5 ms·

How I leared what a decoupling capacitor is for, the hard way

174 points by actinium226 5mo ago

oakwhiz 5mo ago
Seems like a missed opportunity to try adding a capacitor dead-bug style onto the board to see if it cleans it up.
- dragontamer 5mo ago
  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.
  - sebcat 5mo ago
    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 5mo ago
      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-draloric-bc-components/K104K15X7RF5TL2/286538 https://www.digikey.com/en/products/detail/vishay-beyschlag-... Like this or something similar.
      - jacquesm 5mo ago
        I've seen piggy backed decoupling caps straddling chips on some pretty fancy hardware. This lesson is re-learned quite frequently ;)
  - WarmWash 5mo ago
    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.
    - dragontamer 5mo ago
      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.
      - WarmWash 5mo ago
        Yes, but we are splitting hairs at that point. The transient spike is a high impedance voltage that is tripping the high impedance internal protection circuitry of the magnetometer. So whether we have 20mOhms of capacitive decoupling or 500mOhms of inductive decoupling, both are better than the infinite impedance of nothing there. We're not building a precision filter, were cutting the paws off of a paper tiger. No need to let perfect be the enemy of good.
        dragontamer 5mo ago
        This is a circuit with a switching regulator that is, presumably, stabilized with something on the order of a 10uH inductor + 22uF capacitor. So from my perspective, increasing the capacitance from 22uF on that output line to 22.1uF with a 100nF cap will likely do jack diddly shit. It is far more likely that, ex, the author of this post screwed up the regulator design. Ex: did the author mistakenly think that more capacitance is better-er and stick a 100uF cap there, blowing out the phase margin of the feedback of the switching regulator? Was the inductor properly sized? Not just inductance but also saturation current and internal resistance?
  - ErroneousBosh 5mo ago
    > 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 5mo ago
      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.
      - jacquesm 5mo ago
        > 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 5mo ago
        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 5mo ago
        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.
  - PunchyHamster 5mo ago
    you can dead bug SMD caps
hadrietta 5mo ago
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?
- dragontamer 5mo ago
  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.
- actinium226 5mo ago
  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 5mo ago
    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).
- analog31 5mo ago
  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.
userbinator 5mo ago
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_hobby_use https://en.wikipedia.org/wiki/6-meter_band#Radio_control_hob... And "leared" -- the (unintentional?) pun made me click.
- codedokode 5mo ago
  Cannot it be a noise from imperfect switching? The switching occurs at lower frequency, and the noise is high frequency.
  - deleted 5mo ago
    [deleted]
- JCTheDenthog 5mo ago
  >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 5mo ago
    > questionable daycares If they don't find fraud, is it "questionable"?
    - peterfirefly 5mo ago
      If they choose not to look, yes.
    - bavell 5mo ago
      Didn't the guy flee the country after posting bail? Doesn't exactly scream "innocent".
unwind 5mo ago
Meta; typo in title, should be "learNEd".
- xaxfixho 5mo ago
  Quality *Learing* Center 1-800-FRAUD
hilbert42 5mo ago
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.
- Lerc 5mo ago
  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 5mo ago
    "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_Series 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%20-%20Microwave%20Receivers.pdf 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.
nippoo 5mo ago
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=2210&context=ele_comeng_facwork https://scholarsmine.mst.edu/cgi/viewcontent.cgi?article=221...
- laydn 5mo ago
  Great paper!. Anyonw know whether there are any modern tools/software that can simulate this during design?
  - lefra 5mo ago
    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.
- fps-hero 5mo ago
  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 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-spaced-planes https://learnemc.com/decoupling-for-boards-with-widely-space...), or arranging capacitors in alternating or doublet layouts (https://incompliancemag.com/decoupling-capacitor-design-on-pcbs-to-minimize-inductance-and-maximize-emi-performance/ 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.
WarmWash 5mo ago
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.
themafia 5mo ago
The first time I saw a complex number used with units of resistance, I was like, huh?
kreelman 5mo ago
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.
moffkalast 5mo ago
> 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.
PunchyHamster 5mo ago
Datasheet shows 2 (which is a bit unusual, one for VDD and one for VDDIO soooo very much "RTFM" problem
frangonf 5mo ago
To makers that want to play and learn with power converters I recommend you: - Test the converter at various points of load (when prototiping keep some 0ohm resistor/jumper for attaching a resistor load or electronic load). - When you have to measure things, look around app notes/white papers of manufacturers, you will usually find practical actionable info and some examples. Doing proper measurements is really a discipline of its own, but for low frequency you can get far with the basics of craftsman/rule of thumb engineering. [0] [1] For example the author here in the videos is mostly measuring the inductance loop between the positive of the rail and wherever ground is (we cannot even see where the osc negative is??) and how this particular loop responds to a cap, not the real bus. [0] https://www.analog.com/en/resources/app-notes/an-1144.html https://www.analog.com/en/resources/app-notes/an-1144.html [1] https://www.richtek.com/Design%20Support/Technical%20Document/AN079 https://www.richtek.com/Design%20Support/Technical%20Documen...
exDM69 5mo ago
Slightly related note, the pictures in the articles show a handheld digital oscillscope. It's an Owon HDS200 series oscilloscope with signal generator and they are amazing and the lower frequency models are quite inexpensive. I got myself one earlier this year and it does what it says on the tin. It can also be controlled from a computer via USB serial connection using a text based protocol (albeit poorly documented and a bit buggy). I used some python scripts to program the signal generator and then capture some measurements from the scope to check the frequency responses of some analog electronics circuits for guitar. There is a small community around, there are a few repos on GitHub for using them and also this very long eevblog thred. https://www.eevblog.com/forum/testgear/owon-hds-200-handheld-oscilloscope-w-builtin-dmmawg/ https://www.eevblog.com/forum/testgear/owon-hds-200-handheld...
kazinator 5mo ago
I think the author's analysis of the problem is off. He writes about ripple from the regulator: > This switching causes ripples in the voltage line, But what is on the scope is not that ripple: > Take another look at the pictures of the ripples above and notice the “M: 20ns” in the top left corner. This signifies that each vertical dotted line is 20ns apart, so the ripple you see has a frequency of something like 50MHz. The switching regulator does not operate anywhere near 50 Mhz. Those voltage fluctuations are caused by the magnetometer itself: its own internal switching causing rapid current demand fluctuations. Or, possibly, it could be some other nearby device, in which case that device needs the decoupler (also). This is why the decoupling capacitor addresses the problem. The purpose of the decoupling capacitor isn't to filter power supply ripple, but to provide a local, low-impedance current source that can swallow changes in current demand. That's why it's placed close to the device. It not only ensures that the device has smooth power, but also reduces the noise that it generates, protecting other devices.
- Neywiny 5mo ago
  I don't think you're entirely correct here. The regulator will create noise that has a fundamental frequency of 500 kHz. That we agree on. However, this 50 MHz noise can most certainly be caused by the regulator. It's critical to note that it's not a constant 50 MHz noise. It's clearly attenuating quickly and occurring sharply. If the author had zoomed out to the 500 kHz time scale, I'd bet we'd see this noise every time the switches change.

174 points by actinium226 5mo ago

oakwhiz 5mo ago

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

dragontamer 5mo ago
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.
- sebcat 5mo ago
  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 5mo ago
    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-draloric-bc-components/K104K15X7RF5TL2/286538 https://www.digikey.com/en/products/detail/vishay-beyschlag-... Like this or something similar.
    - jacquesm 5mo ago
      I've seen piggy backed decoupling caps straddling chips on some pretty fancy hardware. This lesson is re-learned quite frequently ;)
- WarmWash 5mo ago
  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.
  - dragontamer 5mo ago
    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.
    - WarmWash 5mo ago
      Yes, but we are splitting hairs at that point. The transient spike is a high impedance voltage that is tripping the high impedance internal protection circuitry of the magnetometer. So whether we have 20mOhms of capacitive decoupling or 500mOhms of inductive decoupling, both are better than the infinite impedance of nothing there. We're not building a precision filter, were cutting the paws off of a paper tiger. No need to let perfect be the enemy of good.
      - dragontamer 5mo ago
        This is a circuit with a switching regulator that is, presumably, stabilized with something on the order of a 10uH inductor + 22uF capacitor. So from my perspective, increasing the capacitance from 22uF on that output line to 22.1uF with a 100nF cap will likely do jack diddly shit. It is far more likely that, ex, the author of this post screwed up the regulator design. Ex: did the author mistakenly think that more capacitance is better-er and stick a 100uF cap there, blowing out the phase margin of the feedback of the switching regulator? Was the inductor properly sized? Not just inductance but also saturation current and internal resistance?
- ErroneousBosh 5mo ago
  > 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 5mo ago
    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.
    - jacquesm 5mo ago
      > 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 5mo ago
        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 5mo ago
        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.
- PunchyHamster 5mo ago
  you can dead bug SMD caps

hadrietta 5mo ago

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?

dragontamer 5mo ago
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.
actinium226 5mo ago
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 5mo ago
  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).
analog31 5mo ago
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.

userbinator 5mo ago

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_hobby_use https://en.wikipedia.org/wiki/6-meter_band#Radio_control_hob... And "leared" -- the (unintentional?) pun made me click.

codedokode 5mo ago
Cannot it be a noise from imperfect switching? The switching occurs at lower frequency, and the noise is high frequency.
- deleted 5mo ago
  [deleted]
JCTheDenthog 5mo ago
>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 5mo ago
  > questionable daycares If they don't find fraud, is it "questionable"?
  - peterfirefly 5mo ago
    If they choose not to look, yes.
  - bavell 5mo ago
    Didn't the guy flee the country after posting bail? Doesn't exactly scream "innocent".

unwind 5mo ago

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

xaxfixho 5mo ago
Quality *Learing* Center 1-800-FRAUD

hilbert42 5mo ago

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.

Lerc 5mo ago
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 5mo ago
  "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_Series 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%20-%20Microwave%20Receivers.pdf 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.

nippoo 5mo ago

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=2210&context=ele_comeng_facwork https://scholarsmine.mst.edu/cgi/viewcontent.cgi?article=221...

laydn 5mo ago
Great paper!. Anyonw know whether there are any modern tools/software that can simulate this during design?
- lefra 5mo ago
  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.
fps-hero 5mo ago
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 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-spaced-planes https://learnemc.com/decoupling-for-boards-with-widely-space...), or arranging capacitors in alternating or doublet layouts (https://incompliancemag.com/decoupling-capacitor-design-on-pcbs-to-minimize-inductance-and-maximize-emi-performance/ 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.

WarmWash 5mo ago

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.

themafia 5mo ago

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

kreelman 5mo ago

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.

moffkalast 5mo ago

> 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.

PunchyHamster 5mo ago

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

frangonf 5mo ago

To makers that want to play and learn with power converters I recommend you: - Test the converter at various points of load (when prototiping keep some 0ohm resistor/jumper for attaching a resistor load or electronic load). - When you have to measure things, look around app notes/white papers of manufacturers, you will usually find practical actionable info and some examples. Doing proper measurements is really a discipline of its own, but for low frequency you can get far with the basics of craftsman/rule of thumb engineering. [0] [1] For example the author here in the videos is mostly measuring the inductance loop between the positive of the rail and wherever ground is (we cannot even see where the osc negative is??) and how this particular loop responds to a cap, not the real bus. [0] https://www.analog.com/en/resources/app-notes/an-1144.html https://www.analog.com/en/resources/app-notes/an-1144.html [1] https://www.richtek.com/Design%20Support/Technical%20Document/AN079 https://www.richtek.com/Design%20Support/Technical%20Documen...

exDM69 5mo ago

Slightly related note, the pictures in the articles show a handheld digital oscillscope. It's an Owon HDS200 series oscilloscope with signal generator and they are amazing and the lower frequency models are quite inexpensive. I got myself one earlier this year and it does what it says on the tin. It can also be controlled from a computer via USB serial connection using a text based protocol (albeit poorly documented and a bit buggy). I used some python scripts to program the signal generator and then capture some measurements from the scope to check the frequency responses of some analog electronics circuits for guitar. There is a small community around, there are a few repos on GitHub for using them and also this very long eevblog thred. https://www.eevblog.com/forum/testgear/owon-hds-200-handheld-oscilloscope-w-builtin-dmmawg/ https://www.eevblog.com/forum/testgear/owon-hds-200-handheld...

kazinator 5mo ago

I think the author's analysis of the problem is off. He writes about ripple from the regulator: > This switching causes ripples in the voltage line, But what is on the scope is not that ripple: > Take another look at the pictures of the ripples above and notice the “M: 20ns” in the top left corner. This signifies that each vertical dotted line is 20ns apart, so the ripple you see has a frequency of something like 50MHz. The switching regulator does not operate anywhere near 50 Mhz. Those voltage fluctuations are caused by the magnetometer itself: its own internal switching causing rapid current demand fluctuations. Or, possibly, it could be some other nearby device, in which case that device needs the decoupler (also). This is why the decoupling capacitor addresses the problem. The purpose of the decoupling capacitor isn't to filter power supply ripple, but to provide a local, low-impedance current source that can swallow changes in current demand. That's why it's placed close to the device. It not only ensures that the device has smooth power, but also reduces the noise that it generates, protecting other devices.

Neywiny 5mo ago
I don't think you're entirely correct here. The regulator will create noise that has a fundamental frequency of 500 kHz. That we agree on. However, this 50 MHz noise can most certainly be caused by the regulator. It's critical to note that it's not a constant 50 MHz noise. It's clearly attenuating quickly and occurring sharply. If the author had zoomed out to the 500 kHz time scale, I'd bet we'd see this noise every time the switches change.