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Trigger at second falling edge, not the first

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Trigger at second falling edge, not the first

Postby joshua_morris » Wed Nov 08, 2017 5:46 pm

Hello. I did a quick search of the forum and couldn't find a similar thread. I am using PicoScope 6 software with my USB PicoScope 3425 to collect impact signals from strain gauges. I have been using this configuration for a over a year and have new issue with the trigger. I would like to trigger my signal collection at the first falling edge (-3mV and 20% pre-trigger), however the software has recently started grabbing the second falling edge. This happens every time now. Has anyone seen this issue before or have an idea what may be the cause? I attached a psdata file that shows this occurring. I can't think of any settings that were changed before/after this problem started and the issue is repeatable using different computers. Thanks for any help or insights.
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Re: Trigger at second falling edge, not the first

Postby Martyn » Thu Nov 09, 2017 7:01 am

Is it possible that the time from starting the collection to the time of the first falling edge isn't long enough to collect 20% pre trigger samples ?
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Re: Trigger at second falling edge, not the first

Postby bennog » Thu Nov 09, 2017 4:21 pm

1st i would set the trigger at -40 mv and see if it happens again.
It looks like you have some hysteresis set at the trigger settings, this is the only reason why this could happen.
the trigger happens at the 10th crossing or so if it was a normal trigger.
If it is an hysteresis trigger then it fired at the moment it is triggering.

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Re: Trigger at second falling edge, not the first

Postby joshua_morris » Fri Nov 17, 2017 5:22 pm

Hello and thanks for your replies. I apologize for my very late response. I had been expecting an email notification about replies and had not logged back in until now to manually check. I'll be more vigilant now.

Martyn, I am feeding in a continuous signal so I think there is plenty of signal for the 20% pre-trigger. The signal looks like the noisy bit at the very beginning of the signals until that first square wave arrives. The screen is actually blank white after I set the trigger to "Running" so maybe its not actually collecting at that time? How might I solve this or check to see if this is actually the problem?

bennog, I don't think I have a hysteresis setting for our trigger. I am using a "Simple Edge" trigger. I attached a picture of the trigger window. The "Advanced Edge" trigger introduces a hysteresis field. I am actually not sure what the hysteresis does? I will be back in the lab on Tues and can try the -40mV trigger then.

Thanks for your help.
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Re: Trigger at second falling edge, not the first

Postby Martyn » Sat Nov 18, 2017 8:40 pm

Simple trigger does have some hysteresis built in, otherwise you would get false triggers on noise. -40mv should work for you.
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Re: Trigger at second falling edge, not the first

Postby joshua_morris » Tue Nov 21, 2017 8:05 pm

I tried a few more tests this afternoon and a higher trigger level seems to be working. I was doing some lighter impact tests today so I used -30mV and -20mV instead of -40mV, but they all triggered at the first downward slope as desired. I guess the -3mV I had been using before was too conservative. Thanks for the help. I'll assume the problem is solved for now and keep an eye on it going forward.
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