Mhz digital signals and board noise. - Page 1
Forum section: electronics / projects

author: paulca
author_rank: Super Contributor
timestamp: February 18, 2023, 02:31:05 pm
content: My project is getting close to moving to PCB+Modules phase. Some modules will be soldered into the parent PCB, or on female pin headers. Some will be "off board" on the end of a 4 or 5 core JST cable.On the breadboards with a mix of DuPoint and steel pin jumpers (they don't mix well as the duponts force open the jaws such that the steel pin jumpers don't make, or keep good contact.It's very noisey. I was seeing 400mVpp wide band multi-mhz noise on everything. Thankfully peppering the breadboards with 100nF decoupling caps on anything with power and adding 220uF bulk caps to each breadboard power rail instead of 10uF.... brought the analogue output noise down to 40-50mV wide bank noise.Noise is mostly digital squarewave wideband stuff from 48k up to 24Mhz and then the MCU processor/bus noise all the way up beyond what my scope sees.Part of me thinks, it's outside the audio band, the noise I can find IN the audio band is below my care level. The other part of me realises it's not nice to send multi-megahertz digital noise around one's room on unsheilded, unbalanced audio cables. I don't need to get it certified, but I'd like mine and the neighbours wifi to still workI can filter the analogue out. A few 1Mhz ferrites in strategic places would probably be a good start... also on the power connections.So, checklist for going PCB + Interconnects.* Decouple the wazzo all power rails. 100nF + 10nF* Bulk support each power hungry module. 10uF per device 400uF single bulk. Minimimse current gulps across PCB traces. etc.* Use USB cable for 4 core + sheild (I2S etc). <-- Do I ground the shield? Do I ground it at the least sensitive end, like the MCU and leave it off at the sensitive end (the DAC say).* Keep the signal traces as short as is feasible on the board... and on the interconnect wires.* Is it worth going to 4 layer with stack Power,GND,Signal,GND versus standard 2 layer signal/power + signal/gnd?The case will be 3D printed. The outputs have an "AGND" which I am hoping to couple to a battery powered analogue output section, including a headphone amp. That will come later. The ferrite solution will be my first attempt.* Is it worth lining the insides of the case in a rudimentary way with aluminium foil tape?Any other tips? The PCB is basically going to be a USB power stage (maybe even one of the MCU modules with a 3.3V 1Amp regulator on it). 2 x STM32H7 MCUs. A few audio modules, like optical/i2s bridges, usb bridges, ADC, DAC stuff. Everything is 3.3V until I get to that analouge output stage ... later.At the moment I'm just trying to damp the digital noise "generally". There are no requirements to make it ultra silent and I'm not that fussy. Just want to do what's right.

author: paulca
author_rank: Super Contributor
timestamp: February 18, 2023, 02:56:27 pm
content: Quote from: CHATGPT

author: Kahooli
author_rank: Contributor
timestamp: February 21, 2023, 12:23:15 am
content: If you would like a layout or schematic design review, post the files or send me a google docs link.

author: jonpaul
author_rank: Super Contributor
timestamp: February 21, 2023, 02:50:22 pm
content: Suspect just poor measurement of the noise.impossible to comment without1./ schematic, block , pcb layout2 / photo device, test set-up, scope photos3/ scopes and probes used?4/ exact probe and scope setup, techniquej

author: paulca
author_rank: Super Contributor
timestamp: February 21, 2023, 05:18:45 pm
content: At the moment it's on a breadboard with wires everywhere.It's likely the noise will be negligible when I move it to PCB anyway.This is it, photo take a while back, it's got a bit worse since.On PCB. This is what I drafted at lunch time. It doesn't show much I know. It was mostly the mechanical layout to get board dimensions and see how it might fit.The two header rows are duplicates such that I have a female to install the MCU, but also get access to all the pins for probing extending. Signal noise ... the main enemy is distance. These modules have a fixed size, unless I start going perpendicular/vertical daughter mounts (no!).For the clock generators. There are 3 of them. I put a solder bridge beside each's pin, so only one will be actually radiating on the board. All I can do is keep those traces as short, direct and "starred" as possible.

author: Kahooli
author_rank: Contributor
timestamp: February 21, 2023, 05:53:47 pm
content: Try to not have so many stubs on your clock lines. If you're picking a particular path, make sure its point to point.

author: paulca
author_rank: Super Contributor
timestamp: February 21, 2023, 06:27:17 pm
content: Quote from: Kahooli on February 21, 2023, 05:53:47 pmIt would be simplier if I picked one source clock. All 3 of the input masters have a "phile clock", I should just pick the closet and leave the rest off the PCB entirely. Having the option later, ... a single solder bridge I can hack into later will do.I'm also thinking of dropping the duplicate debug headers. They are a pain to trace up. They occupy a full layer of noise traces that everything has to cross under or around. If I need probe points I can jack the module up, clip onto the headers or solder mod wire where I need. Note. Some of these modules are already "pinned", so not only do I need to be extremely careful of my mirror and flip on those headers, not all of them are "pinned" the same way up. It took me a while to twig it's better to put the pins out of bottom where the labels are, rather than out the top so it's right way up and pretty.Whatever I do, I think it will be 1000 times better than the breadboard with is starting to do my head in. It becomes so twitchy and sensitive at times, goes nuts any time you touch it. The last project that got that way did very well going to a basic module interconnect PCB.I was also hoping to load all the digital signal lines with 100 Ohm series resistors. Just to calm the overshoot/undershoots and provide a little protection.Should I also do the master clocks lines or are they just too sensitive and 100 Ohms + parasitic .... nevermind I'll run it through LTSpice first.

author: paulca
author_rank: Super Contributor
timestamp: February 21, 2023, 06:39:32 pm
content: Quote from: jonpaul on February 21, 2023, 02:50:22 pmYou see I'm never trying to measure the noise. I'm trying to measure something else and I can see the "wall" effect on the scope (caused by persistence). Instead of a line I get a wall about X mV high. I say X as it depends on the day, the signal line in question etc. I had a post in "beginners" I think about how to get rid of it using scope maths. I was measuring a current sense, expecting maybe a 0-4mV signal, but the signal was trashed by a 40mV peak to peak noise band. I could see the signal but only as this wall/band moving up and down.My understanding and you can correct me is the "wall" I am seeing, confirmed zooming in, is a much higher megahertz signal than I am triggering on. The fake persistence on the DSO shows that by ghosting it on the slower trigger.My square waves have "fur" on the top and button of them. Peak 2 Peak often twice VDD around 7V.My clocks no longer look like sad sinewaves they look like messed up triangle waves with fur near the top.My board to board SPI connects are ... very poor and constantly erroring. The dataline transistions are just too noisy. The scope can decode it, but it's probing the source micro.Add in breadboard "fissle" if you move near it.I can do some measurements if you want. I think I'll do this prototype PCB, get everything soldered together with ground planes and see how the noise is then.