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.
Bill....WB6BNQ 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. > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
