Electronics > Beginners
Trigger on AM waveform?
David Hess:
--- Quote from: GregDunn on August 11, 2018, 11:09:09 pm ---Varying the trigger holdoff from the lowest to the highest allowed values made no difference at any point along the way.
--- End quote ---
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:
--- Quote from: T3sl4co1l on August 12, 2018, 09:11:10 pm ---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.
--- End quote ---
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
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