Modulation Amplitude unexpected behavior

[avsteele]

The modulation amplitude does not behave like I would expect. I expected that if I had for example Output 1 set to cos_ref or cos_1f, then a modulation amplitude integer value of -1...8100 would let me select the amplitude of the output wave over its full range. But instead I see the results below:

If I set Output 1 to cos_1f then for Mod Amplitude A Out 1 =-1...8100 Output 1 sine wave amplitude goes from ~0.5 V ... 0.75V

If I set Output 1 to cos_ref then for Mod Amp A Out 1 =-1...8100 the sine wave amplitude goes from 0.5...1.0 V

The only configuration which works like I expect is I use the Modulation Amplitude Out 2, where output 2 is set to Ctrl_A . Then a the modulation amplitude integer -1...8100 causes a sine wave of amplitude ~0V to ~0.5 V to get added to the ramp.

Is this the expected behavior?

Thanks a lot!

[marceluda]

Yes, that's the expected behaviour.

The problem is that it's not documented properly. My error.

The idea is this:

• You can choose any signal for out1 and out2.
• You can add the modulation signal to whatever signal you choosed for each out
• On out1 a cos_ref signal is added
• On out2, a sin_ref signal is added

Here's the code: https://github.com/marceluda/rp_lock-in_pid_h/blob/master/lock_in%2Bpid_harmonic/fpga/rtl/lock.v#L421

Sorry for the poor description.

[avsteele]

Thank you Marcelo. Ok I understand partially.

Normally, I set Out 1 to cos_1f, or sin_1f. Then the demodulation works well and the phase setting's effect on the demodulated output works as expected. (I see from the code that _1f signals get phases added).

The cos_ref and sin_ref do not have a phase added to them, so the demodulation cannot be optimized.

Also, (for reasons I cannot quite understand), using cos_ref, sin_ref (or any other output with these added) do not demodulate correctly at 46 kHz, but seem to work OK at every other lower frequency.

Is there something I'm missing?

[marceluda]

That's right.

What I use to do is add cos_ref on out1 (with the control_A signal assigned) and then use F1 signa for demodulation. So I can tune it for maximum amplitude before the PID input.

[avsteele]

Ah I see now. I didn't realize the which was meant by signal F1o. But now I see it here: https://marceluda.github.io/rp_lock-in_pid/TheApp/instruments/instruments_02_scope/

Thanks!