Page 381 - Complete Wireless Design

Communications System Design

380 Chapter Nine

local oscillator is lower in frequency than the incoming RF signal (low-side
injection), then we can find the image frequency by (2 IF) RF or LO IF.
Taking the first case, high-side injection, the image is any signal (or even noise
at that frequency) that differs from the LO by the amount of the IF—just as
the desired signal does—but is higher instead of lower than the local oscilla-
tor. Subtracting the desired signal from the local oscillator frequency will give
the IF, which will, of course, easily pass through the receiver’s IF amplifiers.
But any frequency (the image) that is higher than the LO by exactly the same
amount that the signal is below the LO will give us the same frequency. This
same frequency will also easily pass through the IF amplifiers, and create
interference and a decrease in the SNR.
As mentioned, the dominant technique for attenuating the image frequency
is by front-end filtration. The filtering can be further assisted by using as high
an IF as possible to move the image as far away from the desired frequencies
as possible. This will make the filtering of the image a much easier task, with
decreased risk of excessive group delay variations caused by a tight filter.
Maintaining this first image far from the desired frequency is assisted by uti-
lizing double- or triple-conversion receiver designs. With these multiple-con-
version receivers, the first IF is at a high frequency, while the second and third
IFs are much lower. These lower IFs will supply most of the selectivity and
gain, since the lower the IF, the more simple, stable, sensitive, and selective
the amplifiers will be. This is mainly due to the circuit’s decreased stray capac-
itances and inductances at these frequencies, along with less of a requirement
to employ special high-frequency components.
We will address typical receiver and transmitter system design issues, the
communications link and its impairments, and the communications system as
a whole.

9.1.2 Receiver design
Most receivers are of the down-conversion type, which takes the RF input and
immediately begins to convert it either to a single lower IF or down to two or
more increasingly lower IFs. The other type of receiver, called the up-convert-
ing superhet, is operated in wide-tuning-range applications, and is especially
dominant in HF SSB ham radios. It takes the incoming RF and converts it to
some higher frequency—typically about twice the highest expected receive fre-
quency—to distance the image frequency from the LO to assure simple RF
front-end filtering. Up-converting is rarely seen at VHF and above.
A standard double down-conversion superhet receiver block diagram is
shown in Fig. 9.1. First, the antenna picks up the electromagnetic waves from
the environment and, because of its natural passive gain, amplifies any signals
that are within its bandwidth. The inductor L short-circuits static buildup on
1
the antenna to ground to prevent it from entering, and possibly damaging, the
delicate LNA of the receiver’s front end. L can also be an inductor within
1
BPF1. The signal is amplified by the antenna and sent into the input of the
receiver’s BPF1. BPF1 is a filter sometimes called a preselector, and is utilized

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