Suggestions for used spectrum analyzer?

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What frequencies do you want to measure? If it's audio band, then Bernhard's suggestion is, I think, the best one- a good soundcard, a small interface box, and an AC voltmeter for calibration/level-setting purposes.

For an all-in-one package, it's hard to do better than an HP3581A. Accurate and reliable down to about -85dBV. Build a notch filter for your fundamental and it's good to better than -100dBV.
 
Bernhard said:
Most affordable used fft analyzers have only max. 14bit adc = 80dB dynamic range, that is enough for low level distortion measurement but not for fs.
Maybe better use a soundcard.

Yup, what Bernhard said. The new soundcards are way better than old spectrum analysers for this kind of work.

You can also improve low-end resolution by building a high gain op-amp circuit, but you have to be very careful about layout and bypassing or low-level noise will swamp any measurement you want to make.
 
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