0195136047.pdf

15.2 ANALOG COMMUNICATION SYSTEMS 697

Bandpass

filter Discriminator

VCO

Low-pass

Received filter

FM signal

Ouput

signal

×

Figure 15.2.16Block diagram of an FMFB demodulator.

Amplifier and

low-pass filter

(loop filter)

Phase

detector

(comparator)

VCO

Input SFM(t)

with input

phase φ(t)

Error voltage

∝ phase shift

Output ∝ f(t)

Output phase φ(t)

Figure 15.2.17Block diagram of a PLL FM demodulator.

Preemphasis

filter

Hp(ω)

Deemphasis

filter

Hd(ω)

Amplifier

of voltage

gain K

FM

modulator

FM

demodulator

f(t) Channel f(t)

....

Figure 15.2.18Schematic block diagram of an FM system with emphasis filters.

EXAMPLE 15.2.2

A common deemphasis filter used in FM broadcast has a transfer function

Hd(ω)=

1

1 +j(ω/W 1 )

whereW 1 / 2 π= 2 .12 kHz. For perfect message recovery, the preemphasis filter must have a
transfer function

Hp(ω)=

1

Hd(ω)

= 1 +j

(

ω

W 1

)

over all important frequencies (out to about 15–20 kHz). The system performance improvement
with these filters for voice-type messages is given by

RFM=

SNR with emphasis

SNR with no emphasis

=

(Wf/W 1 )^3

3

[

(Wf/W 1 )−tan−^1 (Wf/W 1 )

]

whereWfis the spectral extent off(t). However, for broader band messages, such as music audio,
since the channel bandwidth in FM broadcast is limited to 200 kHz, a reduction in the improvement
factor occurs, when it can be shown that

RFMwith bandwidth limitation

RFMwith no bandwidth limitation

=

1

1 +(Wrms/Wf)^2 (Wf/W 1 )^2