Page 390 - Complete Wireless Design

Communications System Design

Communications System Design 389

with an NF of 2.5 dB is usually more than sufficient. This metropolitan envi-
ronment will also mean that there will be many other radios broadcasting—
some at very high power—and some will have fundamental and harmonic
frequencies quite close to our own receiver’s frequency. Moreover, since our
receiver may be of a mobile type, the fixed hub or cell site transmitter our receiv-
er is linked with may be transmitting from a distance that is anywhere from 35
miles to 35 feet away. This can place extreme stress on the receiver’s dynamic
range, causing high levels of odd-order IMD (especially the third order) to drop
into our receiver’s IF passband, producing poor BER performance in a digital
radio, or a decrease in fidelity in an analog radio. We can lessen this problem by
adopting an LNA and first mixer with a high IP3 and/or, in certain circum-
stances, a front-end attenuator controlled by AGC. The front-end attenuator is
used only to reduce the desired input signal levels to an amplitude that will not
overdrive the receiver’s LNA or first mixer; this solution would be unacceptable
for attenuation of undesired adjacent channels, since the frequency of interest—
especially if at a low amplitude—would also be attenuated. The noise figure will,
of course, increase as attenuation is added.
Half-IF spurs can be a problem when a receiver possesses a low-frequency
IF relative to its RF. This type of spur is created by the nonlinearities within
the mixer permitting spurs in the receiver’s IF by:

Half-IF (Hz) [ (0.5 IF) RF] 2 (2 LO)

This formula means that any RF that is at half the IF plus the RF, and that
substantially gets past the first RF filters that are before the first mixer, will
be able to cause IF in-band interference. Half-IF interference is due to the sec-
ond harmonic of the half-IF plus RF mixing with the LO’s second harmonic,
with the resulting difference falling dead in the IF band. To mitigate, any RF
signals that are at half the IF plus the desired RF should be properly attenu-
ated before they exit the first front-end receive filter.

9.2 Transmitters
9.2.1 Introduction
Proper transmitter design is critical, since it is a device that radiates an elec-
tromagnetic signal. This signal can, if the transmitter is improperly designed
or constructed, interfere not only with other wireless communications, but also
with many different types of non-RF electronic equipment.
Harmonic and spurious outputs, wideband noise and phase noise, frequen-
cy and amplitude stability, and the signal’s peak and average output powers
are but a few of the critical parameters that must be addressed before any
transmitter design can begin. Spurious signals generated by the mixing of the
LO and IF (along with their harmonics) are of particular concern, as are two-
tone intermodulation products created by two or more frequency components
mixing together in any nonlinearities—at least up to the fifth order (3F ±2F
1 2

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