"singing" board (audible noise), how to pinpoint? - Page 1
Forum section: electronics / projects

author: roland
author_rank: Newbie
timestamp: February 21, 2020, 05:43:52 pm
content: We have a small board (roughly 50mm * 150mm) with medium to complex design (quad core ARM Cortex A17, display, some LEDs, loudspeaker, microphones, networking/WiFi) etc. which is working nicely but there is one big issue: the board produces a clearly noticable, high-pitched whining tone (about 5kHz). After EMC testing of the product, we needed to fit an EMC shield over the CPU and RAM, and this has now unfortunately amplified this noise...I know the usual suspects, we have checked all inductors and MLCCs near the power supply, DC-DC converters (power companion chip) etc., however we didn't find anything yet that might correspond even remotely to the 5kHz we're seeing (or better, hearing).Does anyone here have a clue about a method how to pinpoint the issue? Is there any kind of measurement we could do (some special directed microphone or something) that might help to find the root cause of this issue? At the moment we're more or less cluelessly probing around with the scope or running over the board with a mobile phone's microphone (and FFT app). However, since the root case element seems to let the whole board vibrate, it is very difficult to find the element that produces the waveform in the first place. I guess if you had a guitar and didn't know that the string actually produced the sound, you could probe around the body quite some time because the sound seems to come out of the resonant body.Any idea is appreciated that might help.

author: Twoflower
author_rank: Frequent Contributor
timestamp: February 21, 2020, 05:47:49 pm
content: Use a small tube as stethoscope and scan the PCB with it.

author: ogden
author_rank: Super Contributor
timestamp: February 21, 2020, 06:06:17 pm
content: Quote from: Twoflower on February 21, 2020, 05:47:49 pmUse PC with microphone. Hint: old phones came with (wired) headset+mic, you may still have such in scrapbox. Otherwise just buy it for buck or two:

author: Siwastaja
author_rank: Super Contributor
timestamp: February 21, 2020, 06:44:55 pm
content: Or, use a toothpick or similar to touch/press components one by one, likely a small amount of compression makes the sound stop, helping you pinpoint the suspect.

author: Bud
author_rank: Super Contributor
timestamp: February 21, 2020, 07:48:05 pm
content: Try using a 1uF ceramic capacitor on a toothpick or on a cotton swab stick as a vibration sensor , wired to the oscilloscope input to monitor the amplitude.

author: Miti
author_rank: Super Contributor
timestamp: February 21, 2020, 07:58:04 pm
content: Drop distilled water selectively from the tip of a toothpick or from a needle.

author: FenTiger
author_rank: Regular Contributor
timestamp: February 21, 2020, 08:32:14 pm
content: Clamp the board somehow so it doesn't vibrate, and your microphone might do a better job of locating the source.Alternatively, look more closely at the waveform, in both time and frequency domains - maybe this'll give you a better clue than the 5kHz frequency alone.Are there many clocks or other fixed frequencies on your board? Put the microphone signal on one channel of a scope and look at the clocks etc on another channel. Is the noise synchronous with one of them? If so, you've narrowed down its source somewhat.

author: KMoffett
author_rank: Regular Contributor
timestamp: February 21, 2020, 08:44:14 pm
content: Get a cheap stethoscope. Remove the head (bell and diaphragm). Just use the tube to snoop out noises. Very handy.https://www.amazon.com/Ever-Ready-First-Aid-Stethoscope/dp/B00HW8EEOG/ref=lp_3777201_1_1?s=industrial&ie=UTF8&qid=1582317693&sr=1-1

author: DaJMasta
author_rank: Super Contributor
timestamp: February 21, 2020, 10:01:51 pm
content: Quote from: FenTiger on February 21, 2020, 08:32:14 pmClamp and even damp if required, some sort of vibration reducing material (foam, rubber, etc.) pressed against the bottom should keep the board from amplifying so it's easier to find the source. You don't even need a directional mic for the test if you're sensitivity is high enough that you can pinpoint it with intensity alone, so most small mics should do the job.

author: jonpaul
author_rank: Super Contributor
timestamp: February 23, 2020, 11:45:45 am
content: As I was a power electronics EE, for decades I may have a clew.Singing SMPS check the on-board regulators for sub cycle oscillation, the SMPS is skipping cycles.Inductor singing is the symptom not the causeJon

author: Ice-Tea
author_rank: Super Contributor
timestamp: February 23, 2020, 11:51:23 am
content: Quote from: Siwastaja on February 21, 2020, 06:44:55 pm^^This.

author: Psi
author_rank: Super Contributor
timestamp: February 23, 2020, 11:51:26 am
content: Could also try a ~5khz disc piezo with a toothpick or wire glued to it.Hold the thing with some foam to let it vibrate freely.Then touch the toothpick to various parts while watching Oscope.if that doesn't work, you could try generating a beat frequency.Pick a source frequency that, when mixed with the 5hz noise, will generate a unique beat frequencyFeed that frequency into the piezo then go around touching parts and listening for the new sound.

author: Psi
author_rank: Super Contributor
timestamp: February 23, 2020, 12:17:20 pm
content: Could also try doing an overclock/underclock on the CPU and other peripheral clocksSee what, if any, affects the sound frequency.Might tell you something.

author: SiliconWizard
author_rank: Super Contributor
timestamp: February 23, 2020, 03:27:42 pm
content: Quote from: roland on February 21, 2020, 05:43:52 pmHow did you proceed then? Because yes those definitely look like the usual suspects.Ceramic capacitors when this happens will transmit vibrations to the PCB and it's sometimes hard to locate the source precisely (until of course you shut it down).

author: RoGeorge
author_rank: Super Contributor
timestamp: February 23, 2020, 04:16:54 pm
content: Take a wooden or a plastic rod, and use it like a soundwaves guide.Touch one head of the rod to the ear cartilage, and the other head of the rod to various parts on the PCB. You'll be surprised how effective this could be. If too loud, use the head bone from outside of the back of the ear.First suspects will be the coils with magnetics (e.g. SMPS coils). MLCC capacitors might sing, too.What tune is it singing, tho'?

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 23, 2020, 10:03:13 pm
content: Do you have any much faster clocks that are about 5 kHz apart?

author: richnormand
author_rank: Unknown
timestamp: February 23, 2020, 11:41:50 pm
content: Years ago I had a similar issue on a large board with some high voltage on it too (3-4kV). I could hear it fine but the high pitch made it hard to locate. It was at a higher frequency but with several lower harmonics that were angle sensitive. I used an electret microphone and spectrum analyser and got a good signal. Due to the high frequency there was a complex pattern in intensity distribution in the far field due to interferences. The interference pattern was pretty sharp however either indicating a near point source with diffraction pattern or multiple sources in phase with each other.Solution: a 6 foot length of 1/4" tygon flexible tubing. One end in my ear and the other one taped to a small stick scanning the pcb at a few mm height of components while using the length of the tube to stand away from the assembly.Turned out to be a ceramic capacitor that was signing.People suggesting using a microphone could consider coupling a long flexible tube to it to probe the nearfield and isolate individual components.Best of luck with it!

author: roland
author_rank: Newbie
timestamp: February 24, 2020, 07:47:43 am
content: Guys, thank you all very much for your numerous great suggestions. I have ordered some stuff (microphone, tubing, stethoscope) and will let you know when I've found the culprit. Thanks!

author: RoGeorge
author_rank: Super Contributor
timestamp: February 24, 2020, 10:39:21 am
content: The rod I was telling can be a simple thin, dry and stiff stick from outside, the tubing can be a simple sheet of paper rolled, it will act as a rod and as a tube in the same time.

author: NiHaoMike
author_rank: Super Contributor
timestamp: February 24, 2020, 01:43:49 pm
content: Quote from: KMoffett on February 21, 2020, 08:44:14 pmOr 3D print one.https://hackaday.com/2018/03/17/3d-printed-stethoscope-makes-the-grade/