Low Frequency Auto Trigger, Agilent vs Tektronix

[Michael J. Linden]

We picked up an Agilent MSOX3014A as our first Agilent scope. At first, the auto trigger behaved in a problematic manner similar to that described in the following thread,
https://www.eevblog.com/forum/index.php?topic=4415.msg58597

  So, I upgraded it to the latest firmware which was supposed to improve auto trigger operation. What I've now found is that it won't auto trigger on a signal below approximately 20 Hz at all. However, all of our other scopes (Tektronix, Cleverscope, BitScope, etc.) easily auto trigger on signals down to 5 Hz.

  I spoke with Agilent about it. Their statement was that the MSOX3014A's auto trigger behaves essentially the same as their other scopes. They state that it's not intended for use on signals below ~50 Hz and that normal trigger mode should be used in such cases. From their perspective, my description of the auto trigger behavior seemed normal to them.

  So, here's my question. Are Agilent scopes known to exhibit this behavior. In other words, they don't auto trigger on signals below 50 Hz or so? Even though Tek's do?

  Thanks, Michael

[ToBeFrank]

Here is the response from Agilent I got about this. He says auto trigger should work on anything >= 20Hz. If it's not, file a bug!

I wouldn’t consider the behavior you described to be a bug, since Auto triggering is not recommended for signals under 20 Hz.  This is because Auto trigger forces a trigger every 50 ms if one does not occur.  With an 18 Hz (56 ms) signal, the 50 ms timeout often occurs, and the scope begins forcing triggers every 2.2 ms (for a faster waveform update rate).  It will do that for about 250 ms and then go back to waiting 50 ms for a trigger for another 400+ ms, at which point the process repeats.

During this 650-700 ms cycle, the scope will usually find one valid trigger, right after it exits the “fast auto trigger” state.  That is when the waveform stabilizes on screen briefly.  You should see this happening about every 700 ms.

I think the reason you don’t see this behavior at 500 us/div until you zoom out and then back in has to do with the scope’s attempt to sync to the signal at different timebases, and the different delays involved with each timebase.

You should never see this behavior with a signal >= 20 Hz.  If this is problematic, you can dial in almost any amount of holdoff to fix it (Mode/Coupling | Holdoff).  This adds delay into the cycle I described and forces the scope to wait longer for a trigger than the 50-ms default.  The best solution is probably to stick with Normal trigger mode for anything under 20 Hz. 

[mikeselectricstuff]

I wonder why scopes never seem to provide any adjustment of auto-trigger behavoiur - I find that my MSO6000 tends to ba a bit too keen to auto-trigger even in the presence of a stable signal.
A simple control to adjust the timeout between a trigger signal and auto-trigger kicking in would be useful.

[Michael J. Linden]

Here is the response from Agilent I got about this. He says auto trigger should work on anything >= 20Hz. If it's not, file a bug!

ToBeFrank,

  You quote was on the mark.

  Thanks, Michael

[Rufus]

Mine auto triggers OK on a 1Hz signal providing the timebase is 200ms/div or slower.

Looks like the auto trigger 'wait' time is something like 120% of the current acquisition time with a 50ms minimum limit.

Perhaps it could work better but if you have something to trigger on you don't need auto anyway.