Not Quite As Simple CSA

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Carl Willis
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Re: Not Quite As Simple CSA

Post by Carl Willis »

Hi Richard,

Thanks for your efforts with this design, and your previous contributions. I'll have to give it a try.

If I had one suggestion from just looking at your design, it would be to increase the decay time constant. Charge collection in proportional counters takes a long time, probably on average a few microseconds if the response is to remain linear in energy. So I'd advocate 30+ uS for the product of R1*C2 for that reason. An unrelated reason to make the decay time longer is that the pole-zero adjustment on commercial NIM amplifiers typically works between 50 microseconds and infinity. I think the only downside of using a few dozen MOhm would be the lack of availability at Radio Shack or in most junk boxes...what do you think?

-Carl
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Re: Not Quite As Simple CSA

Post by Richard Hester »

I have some extra intelligence on that, Carl - I was looking up data on Boron-lined counters, and a group in Washington state is evaluating them as a replacement for He3 tubes in portal monitors due to the He3 shortage (sound familiar?). They were talking about using a short time constant to aid in gamma discrimination. It's something that could be played with, as we don't expect a lot of gamma action with the fusor, and it doesn't take a lot of lead to screen out X-rays and such.The paper is one of the first things that pops out if you do a search for "B10 counter tube" or "B10 proportional tube" or something similar. Anyway, it's easy to tailor the time constant by playing with the discharge resistor. The larger you make that resistor, though, the more chance you have of leakage starting to affect your results. I also don't trust normal -type resistors too much over 10M or so. I'd keep the charge storage cap small because of the low amount of picocoulombs involved. When you have a big, fat pulse like from a PMT, then you can splurge on the picofarads.
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Re: Not Quite As Simple CSA

Post by Richard Hester »

To Doug - I really dont care, either. I messed with CSAs for upwards of a year developing various circuits and know what I've seen. The circuit posted is a simplified version of a CSA available from Bicron - I even posted the schematic in files.
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Re: Not Quite As Simple CSA

Post by nemesistech »

How bout I ask simple questions that I am sure most of you can answer with ease. I am still in the learning stages with building circuits, so forgive me if these questions bore you.
First, should I assume you would want to use metail film resistors instead of carbon?
Second, on Q3 transistor, is that one piggybacked on the other, if so, what is the purpose of this?
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Re: Not Quite As Simple CSA

Post by Doug Coulter »

Richard, sorry, bad day at the office, you know. And for once it's Carl being polite and me abrasive ;~) Not really called for....I do what I do and see what I see too, but when that conflicts with predictions and book learnin, I try and find out why -- I suspected it may have been something not obvious in that case as I do also have a lot of experience with things like this. I agree with Carl on the time constants here -- this stuff is on the way slow side compared to things like PMT's.

To Mike , that last transistor is two in one package, in the Darlington configuration, used as an impedance reducing device (eg it will drive more current with this than without it). In common collector mode, it's just a gain of approximately one for voltage, but has a ton of current gain. This way the design can use that 82k resistor in the pnp for good voltage gain, without having the load you put on the preamp affect that (the output of the pnp is a current which the resistor changes to a voltage), and it will drive a hefty load (which helps with noise pickup downstream, keeps the gain stable, and other good things like that).

So think of it as a gain of one (for voltage) buffer that doesn't need much current to drive it, but will put out far more current than it takes to drive it. You can do this with two transistors in separate packages too, and the net current gain is that of the two transistors multiplied -- so the current gain gets big quickly. Darlingtons are usually slow, particularly in turn off, as there is no resistor to discharge the base-emitter capacity of the second transistor when the first turns off (or maybe there's one inside the package and just not shown in this case, I'm not familiar with that particular number).
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Re: Not Quite As Simple CSA

Post by Starfire »

Less talk more build - suck it and see - general comment not direct at any one in particular
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Re: Not Quite As Simple CSA

Post by Carl Willis »

Hi Richard,

I have a preamp at home, derived from probably the same Bicron preamp you mention, on which I use a regular 10M resistor and a 5.6-pF, NPO-grade ATC cap. The cap and resistor, as well as the protection diodes, form a self-supporting "dead-bug" structure. It works well with proportional tubes in my experience, but I cannot comment on its temperature stability or noise figure or anything like that. I do know that the observed time constant--when fed from a charge-terminated pulser--is not any different from what it should be ideally.

Thanks for clarifying your thinking about the short time constant. I had no idea B-lined tubes were making a comeback.

To your point about construction technique, I'll just add that I think it matters a lot. My feeling is that stray capacitance is probably what causes observable differences from the ideal when building the input stage solely on PCB pads, but it could be surface leakage as well, or some of both. I am in the habit of building the rest of the boards by "Dremel etching" like a total amateur, but the input stage is indeed unpredictable if built this way. Ground loops are a huge issue with preamps. Microphonics are a problem also. Power supply filtration is an issue. Cleanliness is a huge issue especially if the load resistor and HV components are in the preamp also. Leaky BNCs...leaky Victoreen resistors...rosin threads...humidity...all very relevant issues, not easy to avoid. I wash with methanol ("Heet") dried over zeolite pellets to get rid of rosin and humidity. Does not always effect a cure, though.

-Carl
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Re: Not Quite As Simple CSA

Post by Richard Hull »

As I first noted above, I just got used to cleaning and floating gate stuff in electrostatic/electrometer systems I have built and worked with and have seen it done on virtually every single charge preamp from GeLi preamps to 3He and BF3 NIM preamps. I'll do it reflexively until the day I die. Once the faraday case or shell closes, it should be a sleeping dog for a long time and require no revisiting.

I do hope to construct this thing soon, just 'fer grins and googlers.

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Re: Not Quite As Simple CSA

Post by DaveC »

In response to Doug's earlier comments on characteristic impedances.

Hey - I said "75 ohms or thereabouts"


Z0 depends on the Log of the diameter ratios, not ratio directly... thus pretty insensitive to size, right?

Further, within a factor 2 or 3 is "Exact" for physics discussions, and not even bad for arm waving electronics.

So what's your beef, eh?


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Re: Not Quite As Simple CSA

Post by Doug Coulter »

No beef at all Dave -- you are absolutely correct on that one, and here it's just not an issue, as we are far shorter than any length that constitutes transmission line behavior for the risetimes involved anyway.

My apologies, I guess I should more often calm down before posting sometimes.
On email, they can't see you grinning and armwaving, so I get in trouble.

Ever notice that stuff that comes off as mere agitated excitement in person sounds pretty bad (or can be taken as such) on email/print? This one bites me now and again. People who have met me in person know it's just me spouting off, but others can't know. Sorry 'bout that.

I have a two transistor circuit I'm using on the 3He tube (and in a lot of other places) I like better myself, and it doesn't need all the capacitors, which is nice both for complexity and for recovery times on overload. This circuit in that use also has builtin adjustable threshold for gamma discrimination.
As I have two B10 tubes, I may try a version optimized for them (that's a somewhat tougher signal) and let everyone know.

The circuit under discussion seems to be derived from the second one here, also in the same book.
(1980 linear apps handbook)
There are some interesting variations of it that provide for almost zero input capacity, due to bootstrapping the fet G-S cap, and eliminating the G-D miller cap by use of the cascode -- the drain basically is driving an emitter, and there's almost no signal volts there -- it's all current.

Which made it a great place for the inductor -- since that's a low impedance node, you don't need one as large, and using an inductor protects the rest of the circuit from the wild variations in Idss most fets tend to have from unit to unit (if not from moment to moment, and temperature to temperature).

Joe Sousa and I try to eliminate all L's and C's where we can and it makes sense. Old habits -- he's a chip designer where you can't have much of either (but where taking a few transistors to make a current source is nothing), and I'm an old cheapskate with a weird idea of what constitutes "elegant", and who used to have to fix up behind designers of mass market stuff, and guess what made all the troubles -- caps, which at least we could get, and inductors, which in some random TV set or stereo, just try to find out what it even *was* so you know what to replace it with, and more often than not the Q and self resonance was as important as the inductance to the thing working right. Nightmare for a tech, which is what I was, coming up in the '60s. I later graduated to EE, then Scientist, but I didn't forget some fantastic lessons I learned about repeatability and reliability as a service tech. We saw what really worked and of course, what didn't -- many times a day.

I first saw this circuit in a very good early solid state Marantz reciever, used in the tone controls, then some years later another version was in the National Semi apps book used at the other extreme of impedances for a moving coil phone cartridge preamp that was near noise theoretic at room temp.

By changing some speeds and feeds, it's in use on my 3He tubes, my BF3 tubes, and some phototubes, all just by changing some values. Very simple, low noise if tuned right for the impedances involved, cleanly closed loop system that can also be an integrator of the type wanted here if you add a cap across the NFB R. Works like a champ, and is not real sensitive to power supply issues. You can use a fet for the input device if you make some small changes for bias issues.
I don't usually have to, with the current gain of the transistor pair with NFB, the input impedance can be plenty high with plain old bipolars (and therefore more rugged, and in some eyes, more elegant).

Here's a scan from the National book. In most other uses, you don't use their fancy low impedance transistor pair, but the good old 2n3904/3906 kind of thing, much lower currents, and in the case of the tubes that put out a negative pulse, use a pnp in the input, then npn (power supply reversed) since the tube will turn on the pnp better with the negative pulse.... and then you can fool with the bias to put a threshold on there -- no parts. In my practice here, I do that via a simple pot across the signal input (after the input cap) to adjust the tube net gain to match tyhe Vbe threshold with no other input bas at all. (I run my tubes higher volt than spec to get into a good gas-gain region and they drive this fine -- out come 5v pulses at decently low impedance).

So basically, all the R's in the national circuit are much higher value, and for some things I do the NFB differently -- sometimes none if I just want a count/no-count situation and am using input bias (or the lack of it) to set the threshold.

In the third one we see the bootstrapping, which can only work if you have a gain == 1 node someplace to play with (however you get it, I've seen people use dividers off a gainy circuit to make that happen). Pretty cool, actually, and this is close to what we wound up with to take EEGs with pure capacitive input coupling -- no shaved heads and no goo needed.
Same class of problem but at a very different passband.

I have a bias toward real pc board building, since it's really not hard these days, and when I did this for a living, of course that was all there was. I use a freeware software tool (Protel TraxEdit, very old dos program, and IMO better than their current multithousand buck offering by far -- simple), a laser printer, and pre-sensitized PCB (available all over). Even if I'm not real sure the thing will work, it's easier to cut the odd track or add the odd wire, change parts etc, and get going.
And once it's right -- it's right forever, every one you make is the same re all the leakage R's, parasitic C's and interwire coupling....worth the effort for me, because once you get set up, it's not much effort at all. Then I can send the same files to AP circuits in Canada and get them made really nice, pretty cheap, and very fast if I need a bunch. I have zip troubles with leakage unless I'm getting way out there on something. Real high volts and real high R's I air-wire as required -- there it doesn't take much conductance to make trouble, but at 5 volts, nah. In this case, the tube supply resistor wouldn't live on the PCB at all, of course, and neither would the input coupling cap as one end is very hot. At the normal solid state volts, up to about 10 megs, no issues whatever with PCB leakages, even if you need a pretty precise 10 megs. Above that, well then -- first find good enough parts at all, then yes, you have issues especially at high volts where tiny amounts of corona play hob with SNR.
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Re: Not Quite As Simple CSA

Post by Richard Hester »

Attached is the output waveform for the circuit in question built dead bug style on a small piece of copper clad board. The excitation source is an LED/Photodiode pulser similar to the circuit described in "files". The charge storage cap used was a 1 pF +/- 0.25pF NPO disk. The coupling capacitor (4.7 nF) was moved to the input rather than inside the feedback loop to allow greater freedom of choice for the charge storage cap. The voltage rating for the charge storage I chose is unknown (though I suspect it is either 500 V or 1kV rating). The input, output, and summing nodes were built off the board on small standoffs for isolation and/or ease of connection. The picture shows a 1/e time of about 2.9 usec, which is somewhat longer than expected, but not too far off considering expected stray capacitance and the small value of charge storage cap.

Will post more info and pictures tomorrow as time permits. This represents ~15min of effort (the data taking , that is - the circuit took a couple of hours of effort, including digging up the fet for the input). The circuit came right up and required no extra effort. I spent a lot more time trying to find my pulser....
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Re: Not Quite As Simple CSA

Post by Richard Hester »

Here is the actual build of the CSA circuit using dead bug construction. The circuit was constructed on single-sided copper clad board. An additional piece of board was tacked to the main board (superglue works ok) to serve as a Vcc bus. The circuit was laid out on the board to flow from left to right pretty much as presented in the schematic. It's not pretty, but it works. Small standoffs were used for input, output, and summing node connections. The summing node could very well have been an "air connection" (and it would have worked better that way), but I wanted a little more support for the components. The nailhead terminals on the input and output connectors make connection to the board a little easier. The large disc cap at the left hand side of the device is the input coupling cap . It's about as big around as a nickel, which gives you an idea of the overalll size.
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Re: Not Quite As Simple CSA

Post by Carl Willis »

That's a nice little project, Richard.

I'm thinking that on account of its diminutive size and simplicity it might work well as a front-end for CDV-700s, Ludlum 3s, etc. to enable their function with proportional detectors.

Thanks for the update.

-Carl
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Re: Not Quite As Simple CSA

Post by Richard Hester »

Test results are in a post a little farther up the chain - maybe I'll ask Mr. Hull or the Professor to move it down the line so it is in chronological order. I based the charge storage cap size on the numbers given in the Reuter-Stokes data sheet for their He3 detector. If i remember correctly, their charge/event was specified at 600V. As you stated earlier on in the boron tube thread (I think), this number will be higher for higher bias voltages. Getting the storage cap value up to near 10pF would help repeatability, and also make the cap a little easier to obtain. Having said that, a teflon wire "gimmick" with some heat shrink around the wires to preserve the twist would make for a high quality charge storage cap - make up a couple if inches of twist, and cut it to the desired capacitance...
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Re: Not Quite As Simple CSA

Post by Tyler Christensen »

I built this amplifier today and it is working great, another successful build. It is a huge improvement over my previous "pathetically simple pre-amp". I really like the battery power source since it simplifies on getting power to it noise-free.

Worked great on my 3He tube, I'll test it on a 10B tube at some point but I'm sure it will work just as well.
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Re: Not Quite As Simple CSA

Post by Richard Hester »

What kind of pulse heights are you seeing out of the preamp/tube combo and at what bias voltage? Are you using the 1pF charge storage cap or a different value? Some fine tuning may be possible. I threw some values together based on what I saw from a Reuter-Stokes data sheet.
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Re: Not Quite As Simple CSA

Post by Tyler Christensen »

I'm using a 2pF charge storage cap. If I recall correctly, the output pulses are right around 100mV, but I could be mistaken about that, I won't have time to test it for a few days. The 3He bias was at ~1600V.
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Re: Not Quite As Simple CSA

Post by Richard Hester »

Let me know when you get around to testing it. The optimum value of charge storage cap would be nice to know.

I have a charge sensitive preamp that I use with my homemade fast neutron scintillators, and it didn't get proper C values until I had a chance to pair it up with a neutron source at Mr. Hull's lab a few years ago. It was giving squirreley results there because valid neutron events were producing signals large enough to saturate the amp, puttting them outside the range of the SCA I had looking at the output (it was only doing its job...). Doubling the value for the charge storage cap did the trick. An added benefit of that was to push down the amplitude of low-level garbage signals and make them easier to discriminate out. For that reason, we want as high a cap value as we can get away with for the He3 tube preamp. 100mV of valid signal would be just peachy.

I may gin up a simple post-amp and shaper that could be included in the same box, making the output signal higher amplitude and easier to deal with.
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Re: Not Quite As Simple CSA

Post by Tyler Christensen »

I'll work on optimizing that when I get a chance to run the fusor again. So the charge storage cap will determine the amplitude of the signal output? I don't have a storage scope to get traces of the exponential decay wave that others have posted if that is required to optimize it, I just see "blips" on my scope at the voltage when neutrons are going through it. Just optimize those "blips" to be 100mV by changing the storage cap?
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Re: Not Quite As Simple CSA

Post by Richard Hester »

I'd set up the scope with the preamp and set the trigger level on the scope so as not to respond to "grass" from a gamma source, then look at the pulse amplitude with a neutron source. Maybe it's best to set the scope to"norm" trigger mode when looking at the neutron pulses. Set the trigger level to just above the value that rejects the grass, and inch it up as you look at neutron generated signals until you reach a point where the scope doesn't trigger any more. That will give you an amplitude range for the solid signals. You can get an idea of the real average signal level by the number of events you get for a given trigger setting. Too bad you don't have a digital scope, as it makes the job easier. See what you have first with the 2pF cap before messing with it. What we're looking for is a good solid output voltage above the noise with a reasonable value of capacitor - there's no one perfect number. Somewhere around100-200 mV looks like an attainable value.
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Re: Not Quite As Simple CSA

Post by Tyler Christensen »

I'm getting 50mV pulses out of the 3He tube and several volt pulses out of the 10B tube, although that one is a direct function of the bias voltage so it has little meaning.

This is with the 2pF charge storage capacitor.

This is well above the 2-5mV noise floor
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Re: Not Quite As Simple CSA

Post by Richard Hester »

This suggests that you're are getting about 100 nC/event from your He3 tube at your chosen bias voltage. This is a plausible value. It also suggests that a charge storage capacitor of 1-2 pF will be a good starting value for other experimenters. It also sounds like a post amp and shaper located inside the same housing as the preamp would be useful. 50 mV is a usable signal - 0.5 to1V would be better.
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Re: Not Quite As Simple CSA

Post by Richard Hester »

Here is my first PSpice tinkering run for a post-amp and shaping circuit to go along with the preamp. Some of the components are in place merely as probe points, and would not be included in an actual design. The first 2N5457 and the 2N3906 constitute the post amp, and the other two 2N5457s are the shaper. Since the shaper only has two poles, its output doesn't come near an ideal Gaussian pulse, but the peak is broadened sufficiently to make discrimination easier, at the cost of a good deal of amplitude, which will be seen in the next post. The blue probe monitors the input pulse, the red is the output of the postamp, and the green probe monitors the shaper output.
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Re: Not Quite As Simple CSA

Post by Richard Hester »

The picture here shows the input pulse (blue), the postamp output (red), and the shaper output (green) from the PSpice simulation. As can be seen, the shaper smooths and stretches the pulse, at the cost of a good deal of amplitude.This is very similar behavior.to the kind of circuits I've been using for my scintillator-based All-In-One detector setups.
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Re: Not Quite As Simple CSA

Post by Tyler Christensen »

My build of this circuit just stopped working, I'm now getting 0.6V P-P noise straight out of the pre-amp. It just suddenly started doing this the other day. At first it was just a few loud peaks then it went into chaos. I've replaced all three active transistors and it behaves exactly the same. Checked all the resistors, none are burned up. Any guess on what's causing this? This is with the HV off.

UPDATE: Solved it, it was the input diodes (just leaving the message incase anyone has the same problem after driving too much voltage into the input). I didn't initial replace them because they test fine on my diode test meter, interesting.
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