Signal Processing for Wireless Transceivers - Signal Processing Systems - page 201

AGC

LPF I

A/D

RF LNA IF

BPF BPF

I/Q LO

LPF A/D Q

LO

LO

Fig. 20 Principal structure of classical superheterodune radio receiver

structures, namely superheterodyne, direct-conversion and low-IF type receivers.
Useful general literature is this field are, e.g., [ 29 , 80 , 85 ] . We also shortly touch the
subsampling aspects [ 27 , 129 ] where controlled aliasing, instead of explicit mixing,
is used for frequency translation. As in the whole communications signal processing
field, the concept of complex-valued or I/Q signals plays an essential role also here
in designing and understanding different receiver principles.

4.4.1 Superheterodyne Receiver

The previously-described real mixing approach is deployed in the traditional
superheterodyne receiver. A tunable local oscillator is used to select the channel
of interest which is translated to a fixed intermediate frequency using real mixing.
At the IF stage, a highly selective bandpass filter is used to separate the desired
channel signal from the others. Tunability in the local oscillator facilitates the use
of a fixed intermediate frequency, thus enabling efficient implementation of the IF
channel selection filter. Special analog filter technologies, such as surface acoustic
wave (SAW), can be deployed in the implementation. After this, the signal is
traditionally quadrature downconverted to baseband, possibly through an additional
IF stage, and the baseband signal is finally A/D converted. Another more advanced
alternative is to sample and digitize the signal directly at IF and carry out the final
I/Q demodulation using DSP. The overall structure with baseband A/D conversions
is illustrated in Fig. 20 .
As shortly discussed already earlier, a real mixer is equally sensitive to frequen-
cies below and above the oscillator frequency. Thus for oscillator frequency f LO ,
any input signal component at some frequency f c will appear at both f c − f LO and

f LO at the mixer output. Thus in addition to the desired channel signal, also
the so called image band signal will appear at the IF if not filtered away before
the downconversion. For this purpose, superheterodyne receivers always use RF
image rejection filtering. In general, the used LO frequencies can be either below
( f LO

f c +

= f c − f IF , lower side injection) or above ( f LO = f c + f IF , upper side injection)