Page 380 - Complete Wireless Design

Source: Complete Wireless Design

Chapter
9

Communications System Design

Without a solid understanding of the complete communications system all the
way from the transmitter’s modulator input to the receiver’s demodulator
output—including everything in between—and how the selection of various
components, circuits, and specifications can make or break an entire system,
any wireless design will surely fail. The interrelationships of the transmitter,
the receiver, the antenna, the air-interface link, the type of digital or analog
modulation, adjacent channel and cochannel considerations, etc., are critical
to a dependable link for a high data rate at the required BER for digital radio,
or with the expected voice quality for analog radio.

9.1 Receivers
9.1.1 Introduction
The most difficult-to-design element in any communications system is the
receiver. A receiver must have a low noise figure (at VHF and above), low
group delay variations and IMD, high dynamic range, stable AGC, appropri-
ate RF and IF gain, good frequency stability, satisfactory gain flatness across
multiple channels, low phase noise, negligible in-band spurs, sufficient selec-
tivity, suitable BER and—sometimes the most critical specification of all—be
within certain cost constraints.
An important concern of any superheterodyne receiver is the image fre-
quency, in that any signal received within this image band will be amplified by
the receiver’s IF stages and then be unavoidably transferred on to the demod-
ulator to be output as interference. This image frequency can be eliminated
only at the front end of a receiver, before down-conversion, by a filter that
blocks the interfering frequency: the image filter. When the local oscillator is
higher in frequency than the incoming RF signal (high-side injection), the
image is any frequency that is at twice the IF plus the desired RF signal fre-
quency [(2 IF) RF], or at the IF plus the LO frequency (LO IF). If the

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