Great bit of vintage 70's 80's test gear.
I've tried a couple of times in my life to build a THD analyser, before I gave in and just bought one... I think I'd be OK today, but I have a reliable instrument, so it saves me the grief!
It takes the input signal, a sine wave, and buffers it, matches it for voltage with it's own internal oscillator, then notches out the input signal. What then remains is the harmonic distortion and noise. Simple to say, a little more complex to put into practice.
The Q-factor of the notch filter must be high enough so it doesn't notch out the 2nd harmonic (or 3rd, 4th, 5th etc) , which at audio frequencies can be difficult to achieve. The resultant output needs to be measured with a high bandwidth AC RMS meter, sensitive to microvolts, which can also be a tall order!
It's also tricky to keep the whole thing from oscillating.
Thankfully, I picked up this:
It's all discrete, not an op-amp in sight, and just works! It's also auto-tuning, but you still need to set the range and adjust the notch, so I'm not sure how "auto" it really is!
So, when we're designing an oscillator or amplifier, we can check how much distortion it really is producing, and where in the frequency spectrum it is! Useful stuff....
Here's a quick video of the thing in action...
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