[time-nuts] Fluke PM6681 triggering

Rex rexa at sonic.net
Tue Jun 7 13:24:34 UTC 2011

 From what I read in the manual, that filter sounded a bit extreme for 
this signal. With the external filter I can see what the resulting 
signal looks like on a scope. With the internal filter it would require 
some analysis and faith as to what I was measuring.

If the hold-off worked the way i thought it did, it would have been a 
clean predictable solution. But it seems it doesn't do what I thought it 

Thanks for all the comments and suggestions, though. My question was as 
much about understanding the counter features as it was about making 
this particular measurement. I expected that I could clean the signal to 
remove the glitch if I needed to.

On 6/7/2011 5:45 AM, Heinzmann, Stefan (ALC NetworX GmbH) wrote:
> The PM6681 has an analog filter that you can engage with a button on the front panel. Have you tried using that?
> Stefan
> -----Ursprüngliche Nachricht-----
> Von: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] Im Auftrag von Rex
> Gesendet: Montag, 6. Juni 2011 00:42
> An: Discussion of precise time and frequency measurement
> Betreff: Re: [time-nuts] Fluke PM6681 triggering
> I forgot that I had a 50 ohm load on the scope end of my cable for that
> picture. The audio output is cap coupled. That's why the short pulse
> with long exponential tail. If I use 1M input at the scope it is a
> square pulse>200 uS but still has that glitch on the leading edge.
> I put a simple RC filter on the output; the rise time gets stretched but
> the negative glitch becomes a step. Good enough for the counting.
> On 6/5/2011 3:53 AM, Bruce Griffiths wrote:
>> Maybe, but the circuit diagram indicates that the pulse that you see
>> is nothing like what it should be.
>> There may well be a circuit fault.
>> The circuit already includes a monostable.
>> Bruce
>> Rex wrote:
>>> We are getting pretty far afield of my original counter triggering
>>> question.
>>> As far as I know, any specific quenching is only necessary for
>>> achieving the highest counting rates which isn't involved in my
>>> measurements, so far as I know.
>>> Like I said, this is a 1960's CD counter. Pretty impressive that it
>>> still works. They did a pretty nice job with minimal components, I
>>> think. If you really want to see the trivial details, here is the
>>> circuit (updated by someone to make it more logical from it's
>>> original drafted version).
>>> http://www.cs.utah.edu/~hatch/images/lionel.gif
>>> I am counting out of the suboptimal Audio output.
>>> I did change the original "hotdog" tube to a pancake style detector
>>> for these measurements.
>>> On 6/5/2011 2:12 AM, Bruce Griffiths wrote:
>>>> Is the GM tube internally or externally quenched?
>>>> Its necessary to quench each avalanche discharge either by using an
>>>> internal quenching gas (eg a halogen) or to use suitable circuitry
>>>> to ensure the discharge terminates.
>>>> Bruce
>>>> Rex wrote:
>>>>> Bill and Bruce,
>>>>> Clearly, fixing the messed up signal is the proper approach. What
>>>>> you are missing is that I got a shiney new (for me) expensive
>>>>> hammer and I thought that it should be able to drive defective
>>>>> nails. :)
>>>>> I got an off-list reply that suggested that hold-off affects the
>>>>> counter gating -- which either doesn't matter in this totaling app
>>>>> or complicates it. He also suggested using the negative slope of
>>>>> the pulse to trigger. Doh! The negative slope is more gradual and
>>>>> would affect timing accuracy, but that doesn't matter in my
>>>>> counting situation.
>>>>> Oh, and as reply to the question of more detail on where the signal
>>>>> comes from, this is a 1960's CD-700 (civil defense, yellow) gieger
>>>>> counter. The signal is the earphone output. In the future I think
>>>>> I'm going to make my own circuits to connect to a geiger tube or a
>>>>> scintillator/PMT MCA application, but that is even further from
>>>>> playing with the nice new counter.
>>>>> Thanks for the feedback -- any more welcomed.
>>>>> -Rex
>>>>> On 6/5/2011 12:42 AM, Bruce Griffiths wrote:
>>>>>> A Geiger Muller (GM) tube produces an output pulse as a result of
>>>>>> an avalanche discharge in the gas filled tube initiated by the
>>>>>> passage of ionising radiation through the tube.
>>>>>> A high voltage is initially maintained between an outer usually
>>>>>> cylindrical electrode and an inner small diameter wire electrode.
>>>>>> The discharge current develops a voltage across a resistor in
>>>>>> series with the inner electrode. The pulse amplitude is relatively
>>>>>> large and little gain is required to drive a speaker.
>>>>>> Pulse shaping using a suitable differentiating and integrating RC
>>>>>> time constants is typically used to shape the pulses and  maximise
>>>>>> the SNR of signals from scintillators and proportional counters.
>>>>>> For Geiger counters the signal is so large that such shaping to
>>>>>> maximise SNR isnt usually required.
>>>>>> Using a non retriggerable monostable to define the deadtime in
>>>>>> nuclear counters is relatively common.
>>>>>> The pulse risetime for a GM tube is relatively slow so that
>>>>>> something like a 74HC series monostable should suffice.
>>>>>> An HCMOS monostable also has the advantage of a high input
>>>>>> impedance so that little or no amplification should be necessary,
>>>>>> Bruce
>>>>>> WB6BNQ wrote:
>>>>>>> Hi again Rex,
>>>>>>> I should have asked these questions in the first place.
>>>>>>> How are you connecting the Fluke to the geiger counter ?
>>>>>>> Is this a signal that drives a speaker or some other kind of
>>>>>>> noise maker ?
>>>>>>> What happens if you load that line with some capacitance like 1
>>>>>>> uf or more ?
>>>>>>> If the capacitance helps you will have to experiment with the
>>>>>>> value so as to not
>>>>>>> completely destroy the pulse shape.  Never played with a geiger
>>>>>>> counter so have
>>>>>>> no real idea how they do the noise making.
>>>>>>> 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.

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