Avalanche pulse gen on a breadboard, pretty good - Page 1
Forum section: electronics / projects

author: MathWizard
author_rank: Super Contributor
timestamp: March 15, 2025, 08:23:35 am
content: I basically improvised this, inspired by the Jim Williams and Kerry Wong versions. So I haven't calculated much in the way of the coupling between stages, I just used LTspice and then the breadboard. And it works pretty good. The Voltage multiplier is just powerful enough, it's almost running out of juice even unregulated, just w/ the leakage current of a BC547B. And a 10x scope probe or 10Meg DMM probe on the HighVoltage output, steals enough current, to stop the avalanches.With the astable multi-vibrator osc at 87kHz, and having added R13 to the osc. buffer, I get a pretty equal duty for the gate drive. The drain has some ringing, but if I get rid of most of that ringing, w/ a simple high-pass RC filter, then I can barely get up to 120V on the VM. I could up the frequency, but it works good enough with the ringing for now on a breadboard.But it needs over 172V for a BC547. IDK what it really is, w/o the DMM or probe stopping the avalanches. And indeed the diff-amp can regulate the voltage between ~100-175V, and then shut it off all together, as I hoped.There's no real difference in the circuit, between the 70MHz BW SDS804X HD, and the 200MHz SDS2204X+, both used 200MHz probes, but for the 2204X+, I used a ground spring.But remember, this is all on a breadboard, but most of the leads are trimmed, and it's all pretty tidy. I used a 2N7000 mosfet, and 50V rated caps. So far I only tried w/ 3, then 4, 15pF caps in parallel. And besides trying the RC filter on the drain, I haven't much considered any frequency response of the diff-amp yet, it seems fine enough w/o any size C18 or C44.The output load is just 47ohms, and the 200MHz 10x probe.

author: jonpaul
author_rank: Super Contributor
timestamp: March 15, 2025, 11:34:21 am
content: probes ringing.Use 1 GHz analog scope or equiv.We use Leo Bodnar 30-40 pS pulsers,We insert a series 20 dB attn to reduce VSWR.j

author: macboy
author_rank: Super Contributor
timestamp: March 15, 2025, 08:56:47 pm
content: Don't forget that this is really more of an impulse generator, than a pulse generator. For a true flat top pulse, use a charge line instead of a capacitor. A chunk of any decent 50 ohm coax will do. The pulse duration is a function of length of this coax, approx 1 ns per foot. A later Jim Williams app note describes more details of this much improved Avalanche pulser, but curiously it is not one most people talk about. I don't recall the AN number off hand.

author: MathWizard
author_rank: Super Contributor
timestamp: March 16, 2025, 06:41:46 am
content: I made a 1GHz JFET probe with through-hole parts on copper-clad. It seemed to work ok with a 125 MHz signal gen, but mechanically it needs work. I bought some nice SMD JFET's to make an SMD version. I should try that somehow, and compare it to the through-hole parts version. I barely have any BNC cables/connectors/accessories. I can try and make discrete attenuators.Yeah I read about the coax turning an impulse into a step, related to it's length. I want to see that effect with coax, I've barely gotten into transmission lines and lumped vs distributed, this and that.For the APG, I haven't tried different output cap sizes yet on the breadboard. And I'll add another BJT between the high voltage and the diff-amp, so I can use larger resistors, and use way less current.I have copper clad boards, and ring-pad proto-boards. I also have SMD parts of a decent quality. Most all my through whole parts are from ebay/etc. (Someday I'll have to learn to design and order SMD PCB's)I'd probably cut my fingers off trying to cut traces on the copper clad board, for the SMD stuff. IDK if people glue little islands on top, like a piece of some old PCB, and solder the SMD stuff between them.I've made a few things with SMD parts on the ring-pad proto-boards. But I can't remember how they compared to the through-hole versions. Or if just soldering that over a GND plane helps.I do have some BNC terminals, for PCBs, and a bit of coax. Trying to attach anything like that properly, to copper-clad and proto-boards, without making a mess, will be hard. Trying to solder or bond to shielding, is usually a nightmare too.

author: tggzzz
author_rank: Super Contributor
timestamp: March 16, 2025, 09:37:10 am
content: IMHO impulses are only of value in limited circumstances.What you see on a scope is the convolution of the input signal and the scope's response. If you haven't characterised the input signal, you can't use what you see on the scope to determine the scopes response. In practical terms that means a higher shorter input pulse could appear the same as a lower longer input pulse.That problem is avoided if a step is used. Practically that means the step should be long enough that the scope's transient response has died away. Hence a 10MHz scope requires a longer step than a 100MHz scope.You may find the construction details inhttps://electrooptical.net/News/a-high-performance-time-domain-reflectometer/helpful, particularly the "Receive Pulse Generator". The previous article washttps://electrooptical.net/News/150-ps-tdr-for-under-2/"A 150-Picosecond Time Domain Reflectometer for Under $2"

author: jonpaul
author_rank: Super Contributor
timestamp: March 17, 2025, 11:21:54 am
content: TGZZZAll the pulsers do not make an "impulse" which is in theory zero duration, infinite height.Typically they make pulses or square waves with fast edges eg 30 pS at a defined PRF, eg 10 MHz.We use the Leo Bodnar pulses all the time to check and adjust scope vericla amps and plugins.The fast clean edges reveal the transient abberations and allow adjustmet at various times after the edge starts.The spectra and transient overshoot and rise are well soecified.Jon

author: tggzzz
author_rank: Super Contributor
timestamp: March 17, 2025, 11:34:59 am
content: Quote from: jonpaul on March 17, 2025, 11:21:54 amNot sure what you are replying to; please use the "quote" button.If a pulse's duration is much less than the UUT's response time, then it is effectively an impulse. Hence in many circumstances an avalanche pulser without an additional long stub transmission line does create good approximations to impulses.Similarly, if the pulse's duration is much longer than the UUT's response time, it is effectively a step generator.Quote30 pico Siemens (pS)? I think you mean 30 pico seconds (ps)For the purposes you mention, Bodnar's excellent devices are effectively step generators, not pulse generators not impulse generators.

author: MathWizard
author_rank: Super Contributor
timestamp: March 17, 2025, 03:51:11 pm
content: Besides my signal gen, or stuff inside my computer or scopes, I don't really have any fast circuits or parts I guess. Yeah I tried my JFET-probe, I think it's more like a 100-MHz probe, so it didn't look that great. That was with 2N5485's, I have some better SMD JFET's to try too. But I don't think my BJT's are fast enough. I've been looking at the DIY 500-1000 MHz probes on here.I'm making a Tektronix P6202 in LTSpice, but IDK the names of any fast BJT's of JFET's , I should look up more UHF ones. My JFET probe used BC550/560, they aren't consider fast, just low noise. There's plenty more capacitance's in the BJT models, than the 2 or 3 I use tho. I own some S9018's , they are probably the only "fast" BJT's I own. But in LTspice, they didn't seem and better or different, in the JFET probe.Yeah I want to built some more advanced test circuits, like the TDR circuits, and fast as possible circuits. And voltage and current references.

author: TimFox
author_rank: Super Contributor
timestamp: March 17, 2025, 04:01:01 pm
content: Can anyone recommend a good reference for avalanche operation of BJTs for these applications?

author: tggzzz
author_rank: Super Contributor
timestamp: March 17, 2025, 04:02:32 pm
content: Quote from: MathWizard on March 17, 2025, 03:51:11 pmHere's a simple predictable circuit with a clean 2.5V <300ps risetime step into 50ohms.https://www.eevblog.com/forum/testgear/show-us-your-square-wave/msg1902941/#msg1902941Getting basic functionality is easy, but getting a good transition requires thought and care.

author: tggzzz
author_rank: Super Contributor
timestamp: March 17, 2025, 04:03:30 pm
content: Quote from: TimFox on March 17, 2025, 04:01:01 pmPossible starting point: Zetex has transistors with specified avalanche behaviour.

author: TimFox
author_rank: Super Contributor
timestamp: March 17, 2025, 04:11:34 pm
content: Quote from: tggzzz on March 17, 2025, 04:03:30 pmThank you. A reprint of a Zetex app note:https://user-web.icecube.wisc.edu/~kitamura/NK/Flasher_Board/Useful/research/AN8.pdfThe app note includes references to IEEE and Rev Sci Inst journal articles.

author: MathWizard
author_rank: Super Contributor
timestamp: March 17, 2025, 04:46:25 pm
content: Thanks I need to try some pulser circuits with my CD4000 and whatever 74series IC's I have, like hex Schmitt trigger inverter's. I guess I never really tried to make any of them go as fast as possible. I did buy some really fast +1GHz op-amps too, IDK what to use them for yet, plenty of lower frequency theory to learn first.Yeah there's a bunch of little projects I see on here, some of them I want to make a version with whatever I have. And then one of these days I need to copy all good, special part names, and order them all at once, and make better versions. It's a fun way to learn about it all.