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Re: Not Quite As Simple CSA

Posted: Sun Mar 28, 2010 7:08 pm
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

Re: Not Quite As Simple CSA

Posted: Sun Mar 28, 2010 11:27 pm
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.

Re: Not Quite As Simple CSA

Posted: Mon Mar 29, 2010 3:09 am
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.

Re: Not Quite As Simple CSA

Posted: Mon Mar 29, 2010 3:17 am
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.

Re: Not Quite As Simple CSA

Posted: Sat Apr 03, 2010 12:50 am
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.

Re: Not Quite As Simple CSA

Posted: Sat Apr 03, 2010 3:37 am
by Richard Hester
The diodes are there to catch a bullet (like switching on the HV to the detector with the preamp connected) that would normally take out the input FET first thing. Bad diodes also happen, and they will inject noise right into the input. The diodes might have been degraded enough to get noisy in the reverse direction. This may not necessarily show up on a simple diode tester. It would be interesting to put the original FET back into the circuit to see if it survived whatever took out the diodes.

Re: Not Quite As Simple CSA

Posted: Thu Sep 23, 2010 7:15 am
by Richard Hester
I was looking at the postamp/shaper circuit I posted earlier on in this thread, and may have a solution for getting the signal amplitude higher while still keeping in the range needed to operate with a 9V battery. This is an offshoot of work I'm doing with jfet-based phono amps I'm documenting on a DIY audio site. More on that tomorrow when I can get to my simulator results. If I could get 0.5-1V output from a simple 2-pole pulse shaper things would be very happy-making.
A circuit for optically coupling the preamp output to a remote SCA would also be neat, but it may be more expedient to keep the SCA local to the detector housing and optically couple the resulting go/no-go pulse to a remote counter. This might require a pair of 9V batteries rather than a single, but that's still not a biggie.

Re: Not Quite As Simple CSA

Posted: Thu Sep 23, 2010 10:49 am
by Starfire
Richard - I'm getting good results with a 555 using the trigger as input and set up for pulse o/p.

Re: Not Quite As Simple CSA

Posted: Mon Apr 25, 2011 4:10 am
by Richard Hester
Attached is my second stab at a discrete postamp and shaper circuit to tie in with the charge sensitive amp. I relaxed my design constraints a bit and used a different fet for the input, which got me the gain I needed. As before, it runs off of a 9V battery.

Re: Not Quite As Simple CSA

Posted: Mon Apr 25, 2011 4:13 am
by Richard Hester
Attached are the input and output waveforms. From a teensy ~50mV input, you get a solid 500mV pulse with a rounded top that will make it easier for a discriminator to work without needing a blindingly fast comparator.