[time-nuts] Fluke PM6681 triggering

[WB6BNQ]

Hi Rex,

I think Bruce has hit it on the old nail head.  Especially now that I have a
schematic to look at.

You indicated you changed the tube to a different one, is that right ?  I wonder
if it may be a characteristic of the new tube you are using ?  Maybe it needs a
higher voltage to cause a stable avalanche ?

I guessing that at slow rates it starts to avalanche but cannot sustain the the
action due to the voltage being too low.  However, to be clear I do not really
have any experience with these devices and just going off what Bruce stated on
how it works.

Good luck,

Bill....WB6BNQ


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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