Specifications

6
Why measure spectra?
The frequency domain also has its measurement strengths. We have
already seen in Figures 1-1 and 1-2 that the frequency domain is better
for determining the harmonic content of a signal. People involved in wireless
communications are extremely interested in out-of-band and spurious
emissions. For example, cellular radio systems must be checked for harmonics
of the carrier signal that might interfere with other systems operating at the
same frequencies as the harmonics. Engineers and technicians are also very
concerned about distortion of the message modulated onto a carrier.
Third-order intermodulation (two tones of a complex signal modulating each
other) can be particularly troublesome because the distortion components
can fall within the band of interest and so will not be filtered away.
Spectrum monitoring is another important frequency-domain measurement
activity. Government regulatory agencies allocate different frequencies for
various radio services, such as broadcast television and radio, mobile
phone systems, police and emergency communications, and a host of other
applications. It is critical that each of these services operates at the assigned
frequency and stays within the allocated channel bandwidth. Transmitters
and other intentional radiators can often be required to operate at closely
spaced adjacent frequencies. A key performance measure for the power
amplifiers and other components used in these systems is the amount of
signal energy that spills over into adjacent channels and causes interference.
Electromagnetic interference (EMI) is a term applied to unwanted emissions
from both intentional and unintentional radiators. Here, the concern is that
these unwanted emissions, either radiated or conducted (through the power
lines or other interconnecting wires), might impair the operation of other
systems. Almost anyone designing or manufacturing electrical or electronic
products must test for emission levels versus frequency according to
regulations set by various government agencies or industry-standard bodies.
Figures 1-3 through 1-6 illustrate some of these measurements.