Meta; typo in title, should be "learNEd".
How I leared what a decoupling capacitor is for, the hard way
11–20 of 91 posts
Re: How I leared what a decoupling capacitor is for, the hard way
#12This 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…
Cannot it be a noise from imperfect switching? The switching occurs at lower frequency, and the noise is high frequency.
Re: How I leared what a decoupling capacitor is for, the hard way
#13Earlier 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.
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.
Re: How I leared what a decoupling capacitor is for, the hard way
#14This 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…
>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".
Re: How I leared what a decoupling capacitor is for, the hard way
#15This 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…
Cannot it be a noise from imperfect switching? The switching occurs at lower frequency, and the noise is high frequency.
[deleted]
Re: How I leared what a decoupling capacitor is for, the hard way
#16Seems 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.
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.
Re: How I leared what a decoupling capacitor is for, the hard way
#17Having 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?
decoupling is a real issue, but I think you are right in this case.
Re: How I leared what a decoupling capacitor is for, the hard way
#18Ah 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.
Re: How I leared what a decoupling capacitor is for, the hard way
#19This 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...
Re: How I leared what a decoupling capacitor is for, the hard way
#20If 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.