Mechanical vs. Avalanche Pulse Generator - Page 1
Forum section: electronics / projects

author: Andie
author_rank: Contributor
timestamp: August 28, 2016, 11:15:03 am
content: Hello everyone,I enjoyed reading these discussions of pulse generators:https://www.eevblog.com/forum/blog/eevblog-306-jim-williams-pulse-generator/https://www.eevblog.com/forum/projects/transmission-line-avalanche-pulse-generator/In these threads, some readers suggested mechanical solutions for pulse generation. Unfortunately I couldn't find reports about how that approach performs. Some time ago, I made a generator with adjustable pulse width (adjustable by attaching different lengths of 50 Ohm coax as the charge line) from one of several mercury wetted reed relays, which I had lying around. With this post, I want to show results and tell why I finally switched to building an avalanche pulse generator.Mercury Wetted Reed Relay Pulse GeneratorMercury wetted reed relay pulser, schematic.The following picture shows the relay pulse generator at work. A charge line of 0.5m results in a 5ns pulse.Mercury wetted reed relay pulser,1ns/div, 500mV/div, HP54100A (1GHz), 50 Ohm input.The incredible HP54100A with a bandwidth of 1GHz measured a rise time of 1.2ns. But I was not quite happy with this solution. The uncontrolled impedance of the reed switch and the wiring diminished signal quality. It was obvious that I should have put more effort into the setup. Disassembling the relay and separating the reed switch, optimizing the wiring and then using a suitable coil would have helped. But there were further issues with this reed switch. The pulse looked like it suffered from randomly occurring slight impedance mismatch. This varied with even minor tilting of the switch around its prescribed orientation. Other relays of this type behaved the same. While disassembling one relay I found out that the contacts weren't like in an ordinary reed switch where two springy parts move towards each other, but they were three parts with the moving one held in place only by the surface tension of the mercury. Perhaps this moving and in fact loose part is dangling around? The switch did not bounce, but it seemed, that the moving contacts cause variations in on-resistance, despite of being embedded in mercury. Reducing the coil current during the movement of the contacts in order to limit their speed before they finally touch, improved that behaviour, ... but I would have liked something more reliable.So I discarded the idea of using a mechanical switch and decided to build a triggerable avalanche pulser with adjustable pulse width, based on Linear Technology AN47 by Jim Williams. See page 93 ofhttp://cds.linear.com/docs/en/application-note/an47fa.pdfAvalanche Pulse GeneratorInitially I used a 2N2369 as avalanche transistor, which delivered a rise time of 150...200ps, depending on the actually used transistor. Then I tried other RF transistors which were in stock at a local store: BFW425W, 2SC3355, BFP196E, BFP420, BFG541. The best results regarding rise time were achieved using BFG541. Rise time is about 75ps. However, pulse shape was best with 2N2369. With BFG541 the rising edge looks good, but the falling edge is distorted.Avalanche pulser with BFG541, schematic.Detail of pulse generator: Transistor, attenuator (R14, R15, R18)and high voltage supply (R9) are directly mounted to SMA connectors.The following two pictures show the output of the BFG541 equipped generator with 1ns/div and 50ps/div. Charge line length is 50cm. Rise time is about 75ps.Avalanche pulser with BFG541,1ns/div, 100mV/div, DS800 (8GHz), 20dB attenuator, 50 Ohm input.There is some ringing visible, which is the result of an attempted correction of "dribble-up" before knowing what it is. For "dribble-up", see the below section "Trying to adjust the pulse shape".Avalanche pulser with BFG541,50ps/div, 100mV/div, DS800 (8GHz), 20dB attenuator, 50 Ohm input.Same configuration as above. Rise time is about 75ps.Measurement EquipmentMeasuring these short times was possible by using a DS800 8GHz sampling oscilloscope from IBZ Electronics. The following picture shows the DS800 scope and the pulse generator. The three cables attached to the generator are trigger output, pulse output and the charge line. In this photo the length of the charge line is 4cm, which results in a pulse width of 400ps.DS800 oscilloscope and pulse generator.Trying to adjust the pulse shapeWhat was common to all variations of my charge line pulsers, no matter whether relay- or transistor-driven, was the ramping up of the pulse soon after the rising edge. Jay_Diddy_B also encountered this while building his pulse generator and he identified it as "dribble-up". See his posthttps://www.eevblog.com/forum/projects/transmission-line-avalanche-pulse-generator/msg606142/?topicseen#msg606142.In the following picture I've marked a flat portion (region A) and the "dribble-up" (region B).Pulse top,500ps/div.As long as the charge line is short enough, the pulse consists only of the flat portion (region A). The longer the charge line gets, the more of the "dribble-up" (region B) becomes part of the pulse. This distorts automated measurement of rise times, which are measured shorter, when the pulse width is low and are measured longer, when the pulse is wide and the pulse therefore includes "dribble-up". The initial rising slope itself doesn't change significantly with different lengths of the charge line.Before I read more about "dribble-up", I thought that there is something wrong with the pulse generator. The first idea was to put a variable capacitor in parallel to the charge line. This produces an initial peak, extending the rising edge to an adjustable amplitude. This peak makes automated measurements more consistent. However, this doesn't cure "dribble-up", so I removed the variable capacitor. Lacking a clever plan, I tried putting stray capacitances in parallel to the charge line (short wire(s) sticking in the air). Somehow, this lifted the entire flat portion (region A) a little up, partially covering the "dribble-up". However, it added ringing. At least it seems that the averaged pulse top represents the true amplitude.The following two pictures show pulse tops before and after attempted correction.Pulse top, original,500ps/divPulse top, after attempted correction,500ps/divRegarding "dribble-up" I found this:Tektronix Type 284 Pulse Generator. For "dribble-up", see page 2-12.http://bama.edebris.com/download/tek/284/Tek%20284_v6.pdfAEA Technology AN204http://help.aeatech.com/helpdesk/attachments/6005556252Maxim Integrated Application Note 5141http://pdfserv.maximintegrated.com/en/an/AN5141.pdfPicosecond Pulse Labs Application Note AN-3ahttps://kh6htv.files.wordpress.com/2015/11/an-03a-pulse-meas.pdf

author: EmmanuelFaure
author_rank: Regular Contributor
timestamp: August 28, 2016, 12:06:23 pm
content: Nice work, thanksAre you satisfied by your DS800 oscilloscope? Seems to be a nice piece of equipment for the price.

author: David Hess
author_rank: Super Contributor
timestamp: August 28, 2016, 09:02:11 pm
content: TheTektronix 109 Pulse Generatorused a mercury wetted relay to achieve a 250ps edge with a charge line.The chief problems with this concept are the low repetition rate and short operating life. For good high frequency performance, the reed switch needs to be part of a transmission line environment.

author: Andie
author_rank: Contributor
timestamp: August 28, 2016, 09:06:23 pm
content: Quote from: EmmanuelFaure on August 28, 2016, 12:06:23 pmThe hardware is amazing and it works as intended; that's why I am satisfied. The software however, has some room for improvement.

author: KE5FX
author_rank: Super Contributor
timestamp: August 28, 2016, 09:36:10 pm
content: Nice! I wouldn't be surprised if what you're actually measuring is the response of those attenuators and RG174 cables.What are some of the areas of improvement you'd like to see in the sampling-scope software?

author: tggzzz
author_rank: Super Contributor
timestamp: August 28, 2016, 09:50:38 pm
content: Have you looked at tunnel diode step generators? Even a 1976 Tek 1502 manages a 50ps 200mV step. The service manual gives design details, or seehttp://w140.com/tekwiki/wiki/1502

author: Andie
author_rank: Contributor
timestamp: August 28, 2016, 10:11:48 pm
content: Quote from: David Hess on August 28, 2016, 09:02:11 pmThank you for the link to TekWiki!I was aware that Tektronix did this in their Type 109 Pulse Generator. If my switch had been more reliable, I surely would have continued with it - including the attempt to create a 50Ohm evironment for it.

author: David Hess
author_rank: Super Contributor
timestamp: August 28, 2016, 10:36:11 pm
content: Quote from: Andie on August 28, 2016, 10:11:48 pmSo it does not matter now but take a look at what Tektronix did with the reed relays in the7T11 sampling timebasewhich can operate above 10 GHz:http://w140.com/tekwiki/images/4/43/7t11-trigger.JPGhttp://w140.com/tekwiki/images/9/97/Tek-7t11-trigger-back.jpghttp://w140.com/tekwiki/images/5/5a/7t11-cr28.JPGhttp://w140.com/tekwiki/images/c/c2/7t11a-cr28.JPG

author: Andie
author_rank: Contributor
timestamp: August 28, 2016, 11:02:04 pm
content: Quote from: KE5FX on August 28, 2016, 09:36:10 pmI also worried about cables and attenuators. Attenuators are Minicircuits DC...12GHz. Not knowing how far I can get with that project, I used inexpensive and easy to find cables.

author: Andie
author_rank: Contributor
timestamp: August 29, 2016, 01:15:20 am
content: Quote from: tggzzz on August 28, 2016, 09:50:38 pmI did. I was scared of the touchiness of tunnel diodes and their poor availability. Without a doubt it would have needed more than one diode to understand how to use it.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: August 29, 2016, 01:32:30 am
content: Quote from: Andie on August 29, 2016, 01:15:20 amhttp://www.ebay.ca/sch/i.html?_odkw=ds800&_osacat=12576&_from=R40&_trksid=p2045573.m570.l1313.TR12.TRC2.A0.H0.Xtunnel+diode.TRS0&_nkw=tunnel+diode&_sacat=12576164 listingsOr,http://www.ebay.ca/itm/2x-2D524A-Russian-Microwave-Step-Recovery-Diode-100GHz-/191939884210?hash=item2cb08234b2:m:mxyPAGeISZYRHzhbFMkZeVg

author: David Hess
author_rank: Super Contributor
timestamp: August 29, 2016, 01:49:23 am
content: I have avoided tunnel diodes for the same reason. Availability and reliability is questionable.What about step recovery diodes and the use of a non-linear transmission line for sharpening the edge of a pulse? Varactor diodes can be qualified for use as step recovery diodes.

author: Berni
author_rank: Super Contributor
timestamp: August 29, 2016, 06:30:34 am
content: Interesting stuff. I had a go at picosecond pulses before and i found that its actually a difficult task getting under 100ps with common components that one might have laying around.When using a relay perhaps what is more important is the parasitics of the conductor inside the relay. At these speeds an extra 1cm of bare wire can make a significant difference in the signal. Perhaps RF relays that are designed to have a certain characteristic impedance might be something to try?

author: Andie
author_rank: Contributor
timestamp: August 29, 2016, 11:14:30 am
content: Quote from: Alex Eisenhut on August 29, 2016, 01:32:30 amThank you. Quite a lot stuff to experiment with. Probably some time I'll try it.Quote from: David Hess on August 29, 2016, 01:49:23 amI remember having read about non-linear transmission lines in a Picosecond Pulse Labs application note. Being not unexperienced in electronics, I'm new to RF topics. During this pulse generator project I learned incredibly lot (from my point of view). But there is still much more to do. Hopefully some time I will be able to use the mentioned components with myself equipped with a deeper understanding. Thank you for the suggestions!

author: grouchobyte
author_rank: Regular Contributor
timestamp: August 29, 2016, 11:47:26 am
content: Unfortunately, I am on a train right now on my way to Ottawa and cant send you much more than what I have in my dropbox folder. I have lots of experience with avalanche pulsers for lasers and electro-acoustic modulators for Brillioun scattering sytems and metrology instruments, ets. In the 1990's Jim Williams and I collaboarted on his famous pulser ( Its just a rehash of a bell labs experiment 20 years prior)Here is a very clean pulser I did for a client in 2004 that puts out 4-5 volts into 50 ohms with sub 500 ps edges. There is a 20db attenuator at the sample head input so the screen voltage readout is 1/10 of the actual output. You can achieve sub 50 ps edges if the swing is lower.( dv/dt) It does not need an avalanche transistor, lumped delay lines or an SRD. ..... Just fast PECL, an optional sige discrete and some off the shelf components. You can achieve textbook results with attention to detail on the signal integrity,....load, impedance matching s11, s12, decoupling, etc. Rather simple actually....https://www.dropbox.com/s/qplkwsvnbdw2d3s/pulser%20measurements.pdf?dl=0@grouchobyte

author: Marco
author_rank: Super Contributor
timestamp: August 29, 2016, 04:48:02 pm
content: Really the only point of using avalanche at this point is power density and high voltages.

author: Andie
author_rank: Contributor
timestamp: August 29, 2016, 06:46:48 pm
content: Quote from: grouchobyte on August 29, 2016, 11:47:26 amThank you for the oscilloscope screenshots. The signals look remarkable clean.A similar solution (assumed) I considered already, since I have two HMC674LP3E (10 GHz latched comparators) lying around. I needed one of these in order to repair the DS800 oscilloscope of which I fried one channel recently.These comparators are the channel input stages of that oscilloscope. Luckily I managed to replace that part (16 lead 3x3 mm SMT package).The output of the HMC674LP3E drives 400mV into 50 Ohm.You mentioned "4-5 volts into 50 ohms". Sounds interesting. Would you mind posting some schematic?

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: August 29, 2016, 11:40:23 pm
content: You can marvel at thishttp://w140.com/tekwiki/wiki/284or thishttp://w140.com/tekwiki/wiki/S-52

author: David Hess
author_rank: Super Contributor
timestamp: August 30, 2016, 05:21:05 am
content: The wire connection on the S-52 to the tunnel diode is deliberately thin to raise inductance. The wire used in the 7T11 tunnel diode trigger circuit is so thin, it looks like spider silk.I think it would be pretty easy to mount one of those Russian tunnel diodes in a transmission line environment made from brass strip (a coplanar waveguide would be the easiest to construct) but I never pursued it for two reasons, availability and suitability. Most of the available Russian tunnel diodes have a high ratio of capacitance to current which I discovered looking for the ones which can be used as replacements in tunnel diode trigger circuits.One trick Tektronix used for high speed packaging was to bring the bases and collectors out on two leads each; that places them in the middle of a transmission line instead of the end. Unfortunately, no such parts are made today. CML (common mode logic or emitter/source coupled logic) sometimes effectively does this with an internal termination in place of the extra lead.

author: grouchobyte
author_rank: Regular Contributor
timestamp: August 31, 2016, 07:03:41 am
content: Quote from: Andie on August 29, 2016, 06:46:48 pm@AndieI can share some design details but I wont post them to the forum. PM me if you are interested and I will dig out the schematic.