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Bandpass Signaling Principles and Circuits Chap. 4
shown in Fig. 4–18b. For purposes of illustration, it is assumed that v 1 (t) is observed over that
portion of the modulation cycle when the instantaneous frequency
f i (t) = f c + 1
2p
du(t)
dt
(4–91)
is larger than the carrier frequency f c . That is, f i 7 f c in the illustration. Since the modulation voltage
varies slowly with respect to the input FM signal oscillation, v 1 (t) appears (in the figure) to
have a constant frequency, although it is actually varying in frequency according to f i (t). The
monostable multivibrator (one-shot) is triggered on the positive slope zero-crossings of v 1 (t).
For balanced FM detection, the pulse width of the Q output is set to T c 2 = 12f c , where f c is the
carrier frequency of the FM signal at the input. Thus, the differential amplifier output voltage is
zero if f i = f c . For f i 7 f c [as illustrated by the v 1 (t) waveform in the figure], the output voltage is
positive, and for f i 6 f c , the output voltage will be negative. Hence, a linear frequency-to-voltage
characteristic, C [ f i (t) - f c ], is obtained where, for an FM signal at the input, f i (t) = f c +
(12p)K f m(t).
Another circuit that can be used for FM demodulation, as well as for other purposes, is
the phase-locked loop.
4–14 PHASE-LOCKED LOOPS AND FREQUENCY
SYNTHESIZERS
A phase-locked loop (PLL) consists of three basic components: (1) a phase detector, (2) a
low-pass filter, and (3) a voltage-controlled oscillator (VCO), as shown in Fig. 4–19. The
VCO is an oscillator that produces a periodic waveform with a frequency that may be varied
about some free-running frequency f 0 , according to the value of the applied voltage v 2 (t). The
free-running frequency f 0 , is the frequency of the VCO output when the applied voltage v 2 (t)
is zero. The phase detector produces an output signal v 1 (t) that is a function of the phase
difference between the incoming signal v in (t) and the oscillator signal v 0 (t). The filtered signal
v 2 (t) is the control signal that is used to change the frequency of the VCO output. The PLL
configuration may be designed so that it acts as a narrowband tracking filter when the
v in (t) v 1 (t) Low-pass v 2 (t)
Phase detector
filter (LPF)
(PD)
F(f )
v 0 (t)
Voltage-controlled
oscillator (VCO)
v 0 (t)
Figure 4–19
Basic PLL.