Does anybody know what the circuit symbol for spectrum analyzer is? My internet searches for it haven't turned up anything so far. Most of the searches ended up showing the internal circuit diagram for spectrum analyzer.
A spectrum analyzer is a sufficiently esoteric and rare (and expensive!) piece of equipment that there's no generally recognized symbol for it.
Make something up! You could use something similar to the oscilloscope symbol, but with a waveform showing a single peak, similar to what one might see if analyzing a slightly noisy sine wave.