[time-nuts] Fluke PM6681 triggering

WB6BNQ wb6bnq at cox.net
Sun Jun 5 06:12:49 UTC 2011

Hi Rex,

I am not familiar with that counter, but I am wondering if you could use a
"one-shot" to clean up that problem ?  Something similar to the 74121 or 74122.
Make sure you do not select a "re-triggerable" type (74123) as that will follow
the apparent double pulse.  You might see if there is a modern model of those
chips that will follow the fast rise time.  Also make sure, if you use this idea
to set the one-shot output pulse width just shorter then the fastest rep rate of
the geiger counter.


Rex wrote:

> 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:
> http://www.xertech.net/geiger/single.jpg
> 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:
> http://www.xertech.net/geiger/multiple.jpg
> 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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