[time-nuts] Fluke PM6681 triggering

Rex rexa at sonic.net
Sun Jun 5 10:12:18 UTC 2011


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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