Trigger on AM waveform?

[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  ), 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]

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  ), 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?

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,"

[GregDunn]

I thought about changing the frequency ratio last night after I turned everything off; that's worth trying.  I was hoping to avoid using up a channel for just the trigger (this scope doesn't have a dedicated trigger input, like many low cost 4-ch scopes) but that certainly works quite well. 

It's interesting to note that the scope needs so much memory to capture a complex modulated waveform that the sample rate actually drops.  It will trigger stably on the carrier but then you can't get good resolution on the entire waveform at the same time, so it's a bit of a tradeoff.  Anyway, I'll fiddle with it a bit more.

[RoGeorge]

If your Siglent has a low pass filtering option for the trigger, it might work.

[David Hess]

Increasing the trigger holdoff should allow you to get a stable trigger.

[GregDunn]

Setting LF reject on the trigger prevented it from triggering at all, while HF reject made no difference in how the waveform was triggered - still flaky.

Varying the trigger holdoff from the lowest to the highest allowed values made no difference at any point along the way.

Changing the values of either the carrier or the modulation by a few Hz made no difference.

The best result other than using the modulating waveform to trigger the scope was to set the trigger threshold very close to the peak amplitude of the modulated waveform; it still glitches every few seconds.

The goal (learning about how to use the scope for complex waveforms) was essentially achieved. 

[StillTrying]

Noise Rejection ? widens the trigger's edge thresholds.

[David Hess]

Varying the trigger holdoff from the lowest to the highest allowed values made no difference at any point along the way.

Does the trigger holdoff even work?  Or is this another feature like trigger AC coupling which is (was?) broken?

[T3sl4co1l]

Trigger holdoff isn't really a good solution, and varies on some machines.  Analog scopes, it's some time delay, which is fine if the modulation is constant frequency.  TDS4xx's have a trigger edge count holdoff, which is nice, but it resets every time you change the scale, which is ridiculous.  Modern ones I assume are actually usable.

Amplitude modulation, by definition, has no low frequency components, so we can instantly tell that HF Reject won't help at all.

We can use the trigger to demodulate the AM, to some extent.  Adjust trigger level just to the highest peak.  The envelope will still dance about by a few cycles (as it must*!), but you will at least get a good look at it now.

Actually detecting the AM, or starting with the modulating signal, is the only way to get a stable triggered envelope.

*In general, the carrier and envelope are not coherent (i.e., don't line up, and vary independently), so triggering on the peak will vary by a cycle of the carrier, give or take.  Probably more if you can't clip the exact peak, which is very difficult even for fast, accurate triggers and a fine touch on the TRIG LVL knob.  Obviously, this gets worse the more cycles there are near the peak, i.e., at large ratios of Fcarrier / Fenvelope.

The output from a synthesizer may well be coherent, in which case the display might still jitter between segments of the waveform, but only finitely many.

Tim

[David Hess]

Trigger holdoff isn't really a good solution, and varies on some machines.  Analog scopes, it's some time delay, which is fine if the modulation is constant frequency.  TDS4xx's have a trigger edge count holdoff, which is nice, but it resets every time you change the scale, which is ridiculous.  Modern ones I assume are actually usable.

On analog oscilloscopes, there was a limited set of adjustable time constants, often just 3 or 4, which delayed arming the trigger.  One time constant was used for multiple adjacent sweep speeds so within that group, the variable holdoff time was fixed.  Very early DSOs with analog triggers use the same holdoff configuration but I do not know what later DSOs with analog triggers did.

DSOs with digital triggers could do just about anything but I assume they have a variable delay of up to 10s of divisions which would result in a holdoff time proportional to sweep time.

If the holdoff time exceeds the time that the signal is above the trigger level, then with some fiddling, the trigger should lock to the AM modulated signal at least until the sweep speed is changed.  I have never had a problem doing this with any of my analog oscilloscopes or DSOs but none of them have digital triggers.  Maybe something else is going on there like non-real time trigger arming or interaction with a long record length if used.  Based on previous experience with Rigol, I would just assume it is broken or barely works.

Set the sweep so that just over one modulation cycle is displayed, set the trigger level near the peak, and then the holdoff control should work or at least do something.  Try using peak detection with a short record length.

[romons]

Use the other siggen channel with a 100Hz square wave, and trigger on that instead of the display signal.

An AM signal is basically a sine wave thats amplitude is varied, so unless you can trigger on the tippy top of the signal, you won't get anything.

Another poster suggested that you demodulate the signal. You could then trigger on that signal.

Here is a diode detector you could use:

https://www.radio-electronics.com/info/rf-technology-design/am-reception/diode-detector-demodulator.php