SPECTRUM ANALYZER; Why measure spectra?

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 vary concerned about distortion of the message modulated onto a carrier.

Third-order int rmodulation ( 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. Gov rnment 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 k y performance measure for the power amplifiers and other components used in these systems is the amount of signal energy that spills ov r into adjacent channels and causes int rference.

Electromagnetic interf rence ( EMI) is a t rm 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 lev ls versus frequency according to regulations set by various gov rnment agencies or industry-standard bodies.
Figures 1-3 through 1-6 illustrat some of these measurements. 6

 Figure 1-3. Harmonic distortion test of a transmitter

Figure 1-4. GSM radio signal and spectral mask showing limits of 

unwanted emissions 

 Figure 1-5. Two-tone test on an RF power amplifier

Figure 1-6. Radiated emissions plotted against CISPR11 limits as 

part of an EMI test 7