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

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

#51

Post by baq »

hilbert42 wrote: post #45
lefra wrote: post #34

Earlier quoted context omitted.

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,…

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

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

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

#52

Post by hilbert42 »

Lerc wrote: post #21
hilbert42 wrote: post #18

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…

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 the…

"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.

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

#54

Post by PunchyHamster »

nippoo wrote: post #19

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 d…

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

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

#55

Post by PunchyHamster »

elch wrote: post #24
userbinator wrote: post #9

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_ban…

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

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

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

#56

Post by PunchyHamster »

dragontamer wrote: post #3
oakwhiz wrote: post #2

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

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.

you can dead bug SMD caps

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

#57

Post by PunchyHamster »

dragontamer wrote: post #41
ErroneousBosh wrote: post #36

Earlier quoted context omitted.

> 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.

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.

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

That is just straight up not correct

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

#58

Post by elch »

PunchyHamster wrote: post #55
elch wrote: post #24

Earlier quoted context omitted.

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

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

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

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.

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

#59

Post by peterfirefly »

nielsbot wrote: post #28
JCTheDenthog wrote: post #14

Earlier quoted context omitted.

>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".

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

If they choose not to look, yes.

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

#60

Post by vintermann »

madaxe_again wrote: post #27
hilbert42 wrote: post #18

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…

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 f…

> you end up with people building ziggurats atop an ocean of incomprehension.

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.

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