Ah how things have changed. When I was learning electronics we mainly dealt with radio and TV circuits and just about the first lesson one learned was to keep leads short (reduce unwanted inductance) and use decoupling capacitors everywhere.

I recall some years later a young graduate engineer coming into my office with a rather involved circuit consisting of 30/40 TTL ICs and complaining that he'd double checked the circuit and it still didn't work. I took one look at his device then went to the draws of capacitors and handed him a handful of 0.1uF ceramic caps and told him to put them between the ICs' PS rail pins to ground which he did and to his amazement the circuit worked immediately.

He stood in amazement that I should have such insight so as to fix the problem at first glance.

How such critical knowledge can get lost in university training these days just amazes me.

• lefra

  today at 7:06 AM

  My university made us use really crappy power supplies and dev boards. Nothing worked unless you first put a large bulk capacitor on the power supply's output, and small capacitors close to the components.

  Also I got bitten by parasitics in capacitors very early in my career: capacitors of different face value will resonate with each other to effectively kill the decoupling network at a specific frequency (resulting, for me, in an amplifier with a nice hole in its frequency response).

  • hilbert42

    today at 7:45 AM

    Excellent training, especially the parasitic bit. Trouble is somehow many aren't taught that stuff nowadays.

    • baq

      today at 9:13 AM

      Sounds like an opportunity to build a shenzhen i/o prequel

• michaelt

  today at 6:55 AM

  > How such critical knowledge can get lost in university training these days just amazes me.

  It will probably have been taught.... but very briefly. Before going go back to analysing circuit schematics, where connections between components don't show resistance or inductance, and the capacitance of two parallel capacitors sums.

  • st_goliath

    today at 7:16 AM

    This is why lab exercises are important. I remember first building some actual TTL circuits on bread board, I learned very quickly that this whole digital stuff is a lot uglier and messier than on paper or in the simulator.

    With sharp rise times, synced up to a common clock, even after soldering in a whole bunch of capacitors, you can still stick a probe pretty much anywhere and see switching spikes all over the place, from power rails to completely unrelated signals that are supposed to be stable. Using actual TTL, there was another funny lesson what this weird "fanout" value in the datasheet meant.

    A similar lesson I learned that way (and a very memorable one :-)) was about flyback diodes.

    • jacquesm

      today at 7:30 AM

      Ah, but that may well be because of your scope probe's leads! The sharper the edge the more likely that will happen. That's what those shitty little springs are for that come with your scope probe: you disconnect the ground wire and put that spring on the naked scope probe pin around the ground collar. Then where you want to measure you use the pin to go to the signal and the little spring to reach the nearest ground. Presto: clean signal (or at least, much cleaner). Also, make sure to tune your probe (that's what the little plastic screwdriver with metal tip is for, there is a small trimmer in the probe you can reach through a hole and that is critical at high frequencies) and avoid probes with switchable 1/10 like the plague, over time the switches go lame and then you'll be tracking all kinds of weird gremlins.

      • ben_w

        today at 8:11 AM

        This is just reminding me of the time I played with an oscilloscope, touched the probe against my finger and found my body was antenna picking up mains frequency.

• madaxe_again

  today at 6:41 AM

  I feel it’s a function of abstraction.

  You learned when analogue circuitry was the norm. I learned when digital circuitry was simple enough that you could readily take something apart and understand it.

  Now, EE courses often start with cad, simulations, digital electronics, and you end up with people building ziggurats atop an ocean of incomprehension.

  It’s exactly the same thing with software.

  I don’t scorn people for this, rather I see myself as fortunate for having learned in a time when the more fundamental knowledge was still worth learning - and that’s the rub - for a vast majority, it simply isn’t worth the time or energy to explore the full stack, when there’s so much to learn atop it.

  • hilbert42

    today at 7:21 AM

    "You learned when analogue circuitry was the norm. I learned when digital circuitry..."

    What's not taught properly these days is that ALL electronics is analog at the physical/circuit level.

    For you digital types that's OSI Model Layer 1 — Physical layer (look it up on Wiki). Nothing in electronics works unless that's working properly—ICs, tunnel diodes, transistors, inductors, resistors, capacitors, cables and antennas are all analog devices at that level. That includes the heart of the most advanced digital ICs. For example, the upper clock speeds in processors are limited by transit times/electron mobility, inter-electrode and stray capacitances, unwanted inductance, etc.—all of which are analog effects and they must be accounted for.

    Like it or not, the physical analog world is alive and well! The Noughts & Ones Brigade unfortunately seems to have forgotten that fact.

• Lerc

  today at 6:03 AM

  I can see how that happens when people come at things from a conceptual digital side first.

  It probably doesn't help when you have a circuit diagram that while topologically correct doesn't show the relative positioning between components. The first time I saw all the decoupling caps rendered in a single chain on the side of the diagram I was mightily confused. It seemed like utter nonsense until I realised where they actually went.

  • hilbert42

    today at 9:42 AM

    "The first time I saw all the decoupling caps rendered in a single chain on the side of the diagram I was mightily confused…"

    If you've read my other comments here you'll realize I'm concerned that these days EE training doesn't place a strong enough emphasis on shielding, ground loops, decoupling and such that it ought to. For any electrical/electronic engineer these are critical concepts.

    By way of stressing that I'd like to take a sojourn into history and refer you to probably the greatest set of electronic engineering books ever produced: the MIT Radiation Laboratory Series — a massive 28 volume set written nearly 80 years ago to document electronics and microwave/radar research done during WWII.

    Anyone seriously interested in electronics should be aware of this series. Yes, it's dated, heavily weighted towards vacuum tube technology (although klystrons and magnetrons are still current), and it lacks modern semiconductor tech, however this truly remarkable set contains a huge amount of information that's still very relevant today. Moreover, whilst it covers the topics in depth it does so at a level that can be easily understood by undergraduates (explanations are more general than today's very specialized textbooks).

    https://en.wikipedia.org/wiki/MIT_Radiation_Laboratory_Serie...

    Here you'll find links to the Internet Archive where the volumes can be downloaded. Specifically, I would refer you to Volume 23 - Microwave Receivers, — Chapter 6 Intermediate Frequency Amplifiers p155. Now turn to p182 and read 6-10 Practical Considerations.

    Here's the PDF of V23:https://archive.org/download/mit-rad-lab-series-version-3/23...

    This section on decoupling, shielding etc. is just as applicable to today's high speed digital circuits as it was back in WWII. Sure it needs updating but the fundamentals of screening and decoupling have not changed. What's important here is that these physical (analog) effects are set by the fundamental laws of physics, and circuits that do not take them into account will fail to work correctly.

• amelius

  today at 7:27 AM

  https://xkcd.com/1053/

This is probably a good place to debunk the usual wisdom that "decoupling capacitors must be placed very close to the IC pins". If you're using a solid power plane, rather than routing power through traces (and honestly 4/6 layer boards are cheap enough these days) it really doesn't matter where you place decoupling capacitors for most uses - keep the via traces short or ideally in the pad, and you can put all your decoupling capacitors in one place on the boards a way away from the chip and focus on good routing of your signals. Figure 15 on this paper (and the whole paper!) explains it well: https://scholarsmine.mst.edu/cgi/viewcontent.cgi?article=221...

• fps-hero

  today at 7:03 AM

  Loop inductance is what really matters with decoupling. Once you understand that, it becomes really easy to make good decisions. This article explains how you can approximate the inductance for a given layout, so it makes evaluating layouts much simpler. It actually used the data from the paper you referenced in example 3!

  https://learnemc.com/estimating-connection-inductance

  You can even use mutual inductance of vias improve performance, either by having vias spaced close together and in the right order (https://learnemc.com/decoupling-for-boards-with-widely-space...), or arranging capacitors in alternating or doublet layouts (https://incompliancemag.com/decoupling-capacitor-design-on-p...).

  As you say, just having power planes and directly connecting to them is almost always going to be superior to using a trace, despite seeing this all the time, especially in datasheet example layouts. It made sense for 2 layer boards, but not today. Just think, the inductance of the planes is practically zero, and distance to the plane from the components is going to be on the order of 0.2mm, round trip 0.4mm. Is there any way I could place the capacitor 0.4mm away from the pins to achieve an equivalent inductance? And even if you could, you can't add extra vias to lower inductance, and you don't benefit from mutual inductance.

  • raverbashing

    today at 8:23 AM

    Yeah

    The ELI5 for decoupling capacitors is "imagine an energy storage for quick usage"

    The ELI(tired EE student) is more like the explanation above

    And this concept is ok for most of the 'low speed' circuits

    in RF ranges, everything is a capacitor (except when you need one), everything is an inductor (except when you need one) and the intuitive explanations break down and everything looks like dark magic

• PunchyHamster

  today at 9:48 AM

  Well, till it does. Paper talks about frequencies in 200MHz range, not every project can afford solid power planes and putting it next to a chip costs literally nothing. It's like safety helmet, 99.9% of the time it's not needed

• laydn

  today at 6:04 AM

  Great paper!. Anyonw know whether there are any modern tools/software that can simulate this during design?

  • lefra

    today at 6:54 AM

    For approximative simulation, any SPICE simulator works. You'll need to know your capacitors parasitics and power supply output impedance, find a typical via's impedance, and manually compute traces impedances and board capacitance.

    For accurate simulation, the actual board geometry needs to be fed to a simulator that'll compute the actual impedances. Last I checked only Very Expensive Software could do that in a user-friendly way (I had to route a DDR3 bus. I ended up being very cautious so that all traces had the same topology and the same lengths, and cross my fingers. It worked).

    If anyone knows of free alternatives for that, I'd be interested to hear about it.

This signifies that each vertical dotted line is 20ns apart, so the ripple you see has a frequency of something like 50MHz.

Unless you have a 50MHz buck converter (which would be very exotic --- the fastest common ones are around 1/10th that), that looks more like something may be inadvertently oscillating and/or you're picking up strong RF noise from possibly something in...

https://en.wikipedia.org/wiki/6-meter_band#Radio_control_hob...

And "leared" -- the (unintentional?) pun made me click.

• Animats

  today at 6:26 AM

  It's not oscillating at 50MHz. Look at the waveform, with the big spike in the middle. That's a spike at some lower frequency, wider than the screen, followed by ringing. Need to zoom out the time base some more to see the period of the big spikes. It's no higher than 4 MHZ (the screen is 12 units wide) and possibly much lower. (Assuming that M:20ns on the display means 20ns/grid division. The manual is a bit hazy on that part of the UI.)[1]

  The power regulator IC mentioned is normally run at 500KHz. There's a reasonable chance that this is the power regulator spike not being damped out. Easy enough to check with a scope handy.

  [1] https://fotronic.asset.akeneo.cloud/pdfs/media/owon_hds242s_...

• JCTheDenthog

  today at 4:23 AM

  >And "leared" -- the (unintentional?) pun made me click.

  I assume it's a reference to the "Quality Learing Center" in Minnesota, one of the questionable daycares at the center of the alleged Somali daycare fraud scandal. Ever since some of the expose videos about it came out it's become a meme to say "lear" instead of "learn".

  • nielsbot

    today at 6:44 AM

    > questionable daycares

    If they don't find fraud, is it "questionable"?

    • peterfirefly

      today at 10:12 AM

      If they choose not to look, yes.

• codedokode

  today at 4:16 AM

  Cannot it be a noise from imperfect switching? The switching occurs at lower frequency, and the noise is high frequency.

  • today at 4:44 AM

• elch

  today at 6:18 AM

  I guess he also believes that 50 MHz or so signals can be measured reliably on a 40 MHz (on paper at least) scope.

  • PunchyHamster

    today at 9:51 AM

    Most digital scopes have around 5-10 times faster sampling than bandwidth. The one on pic is 250Msps.

    That's more than good enough for the purpose of checking interference

    • elch

      today at 10:06 AM

      But he tries to quantify this interference. Anyway Animats's comment is the one that points IMHO to the most likely cause of the observed waveforms.

      https://news.ycombinator.com/item?id=47931024

Datasheet shows 2 (which is a bit unusual, one for VDD and one for VDDIO soooo very much "RTFM" problem

Seems like a missed opportunity to try adding a capacitor dead-bug style onto the board to see if it cleans it up.

• dragontamer

  today at 1:27 AM

  If it's really 20MHz++ noise that's screwing him, you need something faster than a through hole capacitor IMO to deal with it.

  That being said, I'm not 100% convinced this is a 20MHz++ noise issue.

  • WarmWash

    today at 4:56 AM

    The capacitor doesn't have a concept of "fast enough", it's a passive component. The signal is what determines what it does when it encounters the capacitor. Non-linearities and capacitor species aside, a good ole x7r 100nF would clean this up.

    In general you can just liberally dump 100nF caps all over your pcb power traces and quash most problems like this before even knowing they exist. I joke that you make a circuit then take out your 100nF salt shaker to make it just right.

    • jacquesm

      today at 7:35 AM

      The capacitor has a self inductance. That's why you use low self inductance capacitors with very short leads or traces in this role. 100 nF ceramics are fine, but you may actually need a 100 nF and a 10 nF side-by-side because of that inductance depending on how dirty your power line is. Fast clocked circuitry can be pretty nasty.

    • dragontamer

      today at 6:18 AM

      Look up parasitic inductance.

      Through hole parts cap out at maybe low MHz. Many electrolytic caps frankly cannot effectively decouple signals above 100s of kHz even. Above that value, capacitors become inductors due to lead lengths, parasitic resistance, and other details.

      To make capacitors work faster, we make them smaller and smaller. Surface Mount Caps are the only way to reach 20MHz++ decoupling speeds, and you need crazier tricks if you need additional decoupling beyond that frequency.

  • sebcat

    today at 3:47 AM

    It's an easy test though and it can be an SMD component and some PUR-coated magnet wire or 30 awg single stranded kynar hookup wire.

    Use a small amount of glue from a hot glue gun to fixate it when done, or epoxy if that's your thing. Avoid cyanoacrylate. Not always needed but I imagine a drone moves around alot.

    Bodge wiring is a good skill to acquire - PCBs will not always be perfect. Maybe practice on something else first?

    • dragontamer

      today at 4:18 AM

      True.

      I have a bunch of through-hole parts for these sorts of situations. There are plenty of small through-hole ceramics that have leads if you really want to go there.

      https://www.digikey.com/en/products/detail/vishay-beyschlag-...

      Like this or something similar.

      • jacquesm

        today at 7:37 AM

        I've seen piggy backed decoupling caps straddling chips on some pretty fancy hardware. This lesson is re-learned quite frequently ;)

  • PunchyHamster

    today at 9:51 AM

    you can dead bug SMD caps

  • ErroneousBosh

    today at 7:18 AM

    > If it's really 20MHz++ noise that's screwing him, you need something faster than a through hole capacitor IMO to deal with it.

    That's always worked well enough in the past.

    • dragontamer

      today at 7:30 AM

      That's because you weren't dealing with 20MHz noise.

      Hobbyists are not dealing with 20MHz noise issues. Period. And if you are actually crazy enough to deal with high frequency circuits like that, you would well know that the land of through hole designs is simply insufficient, and that you are probably somewhere with some 0402 capacitors and some tweezers right now.

      • PunchyHamster

        today at 9:52 AM

        > That's because you weren't dealing with 20MHz noise.

        That is just straight up not correct

      • jacquesm

        today at 7:40 AM

        > Hobbyists are not dealing with 20MHz noise issues.

        It happens. Not often, but it does happen and it depends on the hobbyist and what they're up to (but you won't be sticking that together on a breadboard). Also: if you start using HCT, AHC or even G parts where you don't really need them it can happen to you in places where you don't normally expect it. Those things have crazy fast rise times.

        • dragontamer

          today at 7:57 AM

          Real talk: 6 layer oshpark is cheap enough for a hobbyist and there are a bunch of 500MHz / DDR2 parts that can be laid out. Like 0.8mm pitch BGAs can fit and breakout.

          So yeah. Hobbyists can go here. But here be dragons!!

          Nonetheless, I continue to assert that typical hobbyists are making mistakes at 100kHz region rather than the 100MHz region.

          • jacquesm

            today at 8:10 AM

            That's fair. It's just that I have seen some hobbyists doing the most insane stuff and eventually getting it to work. Some HAMs for instance have pretty extreme skills and it is not their profession, they just do it because they like it, not because they get paid.

            And in many of those cases their skills are hard capped by their budget for test gear and simulation software rather than by their actual ability. Keep in mind that until not that long ago anything above 1 G was fair game because 'nobody does anything there anyway' and so HAMs and radio astronomers were pretty much the only ones with experience in that region.

Having 1.5V Vpp ripple on a 3.3V supply rail seems more like an issue with the regulator / bulk capacitance than a decoupling capacitor, I would think?

• actinium226

  today at 1:44 AM

  Yea since writing this I think it has more to do with the regulator circuit. I plan to do a small rewrite and change the title to something like "When 3.3V isn't actually 3.3V" to more accurately reflect the situation. A decoupling cap would probably still help, but there were some mistakes made on the regulator circuit.

  • dragontamer

    today at 1:49 AM

    Switching regulators (and even linear regulators!!) have maximum capacitance ratings.

    Adding more capacitance could, in theory, further destabilize your regulator.

    The overall tank circuit (the inductor + capacitor forming the bulk of the switching circuit) is incredibly fragile.

    It's legend that some old switching designs stopped working as newer tantalum capacitors had less resistance, screwing with the stability of older switching designs. You kind of need to choose exactly the "expected" kind of capacitor (aluminum caps have more resistance, which increases stability of the feedback but slows down the feedback).

• dragontamer

  today at 1:31 AM

  Yeah. Decoupling capacitors are for smaller ripples than that.

  There might be a resonnance point on that regulator, or maybe a maximum capacitance that was violated on the feedback.

  There are a TON of ways to screw up your PDN on a PCB. It's nominally a master's degree level subject.

• analog31

  today at 3:56 AM

  Some small switching regulators go into a low power mode when the output current goes below a threshold. The frequency drops to some "hovering just above zero" level. I've had to artificially load a power supply, to get it to be stable, e.g., with a shunt resistor. Naturally, that's inefficient, so it goes onto the TODO list to improve the design.

• elch

  today at 6:21 AM

  1.5 Vpp ripple measured on a 40 MHz scope - when the waveform is 50 MHz according to him...

• boznz

  today at 4:57 AM

  decoupling is a real issue, but I think you are right in this case.

If getting a cap on the input of the magnetometer is too challenging, a ferrite bead on the output of the caps fed by the switching supply might also do the trick.

You could also try just sticking a 100n and 10n across the smps output too.

leared = learned ? The O'Reilly book "Designing Embedded Systems" covers this pretty well with a story very similar to yours. Great to be able to learn something new.

Meta; typo in title, should be "learNEd".

• xaxfixho

  today at 7:05 AM

  Quality *Learing* Center 1-800-FRAUD

> How I leared what radial magnetic emissions are, the hard way

Another lesson waiting in the wings from mounting a magnetometer in plane and right next to four BLCD motors, lmao.

The first time I saw a complex number used with units of resistance, I was like, huh?