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Bandpass Signaling Principles and Circuits Chap. 4
4–13 DETECTOR CIRCUITS
As indicated in Fig. 4–1, the receiver contains carrier circuits that convert the input bandpass
waveform into an output baseband waveform. These carrier circuits are called detector circuits.
The sections that follow will show how detector circuits can be designed to produce
R(t), u(t), x(t), or y(t) at their output for the corresponding bandpass signal that is fed into the
detector input.
Envelope Detector
An ideal envelope detector is a circuit that produces a waveform at its output that is proportional
to the real envelope R(t) of its input. From Eq. (4–1b), the bandpass input may be
represented by R(t) cos[v c t + u(t)], where R(t) Ú 0; then the output of the ideal envelope
detector is
v out (t) = KR(t)
(4–74)
where K is the proportionality constant.
A simple diode detector circuit that approximates an ideal envelope detector is shown in
Fig. 4–13a. The diode current occurs in pulses that are proportional to the positive part of the
input waveform. The current pulses charge the capacitor to produce the output voltage waveform,
as illustrated in Fig. 4–13b. The RC time constant is chosen so that the output signal
will follow the real envelope R(t) of the input signal. Consequently, the cutoff frequency of
v in (t)
C R v out (t)
(a) A Diode Envelope Detector
Input signal, v in (t)
Output signal, v out (t)
(b) Waveforms Associated with the Diode Envelope Detector
Figure 4–13
Envelope detector.