How I leared what a decoupling capacitor is for, the hard way(nbelakovski.substack.com) |
How I leared what a decoupling capacitor is for, the hard way(nbelakovski.substack.com) |
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.
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).
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.
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.
Everyone does. There's probably a layer below for everyone but the most theoretical physicists. I don't know where the leaks in electronics engineering's abstractions are, but I'm pretty sure they exist.
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.
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.
Phil also recommends this lecture in one of his videos [3], which is still one of my all time favourite lectures ever.
[1] https://www.youtube.com/@PhilsLab
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.
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.
This is utter nonsense. Just ask the layouter where they will be placed. (at the output of the voltage regulator or where he will find empty space on the board, completely missing their function). Where your schematics is bad, the layout will be also bad.
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.
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_...
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".
If they don't find fraud, is it "questionable"?
That's more than good enough for the purpose of checking interference
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...
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).
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.
> 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.
That being said, I'm not 100% convinced this is a 20MHz++ noise issue.
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.
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?
That's always worked well enough in the past.
- 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
[1] https://www.richtek.com/Design%20Support/Technical%20Documen...
You could also try just sticking a 100n and 10n across the smps output too.
Another lesson waiting in the wings from mounting a magnetometer in plane and right next to four BLCD motors, lmao.
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.
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
Putting your decoupling capacitors next to the power pins _does_ cost. Not just in board space, but I've seen and reviewed layouts where the signal traces had to snake around decoupling caps or in some cases through vias because the designer believes that putting the caps close to the pins was the most important thing...
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.
https://news.ycombinator.com/item?id=47931024
PS Now that I’ve taken a closer look, this is even sillier than I first thought.
He’s hunting for 50 MHz ghost signals by connecting his PCB to a breadboard using (crappy) wires that are at least 10 cm long. And he’s connecting the scope probe to the breadboard (or those breadboard wires).
And if I’m not mistaken, he doesn’t even bother to connect the ground lead of the probe.
Inadequate simulator, then, no?
(I imagine analogue RF board-level simulation is a lot more expensive than digital-logic board-level simulation. Might have been impractical way back when, such that we only used to have the digital-logic kind. But we certainly have both kinds today.)
I did read there was some amount of fraud in state sponsored care programs but nothing extensive.
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.
Across distances according to the power available, where ariel orientation makes a big difference, "as expected".
All it does is provide yet more profound questions.
When a kid of about four I found a pair of WWII headphones and took them to my father who pulled out the iron diaphragm and showed me magnetism at work—somehow some magical force was pulling the diaphragm back into the headphone with seemingly nothing in between. Absolutely fascinated, I wanted to know what this invisible 'magic' was. Many decades later every time I look at my fridge magnets I still ask the same question and I don't believe I'm much closer to the truth!
Sure, there are the simple answers everyone's taught, then there's QFT but even that doesn't tell me exactly what's going on. And why does alpha have the value it does, and why exactly does c = 1/(μ0ε0)^1/2? Not knowing and not being able to figure these questions out is, at times, infuriating.
For me, solace of sorts can be found in engineering—I can build an electronic circuit and end up with a tangible working device. On the way I'll curse my electrons for making so much noise that they sound like ball bearings rattling around in an empty oil drum but I'll eventually calm down and apply Johnson–Nyquist to shut them up (well, a little bit anyway).
I got sucked into the infinite perspective vortex from the cosmology angle - a grandmother with a vast collection of pulp sci fi and clear skies over her canal boat. And yes, magnets, what is this devilry and why will nobody tell me how it works?
Upon discovering said imponderables I moved to the woods instead - building infrastructure of all varieties from the ground up, playing with hydropower, that sort of thing - and of course EE gets jammed in all over the place, and when I can get away with something simple and analogue, I do.
I build things because the alternative is spending tracts of time staring into the abyssal fact that explanation is always ultimately internal to the system being explained.
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.
That is just straight up not correct
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.
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.
What makes you think that?
Edit: FWIW I consider 20MHz to be basically audio.
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.
Capacitance value is essentially irrelevant to inductance. The thing that dictates inductance of a decoupling capacitor is essentially only the package size. The way to fix that is to use smaller (physical) sized components. The only reason it may be related is that some larger values of capacitance physically can't fit into tiny package sizes. For 100nF, you have essentially no restrictions though. 0201(i) are easy to find in that value.
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.
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.
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?
MCP6002 jellybean is GBP of 1MHz. Most "jellybean" OpAmps I know cap out at 10MHz GBWP (aka: a useless 10x gain at 1 MHz).
LM358B is 1.2 MHz GBWP or even 700kHz depending on the design. Magnitudes away from 20MHz especially when you want more than 1.0x gain.
If you want even 10x gain (aka around 10% error), you might suffice with 200MHz GBWP at 20MHz. Or maybe get a nice ADC and just go all digital given today's equipment...
Come on man. Typical "high speed" OpAmps are like 100MHz GBWP or less... correlating to only 5x gain at 20MHz. This sort of stuff is well outside "jellybean" amps. And I'm not even sure if a 100MHz amp is very effective at 20MHz.
They cost pennies.
Looks like they're closer to $3+? Like this is more expensive than a lot of microcontrollers for just one OpAmp.
I wouldn't call any Analog Devices / Linear Technologies parts to be a jellybean. Jellybean means matching common specs that any manufacturer can do.
But at $3 per chip, that's very far away from jellybean status. And AD / LT devices are known for high quality that no one else replicates.
> although THD is getting a little rough up there
Yeah, because a 100MHz high speed OpAmps only has 5x gain at 20MHz. So your error is now well in excess of like 20% (best case) or probably way way worse errors in practice.
As I said earlier: Im not even convinced that 100MHz OpAmps have the GBWP to effectively operate on 20MHz signals. You need more GBWP than that to be comfortable.