A

FIGURE 3.29 Modulating a signal

Note that the modulated signal is no longer a pure sinusoid, since the overall transmitted signal contains glitches between the T-second intervals, but its primary oscillations are still around the center frequency fc; therefore, we expect that the power of the signal wil be centered about fc and hence located in the range of frequencies that are passed by the band-pass channel.

By monitoring the polarity of the signal over the intervals of T seconds, a receiver can recover the original information sequence. Let us see more precisely how this recovery may be accomplished. As shown in Figure 3.30b suppose we

(a) Modulate cos(2nfc t) by multiplying it by Ak for (k-1)T < t < kT:

eos(2n/ci)

eos(2n/ci)

(b) Demodulate (recover) Ak by multiplying by 2 cos(27lf, t) and low-pass filtering: Yi (t) = Ak cos(2nf, t)—^ -- Xt (t)

FIGURE 3.30 Modulator and demodulator

FIGURE 3.30 Modulator and demodulator

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