[time-nuts] Fluke PM6681 triggering

Sun Jun 5 10:53:15 UTC 2011

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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