Page 414 - Complete Wireless Design

Source: Complete Wireless Design

Chapter
10

Wireless Issues

There are many important topics in wireless communications that must be
addressed, not only for a full understanding of RF design, but also to be able
to successfully implement complex circuits. Noise, EMI, PCB layout, proto-
typing, FCC rules, etc., are all issues of significance in the world of wireless.

10.1 Noise in Components and Systems
Noise is of crucial concern in radio, since the higher a signal is above the noise,
the higher will be its SNR and the farther away it can be detected with a
desired BER. There are two primary classifications of noise: circuit generated
and externally generated. Both are unavoidable and limit the possible gain of
any receiver’s amplifiers. However, noise can be minimized by careful and cau-
tious circuit and systems design. For instance, if we employ LNAs and tight
filtering in the front end of receivers, and decrease the noise contributions
from local oscillators and the image frequency, and use proper shielding and
layout techniques, we can significantly improve on the noise floor amplitude.
Noise manufactured within circuits and systems produces a haphazard and
fluctuating voltage that varies widely in frequency. White noise (also referred
to as Johnson or thermal noise) is created by a component’s electrons randomly
moving around under the influence of thermal energy. Shot noise, because of
its characteristics, can also be considered another type of white noise, but is
caused by electrons entering the collector or drain of a transistor, and by the
haphazard movement of electrons across any semiconductor junction.
Zener diodes are especially problematic as a noise contributor because of
their shot noise effects. In fact, all zeners will add shot noise, but the diode’s
designers will have minimized other, more complex, noise contributors in “low-
noise” zener types.
External noise is caused by not only artificial sources of electromagnetic
interference, such as dimmer switches, car ignitions, and electric motors, but

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