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Sec. 4–16 Transmitters and Receivers 295
|V LO (f)|
|V in (f)|
|H 1 (f)|
Image attenuation
–1,760 –1,305 –850
f
f c = 850 f LO = 1,305 f image = 1,760 kHz
455 kHz 455 kHz
910 kHz
Figure 4–30
Spectra of signals and transfer function of an RF amplifier in a superheterodyne receiver.
From Fig. 4–30, it is seen that the image response will usually be reduced if the IF frequency
is increased, since f image will occur farther away from the main peak (or lobe) of the RF filter
characteristic, |H 1 ( f)|.
Recalling our earlier discussion on mixers, we also realize that other spurious responses
(in addition to the image response) will occur in practical mixer circuits. These must also be
taken into account in good receiver design.
Table 4–4 illustrates some typical IF frequencies that have become de facto standards.
For the intended application, the IF frequency is low enough that the IF filter will provide
good adjacent channel signal rejection when circuit elements with a realizable Q are used;
yet the IF frequency is large enough to provide adequate image-signal rejection by the RF
amplifier filter.
The type of detector selected for use in the superheterodyne receiver depends on the
intended application. For example, a product detector may be used in a PSK (digital) system,
and an envelope detector is used in AM broadcast receivers. If the complex envelope g(t) is desired
for generalized signal detection or for optimum reception in digital systems, the x(t) and
y(t) quadrature components, where x(t) + jy(t) = g(t), may be obtained by using quadrature
TABLE 4–4
SOME POPULAR IF FREQUENCIES IN THE UNITED STATES.
IF Frequency
Application
262.5 kHz AM broadcast radios (in automobiles)
455 kHz AM broadcast radios
10.7 MHz FM broadcast radios
21.4 MHz FM two-way radios
30 MHz Radar receivers
43.75 MHz TV sets
60 MHz Radar receivers
70 MHz Satellite receivers