[time-nuts] Fluke PM6681 triggering

Rex rexa at sonic.net
Sun Jun 5 04:05:53 UTC 2011

I recently picked up a Fluke PM6681 counter (same as a Pendulum CNT-81). 
Looks like a sweet device.

I was just trying to use it for a not-so-much-timing purpose and was 
hoping to find an expert here who might help me with a triggering question.

I just set it up to count total pulses, over a 5 min interval, coming 
randomly out of a geiger counter. Basically I set it up and it works 
except for a subtlety. The pulses out of the geiger counter are not 
clean. At a low count rate they have a big glitch on the leading edge. 
Here is a picture of the pulse:

The glitch causes the count to increment by two on each event except 
that when the pulse rate gets high the pulse shape changes causing the 
the glitch to smooth out and the peak amplitude to drop, like this:

If I set the trigger voltage on the counter to just above the glitch 
peak I can get proper counts, but finding a sweet spot on the changing 
wave shape is not ideal.

I thought I could use the counter's Hold Off feature to get a clean 
solution but it isn't working as I expected. Reading the Operator's 
Manual I thought that the Hold Off period started at a trigger event and 
would prevent another trigger event until after the hold-off period. I 
thought I could set the trigger level to occur around the middle of the 
glitch rise (about 3 volts) and set the hold-off time for 1 uS or more 
to prevent a 2nd trigger on the big rise just after the glitch. I tried 
hold-off values of 250 nS through 20 uS, but I still see the count 
incrementing by two on the glitchy pulses.

I'm wondering if anyone has experience with this counter and can tell me 
if I have mis-understood the Hold-Off function. Or maybe it has 
something to do with me using Total A-B mode. The Op Manual covers a lot 
of ground, but it isn't the easiest to follow the finesse stuff unless 
you happen to need to do exactly what they are showing in an example.

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