"Tail" pulse generator // particle detector simulator  - Page 1
Forum section: electronics / projects

author: ChristofferB
author_rank: Frequent Contributor
timestamp: June 08, 2020, 10:10:21 pm
content: Hey all!As some of you may know from the previous threads I've been pestering you with, I'm messing around with a few gamma spectroscopy, cosmic ray detection etc. projects.One universal tool I'm lacking is a detector simulator (as calibration with a real detector requires a fairly strong monoenergetic gamma ray source, which is not really an option).Basically, what I need is a pulse generator to simulate particle detection events; sharp rise, slow decay, entire pulse duration somewhere between .5 and 10 ns.Pulse looking something like this:I'd like the pulse event to be triggerable by TTL pulse, so that the frequency can be set fairly easily with a standard function gen (or simple square wave osc).How would one go about designing something like this? I know the old-school way is to discharge a capacitor through a resistor, switched with a mercury wetted relay.If I can find one of those that might still be a possibility.Another option is to just couple a narrow square wave pulse into a charge sensitive amplifier via a small value cap, but I'm unsure if this will ever get very good performance.Thirdly, I've read about people using a photodiode coupled to a charge amp, with a VERY dim LED flashing it briefly.What do you think? Thanks in advance!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 08, 2020, 11:14:21 pm
content: Wouldn't a reasonably fast (maybe not 74HC, but LVC or better?) logic gate have enough ramp rate, and a differentiator would do to generate the pulse? Probably needs to be unipolar, so something would be needed to clamp or control the opposite edge so it's at least not objectionable. (An open-collector output would be handy; 74LVC07 isn't too bad, but an RF transistor might do better.)The differentiator can be tweaked, in general a pulse-forming network (PFN), for whatever waveform is desired. It should probably be attenuated or buffered to remove sensitivity to the load.Variable pulse height could be controlled by varying supply voltage (over a reasonable range; a level translator or analog switch might be needed to cover a wide enough range), or variable attenuation.Timing obviously can be generated from a Poissonian timing generator, to get realistic distributions as well.Tim

author: David Hess
author_rank: Super Contributor
timestamp: June 08, 2020, 11:34:54 pm
content: T3sl4co1l beat me to it; AC couple a suitable fast pulse generator into a 50 ohm load, simmer, and serve. AC or LVC logic will be fast enough to require careful ground plane construction for a clean pulse shape.If you need more than 5 volts, then things get interesting.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: June 08, 2020, 11:44:37 pm
content: That sounds orders of magnitude easier than what I was thinking! Thanks!I'll try a 74AC or 74LVC oscillator AC coupled into either my wanted load, or a shaping net/buffer.Having some binary pi or T attenuators in series would give me a pretty big selection of pulse heights from 5V and down, right?

author: David Hess
author_rank: Super Contributor
timestamp: June 09, 2020, 12:01:23 am
content: Quote from: ChristofferB on June 08, 2020, 11:44:37 pmThat is right. Like T3sl4co1l says, some easy pulse height control can be achieved by varying the supply voltage to the gate from 5 volts (actually a little higher) to 3.3 or maybe even down to 2 volts but this will affect the switching speed.A simple alternative to varying the supply voltage is to use an open drain output like a 74AC125/74LVC125 where a resistor pull-up or pull-down (or a current source/sink) on the output sets the output voltage. This would allow outputs down to millivolts so a much larger range.If you need more than 5 volts, then the same open drain output can drive the emitter or source of a higher voltage cascode RF transistor which could potentially get you to 10 or maybe even 15 volts but now developing enough current into a 50 ohm load will be the limitation. They used to make RF transistors packaged to support this configuration with two base connections and one emitter connection but availability was never good and is worse now.But I suspect an AC or LVC gate operating at 5 or 3.3 volts driving a high pass RC filter, followed by attenuators as needed, will do exactly what you need. The coupling capacitance is low enough that a small trimmer capacitor will have enough range and make it adjustable.

author: jbb
author_rank: Super Contributor
timestamp: June 09, 2020, 12:05:59 am
content: Well, mercury wetted relays could be a bit hard to find nowadays.It might just be a case of “this is my favourite hammer,” but how about a high speed gate driver (Texas Instruments LMG1025 perhaps? 5V Drive, <0.7ns rise time) with some pulse forming RC networks?

author: tggzzz
author_rank: Super Contributor
timestamp: June 09, 2020, 07:59:44 am
content: Here's an example of 74lvc1g* driving 2.5V into 50ohms with a ~300ps risetime;https://www.eevblog.com/forum/testgear/show-us-your-square-wave/msg1902941/#msg1902941As others have noted, you can reduce the amplitude by reducing the Vcc (IIRC to below 1V!), but that increases the risetime. Using a 50ohm RF attenuator is a better technique.As for the differentiator, use RF components and techniques of course. Realise that many small SMD capacitors' capacitance is such a strong function of applied voltage that the exponential curve is visibly non-exponential. I don't know whether that would matter in your case.

author: eliocor
author_rank: Unknown
timestamp: June 09, 2020, 09:35:05 am
content: You can take a look on how the "professionals" do it:https://ludlums.com/documents/product-manualsespecially the M500:https://ludlums.com/images/product_manuals/M500-2.pdfotherwise, on the cheap way, here is the schematic of a very simple pulser (using an arduino). If needed I can give you also the awful code for it.Another implementation from Dario Tortato:https://www.radioactivityforum.it/forum/viewtopic.php?f=69&t=2893&p=12153&hilit=pulser2#p12153https://www.radioactivityforum.it/forum/viewtopic.php?f=46&t=2083&start=10#p10364

author: ChristofferB
author_rank: Frequent Contributor
timestamp: June 10, 2020, 09:44:55 am
content: Thanks all! The picture that's painted is that AC coupling a fast logic gate into a load is the way to go!The only issue I have with that, is that when the logic gate switches the other way, an identical peak would be formed with opposite polarity to the original, right?Strictly speaking that may not be a problem, but it's still not ideal. How could one solve that? A tunneling diode to ground maybe?

author: tggzzz
author_rank: Super Contributor
timestamp: June 10, 2020, 10:27:07 am
content: Quote from: ChristofferB on June 10, 2020, 09:44:55 amA simple reverse diode might be sufficient. A 600mV reverse clamp voltage could be reduced by prebiassing the other end of the diode by 600mV.A BJT or FET clamp might be sufficient, but I'd want to simulate the effects of the various capacitances.There are various devices placed across signal lines to prevent damage from ESD. There are varying power/speed/voltage tradeoffs. Littlefuse makes many such devices.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: July 15, 2020, 03:00:57 pm
content: Just to round off this topic (I know it's a bit old now, sorry!)I ended up buying a proffessional pulser (for like 20$ - and it's ORNL surplus!!) which is based on a mercury wetted relay. I made a brief video on its teardown and operation, I though that might be of interest to others than me.

author: OM222O
author_rank: Frequent Contributor
timestamp: July 15, 2020, 03:45:28 pm
content: You could have also used a small capacitor and a fet.The fet would "short" the capacitor and charge it without resistance which is pretty fast, then when the fet disconnects, the cap would discharge via a small resistor to have the rounded off "tail". By choosing low capacitance fets and reasonably small capacitor and resistor, it should have been possible. Then all you needed was a square wave pulse to control the fet 😉

author: eliocor
author_rank: Unknown
timestamp: July 15, 2020, 06:16:07 pm
content: Here is the schematic of one of the simplest wetted relay pulser: the Canberra 807

author: ChristofferB
author_rank: Frequent Contributor
timestamp: July 15, 2020, 06:29:32 pm
content: Thank you so much! This might actually be functionally identical to my Tennelec unit, as the function is identical, and Canberra bought Tennelec in the late 80's!