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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

#41

Post by dragontamer »

ErroneousBosh wrote: post #36
dragontamer wrote: post #3

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

> 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.

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

#42

Post by jacquesm »

WarmWash wrote: post #16
dragontamer wrote: post #3

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

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 circu…

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.

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

#43

Post by jacquesm »

dragontamer wrote: post #13
sebcat wrote: post #8

Earlier quoted context omitted.

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 el…

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.

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

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

#44

Post by jacquesm »

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.

> 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.

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

#45

Post by hilbert42 »

lefra wrote: post #34
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…

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.

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

#46

Post by dragontamer »

jacquesm wrote: post #44
dragontamer wrote: post #41

Earlier quoted context omitted.

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.

> 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.

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.

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

#47

Post by jacquesm »

dragontamer wrote: post #46
jacquesm wrote: post #44

Earlier quoted context omitted.

> 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.

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.

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.

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

#48

Post by ben_w »

jacquesm wrote: post #40
st_goliath wrote: post #35

Earlier quoted context omitted.

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 plac…

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…

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.

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

#49

Post by raverbashing »

fps-hero wrote: post #32
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…

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 via…

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

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