High pitched whistling coming from a power supply - Page 1
Forum section: electronics / projects

author: tnt
author_rank: Regular Contributor
timestamp: June 01, 2011, 07:51:19 am
content: Hi,Yesterday I finished my adaptater board to be able to plug my Agilent 1152A probe into a Agilent 3000-X.The adapter is mostly just a LDO bringing down the +5V from the scope to +4V needed by the probe. (It has other stuff but not relevant).Now when I plug in that probe, I can hear the front panel (where the probe PSU is on this scope, see the teardown) making a high pitched whistling.I removed it immediately and tested with a support 1158A probe and it works fine with it without sound.I then tried to power the probe using a PC power supply (providing +5V -12V +12V) and when I do that, the PC power supply _also_ starts to emit the same sound.I checked the DC power consumptions and it looks fine according to spec 85mA on +5V, 5mA on +12V and 85mA on -12V.I also confirmed that when powered, the probe works (i.e. it passes the scope calibration just fine).What am I missing ?Cheers,Sylvain

author: mobbarley
author_rank: Regular Contributor
timestamp: June 01, 2011, 08:00:07 am
content: Have you scoped the power output of the PC supply before and after connecting your probe adapter ?

author: Mechatrommer
author_rank: Super Contributor
timestamp: June 01, 2011, 08:23:35 am
content: some high frequency (few KHz) switching/oscillation circuit goin on.

author: tnt
author_rank: Regular Contributor
timestamp: June 01, 2011, 08:35:14 am
content: Quote from: mobbarley on June 01, 2011, 08:00:07 amNot yet no.Quote from: Mechatrommer on June 01, 2011, 08:23:35 amYes but why ?I don't see what would oscillate in a fet probe.The probe head I don't know what's in there but I doubt they'd put a switcher next to the sensing circuit.The little box that connect to the scope is just some power supply filtering (cap / feritte) and an opamp. There is also a big transistor but I'm not sure what purpose it serves ...

author: Rufus
author_rank: Super Contributor
timestamp: June 01, 2011, 05:03:00 pm
content: Quote from: tnt on June 01, 2011, 07:51:19 amYour LDO is unstable and oscillating at a few kHz?

author: Zero999
author_rank: Super Contributor
timestamp: June 01, 2011, 05:45:55 pm
content: Yes, what regulator are you using?The oscillation can cause piezoelectric materials used to make ceramic capacitors vibrate.

author: tnt
author_rank: Regular Contributor
timestamp: June 01, 2011, 08:53:36 pm
content: The regulator is a LT1761 and it has 4.7 uF ceramic capacitors on the input and output.Note that the noise is coming from the PSU powering the probe and not from the probe box (where my circuit is housed).

author: vk6zgo
author_rank: Super Contributor
timestamp: June 02, 2011, 09:07:45 am
content: Make up a passive load drawing the same current,see if you still get the whistling.If something is oscillating in your probe,the average current may be the same,but the peaks may be much higher.I had a similar problem,but on a much larger scale with a radio transmitter used in an ISM application.It had a circuit that set various power levels,& maintained the selected power level by adjusting the RF drive.At some power settings,an Op Amp in this circuit would go into oscillation,& Amplitude Modulate the Transmitter.The average current at this time looked OK,but the peak currents were enough to cook the largeSwitch Mode Power Supply.The first symptom was a loud whistling sound!If the Transmitter had been for Broadcasting or Communications,the problem would have been obvious,but in the service in which it was used,it slipped by.VK6ZGO

author: tnt
author_rank: Regular Contributor
timestamp: June 26, 2011, 10:48:35 pm
content: Sorry for digging up an old thread but since I found the solution ...I hadn't touched the project in a couple of weeks, being busy with other stuff, but tonight I had another look at it and saw the problem.The LDO I use has a 'BYP' input where you can put a 10n cap between it and the output to improve regulation response (less noise).Instead of connecting it between BYP and the output, and connected it between BYP and GND, making the whole thing unstable.When powered from a linear supply, there was no issue but when you powered it from a switcher (like a PC supply or the scope psu) then it oscillate.The PCB was too tight to really fix the traces so since that cap is optional, I just removed it and no whisting anymore. And the probe works.