Cheap Fast Rise Time Oscillator - Page 1
Forum section: electronics / testgear

author: BillyO
author_rank: Super Contributor
timestamp: January 07, 2023, 01:53:05 am
content: The boards for the latest prototype are in and I built up a couple for testing.I got some good results! The last prototype (mentioned in this thread:https://www.eevblog.com/forum/testgear/best-brand-name-for-an-oscilloscope/msg4554020/#msg4554020) was reading about 2.15ns on my SDS2504XP. The latest one reads 1.47ns. Given that the scope has a rise time of about 680ps that would give the oscillator a rise time of about 1.3ns actual.The rise time is dependent on the choice of 74AC14 too. 1.47ns is from the slowest of the two I built. The fastest was 1.37ns or about 1.2ns actual. Unfortunately I don't have a faster scope readily available to get closer to actual rise time. I have a few ideas for the next (and possibly final) version such as using a 4-layer board, slightly shorter trace to the BNC and using micro-USB for power, but I don't expect these will speed it up much. The 74AC technology is just not going to wiggle much faster.

author: tggzzz
author_rank: Super Contributor
timestamp: January 07, 2023, 09:51:56 am
content: If you want <300ps, have a look athttps://www.eevblog.com/forum/testgear/show-us-your-square-wave/msg1902941/#msg1902941

author: BillyO
author_rank: Super Contributor
timestamp: January 07, 2023, 04:56:42 pm
content: Nice! I might try that next using one inverter for a relaxation oscillator. You could build it for about $3!

author: BillyO
author_rank: Super Contributor
timestamp: February 02, 2023, 11:12:37 pm
content: Boards for the latest version are in. Below in purple. Highlights of this version:Smaller boardShorter output traceOnly 4 inverters on output to reduce PD skewCrystal replaced by relaxation oscillator to reduce size and cost to build4-layer board with 2 full ground planes and a full Vcc planeWill build one up tomorrow to test.The 2nd device (black) is a TDR version in early stages. Waiting on the proper parts to properly test the design so far.

author: magic
author_rank: Super Contributor
timestamp: February 03, 2023, 07:15:18 am
content: You will never get good results with DIP or even SO package. Too long pins, too long traces, too much inductance in series with the output, too much inductance in series with the bypass cap. Guaranteed severe overshoot and ringing, easily visible even on 100MHz scopes.For the sort of results shown by tggzzz, SOT-23 (or smaller) package and low inductance layout are essential.And LVC logic appears to be faster than AC. There may or may not be differences between manufacturers too.But nothing is as important as small package and physical construction details.My own attempt, never tested on "serious" gear but you can see its 200MHz performance by scrolling a few posts up:https://www.eevblog.com/forum/projects/compact-74ac14-pulse-generator-pcb/msg3475680/#msg3475680It's an open question whether this or tggzzz's single-gate chips on a pair of power/ground planes would be bettereditAnd by the way, the chips you are using are not TI but Chinese mystery meat, just so you know.

author: tggzzz
author_rank: Super Contributor
timestamp: February 03, 2023, 11:07:17 am
content: Quote from: magic on February 03, 2023, 07:15:18 amJust for fun, my board also has 3*74LVC without series resistors. The overshoot is impressive (+7.5V peak on a 350MHz scope!) - and useless(1V/div, 1ns/div, 50ohms load and 1Mohm//15pF load)QuoteYes indeed.If I ever respin the board, I'll probably opt for 0306 wide caps (not 0603) and will consider a 4-layer PCB with layers 1 and 2 being Vnd and Vcc planes. That will get the planes closer together than 1.6mm of FR4, which will improve the distributed capacitance and decoupling.But I don't have access to time domain equipment that could tell the difference; I only have 21GHz spectrum analysersQuoteYesI suspect the MLCC's capacitancewith Vcc across themwill be at least as significant. For those that don't know, different flavours of MLCC dielectric can "lose" 80-90% of the capacitance when they have the rated voltage across them.

author: BillyO
author_rank: Super Contributor
timestamp: February 03, 2023, 04:06:40 pm
content: Quote from: magic on February 03, 2023, 07:15:18 am?Why the hostility?I got the chips from Digi Key. I didn't know they were selling junk these days.So, it seems my results so far are a bit better than what you got with your SO effort. 1.2ns and minor ringing (voltage wise) that is done in less than 5ns. We'll see what the latest one does when I get around to testing it. Unfortunately I just discovered I don't have the right resistor value, so will have to wait for my next order.In any case my intent here is not to get the fattest possible rise time. Leo Bodnar already did a good job of that. My intent is to design something that anyone can build up for next to nothing that will check out the rise time on their cheap scope or probes or whatever. I'm really just having some fun passing time in little lab, not building the next missile guidance system.What I do see often is people using a too narrow and too long output trace. In this new version here is the bottom layout. The bottom layer is the only layer with traces. I'm also keeping the inductance and resistance low on the supply side low by using two full ground planes a full Vcc plane and a peripheral copper pour on the bottom layer connected to Vcc. I've also taken some effort to make the traces feeding the drive inverters the same length. Not sure if that will help much though.

author: tggzzz
author_rank: Super Contributor
timestamp: February 03, 2023, 04:26:59 pm
content: Quote from: BillyO on February 03, 2023, 04:06:40 pmIt is good to have fun, and that's a worthwhile objective.When measuring risetimes, it is also good to avoid overshoot in the signal. Sometimes the scope setup procedures require/allow a little overshoot in order to get the best useful risetime. Having ringing in the signal will affect those setup procedures, of course

author: BillyO
author_rank: Super Contributor
timestamp: February 03, 2023, 04:50:27 pm
content: Quote from: tggzzz on February 03, 2023, 04:26:59 pmAbsolutely. I take the internal risetime measurement with a grain of salt when looking at less than ideal waveforms (which is most of the time). When I want to be accurate I use cursors.

author: magic
author_rank: Super Contributor
timestamp: February 03, 2023, 05:12:36 pm
content: Quote from: BillyO on February 03, 2023, 04:06:40 pmSo switch to MouserCan you name the manufacturer of the 74AC14 shown in your fist post? Maybe you recognize that logo?And the DIP8 chip on your TDR board also looks like typical fake junk from China.Not necessarily saying such chips can't work, but they may work differently (either better or worse) than specified in original datasheets.Quote from: BillyO on February 03, 2023, 04:06:40 pmSorry, I didn't look at your scope waveforms before saying "never". They look surprisingly good and I have no explanation for thatMy SO14 build was crap with lousy resistors and not so great ground routing. But I have seen many other attempts with DIP/SO and they never looked much better.In particular, rfmerrill's first board (the thread I linked) was almost identical to yours.Quote from: BillyO on February 03, 2023, 04:06:40 pmThe right resistor value is 180~190Ω, depending on RDS(on) of the chip's internal FETs.

author: tggzzz
author_rank: Super Contributor
timestamp: February 03, 2023, 05:26:06 pm
content: Quote from: BillyO on February 03, 2023, 04:50:27 pmHow you measure the displayed waveform doesn't matter. What matters is whatcausesthe displayed waveform to be the way it is.To make the point with a ludicrous example... If you look at the waveforms I added to my earlier post, and use the unterminated waveform with overshoot to twiddle your scope's risetime so that it is nice and sharp+flat, then when you observe a decent edge it would be rather distorted and slower than expected.

author: tggzzz
author_rank: Super Contributor
timestamp: February 03, 2023, 05:31:31 pm
content: Quote from: magic on February 03, 2023, 05:12:36 pm7ohms was about right for a 74LVC1G output. Hence I used a 143ohms resistor to make it up to 150ohms.

author: BillyO
author_rank: Super Contributor
timestamp: February 03, 2023, 07:41:55 pm
content: Well, being an impatient man, I went ahead and built one up anyway using the 220R resistors I have. Yeah, the logo on the 74AC14 looks a little sloppy. I bought a large batch of IC's from Digi Key back in 2010. These, I think, were part of the batch. I don't recall getting any others, but then 13 years is a long time. Look at the last picture though. It shows the logos on the ICs on the TDR version. The '14 and the '02 look like proper TI logos. The 555 did come from China. It's only a 555 though, so meh.However, on my scope with a measured risetime of 660 ps, this version did 1.27 ns. Taking the scope RT in account that's a risetime of about 1.09ns! Not too bad. Yeah, but I'd not use a $5 oscillator to tweak my scope's front end.

author: tautech
author_rank: Super Contributor
timestamp: February 03, 2023, 08:20:44 pm
content: Good work Bill as you can't get much simpler than that !Send me one to look at with this 2 GHz SDS6204A.I had something very similar laid out in Altium and expected ~3ns RT but had a mission trying to do a toner transfer with a new printer (Yep, for some things I'm tight as !) so I gave up on it and ordered a Bodnar pulser that only took ~10 days to arrive in NZ from UK.

author: BillyO
author_rank: Super Contributor
timestamp: February 03, 2023, 09:15:14 pm
content: Quote from: tautech on February 03, 2023, 08:20:44 pmThanks and sure. I'll send you a board from the final version. I have a couple of tweaks in mind.Quote from: tautech on February 03, 2023, 08:20:44 pmMine took a month.

author: jonpaul
author_rank: Super Contributor
timestamp: February 03, 2023, 09:59:03 pm
content: Bravo for the DIY Pulser. Please compare to Leo Bodnar 40ps pulser, BNC and 30 pS SMA versionswe use a 20 db wideband attenuator to reduce abberation due to VSWR.Besides digital scopes, a 500 MHz or 1 ghz CRT analog scope photo is usefulWe use Tektronix scope 7904, 7104 mainframe with 7A29 and 7B10 plug-ins.Bon SoiréeJon

author: BillyO
author_rank: Super Contributor
timestamp: February 03, 2023, 10:49:31 pm
content: Quote from: jonpaul on February 03, 2023, 09:59:03 pmThanks!I'd love a 7104. There is actually one for sale for $800 CDN about 2 hours away from me right now with options 02 & 03 and 7A19, 7B92 and 7B15 plug ins. A nice starter kit!(I must stay strong!)

author: Bud
author_rank: Super Contributor
timestamp: February 03, 2023, 10:50:58 pm
content: Quote from: BillyO on February 03, 2023, 07:41:55 pmIf you still have the Digikey bag they came in, the manufacturer name should be there.

author: magic
author_rank: Super Contributor
timestamp: February 03, 2023, 10:57:17 pm
content: Quote from: BillyO on February 03, 2023, 07:41:55 pmThis version looks more "ringy" than the first one, though.Nevertheless, it's still very good performance for DIP14 package.Maybe it's the Chinese chip being better than original AC logic.One pattern I have seen in knockoff ICs is that they are often die shrinks on more advanced processes; apparently it saves them money somehow.Quote from: Bud on February 03, 2023, 10:50:58 pmMy question about manufacturer was absolutely rhetorical. There is no Western manufacturer with this logo and it isn't TI logo either.It's a vaguely TI-like blob intended to be different enough from TI that it's hard to prove trademark infringement.Common practice in China. There is a new thread with some fake jellybean IC every other months here.This chip probably (hopefully) didn't come from DigiKey, and 13 years is a lot of time indeed

author: Bud
author_rank: Super Contributor
timestamp: February 03, 2023, 11:03:48 pm
content: Actually the logo looks familiar, but can't remember who it was. Given the long time when it was purchased, they may no longer be in business. And 13 years ago counterfeit chips were much less of a problem, i would not think Digikey had a reason to source such chips back then.

author: blackdog
author_rank: Frequent Contributor
timestamp: February 03, 2023, 11:23:03 pm
content: Hi,I tested and few years ago different cmos logic ICs for a nice fast pulse with few abberations.I never managed to get it really nice due to such things as the length of the bonding wire in the IC and the various decoupling capacitors.But the SO enclosure you still get and somewhat better result and think about your PCB pathways, make the output 50 Ohm with multiple resistors of different outputs, e.g. 4x a 200 Ohm resistor in parallel.In the end I bought the Leo Bodnar pulser.PSAnd just like jonpaul pointed out, use an attenuator 10 a 20dB to get the SWR as good as possible.Kind regards,Bram

author: mawyatt
author_rank: Super Contributor
timestamp: February 03, 2023, 11:48:00 pm
content: Quote from: magic on February 03, 2023, 10:57:17 pmRecurring chip cost to the Original Chip Manufacture (OCM) is directly related to the chip area, smaller chip lower cost. Think many generic chips are now fabbed in smaller feature size processes which yields smaller chips, wether the OCM offers this at a lower cost depends on the chip, market, technology and so on but their recurring cost is lower because of the smaller chip area. Of course these chips also benefit from a faster process which may benefit the end user!!Best,

author: BillyO
author_rank: Super Contributor
timestamp: February 04, 2023, 12:36:40 am
content: Quote from: jonpaul on February 03, 2023, 09:59:03 pmI don't have the SMA version, just the BNC version.The Bodnar is the lower trace. Twice as fast as mine and less ringing.

author: tggzzz
author_rank: Super Contributor
timestamp: February 04, 2023, 11:17:24 am
content: Quote from: BillyO on February 04, 2023, 12:36:40 amThe amplitude of the overshoot is pretty similar.The frequency of your "oscillation" ismuchlower; as best as I can see, the peak-to-trough is 0.3div vs 1.1div.Since it is the same scope, I think that points to your construction and components. "Excess" inductance or capacitancesomewherewould, of course, lower the frequency; 10* capacitance or 10* inductance would be necessary to see that. I doubt the capacitance has changed that much, so my bet is on the inductance. That would be consistent with long leads between chip, PCB and decoupling capacitors.

author: magic
author_rank: Super Contributor
timestamp: February 04, 2023, 12:09:15 pm
content: Quote from: Bud on February 03, 2023, 11:03:48 pmOf course it does. It's the logo of WanHungLo Semiconductor ShenzhenFound an example:https://www.eevblog.com/forum/projects/watch-out-for-fake-74ls-logic-icsThey do analog too:https://www.eevblog.com/forum/beginners/lm741-testing/msg4466656/#msg4466656All DIP8 chipshereare also fakes.Quote from: blackdog on February 03, 2023, 11:23:03 pmThat's why I always advocate for using the smallest available package and most compact layout whenever this topic recurs.The mikrocontroller.net thread clearly shows what's possible with a single LVC gate in SOT23.Those things aren't any more expensive or harder to get, so why bother with DIP?

author: jonpaul
author_rank: Super Contributor
timestamp: February 04, 2023, 12:48:51 pm
content: Bonjour, topic seems off a bit this is a fast rise pulse generator, an oscillator is normaly a sinne wave output.See this related EEVBLOG thread re probes, many tests and shots of fast rise pulsers, DIY, Leo B with various scopes and probes.https://www.eevblog.com/forum/testgear/probe-into-probes-whats-up/50/The Leo Bodnar 40 pS BNC pulse gen is low cost, high developed and very fine performance. Was ~~ L 50http://www.leobodnar.com/shop/index.php?main_page=product_info&products_id=295Our Leo B has give fine service for many years.Here are my 2019 Tektronix 7104/&A29/LeoB 40 pS shots with Mini Ckts 20 db 50 Ohm atten.After selection of 7A29 and transient response adjustment both 7A29 and 7104, perhaps the best I can do.Your thoughts appreciated!Lower pix BNC inline atten and 50 Ohm term collection.Vive l'ANALOG! Vive TEKTRONIX! !Jon

author: BillyO
author_rank: Super Contributor
timestamp: February 05, 2023, 04:53:15 am
content: So I maybe not following you 100%, but what I'm getting is that the LB pulser gave you a 10ns rise time when directly connected, but a .79 ns when connected via a 20 db attenuator. Is that correct?Just a bit of confusion.In my experience, the rise time improves with increased current in the load.

author: jonpaul
author_rank: Super Contributor
timestamp: February 05, 2023, 12:58:28 pm
content: Rebonjour, BillyO:1/ Leo Bodnar 40 pS is designed to drive a 50 Ohm termination or 50 Ohm scope input.There is no spec for other Zo, so current is the same with and without the attenuator, and load not the issue.See the spec sheet of the LB 40 pS.2/ A constant impedance atten is a resistive T or Pi network designed to attenuate with same Zo input/output eg 50 Ohms.See spec sheet of Mini Circuits HAT-20 wideband 50 Ohm attenuator.See Tee /Pi attenuatorThus the load on the LeoB pulser is 50 Ohms with or sans the HAT-20 attenuator.3/ Purpose of the attenuator is to the reduce reflected energy from scope t pulser due to VSWR or Zo mismatch.Thus pulse aberrations due to reflections from scope to pulser are reduced and the test aberration is reduced.Pix: Leo B USB utility , allows Vo adjust 50 mV>>1.5 V set at 1.0V P-P into 50 Ohm.Yokogawa DL7440 500 MHz BW, FS = equv 100 G/sec, sweep at 1 nS/divUpper trace LeoB40 direct to scope 50 Ohm inputLower trace LeoB40>>Mini Ckts HAT-20 atten>scope 50 Ohm inputMany previous old EEVBLOG threads on similar like "show us your squarewave" TD pulser, Avalanche pulser, etc.Hope all is now clear!Jon

author: David Hess
author_rank: Super Contributor
timestamp: February 05, 2023, 10:43:36 pm
content: One way to get a cleaner pulse is to use the output of the logic gate to drive the emitter/source of a common base/gate transistor with then has a pull-up or pull-down on its collector/drain to produce the output. Then on the rising or falling edge, the transistor "disconnects" the gate from the output and only the pull-up or pull-down in parallel with a very small capacitance is connected to the transmission line. This essentially duplicates how an ECL output from a differential pair works.Some designs may use a microwave diode in place of the transistor.

author: magic
author_rank: Super Contributor
timestamp: February 06, 2023, 09:21:34 am
content: Do you expect that ordinary 3904 could be good enough to compete with "bare" LVC logic in terms of rise/fall time?Or are we talking some special RF transistors?

author: mawyatt
author_rank: Super Contributor
timestamp: February 06, 2023, 03:29:24 pm
content: The 2N3904 would be too slow for this app, a better solution would be a high Ft SiGe HBT like the BFR840, BFP620, BFU725, and others. These inexpensive SiGe devices aren't designed nor specified for pulse use (they are RF devices), however should yield very fast pulse response if applied properly.Best,

author: BillyO
author_rank: Super Contributor
timestamp: February 06, 2023, 03:38:06 pm
content: Quote from: mawyatt on February 06, 2023, 03:29:24 pmThis then begins to defeat the design criteria of cheap and simple by adding cost and complexity.

author: mawyatt
author_rank: Super Contributor
timestamp: February 06, 2023, 03:51:18 pm
content: Quote from: BillyO on February 06, 2023, 03:38:06 pmThese are about $0.50 HBTsBest,

author: mawyatt
author_rank: Super Contributor
timestamp: February 06, 2023, 03:58:10 pm
content: These inexpensive SiGe HBTs brings up a question IF they would perform well in an avalanche mode for fast pulse edge production?Anyone given this a try?Best,

author: David Hess
author_rank: Super Contributor
timestamp: February 06, 2023, 09:58:17 pm
content: Quote from: magic on February 06, 2023, 09:21:34 amNo, a 2N3904 is too slow.QuoteNothing exotic is required and RF transistors can be inexpensive.