wrote:

"due to progressive attenuation of the harmonics. "

So 100 KHz is the cutoff then ?


** It's the typical -3dB point of the metering circuit.

I never checked. Seems reasonable, that would be the fifth harmonic of 20 KHz.


** Correct - 2nd, 3rd & 5th harmonics plus noise are read by the meter when if 20kHz is the fundamental. By convention, this has long been is deemed adequate to quantify non-linearity in audio circuits.


Next question. This thing FILTERS, not really nulls out the fundamental.

** A notch filter IS a filter, adjusting the centre frequency to match an incoming sine wave is called " nulling". The 339A has a notch filter.


So it gets lovcked to that frequency by something that resembles a PLL at least remotely.


** The auto nulling feature tracks the incoming frequency, long as it varies slowly. Without it, small temperature drifts in the oscillator and notch filter makes manual nulling very tedious or impossible when dealing with low THD percentages.


Now, if I have 1 V P-P sine going in there at 1 KHz, if I took another generator, not locked or anything to the fundamental one, and injected a 2 KHz sine wave into it, the distortion meter should read that accurately, correct ?

** Yep - a pair of say 10kohms in a Y arrangement allows the signals to combine at the input to the 339A. The level of both will be cut in half, of course.


I mean I am putting in a controlled amount of the second harmonic, but it is not locked in any phase relationship and in fact is not even frequency locked.

WILL THAT WORK ?


** Yep.


Anyway, now that I think of it, all I need is a second generator.


** Best set the level of the second generator to not more than say 10% of the main one, so the auto-nulling system is not confused.

..... Phil