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 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.
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 they actually went.
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 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...
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...
It's not oscillating at 50MHz. Look at the waveform, with the big spike in the middle. That's a spike at some lower frequency, wider than the screen, followed by ringing. Need to zoom out the time base some more to see the period of the big spikes. It's no higher than 4 MHZ (the screen is 12 units wide) and possibly much lower. (Assuming that M:20ns on the display means 20ns/grid division. The manual is a bit hazy on that part of the UI.)[1]
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.
>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".
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.
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.
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?
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.
If 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.
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?
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.
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).
Some small switching regulators go into a low power mode when the output current goes below a threshold. The frequency drops to some "hovering just above zero" level. I've had to artificially load a power supply, to get it to be stable, e.g., with a shunt resistor. Naturally, that's inefficient, so it goes onto the TODO list to improve the design.
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.
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.
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".
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.
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.
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?
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.
You could also try just sticking a 100n and 10n across the smps output too.
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.
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).