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3.4 Khuech dai lock-in hai pha (Two-ph ase Lock-in amplifier)
KhuSch dai lock-in hai pha la khuech dai lock-in ket hgp hai bp phat hien nhay pha (PSD) va su dung hai tin hieu chuan lech pha nhau 7t/2. Cau tao cua bp khuech dai duge trinh bay tren hinh 1.12. No la su ket hgp phep do ung voi hai truang hgp ciing pha va vuong pha giua tin hieu va tin hieu chuan.
Vai bp khuech dai lock-in co dp nhay full-scale la Sị va the loi ra trong khoang iVp. Th^ 16i ra V.x va VB sau cac bp Ipc la:
(/^=:F.(JV/5, )cos^ F ^ = ( ; ( F , /5/. )sin^
Trong do V. la the hieu dung cua tin hieu va ty so (> / S/: la he so ty le ciia khuech
dai lock-in. Khi da co cac gia tri loi ra la VA va VB ehiing ta tim duge V^ ty le thuan voi bien dp tin bieu V, va dp lech pha (j) theo cong thuc sau:
(t>^\^n'V,IV, _
Nhu vay khuSch dai lock-in hai pha co thS biSu dicn loi ra trong toa dp dh cac hoac
toa dp ci;c. No thuong duge sii dung d^ do dac trong hai ITnh vuc chinh:
Thii nhat la cac ung dung sii dung thanh ph4n vuong pha cua tin hieu, nhu la phep do cau can bang AC va phep do tong trạ
Thii hai la cac ung dung do bien dp tin hieu ma co su bien doi pha Ion nhu cac phep do tin hieu nho DLTS bay cam ling tir...
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Hinh 3.12 - Khuech dai lock-in hai pha
l/u diem Ion nhat cua khuech dai lock-in hai pha la co kha nang loai bo cac hoa bạ Chat lugng tuong duong voi vice su dung tin hieu chuan dang sin. Tiíc la voi he two-phase chi co mot dap ling xung co ban, bien dp voi thanh phan co ban ciia tin hieu va dp lech pha giu"a tin hieu va tin hieu chuan co the bieu dien vecto (R va (j)) vai dp chinh xac caọ Nhung ta chi co the bieu dien bien dp va pha duoi dang vecto khi tin hieu co dang sin.