The HP339a was a state of the art (SOTA) distortion analyzer in the 1980s.
It is a full featured all-in-one product including a volt meter,
low-distortion oscillator, distortion analyzer, AM detector, and VU meter.
There was an in-depth analysis of low distortion oscillators
on DIYAudio on the following link:
<Low-disortion Audio-range Oscillator>
This was followed by a specific HP339a thread, linked here:
This is a summary of those threads along with the links, conversations, and
posts, about the research and development that went into the suggested modifications to the venerable HP 339A.
Why HP 339A?
HP339A Post #78
Well, a stock 339A gives you a wide band ac voltmeter, an oscillator and analyzer. A stock one will do you pretty well as long as you have no interest in reading below...