Amplitude Modulation - extract modulation waveform on scope

[mkj]

Im trying to simulate an LVDT frequency response.  So I have two amplitude modulated waveforms (analog discover waveform generator) with a 1000Hz carrier and low frequency modulation (1-30 Hz) to simulate the "movement" of the LVDT.    These become my Va and Vb of an LVDT.

I can feed the AD598 (LVDT asynchronous demod chip) with the waveforms and will get the demodulated (Va-VB)/(Va+Vb) ratio of the modulation.  I need to compare phase lag at the modulated frequencies to evaluate phase shift caused by the AD598 filters.

 

How can I get the modulated part of the Waveforms signal on the scope to be able to compare demod vs input phase?

I tried a MATH channel:  modulation = C1/(sin(2 pi fc) - Ac    (C1 is waveform for Va, where fc and Ac are the carrier freq and amplitude)

But this doesnt work... it seems not accurate enough to mix the equation with the actual data - getting spikes etc.

Is there a way to just show the low freq modulation on a the scope?

 

I have the AD original, and the AD3450

Ideally this would be a freq response sweep over 1-30 range of modulations, comparing the demod signal to the command modulate.  But I can run this point by point if I can used the scope.

[attila]

Hi @mkj

See the following:

[mkj]

Thanks - perfect.  Didn't know you could do that equation w/o a channel input!  Awesome.

[mkj]

So I tried this - but what do I set my trigger to?  The math wave will always be at "0" time - the waveform channels will be the composite carrier and modulation so I cant trigger to that.  How do I trigger to the waveform "modulation" part?  I dont want to trigger to the "DEMOD" - if I do it will always be inphase to the math channel.

 

I cant trigger on the initial waveform edge (carrier and mod) since the wave needs to be "on" for  a few second for the demodulation to stabiilize.

Edited March 6, 2023 by mkj

[attila]

Hi @mkj

In this setup the Wavegen generates a trigger event for each AM period so each T0 will be a modulation starting point.
Make sure the AM period (run length) is multiple of the carrier to prevent signal discontinuity.

 

The next software version will bring carrier, AM and FM capture so you won't need such Math tricks with ADP3X50.

[attila]

Hi @mkj

 

[mkj]

Thanks for the answer and I'll try that beta out - looks awesome!

I needed a specific frequency so I created a math C1-C2 (modulation delta) and used to to get the zero crossing vs the demod

C1 is waveform 1 carrier modulated  (Va to AD598)

C2 is waveform 2 carrier modulated - 180 out of phase with C1  (Vb to AD598)

BLUE is C1-C2 

PURPLE is demod from AD598 (Va-Vb)/(Va+Vb) showing the phase lag effect of filters on the response 

 

 

Edited March 7, 2023 by mkj

[mkj]

Installed the beta update and updated the firmware - how do I get the extra channels etc?

Also math C1-C2 is not working.

 

Edited March 8, 2023 by mkj
figured out update

[attila]

Hi @mkj

[mkj]

Sweet!