How does a spectrum analyzer work?

A signal that looks very complex in the time domain behaves quite differently in the frequency domain. Time domain measurements show an impure sine wave. Without frequency domain measurements, it is still impossible to know the source and frequency of the second harmonic. Spectral analysis can show the spectral components individually, thus revealing the source of the interference. While the information provided by the time domain (such as the rise and fall times of signal pulses) is important, the frequency domain allows you to determine the harmonic content of the signal, such as out-of-band emissions and distortion. For more information, read this blog post:Spectrum Analysis Basics Part 1 - What is a Spectrum Analyzer?