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

(nbelakovski.substack.com)

37 points by actinium2262 days ago

3 comments

userbinator4 minutes 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...<a href="https://en.wikipedia.org/wiki/6-meter_band#Radio_control_hobby_use" rel="nofollow">https://en.wikipedia.org/wiki/6-meter_band#Radio_control_hob...</a>And "leared" -- the (unintentional?) pun made me click.
oakwhiz2 hours ago
Seems like a missed opportunity to try adding a capacitor dead-bug style onto the board to see if it cleans it up.
- dragontamer2 hours 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.
 - sebcat4 minutes 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?
hadrietta2 hours 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?
- actinium2262 hours 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.
 - dragontamer2 hours 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).
- dragontamer2 hours 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.