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Sec. 4–13 Detector Circuits 281
This demonstrates that the quadrature detector detects the modulation on the input FM signal.
The quadrature detector principle is also used by phase-locked loops that are configured to
detect FM. [See Eq. (4–110).]
As indicated by Eq. (4–80), the output of an ideal FM detector is directly proportional
to the instantaneous frequency of the input. This linear frequency-to-voltage characteristic
may be obtained directly by counting the zero-crossings of the input waveform. An FM
detector utilizing this technique is called a zero-crossing detector. A hybrid circuit (i.e., a
circuit consisting of both digital and analog devices) that is a balanced FM zero-crossing
detector is shown in Fig. 4–18. The limited (square-wave) FM signal, denoted by v 1 (t), is
LPF
Q
v 2 (t)
v in (t)
Limiter
v 1 (t)
Monostable
R
+
multivibrator
C
v out (t)
(Q pulse width
Differential
T c 1
amplifier
is –– = –– ) –
2 2f c Q
v 3 (t)
R
C
-
LPF
(a) Circuit
1
Waveform for the case of the instantaneous frequency f i >f c where f i =––
T i
T i
v 1 (t)
t
Limiter output
Q
v 2 (t)=DC level of Q
Q monostable output
T c /2
–
Q
–
v 3 (t)=DC level of Q
t
–
Q monostable output
t
(b) Waveforms ( f i >f c )
Figure 4–18
Balanced zero-crossing FM detector.