0195136047.pdf

694 COMMUNICATION SYSTEMS

FM

modulator

PM

modulator

f(t)

Message

signal

Integrator

f(t)

≡

PM

modulator

FM

modulator

f(t)

Differentiator

f(t)

≡

Figure 15.2.11Close relationship between FM and PM.

and frequency modulating the derivative of a message is equivalent to phase modulation of the

message itself.

Generation of wide-band FMcan be done by various means. However, only the most common

and conceptually the simplest one, known as thedirect method, is considered here. It employs

avoltage-controlled oscillator(VCO) as a modulator. A VCO is an oscillator whose oscillation

frequency is equal to the resonant frequency of a tuned circuit, as shown in Figure 15.2.12. The

frequency can be varied if either the inductance or the capacitance is made voltage-sensitive to

the message signalf(t).

One approach to obtaining a voltage-variable reactance is through avaractor diode, whose

capacitance changes with the applied voltage, such as the junction capacitance of a reverse-

biased diode, which depends on the amount of bias. Figure 15.2.13 illustrates the varactor-diode

implementation of an angle modulator. The frequency of the tuned circuit and the oscillator will

change in accordance with the message signalf(t). Varactor-controlled VCOs can have a nearly

linear frequency–voltage characteristic, but often yield only small frequency deviations, i.e., small

ω. Since any VCO is inherently unstable as its frequency is varied to produce FM, it becomes

necessary in many applications to stabilize the carrier’s frequency.

Demodulators for FM

Figure 15.2.14 shows a block diagram of a general FM demodulator, which is implemented by

generating an AM signal whose amplitude is proportional to the instantaneous frequency of the

FM signal, and then using an AM demodulator to recover the message signal. Transforming

Active

oscillator

circuit

Output proportional to cos ωt

where ω =^1

√LC

Resonant

circuit

L C

Figure 15.2.12Oscillator for use

in a VCO.

To

oscillator

circuit

...
...

C 0

f(t)

Cv

+ L

−

Figure 15.2.13Varactor-diode implementation of an angle

modulator.