Page 450 - Complete Wireless Design

Wireless Issues

Wireless Issues 449

As stated, circuit theory’s limitations can plague even the simplest of high-
frequency designs. When working with low frequencies, the answers provided
by circuit theory formulas match well when compared to a final realized circuit.
But as the frequencies increase into the high RF regions, circuit theory calcu-
lations lose some of their validity. In fact, if it takes less than one-eighth a sig-
nal’s wavelength period to traverse a circuit, then circuit theory will gradually
sacrifice its accuracy. This can begin to noticeably affect a normal circuit at fre-
quencies as low as 10 MHz, and is due to stray electromagnetic interactions
between components and transmission lines that cannot be modeled in most
circuit theory software. Another common problem with most simulation soft-
ware is the modeling of internal capacitances and inductances, as well as unde-
sired series resistances, within the passive components. But we can solve this
problem by using passive components that have been modeled as S parameters,
instead of the normal perfect components (with only a selectable Q) supplied
with the average simulator. Thus, we can see that we have some complex RF
effects that can severely degrade circuit operation, resulting in a circuit that is
actually resonant at a lower frequency, and with higher insertion losses and
poorer return losses, than that calculated with only circuit theory. RF design is
not considered “black magic” capriciously.

10.11 FCC Equipment Authorizations
10.11.1 Introduction
The Federal Communications Commission (FCC) is the governing force in the
United States that controls all wireless communications. The FCC does this by
allotting spectrum for the various wireless services, by licensing users of this
spectrum, and by setting the power, stability, spurious signals, and bandwidth
limits of all equipment capable of RF transmissions. Even receiving equipment
is controlled by law, as it must not radiate excessively (this receiver radiation is
due to the LO power reaching the antenna). Keeping electromagnetic interfer-
ence to a minimum is vital so that operators of wireless equipment—as well as
users of other devices (such as telephones, navigation equipment, and televi-
sions)—will be able to enjoy relatively interference-free performance.
The FCC accomplishes this task by mandating that any device you design,
whether it be a transmitter or a receiver, must have some type of FCC equip-
ment authorization if it is to be sold, leased, or advertised to any individual or
company; unless such sale is to the federal government or to a foreign country.

10.11.2 Wireless equipment law
As stated above, all electronic equipment that can emit electromagnetic radiation
into the surrounding space is controlled under the FCC umbrella. The FCC
breaks down all such radiators under three logical sections: intentional radiators,
unintentional radiators, and incidental radiators. Intentional radiators comprise
all RF wireless transmitters within the field of communications. Unintentional

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