Does a capacitor charges smooth, or in stairs? - Page 1
Forum section: electronics / general

author: RoGeorge
author_rank: Super Contributor
timestamp: June 06, 2020, 10:46:59 am
content: Does a capacitor (dis)charges as a smooth exponential, or in stair steps?

author: greenpossum
author_rank: Frequent Contributor
timestamp: June 06, 2020, 10:47:54 am
content: It takes the elevator.Actually it sometimes even goes backwards when you are not measuring it, but as soon as you do, it goes back to normal. Schrodinger discovered these capacitors.

author: RoGeorge
author_rank: Super Contributor
timestamp: June 06, 2020, 11:43:55 am
content: Quote from: greenpossum on June 06, 2020, 10:47:54 amNope, it always takes the stairs.Seriously, it's always in small stair steps, overall resembling the well known (and seen on the casual oscilloscope) exponential shape (and it is stair steps not because of charge quantization, or other quantum effect, it's because of something else entirely). Any guess?

author: Nominal Animal
author_rank: Super Contributor
timestamp: June 06, 2020, 11:52:08 am
content: Charge is carried in units of electron charge, yes. But, in a capacitor, their location with respect to the dielectric affects the electrical potential in the capacitor, and above Planck scale, location is continuous; so the potential across a capacitor even with just a single charge carrier pair is continuous.

author: RoGeorge
author_rank: Super Contributor
timestamp: June 06, 2020, 12:31:17 pm
content: I just said, the stair steps I'm talking are not about quantum physics.As a hint, the small stair-steps has much to do with the geometry of the capacitor.

author: RoGeorge
author_rank: Super Contributor
timestamp: June 06, 2020, 01:10:09 pm
content: Some other topic I was answering today (about transmission lines) reminded me about this curiosity I learned: "capacitors always charge/discharge in stair-steps, NOT smoothly"!It was years ago, and I never thought the video lectures are still on YouTube, but here it is, found it, the great channel of prof. Greg Durgin:https://www.youtube.com/user/profdurgin/videosand the video that tackles the current question is, the answer is stated at minute 43:00 (capacitor problem started somewhere around minute 29:50, and if that doesn't makes much sense, then the whole 5 previous lessons from the TDT playlist need to be watched (from youtube.com/watch?v=7Oz1sazpekM&list=PLULkq8AIoZ7zDTvbaWW9RvXR8Ngnpe0s8).On short, it's stair-steps and not smooth, because the capacitor's plates are also a transmission line, as shown at minute 43:00.

author: Nominal Animal
author_rank: Super Contributor
timestamp: June 06, 2020, 02:29:25 pm
content: Quote from: RoGeorge on June 06, 2020, 01:10:09 pmHe says that a capacitor in a circuit discharges in steps (and by logical extension, charges also in steps), because the circuit is and behaves like a transmission line.Makes sense to me, but that looks at the entire situation at a specific scale, which I don't like; it makes this a trick question. It is not an intrinsic property of capacitors, but a property of capacitors in a practical circuit.If your power source is say a chemical or nuclear battery next to the capacitor, the effective circuit length is neglible (twice the dielectric thickness or possibly less), and the capacitor charges "smoothly".What about the case where the capacitor is discharged not via a single line, but via a million strands of copper of different lengths? Each strand is a separate transmission line, so each step is spread out into a million substeps.What about the case where the capacitor is discharged via a wide flat curved conductor, so that the transmission line length varies continuously between minimum and maximum length? (This happens, because most of the charge carriers in the conductor "travel" on or near the surface of the conductor.) The steps are spread out into a continuous change. This, in fact, can be experimentally verified; you just need a large copper sheet to act as the shorting conductor.

author: RoGeorge
author_rank: Super Contributor
timestamp: June 06, 2020, 03:49:04 pm
content: What he's talking there is about the capacitor's plates themselves. The two plates are also a transmission line, therefore the plates themselves produce those stair steps.The capacitors do charge and discharge in stair steps, and not smoothly, and as the poll shows, only very few of us are aware of the phenomenon. Also, doesn't matter much for the day to day electronics.But yes, my question was maliciously crafted to bamboozle.

author: SiliconWizard
author_rank: Super Contributor
timestamp: June 06, 2020, 03:58:26 pm
content: That was an interesting advanced topic, of course related to the fact we're talking about real, physical capacitors here, and not ideal ones.Thanks for pointing it out - some food for thought.And yes of course it probably doesn't matter in most use cases - outside maybe very high frequency stuff.Would be interesting to actually *show* the phenomenon with relatively "basic" lab equipment, if it is at all possible.

author: Labrat101
author_rank: Unknown
timestamp: June 06, 2020, 04:15:36 pm
content: So why are capacitors used for Smoothing .??If they were doing all this quantum stuff that would make a perfectly good power supply noise.Also the stepping is seen on a Digital scope .. Not on analog .. Maybe because you are seeing digitization ..A supper capacitor charges very fast on the graph show almost a 65deg line over time .the staircase would produce clicking . from any CapacitorTheory and Practice are 2 different animalsLong Live Mains HumAll that stuff was done on a blackboard .. Lets see it . In real life .. As a Real Working Model .. & Not with fake chineeese caps.PS If I got that form on my scope I would send it back for re calibration

author: Nominal Animal
author_rank: Super Contributor
timestamp: June 06, 2020, 04:42:17 pm
content: Quote from: RoGeorge on June 06, 2020, 03:49:04 pmFor a long skinny capacitor (a contrived example, he says himself). For a short wide capacitor, or indeed a capacitor formed from circular discs, you get funky wave interactions, and no clear steps. That is because there is no single reflected wave. Similarly if the electrical connection is not a single point at one end, but multiple points with different path lengths.If you consider something like current ceramic capacitors that are stacks of plates separated by dielectric ceramic, connected at one end, they really don't match that long skinny capacitor model.Something like a big bulk capacitance at the end of a long pair of wires, though; that should be measurable.Quote from: RoGeorge on June 06, 2020, 03:49:04 pmAs I said, I believe that only applies to certain circuits, and/or certain shapes of capacitors. I don't like the claim.Quote from: RoGeorge on June 06, 2020, 03:49:04 pmPower transmission line engineers might disagree. I seem to vaguely recall seeing something about this in that context, but can't be sure.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 06, 2020, 05:02:00 pm
content: Neither: the voltage is a continuous function of time and, for the pure capacitance of a component, obeys I = C dV/dt.Current is unspecified, so an exponential charge or discharge is certainly not assured. Nor is the stepwise motion of an ideal transmission line driven by an ideal step.It's an interesting exercise to perform this measurement yourself: how slow can the step be, how lossy can the transmission line be, before the apparently stepwise motion ceases and ordinary exponential charge/discharge (assuming a resistive source) is apparent?Therein lies the deeper truth: we use ideal models for near-ideal situations, and when one situation ceases to be near that ideal, we typically switch to another; a mixed model is therefore necessary to explain the overall behavior.FWIW, most of my capacitors are fully end connected rolls of metallized film, so exhibit no appreciable transmission line behavior. If they did, it would be with Zo << 1 ohm probably, and T < 100ps. The leads (T ~ 100ps, Zo ~ 100 ohms) clearly dominate, and it looks like a lumped equivalent -- hence the three-element series equivalent traditionally used for capacitors.I think TL behavior is sensible (if insignificant in intended applications) in some film-oil caps, where the connection is usually made with a pair of ribbons to the middle of the roll. Zo is still quite low because the roll is wide, but the electrical length is modest, so that assuming you can get a reasonable step through those ribbons (which again have a high Zo), you can probably observe a small ripple as it rings down.Tim

author: Labrat101
author_rank: Unknown
timestamp: June 06, 2020, 05:23:53 pm
content: I Have seen his videos and Only once did he use a scope CT100 . the picture was on purpose out of focus.Sorry the Theory maybe correct, but in Practice No.I used to work for a large electronic manufacture and this was not ever mention nor did we ever have staircase from any capacitor.That was visible on a scope .. At micro level or down to chemical structure of curtain materials maybe visible. but not on a CT100.Over the plates there will never be a 100% perfect transmission of electron movement. But these small factors are so small that 99.9% will never come into play .Each Step would be few electrons But a scope uses pixels which are 1000s of times larger so the whole staircase would fit in 1 pixel .Need a good eye sight as well.Low grade capacitors will have errors . that's why they are cheap.

author: Labrat101
author_rank: Unknown
timestamp: June 06, 2020, 10:39:04 pm
content: The Earth is also Flatmy spirit level shows it to be true ..Show a video of an actual working model with a schematics so we can all enjoy .

author: Nominal Animal
author_rank: Super Contributor
timestamp: June 07, 2020, 06:45:21 am
content: Quote from: RoGeorge on June 06, 2020, 09:24:14 pmI for one am not booing you, RoGeorge; I am only saying it is not an intrinsic property of all capacitors (and because ofwording, that makes me disagree with your statement/poll).(This is also at the core of my recent one-sided arguments about learning things, even if those feel like rants or useless whining to others.)Let me repeat an example I've already mentioned recently elsewhere in this forum.At least here, in school, kids are taught that electrons orbit atomic nuclei.The core problem is the definition of "orbit". (In physics and chemistry, they're not called "orbits", they're called "orbitals", "orbital shells", and so on.)Yes, the electron does have properties that are analogous to rotating around a center: angular momentum, something like an orbital radius, and even spin (analogous to "direction of rotation").But the fact is that those electrons aren't actuallymoving, they're delocalized around the nucleus. This delocalization can be described using quantum mechanics, so well that the best models we have (most accurate with respect to real-world measurable properties) of how molecules behave and react, are based on the quantum mechanical modeling of just the outermost interacting electrons! No fitting to real-world data, just pure math and some physical constants, and out pops our best predictions of what certain molecules are like, and what their properties are.What does it matter, then? The orbit model gives an intuitive grasp of theproperties– radius and angular momentum in particular; and even spin! – and those are what even physicists and chemists work with, so there's no harm, right?Wrong. When a charged particle like an electron is deflected, accelerated, or decelerated by another charged particle, like an atomic nucleus or another electron, it radiates energy. We call this phenomenonBrehmsstrahlung, or "braking radiation". It happens for all charged particles, and is fundamentally due to conservation of energy. So, it is fundamental to charged particles, including electrons.The problem occurs when one tries to integrate the two, Brehmsstrahlung and electron orbits. Ifearthat most people simply decide that somehow an electron orbiting an atomic nucleus, or flitting about in a metal lattice, is just an exception to Brehmsstrahlung, give a quick, and ignore the dichotomy. That is magical thinking, and leads to the inability of correctly choosing which "model" applies in which situation – so those people can recite information, but not apply it in real world to solve a problem or predict or estimate physical phenomena.But, there is no dichotomy, only an incorrect description and thus incorrect understanding. It would have been not that hard to explain correctly in the first place:delocalizationlooks like the electron is "blurred" across its "orbital", which looks more like a cloud than a ring; but if you measure it, it is like putting a stick into the spokes of a fast-turning bicycle wheel: you'll see one quite clearly. The properties these delocalized electrons have, are almost exactly like if the electron was orbiting the nucleus like a moon orbiting a planet, and that's why we use that analog; but the actual physical description is much weirder, and involves quantum mechanics.Trying to fix the mislearning later requires "un-learning" – or at minimum, accepting you were taught an incorrect thing because someone thought that teaching the actual thing was too hard.Now, in the capacitor case, the staircase effect is not due to any intrinsic property of capacitors, but due to capacitors being part of an electrical circuit. You can extend this into the capacitors themselves, if you model a long skinny capacitor charged or discharged at one end, in which case the capacitor itself becomes part of the circuit – transmission line, really. The model breaks down if you have either very short transmission lines, or if the transmission "line" does not have a clearly defined length, so that the change in the electromagnetic field (in the transmission line) does not have a clear wavefront anymore.I suppose the reason for emphasizing the transmission line model is to break through some preconceptions the students might have, and make sure they understand that the transmission line model applies to even circuits on a circuit board – even if in practice it only matters enough to worry about when delivering power via relatively long transmission lines. That it is not something that applies to, well, actual transmission lines, but is a model that accurately describes what happens in circuits.The laws that we use every day to describe electrical circuits or networks – Kirchhoff's laws, Ohm's law, Norton's theorem, Thévenin's theorem – are models that do describe the behaviour of the circuit or network, but themselves are the result of more fundamental descriptions of charges; they describe the behaviour, not what is nor why. The underlying reality is much, much weirder, but these models give us tools to work with such systems without getting too bogged down into the weirdness.That's basically what my own field, molecular dynamics simulations with classical potential models is, "classical" meaning just "non-quantum-mechanic". We can do QM for maybe a system with a thousand electrons (but that may take a week to a month, depending on how large a computing cluster you can grab), but that's just a pretty small molecule – and we need repeated boundary conditions, too, which can be an issue if we want surface phenomena. With less reliable/accurate but much simpler classical interaction models we can model systems with hundreds of millions to billions of atoms, getting into things like corrosion and damage resistance, defect migration in the large scale (and self-healing materials), ion implantation, sputtering, and so on, and get very useful real-world results.

author: Labrat101
author_rank: Unknown
timestamp: June 07, 2020, 07:44:01 am
content: Hi, Nominal AnimalThat was very Good explanation . Very true and totally correct.I did not want to go into that sort of explanation as it maybe above some of the younger readerthat may read this forum.There are Many factors in transmission lines etc .There is one factor that will always be .. is our constant Mr. "Time" how ever we look at it will effect.And I think our friend "RoGeorge" has some interesting ideas . and it gives him a Job.I remember that Dave made some videos a while back on similar .A square orbit would be more interesting . as it would allow 1 to cut corners (joking of course)

author: Someone
author_rank: Super Contributor
timestamp: June 07, 2020, 09:03:58 am
content: Quote from: bdunham7 on June 06, 2020, 03:56:25 pmImage sensors with sub-single-photon noise.

author: StillTrying
author_rank: Super Contributor
timestamp: June 07, 2020, 02:32:48 pm
content: If I use 2 1/2 meters of coax as the capacitor I can see the steps. LOLCharged and discharged through a 390R resistor.

author: RoGeorge
author_rank: Super Contributor
timestamp: June 07, 2020, 04:44:07 pm
content: Quote from: bdunham7 on June 06, 2020, 03:56:25 pmI will love to see an oscilloscope screen capture with some stair steps or at least some undulating exponential, where the undulations can be proved as depending of the capacitor's geometry. I tried once with an 100MHz oscilloscope and failed miserably.If anybody have the time and the equipment to experiment, please share the results.A clean experiment setup is much harder in real life than it is on paper. That's why I never blame those that like to simulate. However, even if I'm aware of a simulation's advantages, I'm not a big fan of it. The best "comments wisdom quote" I can remember (related with my feelings about simulation) is about a software that was simulating robots (mainly a physics engine).Some random dude commented "Simulated robots, simulated fun!"Quote from: Nominal Animal on June 07, 2020, 06:45:21 amWould love to chat on each paragraph, but I'm a slow typer.- I know nobody is booing, and I know that is not some fundamental capacitor's property, just steering the hype- about your rant regarding the educational style, maybe it's OK to make things easy to comprehend, at first, then unlearn that later in order to achieve greater levels of detail, simply because one can not just suddenly jump to the most complicated description of reality- even if the jump to the most complex explanation we have for now would be comprehensible, never forget that there is no ultimate explanation. Don't fall for "my theory can make so manycorrect predictions, therefore it must be the correct one". Remember the Greek legends about seasons changing? Those myths were able to predict seasons, and when to start working the land. Does that makes them true? No. Another example: the religious ideas about the Earth being the center of the Universe apparently was good enough to predict some planets trajectories, and many other religious/social traits. Those ideas look pretty barbaric now, isn't it? So don't count of "predictability implies the ultimate truth". After all, prophets are still dead-wrong in their belief, even if their prediction became true.- let's set aside that capacitor transmission line, you want to talk about the simulation you are developing. Put you Viking helmet on (I've just read that Finns are not Vikings) because my respect for most of the quantum theory (and other latest century physics) might be pretty low, I mean far, far away from the quantum praising we see in the media nowadays. Won't go into it, because having a talk around such ideas will require to "unlearn" as you said, or at least to put aside for a while a lot of ideas we assume unconditionally correct right now just because "they can predict season changes and correct months for seeding and harvesting" as in the Greek mythology about Persephone.- OK then, but do you have some other better theory to replace what we have now? Nope. I'm just generally suspicious against any claims or theories, and since I don't have something better to completely replace the shoddy ones we have now, instead of trying to demolish what others are doing right now, let's try to build on what we have:Can your software run a simulation where electrons are replaced by muons?Asking because if we replace electrons with muons, then the same matter will become much heavier. If it's heavier, then it will move slower (for a given temperature). If it moves slower, then it will stay for longer in the proximity of another piece of matter, or atom/molecule whatever. If it can stay for long enough and close enough, some tunneling phenomena will be expected to be seen, therefore "tunneling cold fusion", and that'll be your Nobel prize.(Just to be clear, not my original idea.)

author: rhb
author_rank: Super Contributor
timestamp: June 07, 2020, 05:45:46 pm
content: Quote from: StillTrying on June 07, 2020, 02:32:48 pmVery nice demonstration.If the scope had a good step response it would be a very neat step approximation of the exponentials. With a multisection trombone line you could make the steps smaller and smaller until they disappeared.Rather like the fundamental theorem of calculus.Would you post a schematic of the setup you used? I'd like to try doing it with my 1 GHz Tek 7104 analog scope and one of Leo Bodnar's <40 ps edge pulsers. Being traditional Tek it has a good step response even if it's out of adjustment.Have Fun!Reg

author: jmelson
author_rank: Super Contributor
timestamp: June 07, 2020, 05:56:39 pm
content: Quote from: Nominal Animal on June 07, 2020, 06:45:21 amAHA! I know this one, that's a coaxial cable! And, I've sure studied those with TDR techniques to detect where the bad spot is, etc.I've also seen "capacitor quakes". We built a rig to detect neutrons, back in days of the Pons-Fleishman madness. It had a bunch of He-3 detectors in a tub of paraffin moderator. These detectors needed a 1500+ V bias, and then the signal was capacitively coupled to electronics. When you turned on the bias, there were large pulses for several hours which slowly decreased in frequency. This turned out to be the ceramic capacitor dielectric being squeezed by the electric field, and "creaking". It made the whole setup very unreliable.JonJon

author: RoGeorge
author_rank: Super Contributor
timestamp: June 07, 2020, 06:09:47 pm
content: Quote from: jmelson on June 07, 2020, 05:56:39 pmDid you say Helium 3?!

author: Nominal Animal
author_rank: Super Contributor
timestamp: June 07, 2020, 06:38:02 pm
content: Quote from: RoGeorge on June 07, 2020, 04:44:07 pmGood, because even if it is not a good model for a capacitor per se, it is/can be very useful model for a capacitor in a circuit.Quote from: RoGeorge on June 07, 2020, 04:44:07 pmVery well could be; I don't know. All I know is that some people I've tried to help learn struggle hard with that unlearning. Best case scenario, in my opinion (and the core of that "rant") is to make sure your explanation is rooted in a verifiable random-access context.Quote from: RoGeorge on June 07, 2020, 04:44:07 pmVery true, and that makes it even more important to teach kids critical thinking. And to not shy away from opposing viewpoints, but to explore them, to see what the core or basis for that viewpoint is.Me, I'm often wrong. Understanding what others base their opinions on, helps me evaluate the reliability of my own. It being just the current working set of assumptions and opinions, after all.Quote from: RoGeorge on June 07, 2020, 04:44:07 pmI'm pretty jaded at anything 'nano' myself: it's typically just a buzzword.QM simulators, likeVASP(ViennaAb InitioSimulation Package) are typically based ondensity functional theory, and really just model the electrons only, using Schrödinger equation. Using just the theoretical models of how electrons (or charges) interact, the minimum energy configurations these simulators find seem to describe actual molecules and lattices very accurately.Classical potential models treat each atom as a separate particle, with interactions between particles described using a potential function – typically between two, three, or four particles; metals (except some like chromium) typically work well with just two, biomolecules use models with three or four. Forces are derived from the potential function (literally, hah!), as the negated gradient of the total potentialisthe force acting on a particle. These usually have an algebraic form derived from how the outer electrons in these atoms interact, with constants fitted to real-world measurements.Water is surprisingly one of the nastiest to model.Currently, the simulators' core structure is straight off the seventies, except with CUDA acceleration bolted on. That's what I'd like to improve on, making it easier for non-programmer physicists to efficiently try out new potential models, parallelize the simulations better, and allow mixing particle trajectory simulations with Monte Carlo methods (so that long-term equilibrium states can be found more efficiently, following some kind of simulated event or process, without switching simulators). Oh, and to make Monte Carlo simulations distribute/parallelize better. I like doing that kind of work, and watch and see what others can discover using those tools.Quote from: RoGeorge on June 07, 2020, 04:44:07 pmNot mine, but I think it should be doable in VASP or some of the other Ab Initio ones, by modifying electron mass; that should "pull" the muon orbitals much closer to the nucleus, so the modifications should be easy to verify with a muonic hydrogen atom. Pity VASP isn't open source. Oh darn,Daltonis since 2017, and I hadn't noticed! I'm out of touch.Quote from: jmelson on June 07, 2020, 05:56:39 pmWhat I'd like to know, is how long did it take to discover the culprit!I've always had a soft spot for LENR, just because it would be so useful if it'd work.. although we're starting to have really interesting SMRs, actually. Pity the public does not realize coal power releases more radioactive particles into the atmosphere than nuclear power does, even including all past accidents.For a hobby project, I'm trying to help put together a sensor suite for a fusor; specifically, a robust subsystem for temperature, pressure, etc. monitoring. Simple, really, except that it should be maintainable and understandable by physicists with minimal electronics or programming background; and I just don't believe cobbling together something on top of Labview is a good idea. It's always the human issues that make things difficult!

author: Labrat101
author_rank: Unknown
timestamp: June 07, 2020, 09:35:10 pm
content: Hi allI am running this experiment on 320cm (3mtr 20) satellite cable which has a capacitance of 0.184nf .Cat 5 series 59 , 20AWG .Using the same more less setup as in our friends "StillTrying" picture I could reproduce this image...I stopped and thought for a while . this can't be right .. and I had this vision of Dave Johns whacking me with a baseball bat.So I ran some test on my HP Agilent scope . Ran a frequency scan to find the tuned frequency of this cable. with a sine wave. these steps are there .. Miss match!! change the resistor to match the cable . and ran a 50ns pulse . I got a square wave back.. the staircase vanished .What you are seeing is the reflected pulse harmonics back adjusting the frequency up or down adds more steps .From what I am seeing its not a capacitive charge in a staircase, but a miss matched independence .When the cable is matched to the scope .also the probes have a capacitance my one is 13pf has to be add to the equation .Also on our friends ''StillTrying'' scope picture had ringing on his pulse which would also be reflected ..I am not knocking your work or the scope picture it was good .I would like to see someone else's work on this ..I started with 390 ohm in series with the centre core and the screen to ground . 1 probe 200mhz @x10 on core after the 390 ohm . Set 5Mhz pulse @ 5v . 1st attempt .also tried low freq @ 100khz @ 2v , 10mhz @ 5v , 12.5mhz @ 5v , 20mhz @ 5v , & 25mhz @ 5v pulseI also put another scope at the end of the cable ..I only go this staircase affect when the cable was Not match to the scope .I am awaiting for some new parts for my function Generator so I can reinstall the OCXO and have 0.02 PPbat the moment its only 200ppm but good enough for this test .

author: StillTrying
author_rank: Super Contributor
timestamp: June 07, 2020, 10:06:46 pm
content: Quote from: rhb on June 07, 2020, 05:45:46 pmThere's not a lot to it! The 5V square is from the same thing I was using to apply 1us pulses to 5mm LEDS, the 390R was the first one I picked up around 500R, and the coax just happened to be ~2.5m long.There's no GND wire or clip on the X10 probe (SP) at the coax, just the GND ring resting on the 25mm of shield and the probe tip resting on the 390R and core junction.Captured at 5 Giggle Sa/s I don't think there's anyscope ringingin the yellow waveform, I think any ringing is real coming from the large amount of real 68MHz ringing on the 5V square.

author: Labrat101
author_rank: Unknown
timestamp: June 07, 2020, 10:30:22 pm
content: Quote from: StillTrying on June 07, 2020, 10:06:46 pmI did not use the inverter I just connected in series but got the same results as you.but if you turn on the FFT you can see there are aliases . If you use a 50 cable it must have the same impedance as the scope or you will see all sorts of weird stuff .My cable was 56ohm when matched it lost all the steps you are not seeing what you think .. Its late we are 2 hrs different tomorrow . also Dave did some videos on this sort of thing its distortion call what you like but its notdue to the charging effect.

author: StillTrying
author_rank: Super Contributor
timestamp: June 07, 2020, 10:38:16 pm
content: Quote from: Labrat101 on June 07, 2020, 09:35:10 pmKnock all you like, it was just a 5 min. experiment.I can't see any images in your post, or even find links to any in the html.Yes it's just a transmission line rather than a capacitor.Vid from previous pagehttps://youtu.be/gdiPwbZm0aM?t=2256

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 07, 2020, 11:56:35 pm
content: Now consider what happens when:1. Transmission line impedance is very low, and length is very short. This is roughly descriptive of typical film and ceramic capacitors, where layers are stacked or rolled up, and the ends are shorted across; the electrical length is approximately the component length, no more.2. Connecting leads or traces have high impedances, and comparable lengths.3. When these TL segments (such as they can be considered to be) have high HF losses, so that if any oscillations do appear, they dissipate quickly.What does it mean for a "wavefront" event to dissipate?!4. When the signal source and/or scope have much less bandwidth than the electrical lengths in #1 and #2.The TL model doesn't go away, it just becomes more complicated; and potentially less useful -- instead of tracing one seemingly discrete event back and forth (like for the one-end-driven TL stub), there are myriad reflections to keep track of, and the slow-moving signal can no longer be considered as a discrete wavefront, but must be handled as a continuous, tsunami-like flow of charge.In general, we can split a connection into two ports of some impedance, and consider the incident and reflected waves. Which are no longer discrete wavefronts, but themselves continuous functions of time -- waveforms, just like any other. This leads to peculiar cases, like a loose battery forever transmitting continuous power, only for that power to immediately and constantly reflect back, in phase, to the battery, hence deliver no charge.This is a perfectly equivalent description of current flow, and an interesting one to wrap your head around! Thought-experimenting with ordinary cases like DC and mains power supplies, is useful for practice itself, if not very useful for practical calculation.Ed: typosTim

author: rhb
author_rank: Super Contributor
timestamp: June 08, 2020, 02:21:16 am
content: Quote from: StillTrying on June 07, 2020, 10:06:46 pmWhere was the source, where was the resistor and where was the DSO on the transmission line? I've got sources which don't ring so badly. The 11801 calibrator at <20 ps rise time, the <40 ps Bodnar BNC pulser and a Tek 7104 to check them against :-)Reg

author: jmelson
author_rank: Super Contributor
timestamp: June 08, 2020, 03:56:42 am
content: Quote from: RoGeorge on June 07, 2020, 06:09:47 pmYes, indeed. And, right after the Pons/Fleishman disclosure, it was VERY hard to get He 3 detectors, the only company in the US that made them had a huge backlog. My boss had to pull some strings to get a few.Jon

author: jmelson
author_rank: Super Contributor
timestamp: June 08, 2020, 04:05:25 am
content: Quote from: Nominal Animal on June 07, 2020, 06:38:02 pmWell, embarrassingly long, I'm afraid. We were in a hurry to get some measurements, and just hooked it up and started counting. But, we also hooked the signals up to a scope. We expected to see signals all of about the same energy, but they were all over the place. That should have been the clue.Anyway, eventually, I had to go back and be methodical. So, we hooked up the bias supply, WITHOUT the detectors. All should be quiet, right?But, no, we got the same pattern of pulses. Then, we saw that they decreased in frequency, as time since bias was turned on increased. Well, all we had in the circuit was a known clean bias supply, series resistors and coupling capacitors. A quick look in the literature showed that pulses from high voltage ceramic capacitors was a fairly well-known phenomenon. But, where are you going to get a .01 uF 3000 V cap EXCEPT ceramic?Jon

author: EEEnthusiast
author_rank: Frequent Contributor
timestamp: June 08, 2020, 04:33:39 am
content: Yes indeed it follows steps, when you measure it using any digital equipment. The voltage and currents are quantized by these measuring instruments.When you use math, an analog scope or analog meter, they follow an exponential decay.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 08, 2020, 05:06:17 am
content: Hah... I'd have gladly given you this one, if I had salvaged it about three decades earlier?...Tim

author: Labrat101
author_rank: Unknown
timestamp: June 08, 2020, 07:40:03 am
content: Quote from: StillTrying on June 07, 2020, 10:38:16 pmFirst you miss read I said I was "NOT KNOCKING"I spent 5 hours late at night running Tests to simulate theory .I got a lot of interesting results . But when I have finished I will show the screen shots .But at the moment my results are not showing what Prof said . I have got dam close.But I am not going to do the tests @ 200v so it has to be scaled down to give the same results.our Prof in the video also mentioned the reflections .('house insurance does not cover Against... 'Mad Scientists' )I have a good stock of Caps & Cables etcIts been a many years since I worked on transmission lines . so I want to see if this experimentcan be setup as per his drawings . With different caps .If one of you guys are Radio hams . You will be better equipped with transmission

author: StillTrying
author_rank: Super Contributor
timestamp: June 08, 2020, 09:57:07 am
content: Quote from: Labrat101 on June 08, 2020, 07:40:03 amI know you weren't, I was saying you could if you want, I wouldn't mind, it might lead to me making an easy minor improvement that I've missed.Assuming there's was a long thin plate capacitor, I don't see the problem in it being a just a capacitor at very low speeds, and just a transmission line at very high speeds, the problem would be in the middle were it's it a bit of both.

author: Nominal Animal
author_rank: Super Contributor
timestamp: June 08, 2020, 11:33:32 am
content: Something in all this reminded me of a funny thing when dealing with heat flow (as opposed to equilibrium). You get similar impedance mismatches that affect how and where the heat is transmitted; it too behaves in many ways as a wave. Feels very unintuitive that you can change how heat flows from hot to cold by just slipping in a very thin sheet of material with a different heat transfer characteristic, but that's what happens in real life... impedance mismatch, really.It's too easy to forget that a lot of thermodynamics applies to equilibrium states, and the dynamics are much more complicated and interesting.Same here. I can imagine a circuit board having a semicircular disc shaped trace when dealing with a high frequency signal where the staircase is undesirable and a smoother behaviour wanted; the trace then acts sorta like a spectrum spreader – not a filter because it does not dampen any frequencies, it just retards some of the wave so that the arrival of the wavefront is spread out in time.I'm beginning to think that even if this does not matter for many/most practical circuits, as an educational approach to ensure the students grasp how different models are needed to describe theimportantfeatures depending on the situation/context, this is superior.

author: GlennSprigg
author_rank: Super Contributor
timestamp: June 08, 2020, 11:57:05 am
content: Wow... I chose to not read beyond the 1st few replies...Let's talk the most simple basics...Capacitors charge/discharge continuously/smoothly on an 'ascentotic curve'...which never actually peaks!! I don't care how long it has been charging, or what'curve' you draw, it is ALWAYS still charging, but by lesser & lesser & lesser amounts.There are no 'STEPS'. Just a continuous analog reductive progression. But there's a problem.Consider a ball thrown perfectly vertical, which eventually comes down again. OK...Did it ever STOP at the top?? NO!! It passed ZERO 'speed' for ZERO time!!So it never stopped.I know where I'm going here mathematically... but do you ??

author: Labrat101
author_rank: Unknown
timestamp: June 08, 2020, 12:28:33 pm
content: Ok I got something.I change some or the resistor to try and get a better results.The Cap is 10nf @ 1.6kv the resistor is 1k ohm .I tried 100ohm it worked but as using lower voltage was hard to get a clear picture.the pulse Gen is set @ 2Mhz .. 9volt .. 100nsThe resistor is in parallel with the cap.I did not show the pulse as a second channel as the probe was loading the circuit .The Sharp peek is the pulse .I used the analogue as it has a faster response .Is this the staircase or is it something elseor dodgy cap ...

author: DannyTheGhost
author_rank: Contributor
timestamp: June 08, 2020, 12:30:34 pm
content: This reminded me about one thing I was studying in earlier university courses.In mid 80s scientists started developing SETs - single-electron transistors.They were creating thin films of Au, typically, above some dielectric thin film on metallic substrate, but they stop the process before Au creates uniformal coating, so there is islands of Au clusters, typically ~10nm. That gives it around tens of aF (attoFarads) of capacitance, which in turn, started creating Coulomb Blockage effect. Then they were using scanning tunnel microscope (STM) and its tip acting like electrode. Coulomb blockade occurs by 'charging' cluster with STM tip and tunneling effect, and because of really low volume of Au cluster when it charges by a 'single' electron tunneling effect stops and there is no current. And, of course, it creates steps in voltage needed to charge or discharge this cluster.In this extreme example 'capacitor' indeed charges in steps

author: RoGeorge
author_rank: Super Contributor
timestamp: June 08, 2020, 01:09:40 pm
content: Quote from: GlennSprigg on June 08, 2020, 11:57:05 amThat's only on paper. In the real world, at some moment the last electron will take its seat on the capacitor plates (or will leave the capacitor if we are discharging) and that's it. The process stops, because we can not have a charge smaller than an electron.As a thumb rule for the real world:- infinite and zero does not exist,- forever and nothingness does not exist,- everything comes in "lumps",- at some point those "lumps" can not be divided into even smaller lumps,- there are small and apparently random fluctuations in everything, a background noise that is all over the place, and it seems to be coming out of nowhere, and it can not be dialed down to zero, to me that's the most intriguing feature of this Universe: the noise and it's randomness. (not talking here about the microwave background radiation, but about any kind of random fluctuations, either at a quantum or at a macroscopic level)With enough randomness, I think a whole Universe can be built: the equivalence between mass and energy is already known, there is also an equivalence between energy and information, therefore it should be possible to generate matter out of information. We might still need two essential ingredients, space and time. If you ask me, matter emanates space (similar with a light source that emanates light), and that will also fix the need for black matter and black energy problem, but that's way out of mainstream physics. The only ingredient left to get is time, and if we buy the idea of spacetime as a continuous, than we can get the time for free, time will be generated by mass, too, together with space.In conclusion, from randomness alone, we can get some unintended structure here and there, or else said information, and from information we can get energy, then matter, then space and time, and if we wait long enough, we get dinosaurs, and monkeys, and us.Now, where from is all this randomness coming, I don't know...

author: Labrat101
author_rank: Unknown
timestamp: June 08, 2020, 01:54:25 pm
content: Quote from: RoGeorge on June 08, 2020, 01:09:40 pmNow You are saying when my Capacitor starts to bulge & smoke . This is How the dinosaurs were created ..The BIG bang TheorySome How the subject has turned into something else.And infinite zeroes is possible but not on your Pay Checkmeans I think you Have been Drinking the IPA again and strayed from the original subject at Hand ..- everything comes in "lumps", Yes you got that right 1 coffee 2 lumps of Sugar.I would Like to know were I can BUY some time from ..

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 08, 2020, 07:11:12 pm
content: Quote from: Labrat101 on June 08, 2020, 12:28:33 pmA core on the ground lead(s)? What?At least draw the lumped equivalent circuit first!Tim

author: Labrat101
author_rank: Unknown
timestamp: June 08, 2020, 08:40:57 pm
content: This is special for our Friend T3sl4co1lQuote

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 08, 2020, 10:23:04 pm
content: A mechanical schematic, showing the physical components and connections yes, but that's not what I asked!Tim

author: Labrat101
author_rank: Unknown
timestamp: June 09, 2020, 08:04:13 am
content: Quote from: T3sl4co1l on June 08, 2020, 10:23:04 pmI suggest you take this question to RoGeorgeWho's idea it was. We have just just tried to figure out if his hair brain theory is correct.There are No 2 components that will beAnatomically the same. . (Close enough.)You would have to use the exact same components for all experiments.Watch his videos. And you can decideHow he makes a square peg fit in a round hole .My experiments proved nothing other than I can pulse a capacitor to failure point.RNS.I know what you want .. but these things were not give in any of his documentation.So that's why his theory is not possible to solve . .Even two lengths of coax from 2 different rolls Will not be exactly the same...We would all be the same ..Enjoy.

author: StillTrying
author_rank: Super Contributor
timestamp: June 10, 2020, 02:32:31 pm
content: Quote from: Labrat101 on June 08, 2020, 12:28:33 pmI think it's probably something else, maybe oscillations (which happen all the time) superimposed on the the discharge curve.The middle of the video is definitely describing transmission line effects, along the plates of the capacitor, so I chose a capacitor that was 2.5m long.To get a good view of effects only a few ns wide you have to get rid of all those long wires. To get the GND and resistor as close as possible to the GND and gates of the 74AC14 I soldered them to the back of the board, and arranged the end of the cable so I didn't have to use any GND clip on the scope probe.

author: rhb
author_rank: Super Contributor
timestamp: June 10, 2020, 03:34:01 pm
content: Thanks for the photo. I shall repeat the experiment with either a 1 GHz 7104 and 36 ps step or an 11801 with a <20 ps step and 20 GHz sampling head and varying lengths of coax. Likely the 11801 as I have some other experiments I need to set it up for.Both of those should give nice neat staircases as they have very little overshoot.Have Fun!Reg

author: rhb
author_rank: Super Contributor
timestamp: June 10, 2020, 06:50:07 pm
content: OK These are with a Tek 11801 & 20 GHz SD-24 sampling head. I fed the coax stub through a 150 ohm resistor.10-12 ft coax50 cm16 cm(sorry about the burred image. I didn't notice it until I posted it)The 250 mV step has a <20 ps rise time. The only thing changing is the travel time and the capacitance. As the coax has approximately the same capacitance per foot, the number of steps stays constant and the only thing that changes is the time delay for each step. If I expand the vertical scale a lot I can count about 13 steps before the steps fall below the noise and quantization.Q.E.D.Quod erat demonstrandum.It has been shown.Have Fun!Reg

author: rhb
author_rank: Super Contributor
timestamp: June 10, 2020, 07:04:41 pm
content: Just for fun, here's a close up of the trailing part of the stairstep for a 30 cm piece of high quality RG402 with >1024 sweeps averaged and at maximum gain. The little bump at the top of the step is connector saver and wear on the 3.5 mm connector of the SD-24.Reg

author: StillTrying
author_rank: Super Contributor
timestamp: June 10, 2020, 07:35:06 pm
content: Are you viewing the open circuit end of the cable rather than the driven end, I don't know why your first step is so high in Reply#49.

author: RoGeorge
author_rank: Super Contributor
timestamp: June 10, 2020, 08:18:27 pm
content: Quote from: rhb on June 10, 2020, 06:50:07 pmWow, the cleanest stair-steps reflections I have ever seen!Can you repeat the experiment with a real life capacitor, too, to see if the stair-steps on a capacitor are as visible as the ones seen on the coax cable, please?Not sure what type and value of a capacitor will be the most advantageous for the experiment, maybe one with large physical dimensions, and with the plates rolled in a cylinder, rather than a tantalum or a MLCC.

author: rhb
author_rank: Super Contributor
timestamp: June 10, 2020, 09:04:35 pm
content: Quote from: StillTrying on June 10, 2020, 07:35:06 pmThis is the reflected signal seen at the driven end. The other end of the cable is open. So each step is a 2 way trip down the cable and back to the source end. Without the resistor it would be two big steps. This is the corner of the 2nd big step produced by an open circuit line. Take a look at my "Testing RF connectors and cable" thread. There are lots of examples of various tests there.RoGeorge, Let me think about how to make a test that is not obscured by other artifacts. As the line length gets short things start getting complicated by parasitic inductance.If I double the resistance it should double the time constant and the number of steps. I think that a 1500 ohm resistor and the same piece of RG402 as my last photo would be the best. That should show >100 steps and still be resolvable.Have Fun!Reg

author: rhb
author_rank: Super Contributor
timestamp: June 10, 2020, 10:19:56 pm
content: I don't think any capacitors are designed to be constant impedance. Certainly rolled foil capacitors cannot be as the diameter is constantly changing. So both the capacitance and inductance per turn is changing constantly from one end to the other. In addition there is inductive and capacitive coupling from layer to layer.The changing impedance is going to produce a continuous series of small reflection events rather than large discrete events at the ends. That will tend to make the response look like a piece wise linear approximation to an exponential rather than a stairstep approximation and thus very difficult to show clearly. The stairstep is the result of the derivative of the impedance being zero except at the ends.A flat microstrip transmission line of trapezoidal shape would be the simplest way to demonstrate the effect of changing impedance along the transmission line.The point here is that the exponential is a series of small steps once one takes the finite dimensions of the capacitor into account. And in the limit as the size becomes infinitesimal, it becomes an exponential.It's probably easier to calculate the response of a hypothetical capacitor than to measure the effect in a real one.Reg

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 10, 2020, 10:23:19 pm
content: Like I said, the electrical length of a typical film or ceramic capacitor is so short, and the impedance so low, that you'll be lucky to observe anything, even with a ps scale generator and scope.Tim

author: eti
author_rank: Super Contributor
timestamp: June 10, 2020, 10:57:28 pm
content: Capacitors are incredibly versatile; they can charge on stairs, in hallways... basically anywhere.

author: rhb
author_rank: Super Contributor
timestamp: June 10, 2020, 11:43:44 pm
content: Just for grins I tried a 750 ohm series resistor. The impedance mismatch between the source and the resistor became so large that it rings like mad between the source and the resistor. Bear in mind that this is a thru hole metal film resistor, so it also has significant inductance at 20 GHz.The first photo is the full step response:The second is a detail from the 2nd step showing the effect of the reflections during the RC constant:As you can see, there are a lot more steps visible because I increased the RC constant by 5x.As has been pointed out, the impedance of a capacitor is generally low, though not at all frequencies. This is why high gain stages need to have the electrolytic caps bypassed by a small ceramic to prevent oscillation. It would well nigh impossible to match a capacitor from DC to 20 GHz unless it was made from a very good piece of coaxial cable as in my examples.The next step for these two resistors is to examine them with the nanoVNA.In general, capacitors do not resemble transmission lines and therefore do not behave the same way. They can be accurately described in terms of transmission line theory, but they won't *look* like a transmission line capacitor.I'd like to add that these sorts of tests are wildly sensitive to the connection make up and often not easily repeatable.Have Fun!Reg

author: StillTrying
author_rank: Super Contributor
timestamp: June 11, 2020, 12:22:13 am
content: Quote from: rhb on June 10, 2020, 09:04:35 pmIf I simulate a 40cm 50R cable driven by 150R I still can't see where your very large first step comes from, it's about 4 times higher than what would fit anywhere.

author: rhb
author_rank: Super Contributor
timestamp: June 11, 2020, 12:42:26 am
content: Read an app note about sampling scopes and step responses. The open end produces a reflection of +1. So you see the initial step and then a 2nd step. A short produces a reflection of -1, so you see a step up and then down to form a single pulse.The latter was the way short pulses were created for many years using a coax stub and a mercury wetted relay.Reg

author: StillTrying
author_rank: Super Contributor
timestamp: June 11, 2020, 03:58:14 am
content: Your very large first steps must be due to probing the wrong end of the 150R, Pulse_Out rather than Driven_End, of the cable.

author: rhb
author_rank: Super Contributor
timestamp: June 11, 2020, 01:00:42 pm
content: There is *nothing* wrong. Take a look at the photo of the short piece of RG402 with no connector at the end. That's the circuit. It's a series resistor and capacitor using transmission line for the capacitor. There isn't any place to connect the sampling head at the far end of that short piece of RG402. If it were connected there wouldn't be a +1 reflection coefficient and thus no stairstep.The signal is a 250 mV, 200 kHz square wave with a <20 ps rise time. The sampling head has a <15 ps rise time. Look at the time scales.The SD-24 head generates a square wave and displays the outgoing and reflected signal appearing on the input. The first big step is the outgoing pulse. The 2nd big step is the reflected pulse from the open circuit at the end of the transmission line. The staircase is a bit of rounding of the edge of the open circuit reflected step as the signal bounces in *just* the coax. The large steps are the delays in the sampling head circuit getting to the 3.5 mm input connector and back.This is basic time domain reflectometry with a high end instrument designed specifically for the task. As I suggested before, find an app note on TDR. It will be quite educational. I've had more fun with the 11801 & SD-24 than any other instrument I've bought (except guitars and harmonicas ;-)Have Fun!Reg

author: Labrat101
author_rank: Unknown
timestamp: June 11, 2020, 01:51:40 pm
content: QuoteYou are all chasing a Rabbit down a Hole.Sending a pulse or a signal down a coax ,50 ohm or what ever. That is not terminated the signal has to go some were .. so it will go backwards. Echo . So there will be a reflection.There will be a time difference. There fore you will see steps . SWR will not be 1:1 on any open Transmission line .Pulse . or Square wave . is made up of all waves . and hence it will break down back to a sine.There is only one true wave form and it a sine .. That why light can't go round corners .withoutReflections call what every you like but its the same.It's like ripping water when a ripple gets to an edge it bounces back. Whether you see it or not it's there.. And there is never a square wave in Water .. or light etc. etc.Only thing at the bottom of a Rabbit hole is ...€¥)^(¿Also Notice How the original Author Who Himself said it was Only practical on paper.As for all Us Nerds . including myself . .Paper theories are very good as long as they stay in the right place .You can always work it out with a pencil ... but taking medicine will also work ..PS. one nanosecond light travels about 0.3 meters measure the distance between the stepsand you will find the refection point in your coax . or the the dialectic failure point .if the coax is perfect with no defects it will give you the length of the coax being tested.A tape measure would also work without the complications.This Method is good for finding bad connections in long data lines and has a high accuracyto locate the damage section of cable and repair it . used it myself to find bad broken transmission line .. . but it does not prove that a Capacitor charges in steps .it like a saying a fish does not breath air . No it removes it from the water (H20.)Still a large Rabbit Hole ..

author: rhb
author_rank: Super Contributor
timestamp: June 22, 2020, 11:06:34 pm
content: I received a Rigol DS1202Z-E today.First the full step response using one of Leo Bodnar's <40 ps 10 MHz square wave generators and a 50 ohm thru termination.Then a vertical zoom on the upper edge of the step:What's really significant here is that Rigol is dithering the sampling clock and the jitter is under 100 ps so it shows a <100 ps step quite clearly.That in turn implies that the AFE has a *real* BW of 3.5 GHz!!!Have Fun!Reg

author: StillTrying
author_rank: Super Contributor
timestamp: June 23, 2020, 01:28:20 am
content: The steps are 1ns apart so it's just some sampling thing.What's it triggering on 12 screens away.This Siglent pseudo ETS DOTS mode shown by rf-loop looks interesting.https://www.eevblog.com/forum/testgear/siglent-1104x-e-or-2204x-e/msg2118325/#msg2118325I don't know if it still works with 500MSa/s on a fast edge.

author: rhb
author_rank: Super Contributor
timestamp: June 23, 2020, 02:10:16 am
content: If the sampling is dithered at 100 ps intervals, then the result is *exactly* what one would get of an AFE with >1.75 GHz BW and no anti-alias filter.Whether that is the case or it is some mundane programming error is unknown at present. But it very much looks to be sample time dithering rather than a programming error. I do *not* get the same result from my DS1102E with the same setup.rf-loop's post is meaningless. There is no indication of the actual input waveform. I'm using a *well* vetted signal source. My <40 ps pulser came with a CSA803A & 40 GHz SD-30 sampling head step response plot showing a 37 ps rise time. I can easily feed the <20 ps 11801 calibrator pulse to the Rigol. It won't change the result. I've used the Bodnar step generator to test my 1 GHz Tek 7104 analog scope. And it shows a <350 ps rise time.In short, I have a wide range of hammers from an ounce or so to 30 pounds. So I can beat on things rather hard. Naturally I don't smack electronics test gear with a 30 pound stone boat, but I have the electronic equivalents.Please look at my posts to the thread RoGeorge started. The 11801 & SD-24 were *very* expensive kit in the early 1990's. If you think those are erroneous you need to try harder.Have Fun!Reg

author: StillTrying
author_rank: Super Contributor
timestamp: June 23, 2020, 11:48:42 am
content: Quote from: rhb on June 22, 2020, 11:06:34 pmTo me the <100ps rise times imply timing errors rather than a 3.5GHz AFE in a £400 scope.I've estimatedwhere the trigger XY point is in comparison to the visible screen. for your 1ns steps capture.

author: rhb
author_rank: Super Contributor
timestamp: June 23, 2020, 01:43:15 pm
content: I was playing with the pulser feeding a tee and coax this morning. There is something odd going on with the vertical settings. Here it is with the trigger on screen. So it is *not* related to the trigger position.I'm going to feed the SD-24 to the AFE and look at the TDR response. That should establish whether it's ringing in the input transmission line or some artifact of the DSO implementation.BTW My calculation was wrong. The pixels are at about 100 ps spacing. So the calculation should have been 0.35/200 ps.My Keysight 33622A has a spec of <1 ps jitter, so if I set up a low amplitude square wave and use the external trigger input I should be able to measure the DSO timebase jitter.Have Fun!Reg

author: RoGeorge
author_rank: Super Contributor
timestamp: June 23, 2020, 02:57:36 pm
content: That wave shape is so weird because of 3 things:- DSP processing on the oscilloscope side - they applied all kind of filters to make the image look good, including non causal filters (e.g. Fourier transform can not be applied to causal signals, therefore it should NOT be used for real time signals, Fourier produces "artifacts" because the resulting spectrum is non causal) - might seem like gibberish talk, but this is how it works. For example, the downwards peak, seen just before the signal starts to raise, is fake. There is no reason for why a signal will first go down before going up, that dip before the signal goes up must be the result of non-causal filters (or other kind of non causal digital processing messing with the samples), that sort of "premonition" artifacts can never to be seen in physical systems, like in an analog oscilloscope. Our physical world is causal.- jitter (phase noise) in the oscilloscope's clock- in Rigol, the triggering moment is detected in softwareReducing the delay between the triggering point and the piece of shown waveform on the display helps reducing the jitter in the final image (for a given phase noise, the jitter accumulates in time).LATER EDIT:------------------One more thing, the 0.35 factor to determine the ratio between raise time and bandwidth of the oscilloscope, it is NOT true any more for digital oscilloscope. That formula is valid only for analogue oscilloscopes, with analog filters and normal roll of (analog filters are always causal). With DSP filters, waveshape corrections are made non-causal (in anticipation of a signal or an edge that is yet to come), and that 0.35 formula is now unrelated with the raise time shown in the "fake" waveform.The Rigol screen capture speaks for itself how fake that signal is.In this Q&A from Tektronix, the factor is 0.45 and is valid for the kind of DSP they apply, but in general, the coefficient depends of the filters, and since DSP filters can be of any kind, the formula is not valid any more:https://www.tek.com/support/faqs/how-bandwidth-related-rise-time-oscilloscopesIn short, a low range digital oscilloscope, like a Rigol DS1202Z-E, can not deal correctly with that kind of signal, and is in no way a match for the performance of a high end 20GHz Tektronix SD-24 analog sampling head.

author: Labrat101
author_rank: Unknown
timestamp: June 23, 2020, 04:33:50 pm
content: Quote from: StillTrying on June 23, 2020, 11:48:42 amI have to agree your trigger is off the screen . and that is an acquisition error or the scope is set up wrong.RNS

author: rhb
author_rank: Super Contributor
timestamp: June 23, 2020, 05:16:26 pm
content: The trigger point is shown in the figure in reply #67 and is on the screen.The use of zero phase, non-causal filters in DSOs is a major complaint of mine. It's not the proper filter. Moreover, the correct interpolation filter is *not* a sinc(t). The correct filter is the minimum phase impulse response of the actual anti-alias filter used. To avoid excessive ringing the corner frequency for the anti-alias filter should be no more than 67% of Nyquist and for really good step response 50% of Nyquist.The "Scope wars" thread will measure jitter among many other things. I'm going to show what they are doing wrong and how it should be done. The failure of a DSO to produce analog scope trace quality is a design failure. It is not inherent in the scope being digital. It's driven by the desire of sales and marketing to advertise a scope as having twice the BW it would have if it had a decent step response. And the naive buyers who don't know enough about the Fourier transform to realize they are being swindled.My LeCroy DDA-125 has a claimed 1.5 GHz BW and has the measured rise time for the 0.35/BW to match that. It also has 20% overshoot on a fast step. My DDA-120 has a 350 ps rise time on a stated 1 GHz BW and 17% overshoot. It's worth noting that Nyquist for the DDA-125 is 1 GHz in 4 channel mode. Can you say "aliasing"?The EE "ideal low pass filter" is only ideal if you don't care about the time domain behavior. If you care about the time domain behavior you must apply a long taper to the BW. If you make the corner 50% of Nyquist, the time domain step response is a minimum phase sinc(t)**2 which suppresses the sidelobes very well. One can do better with a Gaussian or sech(f) taper which produce no ringing, but at the cost of slower rise times.With regard to Tektronix I've been waiting since late April for a step response from the 3 series scopes. I stated I didn't want to bother with a demo unless it had less than 3% overshoot on a <40 ps step.Have Fun!Reg

author: StillTrying
author_rank: Super Contributor
timestamp: June 23, 2020, 06:18:06 pm
content: Quote from: rhb on June 23, 2020, 01:43:15 pmOn my trigger diagram I was thinking maybe the trigger's low Y position was an even a bigger problem than the X position.If the trigger position was going to do some clever sub 1ns position timing it wouldn't have any slope to work with if every ADC sample at that trigger low Y position was 00, - other than wait until the input V and ADC values start counting up from 00 - at some random time.IF it does work like that I can imagine(why not!) up to 1ns of jitter creating some nice stairs somewhere.On your latest capture, a 1 Vpp ? arriving at the scope on 10mV/div. shouldn't overload or out of range anything, - even the trigger!- in theory.DS1202Z-E 1 mV/div to 499 mV/div: Offset Range ±2VWhile I'm there:Trigger Level Range Internal ±5 div from center of the screenBut even with the trigger's X and Y within the screen area you've beaten your record near infinite rise times with some slightly negative ones.Whatever it's doing I don't think a scope showing any negative rise times on relatively simple waveforms is much use, I hope you've still got its box!

author: Labrat101
author_rank: Unknown
timestamp: June 23, 2020, 06:49:03 pm
content: QuoteTry using a good analog scope and then all those filters are not needed.You see what is really there and not a computation Of the signal .Adding the wrong filter in the wrong place does not solve a problem.Tipex would also work . Unless you have a high end DSO the signal is being processed by simulation. So if you play enough with filters etc you can get what looks good.But is it correct . ?

author: rhb
author_rank: Super Contributor
timestamp: June 23, 2020, 07:50:04 pm
content: Is a Tektronix 1 GHz 7104 "good enough"? There is implicitly a BW filter on every analog scope. The step response tells you what that filter shape is.Is a 1.5 GHz LeCroy DSO "high end"? The LeCroy has all the jitter analysis tools enabled, but I've not tried them out yet. Not really much point until I have a GPSDO clock input set up to feed it.I spent 30+ years in the oil industry writing and fixing seismic DSP codes. That is far more exotic DSP than anything the EE community does. Always has been. In the 60's when ADCs barely existed we could live with 4 ms sample rates and had the money to pay for them and the computers to process the data. A common seismic imaging operation today takes 10,000 to 20,000 core clusters 7 to 10 days to complete. The input is 10-15 TB. Each output sample takes several gigaflops to compute. What's more, this is not a one shot. It's iterative. You run the job, look at the results, update the velocity model and repeat.Texas Instruments was a wholly owned subsidiary of Geophysical Services Inc created specifically to build seismic recording equipment.The trigger point is shown on the last plot I posted. The signal is about 900 mVpp. It's adjustable, but I have to run a program on the PC to do that and I don't have a Windows machine I can boot easily at the moment.I'm afraid I don't grok "negative rise times". Where is that from? I think the persistence for that plot was 1 second.As best I can tell what's happening is the scope is triggering and the steps are slightly less than 200 ps. So the vertical segment is samples at various points on a slope which then gets displayed at the same time. As I remarked before, the 11801 will tell the truth about what the front end looks like. If the steps are ringing in the front end they will show up as reflection events on the 11801 & SD-24.Have Fun!Reg

author: StillTrying
author_rank: Super Contributor
timestamp: June 24, 2020, 01:17:36 am
content: Quote from: rhb on June 23, 2020, 07:50:04 pmWell it's not going back in time by a massive amount.The first part of the trace in the bottom left very near the trigger point looks about right and sharp, but the thin spike rising from there has no fall time, - at least not in the forward time direction.Perhaps 1sec of persistence wasn't enough to catch any of the fall time, but there's nowhere else where a full division or even part of the trace is missing.At 2ns/div I'd expect the next rising edge in the middle to look like it's always sloping slightly forward in time even if the slope is only around 5 degrees, at least to me it looks like it's curving back in time but only by about 2 or maybe 3 pixels, I make it 1 pixel width = 25ps.Perhaps it is doing quite a bit of 25ps to 100ps dithering to capture every bit of the waveform, but they're forgetting to correct the dot sample positions by 1 to 4 pixel widths.Could it be some interaction between the charging steps and sample rate both being 1ns wide.Perhaps it's all fine and only some of us are seeing the verticals as unrealistic!

author: rhb
author_rank: Super Contributor
timestamp: June 24, 2020, 02:24:01 am
content: I think that is just jitter.What caught my eye is the change from linear interpolation of pixels on the DS1102E to what looks like clock dithering on the DS120Z-E.Leo's pulser has a few ps of jitter. Not specified, but he's tried to minimize it. He's also working to a pretty brutal price point. But I have other reference sources with less than 1 ps of jitter. So just give me time.More in the "Scope Wars" thread.Have fun!Reg

author: Labrat101
author_rank: Unknown
timestamp: June 24, 2020, 07:06:35 am
content: This time error showing a negative back step is only a few pixels. I would think this is due to time problem between the speed of the screen video timing and the main time base are out of sync by a few pixels.Could be a component delay or a small error in the Dac division..If its real you have found a way to travel back in timewhich is really good .I think you are pushing your scope .And with all electronics there will be one part that may be not 100% up to spec.I would check your video memory maybe getting hot..Have funRNS