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Sec. 4–10 Limiters 265
v out
Ideal limiter
characteristic
v lim (t)
V L
V L
v in
V L
t
t
v in (t)
Figure 4–7
Ideal limiter characteristic with illustrative input and unfiltered output waveforms.
an ideal comparator with a zero reference level. The waveforms shown in Fig. 4–7 illustrate
how amplitude variations in the input signal are eliminated in the output signal. A bandpass
limiter is a nonlinear circuit with a saturating characteristic followed by a bandpass filter. In the
case of an ideal bandpass limiter, the filter output waveform would be sinusoidal, since the
harmonics of the square wave would be filtered out. In general, any bandpass input (even a
modulated signal plus noise) can be represented, using Eq. (4–1b), by
v in (t) = R(t) cos [v c t + u(t)]
(4–54)
where R(t) is the equivalent real envelope and u(t) is the equivalent phase function. The
corresponding output of an ideal bandpass limiter becomes
v out (t) = KV L cos [v c t + u(t)]
(4–55)
where K is the level of the fundamental component of the square wave, 4p, multiplied by the
gain of the output (bandpass) filter. This equation indicates that any AM that was present on
the limiter input does not appear on the limiter output, but that the phase function is preserved