Signal Processing and Linear Systems-B.P.Lathi copy

Wt t he output is t he desired signal m t multiplied

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: he viability of demodulation of AM by an envelope detector. When A < m p , Eq. (4.76) shows t hat J1 > 1 (overmodulation). In this case, the option of envelope detection is no longer viable. We t hen need t o use synchronous demodulation. Note t hat synchronous demodulation can be used for any value of J1 (see P rob. 4.7-4). T he envelope detector, which is considerably simpler a nd less expensive t han t he synchronous detector, can be used only for J1 :::; 1. • E xample 4.19 Sketch 'PAM(t) for modulation indices of J1 = 0.5 (50% modulation) and J1 = 1 (1~0% modulation), when m (t) = B cos wmt. This case is referred to as t one m odulatIOn because the modulating signal is a pure sinusoid (or tone). In this case, m p = B and the modulation index according to Eq. (4.76) is Hence, B = J1A and m (t) = B cos wmt = c A M signal R Envelope detector output R C too large 'PAM(t) = [A I +0.5 cos (Om t "/ + m(t)] cos wet = ~ " >. ~" + J1cos wmt] cos wet 2A (4.78) .., .... ( b) .... F ig. 4 .36 .. ' ... ,.. K, . i ' 1'" .... ;;", 0 ~~f'-" J1Acos wmt A[1 \ ..... , '1" ,< ~~nvelOpe 1 ", .,., Therefore ( a) .... " ..... "' . Envelope detector. l +cosoom t 3A/2 Demodulation of A M: T he Envelope Detector We shall consider here one of t he noncoherent methods of AM demodulation, the e nvelope d etection. t I n a n envelope detector, t he o utput of t he d etector follows t he envelope of t he ( modulated) i nput signal. T he c ircuit illustrated in Fig. 4.36a functions as a n envelope detector. During the positive cycle of t he i nput signal, t he diode conducts and t he c apacitor C charges up t o t he p eak voltage of t he i nput signal (Fig. 4.36b). As t he i nput signal falls below this peak value, t he diode is c ut off, because t he c apacitor voltage (which is very nearly t he p eak voltage) is g reater t han t he i nput signal voltage, a circumstance causing t he diode t o open. T he c apacitor now discharges through t he resistor R a t a slow r ate ( with a time constant R C). D uring the n ext positive cycle, the same d rama r epeats. When t he i nput signal becomes greater t han t he c apacitor voltage, t he d iode conducts again. T he c apacitor again charges to t he p eak value of this (new) cycle. As t he i nput voltage falls below t he new peak value, t he diode cuts off again a nd t he c apacitor discharges slowly during the cutoff period, a process t hat changes t he c apacitor voltage very slightly. T he A M signal can be demodulated coherently by a locally generated carrier (see P rob. 4.7-4). However, coherent, or synchronous, demodulation of AM (with J1 :::; 1) will defeat t he very purpose o f AM and, hence, is r arely used in practice. tThere are also other methods of noncoherent detection. The rectifier detector consists of a rectifier followed by a lowpass filter. This method is also simple and almost as inexpensive as the envelope detector4 . The nonlinear detector, although simple and inexpensive, results in a distorted output. A A...
View Full Document

Ask a homework question - tutors are online