Short note: Analog spectrum analyzer

An analog spectrum analyzer is an instrument that displays the amplitude (or power) of the frequency components of a signal against frequency on a screen, i.e. it shows the signal in the frequency domain. The usual type is the swept-tuned superheterodyne analyzer.

 Input->[Atten]->(X)->[IF filter]->[IF amp]
         Mixer    ^      (RBW)         |
                  |               [Env. det]
                [VCO]                  |
                  ^              [Video filt]
                  |                    |
 [Sawtooth gen]---+--> X (horiz)   CRT Y (vert)

Working:

1. The input is attenuated to a safe level and low-pass filtered.
2. A sawtooth generator sweeps the frequency of a voltage-controlled oscillator (VCO) over a range; the same ramp drives the horizontal deflection, so the x-axis represents frequency.
3. The mixer shifts each input component to the fixed IF. Only the component for which $f_{LO}-f_{in}=f_{IF}$ passes the narrow IF filter; its bandwidth is the resolution bandwidth (RBW).
4. The IF output is amplified (often logarithmically, for a dB scale), envelope detected and smoothed by a video filter, and applied to vertical deflection.
5. As the sweep progresses, each frequency component appears as a vertical line whose height is its amplitude.

Key points: narrower RBW gives better resolution but needs a slower sweep. Analyzers are used to measure harmonics, modulation sidebands, bandwidth, noise and interference. A filter-bank (real-time) analyzer uses many parallel fixed band-pass filters instead of sweeping.