The original version of signal analysis, spectrum analysis measures the amplitude and frequency of a signal, with no phase considerations. Taking these measurements in the frequency domain is done using one of two kinds of methods:
Fast Fourier Transform (FFT) uses mathematic formulas to acquire the frequency spectrum of a signal. The signal is sampled in the time domain then converted to a digital format. It is then transformed using Fourier analysis to display the signal in a frequency domain. This is spectrum analysis using digital technology.
Swept-Tuned Analyzer with a superheterodyne receiver. A sweep generator is set on the device to control the local oscillator (LO). This and the input signal are mixed in order to look at only the set frequency range as determined by the sweep generator. An amplifier and filter is then used on the signal that is mixed in order to obtain the spectrum. This is spectrum analysis using analog technology.