Answer Happy

2.18 Consider the asynchronous receiver of Fig. 6.4. The message could be reconstructed from the output of either envelope detector, as we did in part 6.4 of the experiment. For example, if the MARK signal is available then the SPACE signal is its complement. So why have both envelope detectors?

1 = n) H. Envelope Detector FSK 1 = n1 r() Output = 0 if r(t) <VA Threshold Device Output = 1 if r(1) > VA r() HO Envelope Detector Stage 1 Stage 2 Fig. 6.4 Asynchronous Demodulation of FSK The receiver of Fig. 6.4 takes advantage of this. The FSK signal has been separated into two parts by bandpass filters (BPF) tuned to the MARK and SPACE frequencies. The output from each BPF looks like an amplitude shift keyed (ASK) signal. These can be demodulated asynchronously, using the envelope. The decision circuit, to which the outputs of the envelope detectors are presented, selects the output which is the most likely one of the two inputs. It also re-shapes the waveform from a bandlimited to a rectangular form.