Unwanted Noise

[imk]

Hello,

I am trying to debug a STM32 micro controller robot project and keep find this noise.
So i have switched off most stuff, and with a few inches of wire as an aerial in the scope input I get this noise.
I have tried it on two scopes and old Tektronix and a new hantek and can see the same signal on both of them.
it is about 806 khz Any ideas please?
I live in UK North West about 53n 2w
Many thanks in advance imk

[T3sl4co1l]

Oh hey, the tone bursts are opposite directions, it's actually 403kHz (well, a hair lower, 390kHz maybe?).  But yeah, same idea, you'll likely get all the harmonics/sidebands (i.e. spikes every 390kHz or so, clustered around the, 10MHz or so tone burst?) so it's not much of a distinction.

Jesus, it's actually a whole volt?  From a "few inches" of wire?  Even if there's something nasty plugged in elsewhere (reaching the scope by mains conduction), it'd be hard pressed to be that nasty I think.  Gotta be the scope itself?

Seems like it comes from a symmetrical square wave, and it's fairly fast suggesting a resonant power supply or something like that; maybe CCFL lights (which wouldn't be the scope, that'd use LED backlights right?), possibly a PoL (point of load DC-DC) converter but it would have to be something very luckily near 50%, like a 5 to 2.5V buck converter or something; even then, the rising and falling edges are rarely so symmetrical.

Tim

[imk]

Much searching:
Turning off mains socket circuits with built in usb chargers.
Turned of all switch mode PC's monitor, PC's, router, multi drop wireless phone, mobile phone, ethernet over over mains, cooker, microwave,  mains clocks, power circuit to workshop etc etc.
And it was the  Class D amplifier i use for my PC sound, think speakers are massive aerial for the 800kHz switch :-(
Going back to class AB :-)

[ConKbot]

That much from speakers?  Did they 'optimize' the output filter out of the BOM for that amp? Jeeze.

[imk]

lack of "output filter" on amp is problem.
I have put one of those little Yamaha's in it's place temporally and can now look for the real noise problem i have in this project.
imk