What are those spikes after transients? - Page 1
Forum section: electronics / beginners

author: uliano
author_rank: Regular Contributor
timestamp: September 24, 2021, 11:04:51 am
content: Hello,I've returned to this hobby after maybe 20 years. Today I was playing with an UART and I wanted to test my new scope decode ability (fantastic!). I noticed those overshoots after digital transition and I was wondering if they are due to the breadboard arrangement (or breadboard itself).Is my guess correct or should I worry for something else? Will they disappear in a reasonably routed PCB?

author: rs20
author_rank: Super Contributor
timestamp: September 24, 2021, 11:11:50 am
content: I'm sure a more thorough answer will follow, so just a few basic points:That could be real overshoot/ringing in the circuit, or it could be improperly compensated probes (looks more likely to be improperly compensated probes)Even if this is a real effect, it's not a problem at all. That's a very clean waveform, the receiving system will have no trouble at all understanding it. You need not be so worried about the corners of your nominally square signals being so perfectly square, and whether it carries over to your PCBdoesn't matter!!But it's still good to learn and understand, of course.Try out

author: uliano
author_rank: Regular Contributor
timestamp: September 24, 2021, 11:27:10 am
content: I don't think it is the probe compensation, here I touched nothing since beforeMy worries are for voltages over 5V and under 0V, can they cause problems to components?

author: Marco
author_rank: Super Contributor
timestamp: September 24, 2021, 12:00:30 pm
content: Quote from: uliano on September 24, 2021, 11:27:10 amIt's one minute to test.

author: uliano
author_rank: Regular Contributor
timestamp: September 24, 2021, 12:24:22 pm
content: Yes, I did it in the previous post. I didn't touch the cap since the UART probing and the 1kHz square wave was as reported in the picture (good to me)

author: timenutgoblin
author_rank: Regular Contributor
timestamp: September 24, 2021, 12:29:32 pm
content: Quote from: uliano on September 24, 2021, 11:04:51 amMaybe the ground lead of your probe is causing the problem due to the stray inductance in the short lead picking up EMI? Perhaps you need a tip barrel style ground lead. Do you have one?Source: e2e.ti.com (TI E2E™ design support forums)

author: uliano
author_rank: Regular Contributor
timestamp: September 24, 2021, 12:44:16 pm
content: You are on the right track!No, I don't have one, yet!I tired to arrange some surrogate and it got definitely better!maybe the remaining overshoots are mostly due to the flying connection (orange wire to FTDI chip in the firs post)?

author: uliano
author_rank: Regular Contributor
timestamp: September 24, 2021, 01:00:35 pm
content: Actually I have the tip, I just didn't knew what it was (it was in the probe accessory bag).Also, removing the orange flying wire didn't improve anymore.

author: rs20
author_rank: Super Contributor
timestamp: September 24, 2021, 01:41:52 pm
content: Quote from: uliano on September 24, 2021, 11:27:10 amEven if the ringing/overshoot was real, it is basically impossible (for non-contrived layouts at least) for any damage to occur due to this effect.

author: Terry Bites
author_rank: Super Contributor
timestamp: September 24, 2021, 04:17:12 pm
content: As long as you meet the logic thresholds in the circuit that follows, the minor over/ undershoot won't be a problem. I imagine its an inadequtely bypased or overly indcutive power rail. Or as pointed out a poorly adjusted scope probe.

author: zburmeister
author_rank: Newbie
timestamp: September 24, 2021, 11:20:23 pm
content: My first thought was what was discussed above being an improperly tuned probe. But assuming that is taken care of...Between your solderless breadboard interconnects and lengthy connecting wires, there is a lot of stray inductance that will rear its ugly head in the form of voltage spikes on fast current transients. Remember voltage across an inductor (or long connecting wires in this case) is equal to L*di/dt. And at the edge rates you are showing in your scope capture, the overshoots could be partially caused by di/dt transients. I would suggest reviewing the path of the UART connections and try to shortening your wires. This will help decrease your return path distance and therefore reduce the amount of loop inductance.Edit: Just a side note, I've had issues in the past with that specific type of wire you are using. I believe the tip that goes into the board is just crimped onto the wire. I've spent lots of time debugging circuits to find that it was a bad wire. Definitely won't touch those again

author: David Hess
author_rank: Super Contributor
timestamp: September 25, 2021, 12:02:55 am
content: The spikes are caused by wiring inductance, including the probe's ground lead, and a common problem with breadboard construction.I think that microseconds long settling time anomaly is caused by reflections between the fast switching load and power supply. A local bulk decoupling capacitor should remove it.

author: tggzzz
author_rank: Super Contributor
timestamp: September 25, 2021, 08:50:11 am
content: Quote from: uliano on September 24, 2021, 11:04:51 amWelcome "back". Nothing much has changed in the past 20 years, except components have got smaller and cheaper. (Exception: nanopower, ADC/DAC performance)The overshoots are due to a combination of the inductance of wires and solderless breadboard and probing technique.Ifthe transients are real (not a measurement artifact) then since they are outside the supply rails they could damage components.The basic theory and practice of digital signal probing technique:https://entertaininghacks.wordpress.com/2015/04/23/scope-probe-accessory-improves-signal-fidelity/https://entertaininghacks.wordpress.com/2016/09/17/scope-probe-accessory-higher-frequency-results/Why a digital signals' bit rate is usually unimportant, and what does matter:https://entertaininghacks.wordpress.com/2018/05/08/digital-signal-integrity-and-bandwidth-signals-risetime-is-important-period-is-irrelevant/Why solderless breadboards are the work of rhe devil, and better cheaper faster alternatives:https://entertaininghacks.wordpress.com/2020/07/22/prototyping-circuits-easy-cheap-fast-reliable-techniques/

author: rs20
author_rank: Super Contributor
timestamp: September 25, 2021, 12:54:15 pm
content: Quote from: tggzzz on September 25, 2021, 08:50:11 amI call 'citation needed' on this claim, or at least that this claim is relevant in any remotely normal/common situation. For a start, most ICs can tolerate voltages up to 0.7V beyond the rails just fine, and even then there needs to be enough 'oomph' (i.e. current/charge) behind those spikes to burn out the parasitic diodes in the IC for actual damage to occur. With that in mind, I think it'd be quite a task tofindan IC that could be damaged by the signal showed in the OP's oscilloscope shot?

author: Microdoser
author_rank: Frequent Contributor
timestamp: September 25, 2021, 12:58:14 pm
content: Quote from: uliano on September 24, 2021, 11:04:51 amThat was my first thought, seeing the scope

author: Doctorandus_P
author_rank: Super Contributor
timestamp: September 25, 2021, 01:14:24 pm
content: The duration of the "spikes" is in the micro seconds or less, and probe compensation works in the milli seconds range, so that is definately not the cause.Usually it's stray inductance and the long GND lead with clip on the scope is the culprit, and why the short spring type clip gives much better results.There is very little energy in those measured spikes, and they're caused by the stray inductance somewhere between the breadboard and the scope, as was already mentioned. This also means that those spikes are not even present on the breadboard as long as you do not touch it with your oscilloscope.Those small spring type clips are vital for accurate measurements, but a nuisance to work with.I almost always put the 20MHz bandwidth limit on my oscilloscope on, just because it filters out most of the noise I do not care about. Especially when working with breadboards and digital circuits. Usually I also extend the oscilloscope probe with a "dupont wire" that goes to the breadboard. This makes the probing even less accurate, but it does make it easier to probe. My probes are usually laying flat on the table, with both the GND wire clipped to the breadboard and a dupont wire going elsewhere on the breadboard.Usually it's also possible to directly put the scope probe in the breadboard, but this has a risk of the tip of the probe breaking off during accidents, for example when a book falls on your breadboard or the scope probe is pulled by it's wire because it's snatched behind something.This probing is adequate for 95% of the things I do with my scope.It is important to realize when you are getting into the 5% where more accurate probing is needed.

author: uliano
author_rank: Regular Contributor
timestamp: September 25, 2021, 03:34:58 pm
content: Quote from: tggzzz on September 25, 2021, 08:50:11 amWell, cheap scopes have gotten way better! I resumed my previous and, as I suspected, I can't see anything on it.Quote from: Doctorandus_PActually much less than thatQuote from: Doctorandus_PSeems wise to me.Thanks everybody for all contributions, I'm really appreciating the feedback!I may (or may not, is it worth the learning?) try to reduce the stray inductance by removing the breadboard and solder a minimalist connections directly to an IC socket, or should I do directly on pins> (I've the feeling that they're quite fragile)

author: David Hess
author_rank: Super Contributor
timestamp: September 25, 2021, 07:50:19 pm
content: Quote from: rs20 on September 25, 2021, 12:54:15 pmAs you point out, the ESD protection structures connected to supply and ground will protect against these spikes however this depends on the low impedance power supply decoupling absorbing the spikes, and if the decoupling is of poor quality with large values of series inductance as in this case, protection will be compromized.The danger of exceeding the absolute maximum supply voltage or input voltage for *any* duration is real. In extreme cases CMOS logic can outright fail but even worse is when it becomes unreliable and fails in some obscure way after an indeterminate time. I have seen this happen with 5 and 3.3 volt CMOS logic and 16 volt analog CMOS. I had a 12 volt audio power amplifier which used 15 volt CMOS logic with this problem and ended up adding separate supply regulation at the operational amplifier to keep it from failing.Check out Jim Williams' discussion on this subject in connection with the supply voltage for Flash memory programming on pages 3 and 14 of Linear Technology application note 31.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 25, 2021, 07:57:03 pm
content: The problem is not breadboarding per se, but it must be used responsibly.Not terribly, terribly much has changed in CMOS since the 90s; the main difference being, bog-standard logic is indeed faster. You might've been used to 74/LS TTL, or CD4000 CMOS, neither of which is prone to signal quality issues. (The latter basically at all, ever; the former can be, but is often tolerant of poor conditions.)Whereas today, the average 3.3V CMOS pin driver, is somewhere between 74HC and LVC families, in terms of speed and drive strength. Edges of 1-3ns and source impedance of 30-100 ohms is typical. You don't need fancy MCUs to get there: you can see signal quality issues on a breadboard, even with a boring old 5V ATMEGA328 (Arduino, etc.) running at 20MHz.So you might well have spent time with these sorts of circuits back then, and never noticed these effects. Or you simply don't remember -- and honestly I can't remember a whole lot from that long ago, nothing wrong with that.As for probing, avoid ground-return length. The signal is likely nice and square at the device itself. Try probing between its GND and output pins. And BTW, it's not supply bounce (poor bypassing), notice it's symmetrical -- if it were that, it should only be doing it on the top. But in any case, make sure there's a bypass cap at the device, between nearest VDD/GND pins; if need be, span it over the chip, since you can get away with moves like that on the breadboard.The same goes for anything else that connects to the device. If local switching currents (from each respective chip) are well contained, then you only have to deal with pin/wire currents, and those can be limited with series resistance (preferably 33 ohms or more, as close to the driving pin as possible -- if a shared bus, this applies to every driver!). Or for longer wires/jumpers, a ferrite bead perhaps (same idea, resistance at high frequency).The supply rails on a breadboard, are pretty thin and narrow, they're easily driven to some voltage at high frequencies -- 100s of mV of ground bounce is easily produced by a chip. Keep distances short (between VDD/GND, use adjacent pairs not opposite, around a row of ICs), try to avoid lots of signals switching all at once (parallel buses with fast edge rates are killer; many a breadboarded computer, even just with TTL or NMOS (e.g. 6502, Z80, etc.), had random glitches, likely due to this). Which is nice again these days, as serial buses (I2C, SPI) are predominant, parallel buses only being needed for legacy interface, or the highest bandwidths (memory, display drivers, etc.).Or it can still simply be scope/probe effect. There's often a high frequency compensation adjustment, and maybe it's out of whack, or the probe is mismatched to this scope. (This is probably more of an analog scope issue, and I don't think DSOs usually have this as a problem; but equally so, I don't think they usually have an adjustment for it. I guess that should put the blame on the probe -- maybe try one recommended by the scope manufacturer?) I guess a reference source would be needed to confirm. As for the TDS210, they're pretty slow and chunky, you might simply not be seeing it, whether at this zoom (gotta get a close-up of the edge) or in general (60MHz BW?).Tim

author: tggzzz
author_rank: Super Contributor
timestamp: September 25, 2021, 08:04:36 pm
content: Quote from: rs20 on September 25, 2021, 12:54:15 pmI'm afraid it isn't as simple as you imagine. There are many ways an IC can be damaged, and the damage isn't necessarily immediately apparent nor obvious. Typically higher performance and smaller devices are more susceptable to parametric shifts (including complete failure).Apart from that, please re-read what I actually wrote, paying particular attention to all the caveats.

author: sicco
author_rank: Regular Contributor
timestamp: September 25, 2021, 08:29:55 pm
content: I guess that 20 years ago your scope was not sampling 1 Gs/s. The spikes likely disappear after you disconnect the ribbon cable or other wires that are connected to the digital output. It’s the transmission line (waveguide) effect. Because the other end of the long wire is not terminated with the waveguide characteristic impedance, the steep edges (they are much steeper than 20 years ago, mostly due to Moore’s law) in the signal will bounce back at the other end of the wire, and then the voltage doubles for a short moment at the edge source.Assume pulse propagation time is speed of light, so 300000 km/s. That’s 30 cm in 1 ns.Zoom in on the overshoots (DC coupled, level trigger). Zoom as far until 1 pixel = 1 sample = 1 ns.Count the nanoseconds. Now you should be able (roughly) to tell how long the wire connected to the output is.Nothing to do with scope probe adjusting or power supply decoupling caps. Those caps still needed though.

author: David Hess
author_rank: Super Contributor
timestamp: September 26, 2021, 05:31:10 am
content: Quote from: T3sl4co1l on September 25, 2021, 07:57:03 pmI have a whole collection of 10uF aluminum electrolytic and 1uF solid tantalum capacitors in parallel with 0.1uF ceramic or film capacitors for insertion directly above ICs placed in breadboards.Quote from: sicco on September 25, 2021, 08:29:55 pmThe spikes and probe are not fast enough to require 1 Gsample/second sampling rates, and even more than 30 years ago DSOs were plenty fast to work in this application if they supported peak detection.

author: tggzzz
author_rank: Super Contributor
timestamp: September 26, 2021, 08:03:10 am
content: Quote from: sicco on September 25, 2021, 08:29:55 pmThe sampling rate is completely irrelevant; the important specification is the front end's analogue bandwidth.Example: 30 years ago I was using a high-end HP digitising scope. Its sampling rate was 25MS/s, but I could see <1ns risetimes since the bandwidth was 1GHz.Example: I have a sampling scope with ~4GHz bandwidth which can see risetimes of ~100ps. It has a sampling rate of ~40kS/s.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 26, 2021, 09:47:46 am
content: More specifically the equivalent sample rate, whatever that might be. Like my old TDS460 has a 100MSps rate but reports up to I think 50GSps at the highest sweep (however, it fills in DREADFULLY slowly, by random sampling).Tim

author: Doctorandus_P
author_rank: Super Contributor
timestamp: September 26, 2021, 10:40:43 am
content: There is a phenomena called "latch up".In short this means there are parasitic thyristors (SCR) built into semiconductors, and these can get triggered by I/O signals which are outside of the power supply rails. This was a real problem during the early days of semiconductors. These SCR's can not be eliminated completely, but they're made as in-sensitive as possible by doing some tricks with the doping.When this thyristor is triggered it forms a short between the power supply rails and the chip starts heating up, possibly until the magic smoke is released. The only way to stop the current though this thyristor is to turn the power supply off.I'm suspecting that the Chinese clone of the MAX7219 (often sold with 8 * 8 dot matrix display) is very sensitive to this. It quite often starts up in a state that it turns on a lot of LED's, grows quite hot and does not respond to commands.I have some memory that you have to pull more then 30mA out of an I/O pin to trigger this condition, and it's unlikely this would be very relevant here, but the phenomena is real and I thought it's interesting enough to mention it here.https://en.wikipedia.org/wiki/Latch-uphttps://html.duckduckgo.com/html?q=cmos+latchup

author: tggzzz
author_rank: Super Contributor
timestamp: September 26, 2021, 10:49:34 am
content: Quote from: T3sl4co1l on September 26, 2021, 09:47:46 am... and is still only a 400MHz scope

author: Jwillis
author_rank: Super Contributor
timestamp: September 26, 2021, 07:10:37 pm
content: I'm not an expert on this by any means . Only just begone to delve into this area of electronics . By the micro chip label I'm guessing It's a AVR128 series chip .This looks a lot like a bread board technique issue . Connections to the chip are to long with minimal decoupling . Decoupling and filtering needs to be a close to the chip as possible . There are a lot of very long wire connections that create a lot of parasitic inductance . Found this out with similar devices . Any supply decoupling needs to be as close to the chip pins as possible to reduce current looping.General Guidelineshttp://ww1.microchip.com/downloads/en/DeviceDoc/40002183A.pdfPage 20 Part 4http://ww1.microchip.com/downloads/en/appnotes/atmel-1619-emc-design-considerations_applicationnote_avr040.pdfhttp://ww1.microchip.com/downloads/en/AppNotes/AN2519-AVR-Microcontroller-Hardware-Design-Considerations-00002519B.pdfJust my take on this .I'm learning to so if anyone could chime in I would appreciate the input .

author: StillTrying
author_rank: Super Contributor
timestamp: September 26, 2021, 09:37:46 pm
content: Quote from: tggzzz on September 26, 2021, 10:49:34 amThe 100MHz CML+'s ETS does up to 50 GSa/s per channel, 250 samples per division, there's no difference between between dots and sinx.BW is around 150MHz in ETS mode.

author: David Hess
author_rank: Super Contributor
timestamp: September 27, 2021, 03:41:00 am
content: With a probe configured like that even 100 MHz performance is questionable so with a 3.5 nanosecond transition time, sample rates beyond say 300 Msamples/second do not have much meaning, and in practice much lower sample rates are completely sufficient.Before I learned better, I saw waveforms like that with breadboard construction and a 100 MHz analog oscilloscope but a 100 MHz DSO with 10 nanosecond peak detection from a 100 Msample/second sample rate (Tektronix 2232) or even 20 nanoseconds and 50 Msamples/second would work just fine, and without the need for ETS at 2 Gsamples/second or higher.I think the sweat spot is 200 MHz bandwidth with a suitable sample rate because that is just at the limit where high impedance passive probes are still viable and probing is not too difficult, although spring clips or coaxial connections are still needed to get full performance.Does that waveform matter? It does not in a prototyping environment, and even in a production environment it would be unlikely to cause problems, but I would not accept it considering that two or three decoupling capacitors would fix it. I have seen worse in real products.

author: BrianHG
author_rank: Super Contributor
timestamp: September 27, 2021, 05:08:33 am
content: If you want the absolute minimum transient:Solder a 0.1uf ceramic cap right onto the driving IC's VCC & GND, then place the IC into the breadboard with the output pin bent out not going into the breadboard.On your probe's GND spring contract, press it into the same hole as the IC's GND pin and touch the IC's output you wish to measure.

author: uliano
author_rank: Regular Contributor
timestamp: September 27, 2021, 05:18:44 am
content: Quote from: Jwillis on September 26, 2021, 07:10:37 pmI rearranged the breadboard using much shorter wire for any "signal" connection (here just UPDI & UART) while "DC" connections (supply rails, pushbutton and LED) have been left as they were.I added 100n across each supply injection into the chip and also across FTDI breakboard (which is the supply source for the board).I could have shortened some *but not all* supply connections, instead I went along with the (possibly very wrong) thinking "it's just DC".All in all nothing (significant) changed since the coil barrel GND probing has been introduced some post above.

author: tggzzz
author_rank: Super Contributor
timestamp: September 27, 2021, 07:44:19 am
content: Quote from: uliano on September 27, 2021, 05:18:44 amYou need to understand that a wire is not a wire: it is a component which can be modelled as series resistance, series inductance, and parallel capacitance to other wires. The implication is that there is no such thing as "ground" where everything is at the same voltage. In this case the most important element will be the series inductance.So, what is thetotallength of copper in the loop between probe signal tip to pin output to pin power supply to probe shield? That will determine the series inductance. It is useful that you included a decent photo, since it appears that there may be a long blue wire in that loop.The best probing technique is to have the probe's tip and coil barrel shield connected to the IC pins. The best ground is a solid copper plane. Both of those can (with care) be achieved using "manhattan" prototype techniques, but are more or less impossible with solderless breadboards.

author: uliano
author_rank: Regular Contributor
timestamp: September 27, 2021, 10:07:08 am
content: Quote from: tggzzz on September 27, 2021, 07:44:19 amThere are many "points" that can be assumed as ground: Earth, AC plug ground, PC PS supply ground, the - coming out from the serial breakboard, the blue line at a side of the breadboard, the (two) pins connected to gnd of the MCU...To which ground should I minimize the path from coil tip? To the ground of the chip? To the "digital" one (the one that is in the upper middle in the picture) as the output pin is in the "digital" domain?QuoteI'm reaching awareness that I'seeing a probing epiphenomenon, and won't certainly step up the prototyping technique for a simple MCU circuit.

author: sicco
author_rank: Regular Contributor
timestamp: September 27, 2021, 10:54:24 am
content: uliano,You’ve still not disclosed to us how long the wires are… Share a picture of the entire setup. So a picture that shows also where the other end of your grey ribbon cable sits.Share a scope screenshot also of the same setup after disconnecting all of those grey ribbon cable signal wires. As in no longer physically connected to the digital outputs that you are looking at.Your scope likely has more than one channel. Measure with the second channel what your chip GND (VSS) pin looks like. Same bandwidth, same V/DIV, same everything. GND probe clip on the same spot as your first channel.If you have more scope channels, then do the same for the VDD pin of the chip.

author: uliano
author_rank: Regular Contributor
timestamp: September 27, 2021, 01:42:11 pm
content: the gray ribbon is the UPDI connection with the ICE, maybe 15 cm longhttps://www.microchip.com/en-us/development-tool/ATATMEL-ICEwhich is connected to an USB hub toghether with the USB cable coming from the red breakboard with the FTDI chip.

author: tggzzz
author_rank: Super Contributor
timestamp: September 27, 2021, 02:43:06 pm
content: Quote from: uliano on September 27, 2021, 10:07:08 amNone of which will be at the same potential. Even the earth won't be at the same potential as the earth: consider the case where someone is standing with feet apart on the ground and there is a nearby lightning strike. There can be sufficient potential difference that current flows up one leg and down the other!QuoteWhichever one matters for that part of the circuitFor outputs you consider the current flow, so in this case it wll be the digital 0V associated with the output.For inputs you consider the points at which the receiver will interpret the analogue voltage as a digital signal. For "simple" signals that will be the digital 0V associated with the input. For differential signals it will be the difference between the two inputs, but you can't put a scope's shield on either of them and still have to put the shield on the digital 0V associated with the input. ignoring using currents to convey digital signalsQuoteIt probably is a probing problem, but are you sureAll that matters is what the other components in the circuit "see" and how they "interpret" it

author: uliano
author_rank: Regular Contributor
timestamp: September 27, 2021, 03:59:00 pm
content: Quote from: tggzzz on September 27, 2021, 02:43:06 pmI know, I was just making fun of myself.QuoteI tried to shorten the wire to ground but, alas, to no availThe wire is coming out directly from the digital gnd.

author: Jwillis
author_rank: Super Contributor
timestamp: September 27, 2021, 04:23:11 pm
content: Just a novice idea.Did you configure the I/O pin output drive strength of the FTDI ? Disable the high drive level on the UART and CBUS I/O pins . High drive currents can induce over and under shoot . Also try adding a resistor between the FTDI and AVR128 .

author: uliano
author_rank: Regular Contributor
timestamp: September 27, 2021, 05:25:53 pm
content: The pin I'm probing comes out of the MCU and goes into the FTDI not the other way around.I gave a quick look at the datasheet and, if I didn't miss anything, for output pins I can only configure a slew rate limit (of a whole port).I tried inserting a resistor (I had no clue so I randomly choose 47 Ohm, if you have your preferred value I can test) between MCU and FTDI and again nothing really changes: 13%overshoot 18% undershoot.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 28, 2021, 03:41:35 am
content: Heh, looks worse, wonder if that's because of common mode. Hard to say of course as these are nanosecond things, it's hardly a pixel at this scale.Tim

author: uliano
author_rank: Regular Contributor
timestamp: September 28, 2021, 04:32:45 am
content: Quote from: T3sl4co1l on September 28, 2021, 03:41:35 amsorry, I didn't post the zoomed-in view. Worse here means half a microsecond!

author: tggzzz
author_rank: Super Contributor
timestamp: September 28, 2021, 08:10:02 am
content: Quote from: uliano on September 28, 2021, 04:32:45 amLooks like ~10MHz/100ns "ringing", which is a relatively long period.If you have a clean ~1ns risetime source, it would be interesting to see what that shows.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 28, 2021, 08:39:41 am
content: Most likely stuff with that kind of time constant, is couple-meter wires from PS to board, scope probe, uh USB if this thing is connected, etc. I'd expect bypass caps to be slower, and immediate signal quality things to be faster (~ns, and probably not visible on the scope at all, or at least very well).It also keeps ringing for a while, which suggests a high Q, low loss -- if it's bouncing between cables and boxes, it's doing so for a fairly long time. In that case, a ferrite bead somewhere will definitely cut down on the duration, though not so much the peak amplitude right after the edge (except if it's CM up the probe, in which case it may).Tim

author: uliano
author_rank: Regular Contributor
timestamp: September 28, 2021, 08:54:51 am
content: Look also at reply #16 where i didn't pull out the wire gnd directly from the pin but from breadboard rail.There the time constant was quite shorter.

author: iMo
author_rank: Super Contributor
timestamp: September 28, 2021, 09:01:34 am
content: Do not expect perfect edges with your setup.You work with a breadboard with 3.5pF parasitics.Your blocking capacitors are foil - not suitable for decoupling in your case - you have to use ceramics instead.Also mind 10mm of a straight wire is 10nH.Considering that your ringing seems to be normal..PS: foil capacitors are good for audio or "low" frequency apps.For decoupling of fast transients in digital circuits you want capacitors with low parasitic inductance - the best are smd ceramic capacitors (like 10nF, 100nF), or through-hole one..

author: sicco
author_rank: Regular Contributor
timestamp: September 28, 2021, 09:14:22 am
content: Is it an ordinary probe, 10x, 10 Mohm (1M after the scope’s BNC, 9M in the probe), and a long coax transmission line between scope and probe head?If yes, then the transmission line (the probe’s coax) has an open end (1Mohm is not what the coax characteristic impedance can be…). Whatever was propagating from probe to scope would bounce back from scope to probe. And in the probe that 9M is another open end, so all that bounced at the scope again bounces back. And that’s what you’re seeing here quite likely. It’s not really ringing as in an LC circuit. It’s echoing, it’s standing waves.Do your spikes disappear when you use a 1x probe, and switch to 50 ohms input for the scope channel? (add a small C in series because your chip output probably collapses when DC loading with 50 ohms)Does the ‘ringing’ period change if you add a BNC extension between probe BNC and scope BNC?To confirm, connect the same probe and scope to a perfect square wave generator (50 ohms source, terminated, probe at the termination 50 ohms). Do you still see ~10% amplitude ‘ringing’?see alsohttps://phet.colorado.edu/en/simulations/wave-on-a-stringand give a unit step with the wrench. Wrench is your probe input. Other end of the string is selectable (fixed, free, no end) which translates to, short circuit, 1M or 50 ohms termination at your scope’s BNC input.

author: perieanuo
author_rank: Frequent Contributor
timestamp: September 28, 2021, 09:45:37 am
content: those spikes measured with the barrel mod are small enough for that spider pcb, with careful routing you're fine

author: sicco
author_rank: Regular Contributor
timestamp: September 28, 2021, 09:45:58 am
content: http://jahonen.kapsi.fi/Electronics/DIY%201k%20probe/on how to get a better probe.

author: tggzzz
author_rank: Super Contributor
timestamp: September 28, 2021, 10:03:53 am
content: Quote from: sicco on September 28, 2021, 09:14:22 amYou don't understand how 10* high impedance probes work! It isn't an ordinary transmission line, but is a very lossy transmission line for that reason. Having said that, if the probe is excessively badly manufactured, I suppose they may have skimped on that part.(In addition, a 1* probe is inappropriate for this)For further details, see the references I gave earlier in this thread.

author: sicco
author_rank: Regular Contributor
timestamp: September 28, 2021, 12:27:48 pm
content: Ah, tggzzz has alternative facts…So what then is the characteristic impedance of your ‘very lossy transmission line’ in your probe cables? 1 Mohm??

author: uliano
author_rank: Regular Contributor
timestamp: September 28, 2021, 12:36:39 pm
content: Thanks everybody, lots of stimulating info here!I'm waiting for external 50 Ohm (chinese) terminators as cheap scopes are 1M only. Then I would very much try the "alternative probe"

author: David Hess
author_rank: Super Contributor
timestamp: September 28, 2021, 12:52:10 pm
content: Quote from: imo on September 28, 2021, 09:01:34 amIf those are SMF (stacked metal film) capacitors, then they are fine for decoupling. The construction and layout on a breadboard dominates anyway.

author: tggzzz
author_rank: Super Contributor
timestamp: September 28, 2021, 01:54:14 pm
content: Quote from: sicco on September 28, 2021, 12:27:48 pmThe central conductor's resistance is around 300ohms/m.Don't believe me, cut open a probe and measure it with a multimeter.Basically your understanding is based on, to repeat your phrase, alternative facts.Don't believe me, but do believe Tek, p14 ofhttp://www.davmar.org/TE/TekConcepts/TekProbeCircuits.pdfThis a good reference for other classes of probes, too.

author: uliano
author_rank: Regular Contributor
timestamp: September 28, 2021, 02:58:54 pm
content: Maybe it was not probing, after all!Same probing. (also, no caps at all!)Different outcome. (with a 100 Mhz scope and a 150 Mhz probe I guess I can't see anything steeper than this)

author: sicco
author_rank: Regular Contributor
timestamp: September 28, 2021, 03:39:19 pm
content: Quote from: tggzzz on September 28, 2021, 01:54:14 pmYes, great Tek manual from the old days. The resistance of the coax center wire is there in decent probes and helps in certain scenarios. But it cannot prevent the reflections at the two impedance jumps (the two ends) in the transmission line. Assuming the coax is still 50 ohms characteristic impedance (or maybe it's 20 or maybe 200 ohms or whatever - but it will not be a Z of kohms or Mohms, and the resistance of the center wire is not to be confused with characteristic impedance!), then at both ends, seen from a wave travelling within the cable, a wave that's about to hit the impedance jump, the refection back is still huge. Reflection coefficient is (Zload-Zcable)/(Zload+Zcable). Only way to get that to zero is when Zcable = Zload. There are no cables with a Z of 1 Mohm...Assuming A is the probe and B is the scope's BNC, 1 Mohm input, at say one or two meters displaced from A, one cannot claim that the signal goes beautifully, unchanged, from A to B, with nothing bouncing back into the same cable at B. What bounced at B will propagate back to A (if the wire resistance didn't kill it on the way out, then why would it kill it on the way back?), and there again an impedance jump, so again a reflection back into the cable. Plain physics.Only matched impedance at B would do the trick. The Finnish DIY probe does approach that. The Tek Z0 probes with 50 ohms cables into a 50 ohms scope input do that. But no probe into a 1 Mohm input can do that, no matter how thin you make the center wire in the coax cable.

author: sicco
author_rank: Regular Contributor
timestamp: September 28, 2021, 04:17:13 pm
content: Quote from: uliano on September 28, 2021, 02:58:54 pmIs that the same ic as signal source as before? It doesn’t look like it is. Different ic = different output drive characteristic = different rise and fall times in the transients.

author: uliano
author_rank: Regular Contributor
timestamp: September 28, 2021, 04:53:59 pm
content: No, not the same, but I didnt'expect so much difference!they're both recent 8 bit avr MCU

author: tggzzz
author_rank: Super Contributor
timestamp: September 28, 2021, 05:37:34 pm
content: Quote from: sicco on September 28, 2021, 03:39:19 pmI'm impressed that you know more than Tek engineers, and look forward to seeing your explanation for howanystandard *10 "high" impedance probes caneverwork at up to 500MHz.Start by explaining these measured results (400MHz scope, ~300ps source risetime, 250MHz probe)...FFI:https://entertaininghacks.wordpress.com/2016/09/17/scope-probe-accessory-higher-frequency-results/