Sorry for delay... was ill over weekend.
So, I'm still trying to understand what the
exact phase is required for. Thinking about what's been in the thread so far, it seems like the plan is to:
- begin with excellent 1 kHz sine wave
- pass through some Device Under Test
- generate an n*1 kHz signal for use as a Local Oscillator (LO)
- mix the DUT output Vout with the LO
- low pass filter mixer output down to DC, i.e. select the nth harmonic
- measure said DC signal with multimeter to determine harmonic amplitude
Is that right?
If so, I can see why phase control is required; should the LO phase happen be 90 degrees away from the harmonic out of the DUT, you'll get zero.
With that in mind, how about this idea?
- begin with excellent 1 kHz sine wave
- pass through some Device Under Test
- generate an (n*1 kHz) - 55 Hz signal for use as a Local Oscillator (LO)
- mix the DUT output Vout with the LO
- band pass filter mixer output to select the 55 Hz output, i.e. select the nth harmonic
- measure said AC signal with multimeter to determine harmonic amplitude
Helpfully, you no longer need to fuss about DC offsets. And you might be able to place the frequency of interest a bit higher than the 1/f noise corner of whatever post-mixer amplifiers you need.
I'm not certain that 55 Hz is exactly the right frequency offset here; I wanted to pick one that isn't an integer division of 1 kHz (to help mixer rejection) and lies between the 50 Hz and 60 Hz calibration points of the multimeter. You'd need to do some mixer harmonic analysis to check out all the mixer harmonic products.
I think in this case that you
might be able to get away with an LO that isn't frequency locked to the 1 kHz tone, i.e. use 2 benchtop generators
without clock synch.