An instrument that measures the frequency components contained within a given signal. Any complex periodic signal can be decomposed into a set of single-frequency sinusoidal components, as shown by Fourier analysis; the spectrum analyser plots the amplitude versus frequency of each sine wave in the signal’s spectrum. The time domain and frequency domain representations of a simple signal are shown in the diagram. The spectrum analyser displays the frequency domain response.

Spectrum analysers use the heterodyne principle (see heterodyne reception) to obtain the frequency domain response. Several mixing stages and intermediate frequencies are generally employed. The first local oscillator is a voltage-controlled oscillator driven by a sawtooth control voltage; this sweeps the oscillator across the frequency range desired for the frequency response measurement. The sawtooth control voltage also drives the x-display or timebase signal of the spectrum analyser CRT (cathode-ray tube) display, so the x-axis of the CRT corresponds to the frequency. When the first local oscillator is at such a point in its sweep that mixing with the incoming signal produces a response at the first intermediate frequency (see mixer), then a signal appears at the appropriate location on the CRT display. Subsequent IF stages define the sensitivity and bandwidth of the displayed signal, the final stage defining the resolution of the analyser. This final filter stage can be varied over a wide range of bandwidths. The final displayed signal is a function of the resolution bandwidth, which in turn depends upon the swept frequency range and sweep time (period of the sawtooth control waveform). Modern spectrum analysers often use digital filtering for the narrowest resolutions, as this can improve the speed of response at narrow bandwidths.

Relationship between time and frequency domains