Question

1. A modulating signal m(t) is given by m(t) = 5 (cos(20nt) – cos(40nt)) a. Find and sketch the spectrum of DSB-SC signal 2m(t)cos (100nt) b. Verify that the DSB-SC modulated signal can be recovered using coherent detection.

Answer 1

Note: coswat o mit) costante os [-ccm) +stwortom) į (M6w-60) + Mcworlon] Given, metsa ſcos potit -cos torit) MW) = ( (60-201) + 8(6+2011) - $(W-40TI) -816+4011) Moll | T/2 + 13 45 - 2T40 LET DSB-SC signal I t) = 2mct) Cos 100 t Xosp(w) = 2 [+][ M-1001) + M1L0 41001) - I sw-1200) + 6(6-80T) -816-14011) 1 -86-601)] + T' I sw+801) +8660+12011) 4 -8660 +601) -8(6+128) 1/2

Spectrum of DSB-56 : 0 bot 199 ! 120 SOT -80T -ILOT FT b Block diagram of coherent detectory 46) pit) modulated at message signa signal required fciter mostly LPF Canter signal 200) = m(t) cos too mit Y6) - [X(t) ] [ 105 100 11t ] = 2 mil cosloo Tit 9[t] MU ( 1 + 105200 Tit) Emitt mct) COS20071 Y(6) - MW) + Mw) ( S20-2001) + 8(6+2000)) after equal the passing through LPF having cut off frequeny to signal Band width. out put of LPF Plu) = MW) Plt) = m(t)/= message signal. - coherent detector is used to detect DSB-se signa