[time-nuts] Fluke PM6681 triggering

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sun Jun 5 09:12:52 UTC 2011

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.


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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