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Sec. 4–16 Transmitters and Receivers 293
The TRF receiver consists of a number of cascaded high-gain RF bandpass stages that are
tuned to the carrier frequency f c , followed by an appropriate detector circuit (an envelope detector,
a product detector, an FM detector, etc.). The TRF is not very popular, because it is difficult
to design tunable RF stages so that the desired station can be selected and yet have a narrow
bandwidth so that adjacent channel stations are rejected. In addition, it is difficult to obtain high
gain at radio frequencies and to have sufficiently small stray coupling between the output
and input of the RF amplifying chain so that the chain will not become an oscillator at f c . The
“crystal set” that is built by the Cub Scouts is an example of a single–RF-stage TRF receiver that
has no gain in the RF stage. TRF receivers are often used to measure time-dispersive (multipath)
characteristics of radio channels [Rappaport, 1989].
Most receivers employ the superheterodyne receiving technique as shown in
Fig. 4–29. The technique consists of either down-converting or up-converting the input signal
to some convenient frequency band, called the intermediate frequency (IF) band, and
then extracting the information (or modulation) by using the appropriate detector. † This
basic receiver structure is used for the reception of all types of bandpass signals, such as
television, FM, AM, satellite, cellular, and radar signals. The RF amplifier has a bandpass
characteristic that passes the desired signal and provides amplification to override additional
noise that is generated in the mixer stage. The RF filter characteristic also provides
some rejection of adjacent channel signals and noise, but the main adjacent channel
rejection is accomplished by the IF filter.
The IF filter is a bandpass filter that selects either the up-conversion or down-conversion
component (whichever is chosen by the receiver’s designer). When up conversion is selected,
the complex envelope of the IF (bandpass) filter output is the same as the complex envelope for
the RF input, except for RF filtering, H 1 ( f), and IF filtering, H 2 ( f). However, if down conversion
is used with f LO 7 f c , the complex envelope at the IF output will be the conjugate of that
for the RF input. [See Eq. (4–61c).] This means that the sidebands of the IF output will be
RF inputs
v in (t)
RF
(radio-frequency)
amplifer, H 1 (f)
Mixer
LO (local oscillator)
IF (intermediatefrequency)
amplifer, H 2 (f)
IF
out
Detector
Baseband
amplifer
Baseband
output
(to speaker
CRT, etc.)
Figure 4–29
Superheterodyne receiver.
† Dual-conversion superheterodyne receivers can also be built, in which a second mixer and a second IF stage
follow the first IF stage shown in Fig. 4–29.