563489578934

Sec. 4–13 Detector Circuits 277
Detectors may also be classified as being either coherent or noncoherent. A coherent
detector has two inputs—one for a reference signal, such as the synchronized oscillator
signal, and one for the modulated signal that is to be demodulated. The product detector is an
example of a coherent detector. A noncoherent detector has only one input, namely, the
modulated signal port. The envelope detector is an example of a noncoherent detector.
Frequency Modulation Detector
An ideal frequency modulation (FM) detector is a device that produces an output that is
proportional to the instantaneous frequency of the input. That is, if the bandpass input is
represented by R(t) cos [v c t + u(t)], the output of the ideal FM detector is
v out (t) = Kd[v ct + u(t)]
dt
= Kcv c + du(t) d
dt
(4–80)
when the input is not zero (i.e. R(t) Z 0).
Usually, the FM detector is balanced. This means that the DC voltage Kv c does not
appear on the output if the detector is tuned to (or designed for) the carrier frequency f c . In this
case, the output is
v out (t) = K du(t)
(4–81)
dt
There are many ways to build FM detectors, but almost all of them are based on one of
three principles:
• FM-to-AM conversion.
• Phase-shift or quadrature detection.
• Zero-crossing detection.
A slope detector is one example of the FM-to-AM conversion principle. A block
diagram is shown in Fig. 4–15. A bandpass limiter is needed to suppress any amplitude variations
on the input signal, since these would distort the desired output signal.
The slope detector may be analyzed as follows. Suppose that the input is a fading signal
with frequency modulation. From Table 4–1, this FM signal may be represented by
where
v in (t) = A(t) cos[v c t + u(t)]
t
u(t) = K f m(t 1 ) dt 1
L
-q
(4–82)
(4–83)
v in (t) v 1 (t) v 2 (t) v out (t)
Bandpass
Differentiator
Envelope
limiter
detector
Figure 4–15
Frequency demodulation using slope detection.