BI TP CHNG 2: LY MU V KHI PHC TN HIU. 2.1 V hai tn hiu Sine tng t c tn s 20Hz v 100Hz,tn hiu c ly mu vi tn s l 120Hz. a. Tn hiu khi phc t cc mu l tn hiu g? b. Lp li cu hi a vi tn s ly mu l 220Hz. 2.2 Mt mch ADC ly mu tn hiu tc mt mu trn mi 10s,Tm tn s tn hiu tng t cao nht m khi c ly mu khng c hin tng chng ln ph xy ra. 2.3 Xem sng ngang m thanh dao ng theo hnh Sine tc 1000radian/m (bin thin theo khng gian thay v bin thin the thi gian).Tm khong ly mu ti a c th khi phc li dao ng ban u t cc mu thu c tng ng. 2.4 Cc tn hiu sau c gy ra hin tng chng ln ph khng khi ly mu vi tc tng ng l 100 mu /s: Sin10 t; Cos320 t; Cos 400 t ; 2.5 Tm cc tn s s gy ra hin tng chng ln ph khi tn s ly mu tng ng l 10 mu/s: a) Cos 0.4t c) Sin0.8t b) Sin0.6t d) S in1t 2.6 Cho tn hiu tng t x(t) nh sau: x(t ) = 4Sin 2 t + 4Sin6 t + 2 Sin12 t ;(t = s ) c ly mu tn s 5Hz,tm tn hiu khi phc t cc mu thu c. Lp li vi tn s ly mu l 10Hz. 2.7 Cho tn hiu tng t x(t) nh sau: x(t ) = 3Cos8 t + 2Cos 4 t Cos6 t ;(t = s ) c ly mu tn s 5Hz,tm tn hiu khi phc t cc mu thu c. Lp li vi tn s ly mu l 9Hz. 2.8 Tn hiu tng t x(t) nh sau: x(t ) = Sin6 t[1 + 2Cos 4 t ];(t = ms) Tn hiu c ly mu vi tn s 4Khz,Xc nh tn hiu khi phc(Gi s qu trnh khi phc l l tng). 2.9 Xem tn hiu m thanh nh sau:xa (t ) = Sin10 t + Sin 20 t + Sin60 t + Sin90 t +;(t = ms )

Tn hiu trn c a qua b tin lc c p ng tn s l H(f),sau c ly mu vi tn s tng ng l 40Khz.Cc mu sau c a qua mch khi phc l tng.Xc nh tn hiu khi phc c cho cc trng hp: a. Khi khng dng b tin lc( tc l H(f) = 1).

Hnh v bi tp 2.9

b. Khi b tin lc l mch lc thng thp c tn s ct l fs = 20Khz.

c. Khi b tin lc l mch lc thc t c p ng nh hnh v: 2.10 Khong tn s quan tm trong tn hiu ting ni l [0; 3.4 Khz]. Bn ngoi khong ny tn hiu suy gim dB/decade.Tn hiu ny c a qua b tin lc c p ng phng n fM, ri suy gim dB/decade. Hy chng t rng, mc chng ln ph vo di tn quan tm nh hn A dB th tc ly mu ti thiu l:

fs = fM+10A/(+)fM.2.11 Mt tn hiu tng t c di tn quan tm [0,20Khz]. v c ph c m t nh sau :| X a ( f ) |= 1 1 + ( 0.1 f )8

, f : Khz

Tn hiu c ly mu tc fs. Ngi ta mun mc chng ln ph vo di tn quan tm phi nh hn 60 dB. Hy xc nh gi tr ca fs tha mn yu cu trn nu khng dng b tin lc. Mt tn hiu tng t sau khi qua b tin lc c ly mu tc fs = 8 Khz. Tn hiu s sau c lc dng b lc s thng thp l tng fc = 1 Khz. Tn hiu s ng ra c a n mch khi phc hnh thang ri n b hu lc. Hy xc nh cc thng s ca b hu lc mc ph nh c gim t hn 40 dB.