Electronics > Beginners

Trigger on AM waveform?

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GregDunn:
Again, this is probably not entirely a newb question, but it's something I haven't seen addressed in other discussions - so maybe it'll be helpful to others.

When I was using ancient Tek 500 series scopes, we didn't have a lot of complex signals to analyze (just lots and lots of sine and pulse waves  ;D ), so I never tried this.  But now, having access to better signal generators and more complex circuitry, I'm experimenting more and have run across something that isn't yielding to simple setting changes.  I'm generating an AM waveform using a FeelTech FY6800; just to make it easy to see, I'm using 20 KHz sine for the carrier, and 100 Hz sine for the modulating waveform.  It does a nice job - but I can't get the signal to trigger stably on my Siglent 1104X-E.  No matter what I do with respect to level, slope and offsets, it drifts slowly.  DC coupling, AC coupling, delay, etc. - nothing helps.

If I run the modulating signal into a second channel and trigger on that, it locks immediately and I get a good display.  If I set a level trigger window somewhere between the peaks and troughs of the modulation on the AM signal, it will lock for a few seconds, then drift by about 1 cycle of the modulation signal, and lock again for a few seconds.  No amount of adjustment will make it completely stable.

What's the trick to get an AM signal to trigger stably?  I've resorted to stopping the acquisition so I can look at the signal, but what I really want is to be able to adjust parameters in real time and see the results.  Am I overlooking something obvious, or have I found an edge case that is beyond the scope's abilities?

jeroen79:
Just use the original modulator for the trigger.
If you don't have access to that then demodulate the AM signal.

JS:
Delays don't usually work as they depend on memory depth, horizontal scale, etc. So you move one and you are screwed.

I got to trigger something similar to yours using something like over threshold after X time under. That way it can't go drifting by jumping periods of the carry signal. It might jump one if there wasn't a sample right where it should but tweaking the level a tiny bit that should go away. Start with the trigger level right at the middle of the modulation amplitude, where changes are bigger.

All that makes sense if you want to learn advanced trigger, or you dont have access to a second trigger signal and dont have a demodulator at hand. Otherwise, as is your case, use a separate triggering signal! If you can't spare a sig gen or scope channel use the trigger output and trigger input, thats what they are for.

JS

vk6zgo:

--- Quote from: GregDunn on August 11, 2018, 03:20:33 am ---Again, this is probably not entirely a newb question, but it's something I haven't seen addressed in other discussions - so maybe it'll be helpful to others.

When I was using ancient Tek 500 series scopes, we didn't have a lot of complex signals to analyze (just lots and lots of sine and pulse waves  ;D ), so I never tried this.  But now, having access to better signal generators and more complex circuitry, I'm experimenting more and have run across something that isn't yielding to simple setting changes.  I'm generating an AM waveform using a FeelTech FY6800; just to make it easy to see, I'm using 20 KHz sine for the carrier, and 100 Hz sine for the modulating waveform.  It does a nice job - but I can't get the signal to trigger stably on my Siglent 1104X-E.  No matter what I do with respect to level, slope and offsets, it drifts slowly.  DC coupling, AC coupling, delay, etc. - nothing helps.

If I run the modulating signal into a second channel and trigger on that, it locks immediately and I get a good display.  If I set a level trigger window somewhere between the peaks and troughs of the modulation on the AM signal, it will lock for a few seconds, then drift by about 1 cycle of the modulation signal, and lock again for a few seconds.  No amount of adjustment will make it completely stable.

What's the trick to get an AM signal to trigger stably?  I've resorted to stopping the acquisition so I can look at the signal, but what I really want is to be able to adjust parameters in real time and see the results.  Am I overlooking something obvious, or have I found an edge case that is beyond the scope's abilities?

--- End quote ---

It should be possible.
We often did it with analog 'scopes.
It isn't quite as stable as separately triggering from the modulating signal, but is quite useable.
I can't see why a modern DSO can't achieve this.

StillTrying:
I think it could be a blind time aliasing effect, try changing the 100Hz to 97 or 103Hz. :)
and  "Start with the trigger level right at the middle of the modulation amplitude,"

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