0195136047.pdf

15.2 ANALOG COMMUNICATION SYSTEMS 695

the FM signal into an AM signal can be achieved by passing the FM signal through an LTI
system whose frequency response is nearly a straight line in the frequency band of the FM
signal.
FM demodulators mainly fall into two categories:frequency discriminatorsandlocked-loop
demodulators. While both give the same performance for relatively high signal levels, locked-
loop demodulators provide better performance than the discriminator when signal levels are
low and noise is a problem. The discriminator produces an output voltage proportional to the
frequency variations that occur in the FM signal at its input. Locked-loop demodulators are more
cost-efficient when implemented in IC form.
Abalanced discriminatorwith the corresponding frequency characteristics is depicted in
Figure 15.2.15. The rising half of the frequency characteristic of a tuned circuit, shown in Figure
15.2.15(b), may not have a wide enough linear region. In order to obtain a linear characteristic
over a wider range of frequencies [see Figure 15.2.15(d)], usually two circuits tuned at two
frequenciesf 1 andf 2 [with the frequency response shown in Figure 15.2.15(c)] are used in a
balanced discriminator [Figure 15.2.15(a)].
Figure 15.2.16 shows a block diagram of an FM demodulator with feedback (FMFB), in
which the FM discrimination is placed in a feedback system that uses a VCO path for feedback.
The bandwidth of the discriminator and the subsequent low-pass filter is matched with that of the
message signal, which is the output of the low-pass filter.
An alternative to the FMFB demodulator is the use of a PLL, as shown in Figure 15.2.17,
in which the phase of the VCO’s output signal is forced to follow (or lock to) the phase of
the input FM waveform with small error. Since the VCO acts as an integrator, and phase
is the integral of frequency, the amplified error voltage appearing at the VCO input will be
proportional to the message signalf(t). The filter is selected with a closed-loop bandwidth that
is wide enough to yield demodulation with minor distortion off(t), and narrow enough to reject
noise.
The signal and noise components, particularly at low SNRs, are so intermingled that one
may not be able to distinguish the signal from the noise. In such a case, amutilationorthreshold
effectis said to be present. There exists a specific SNR at the input of the demodulator known as
thethreshold SNR, beyond which signal mutilation occurs. The threshold effect then places an
upper limit on the tradeoff between bandwidth and power in an FM system. Since the thresholds
for locked loops are lower than for the discriminator, loop-type receivers, operating at smaller
signal-power levels, find wide application in space communications where transmitter power is
at a premium.
At the output of the discriminator in an FM receiver, higher frequency components of output
noisepower are accentuated. A low-pass filter, known as adeemphasis filter, is added so that
the large-amplitude noise can be greatly reduced and the output SNR increased. Since the filter
also acts on the message, causing distortion, the message at the transmitter is passed through
a compensating filter, called apreemphasis filter, before modulation occurs. It accentuates the
higher frequencies in the message so as to exactly compensate for the effect of the deemphasis
filter, so that there is no overall effect on the message (see also Section 14.3). The scheme is
illustrated in Figure 15.2.18.

FM to AM

convertor

AM

demodulator

FM signal

input

Output

signal

AM signal Figure 15.2.14Block diagram of a general FM

demodulator.