A recent trip to the attic reminded me of the long-forgotten ritual of an annual proof-of-performance, required of all stations by the FCC. Sitting in a dusty corner was a B&W 410 distortion and voltmeter. Time to bring it down for a general cleanup and photoshoot.
The theory of total harmonic distortion measurement is simple. Feed a low-distortion sine wave into the device under test. Connect the output to the distortion analyzer. Filter out the fundamental frequency. What's left is second and higher order harmonics and noise. Of course, when theory is put into practice, things can get complicated.