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Trigger Holdoff

Forum for discussing PicoScope (version 5)

Trigger Holdoff

Postby mikel » Tue Feb 01, 2005 11:45 pm

I'm using a PicoScope 3205 to capture data wavefoms (amongst other things). One waveform is a burst of data about 10ms long followed by a 90ms gap. I've set the scope to repeat triggering and 2ms per division in the expectation that it will then trigger on the beginning of the burst of data. Much of the time it triggers as expected. However some of the time it misses the beginning of the data and triggers later in the data burst. If I pre-trigger, then it is clear that when the trigger is late it is not that the data has been missed - it's just that the trigger is late.

Any ideas for ways of avoiding this behaviour so that the trigger is more reliable?

Most conventional scopes have 'trigger holdoff' which helps resolve such triggering difficulties. Is there something similar available here?

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Joined: Tue Jan 11, 2005 9:29 pm

Postby Sarah » Wed Feb 02, 2005 11:42 am


Thank you for your post.

This problem that you are seeing is due to the fact that the scope will start sampling at an arbitrary point in time and therefore the trigger circuit will be armed at an arbitrary point in time.

Most of the time the trigger circuit will be armed in the 90ms gap
between data bursts because the signal being measured is in this
state for the longest period of time and therefore it's more likely that
the trigger will be armed in this period. In fact, because it's a 9:1 ratio of data to no-data, it should trigger in the wrong place 1 in 10 times on average.

There are two possible ways that you could overcome this:

1. If the data can be turned on and off then one possibility is to turn
the data off and set the scope running. The trigger point will therefore definatley be armed when the data is reactivated.

2. Increase the timebase and number of points that are acquired. The trigger point should be set to the middle of the screen and the sample rate increased so that no matter where the trigger catches the data, the entire burst is captured and displayed on the screen. If the number of points are now increased (512kS max for PS3205) then the acquired data can be zoomed in on.

Hope this helps

Best Regards

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