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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