Page 415 - Complete Wireless Design

Wireless Issues

414 Chapter Ten

also by natural sources, such as the static created by atmospheric lightning.
External noise also includes space noise, produced by solar flares and
sunspots, and cosmic noise, induced by the stars radiating interfering signals
in all directions.
Circuit-generated noise power, in watts, can be calculated by the formula
shown below. This simple formula states that the two contributors to noise in
a circuit are the temperature and the bandwidth of the circuit; the lower the
temperature and the lower the bandwidth, the lower the noise contribution.
The actual carrier frequency of the signal itself has absolutely no effect on the
production of this noise:
P KTB
N
where P noise power, W
N
K Boltzmann’s constant, 1.38 10 23
T circuit temperature, K
B circuit bandwidth, Hz

A related noise contributor is from an outside origin, such as a signal source,
and is created by the same mechanism as above. It is referred to as source
noise, and can be calculated by:

P
NO

NF 10 log at 290 K
10 P
NI
where NF noise figure, dB
P output noise power, W
NO
P input noise power, W
NI
290 K the reference temperature used in most measurements, in
kelvins.
10.2 Electromagnetic Interference
10.2.1 Introduction
Attenuating electromagnetic interference (EMI) and radio-frequency interfer-
ence (RFI) to the lowest levels possible is a major requirement of most wireless
designs. Undesired electromagnetic radiation escaping from a radio’s enclo-
sure will not only interfere with neighboring wireless equipment, but small RF
levels flowing within the radio cabinet itself can destroy the proper operation
of an otherwise solid design.

10.2.2 Designing for EMI suppression
Analog EMI suppression. Suppression of EMI is necessary for all designs—
both wireless and nonwireless—because of rigid European and strict FCC reg-
ulations, as well as for protection of your design against improper operation.

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