dynamic reserve thuc le phu thupc vao tan (adsbygoogle = window.adsbygoogle || []).push({});
so nhieụ