0195136047.pdf

15.2 ANALOG COMMUNICATION SYSTEMS 701

image response is suppressed by using the antenna and other RF-tuned circuits as a filter. By
limiting the bandwidth of the RF amplifier to the rangeBc<BRF< 2 fIF, whereBcis the
bandwidth of the AM radio signal (10 kHz), the radio signal transmitted at the image frequency
(fc′=fLO+fIF)is rejected. A similar behavior occurs whenfLOis lower thanfc. Figure 15.2.21
illustrates the AM station and image frequencies for a high-side and a low-side local oscillator,
whereas Figure 15.2.22 depicts the frequency response characteristics of IF and RF amplifiers for
the casefLO>fc.
Amplifiers in the IF circuits provide most of the gain needed to raise the small antenna
signal to a level sufficient to drive the envelope detector. The output of the detector contains a
dc component proportional toAcand a component proportional to the audio messagef(t), the
amplified signal of which is used to drive the loudspeaker. The dc component is utilized in an
automatic volume control(AVC), otherwise known asautomatic gain control(AGC), loop to
control the gain of RF and IF amplifiers by controlling their operating bias points. The loop action
is to maintain nearly a constant IF level at the detector’s input, even for large variations in antenna
voltage.
The IF amplifier, with its narrow bandwidth, provides signal rejection from adjacent channels,
and the RF amplifier provides signal rejection from image channels.
AnFM radio superheterodyne receiveris shown in block diagram form in Figure 15.2.23.
The part consisting of the antenna, RF amplifier, mixer, and local oscillator functions in a manner
similar to that of an AM receiver, except that the frequencies involved are different.fIF= 10. 7
MHz in FM, so that the image is 21.4 MHz from the carrier frequencyfc. The RF amplifier must
eliminate the image-frequency band 2fIFaway from the station to which the receiver is tuned.
The IF amplifier is generally divided into two parts. The higher level stage is set to limit at
a proper level to drive the demodulator. More expensive FM receivers may have AGC added to

0 500 1000 1500

540 1600

AM station frequencies

RF-tuned circuit response

2000

(a)

(b)

f, kHz

fLO = fc + fIF

fLO = fc − fIF fc

fimage = fc + 2 fIF

fimage = fc − 2 fIF

fc

0 500 1000 1500

540 1600

AM station frequencies

2000

f, kHz

Figure 15.2.21AM radio station

and image frequencies.(a)High-

side local oscillator,fLO >fc.

(b) Low-side local oscillator,

fLO<fc.