bug in agilent dsox3014a firmware?

[ToBeFrank]

I'm running the latest firmware (2011-06-30) in my dsox3014a, and I think there is a bug in the edge triggering. I have a signal on a bus that repeats every 50-100 milliseconds. I have the trigger set up and I can see the signal very well. However, when I change the horizontal time, it almost always starts failing to trigger. The scope is flashing "Auto?", which according to the manual means it is not finding the trigger condition. If I adjust the trigger level slightly (.02V), it will start to trigger again. Has anyone else noticed this?

[Rufus]

I have a signal on a bus that repeats every 50-100 milliseconds. I have the trigger set up and I can see the signal very well. However, when I change the horizontal time, it almost always starts failing to trigger. The scope is flashing "Auto?", which according to the manual means it is not finding the trigger condition.

Auto? means it is in auto mode and triggered automatically because it didn't see a trigger within some time period since the last trigger. That period depends on the timebase setting so it is possible to have a stable trigger in auto mode which becomes unstable when you decrease the timebase. You shouldn't be using auto mode when you know you have something to trigger on. That's my understanding anyway.

[ToBeFrank]

You are correct. Auto is what was doing me in. Thanks much.

[ToBeFrank]

Auto? means it is in auto mode and triggered automatically because it didn't see a trigger within some time period since the last trigger. That period depends on the timebase setting so it is possible to have a stable trigger in auto mode which becomes unstable when you decrease the timebase. You shouldn't be using auto mode when you know you have something to trigger on. That's my understanding anyway.

After thinking about it and reading the manual, I still don't understand why the auto wouldn't be triggering. When I change the timebase, the period between forced triggers changes like you said, but that period only comes into play when it didn't meet the trigger condition. In this case, the trigger condition was there so it should have triggered. It's like it stops looking for the trigger condition when it enters the forced trigger state and stays that way until a slight change is made to the trigger level. Shouldn't it still be looking for the trigger condition?

[Rufus]

I still don't understand why the auto wouldn't be triggering. When I change the timebase, the period between forced triggers changes like you said, but that period only comes into play when it didn't meet the trigger condition.

If the trigger condition occurs while it is sweeping it will be missed. If the sweep + auto give up waiting time is less than the period between triggers I would expect it to trigger on a proportion of the trigger events. I think you could only miss them all if your trigger is synchronised with the sweep + give up waiting timing.

I just looked and there does seem to be an issue with auto trigger mode. Triggering on an 18Hz squarewave some timebase settings (500us/div being one) will show it alternating between a stable and unstable trigger a bit less than a second each with alternating Auto and Auto?. If you wind the 18Hz down a bit the alternation speeds up. If you wind the timebase down 3 clicks and back to 500us/div the alternation will be there maybe 3 out of 4 times. Triggering is inhibited while you change trigger level and when it starts again it is usually stable.

I would say it is bugged. Don't know if the 2011-06-30 firmware introduced it.

[ToBeFrank]

I just looked and there does seem to be an issue with auto trigger mode. Triggering on an 18Hz squarewave some timebase settings (500us/div being one) will show it alternating between a stable and unstable trigger a bit less than a second each with alternating Auto and Auto?. If you wind the 18Hz down a bit the alternation speeds up. If you wind the timebase down 3 clicks and back to 500us/div the alternation will be there maybe 3 out of 4 times. Triggering is inhibited while you change trigger level and when it starts again it is usually stable.

I would say it is bugged. Don't know if the 2011-06-30 firmware introduced it.

Yes, this is what I was talking about. When I have a stable display of my signal with auto trigger and then switch timebase and then switch back, it is never stable again until I adjust the trigger level.