Question

AM Modulator AM Demodulator m(t) SAM-DSB-sc(t) mdemod(t) LPF cos2nfct cos21fct S Figure 1 P1. For the system shown in Figure 1, sketch the signals mſt), mde mod (t) in the time and frequency domain c(t), and AM-DSB-SC

Assume m(t) is a 10 Hz square wave, with amplitude 1, and that AM-DSB-C is the modulation scheme used with fc = 100Hz.

Answer 1

mla fit Modulation Property of Ger(@tfet).: If any signal mA) with M(H) as Jourier tramform, then M11) *[M(+-te) + M H+le)] le frequency component will be sheft u de 1 Then m(t) cosantet ୧. A m(t) A MCH А kom f Im M(1), Goldalet Al2 le de Wettm) -Jet/m). de-lm) -He-im) - Will ure the above property of corrJet) to draw the signals. Abo, the fouries bromform (F.T) of square ware is AT given as m(H) А HAT Sa(47) t I & Hog T I'm 4-lo simo know as Sampling funchon . cs anded VTe to Time Pevod: Jon : Prenage frequency. where Sa (0) o ! CamScanner

2 now we are ready to draws the diagrom and sketch of syslem. AM Demodulator AM Modulator mit) SAM-pse-sell) mdemod (d) X X LPF Corantet Conect Modulaton Scheme : DSB-SC : Double onde band with Suppressed Carries. Time domain and treg domain Yraph w oketched on next page. Mit is menage signal and M17) abb fot as discussed. and C(4) Gobla let) is Carmies signal and the foT given by C(= [Slt-te) + 81t+de)] Le too impulbe at to tfc. Next is Modulaled Dignal as we have disewned in moduation prop. of Cosantet. Menage signal of BW fm is trammitted at I de. The Amplitude is decreased by half Le T/2. BW becomes Double le & fm. There is no camories component in OSB-SC as it suppresses the carries component le impulses and I de is only shift the menage agnal at 'Ite. CS

3 M(A) Amet) T ܝܙ F. T gy >t - tm fm (1012) T cih ↑ Costante (H) ^CLH F.T -tel-100Hz) ++e(10012) SAM-Dse-se (1) SAM.Dse-Sc(H) WILL АЛМ E.T T12 >t -Vetto) tealte-In) 100 - 90 (Je-lm) de textom) go 100 110 -110 SLPF T/2 T LPF -Jet Im m demod (1) tm feut Mdemod (1) FT 1/2 T/2 Im (1042) T (-1042) Im + Time domain. frequency Domain Scanned with CamScanner

Il » Now the DSB-Sc signed in agein multipled with Confale t), then Sam-868-sekt) le frequency Domcun Signal is shift by t de amount. It #te" component shifhs by -ti" amount it comes to origin If "de" component of Sam- DSB-se (f) shifts #te" units, il abo comes to origin. - Hence both signab add af oorgin before the Low Pan filter (LPF) and looks like SLPF- Let LPF LPF Cut off frequency teuts tm, Will Recome our ongin al signal. M demod It is same ml A) only ifb amklitude is seduced by 112. Giren fe = 100 Hz fm = 10 Hz terum dc-fm 90 Hz. ~ Now Let le 100 +10 · 110 HZ 100 -10 = CS Scanned with CamScanner