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. > >>> > > > > > > _______________________________________________ > > time-nuts mailing list -- time-nuts@febo.com > > To unsubscribe, go to > > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > and follow the instructions there. > > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.