[time-nuts] Fluke PM6681 triggering
bruce.griffiths at xtra.co.nz
Sun Jun 5 07:42:24 UTC 2011
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,
> 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.
> 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:
>> 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:
>> 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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