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Sec. 5–6 Phase Modulation and Frequency Modulation 333<br />

Time-varying phaseshift<br />

circuit<br />

RF<br />

oscillator<br />

(frequency=f c )<br />

i(t)<br />

RFC<br />

C 0<br />

L<br />

s(t)<br />

PM signal out<br />

m(t)=modulation input<br />

Varactor diodes<br />

(a) A Phase Modulator Circuit<br />

RF oscillator circuit<br />

(frequency=f c )<br />

m(t)<br />

Modulation input<br />

RFC<br />

C 0<br />

L<br />

Oscillator<br />

tank<br />

circuit<br />

s(t)<br />

FM signal out<br />

Varactor diodes<br />

(b) A Frequency Modulator Circuit<br />

Figure 5–8<br />

Angle modulator circuits. RFC = radio-frequency choke.<br />

The frequency deviation from the carrier frequency is<br />

f d (t) = f i (t) - f c = 1<br />

2p c du(t) d<br />

dt<br />

and the peak frequency deviation is<br />

¢F = max e 1<br />

2p c du(t) df<br />

dt<br />

(5–41)<br />

(5–42)

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