[time-nuts] Fluke PM6681 triggering

Heinzmann, Stefan (ALC NetworX GmbH) Stefan.Heinzmann at alcnetworx.de
Tue Jun 7 12:45:02 UTC 2011


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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