Mains bourn noise?

[peteb2]

So i've read through as many posts here that make mention of problems with the Mains but this time i am flummoxed!

The background: I set up a fairly good workbench arrangement in my home and purchased a few bits of test gear. This would enable me to make repairs on the sorts of gear i would normally take care of in my usual job at my employer's office.

The thing i cannot understand is why do i have such an obvious level of noise superimposed on just about every kind of signal i scope? Much of it is audio preamp level stuff & 'scoping any medium level of signal on any of my  'scopes we see random stuff on top that signal. Today i'd had enough & systematically went through the whole house and powered off every appliance & unplugged it. I found one cheap LED table lamp that was 'radiating' plenty of spikey hash & promptly thought gotcha but no it only cleared things up by a tiny margin.

I notice too (and this has only begun to happen recently) that if the Hifi is powered on but in silence with the volume left up just a tad or the main PC that runs off mains has it's speakers turned up but 'running silent' i get to hear random bursts of noise from time to time or worse the Ripple Control racket come through  every morning at 6am...

The upshot is that tracing any kind of a lowlevel signal with the my new scope or the old Tek is useless now because there will be 'fuzzies' on top that signal...

I'm going to set the new scope up to screen capture the utter hash that's screwing things up by terminating a channel input with a direct connection to say a bench PSU but compare unplugged/ plugged etc & powered on or off.... and post an image or two here.

Basically i'm at my wits end. Most of what i do is low level signal audio. My only other thought is whether the hash i'm seeing (that has a periodicy about it) as 'packets' of noise move through a scope signal display could be data originating from the recently installed mains SmartMeter, that apparently communicates over the mains wiring.

Any ideas truly appreciated.

[Phoenix]

I've had a radiated noise problem from a mid-range LeCroy scope (Siglent OEM). It radiated so much noise from its own power supply (100mV+ with 10x connected and looped) it was basically unusable. Had to return it and bought a different scope. We didn't hear any feedback from Siglent. The replacement scope we got from Siglent has some PSU noise but suppressed enough with a ferrite.

Set your scope to line trigger - that will tell you if the noise is anything synchronous to the mains. Move a looped probe closer/further from the scope itself to see if it's the scope itself.

[floobydust]

RFI in a building is complicated to track down.
Beeps and clicking sounds every minute or so are from cellphones being demodulated by audio gear. Put a cellphone near your loudspeaker wires and see what a typical power amplifier does. Beep. Boop. I hear that when a text message comes in or a SMS poll is happening.

I had a lab near a cell tower and it was really noisy, scopes and equipment constantly showing carriers and op-amp's offsets moving around. Check for any cell repeater antennas in the neighborhood. First  narrow down if it's airborn verses coming in on mains., or both. Looking at the noise on the scope, you can see if it's AM or FM. I also use an SDR to check the spectrum for local culprits. SmartMeter burst is every 10 minutes at 433MHz I notice.

The bonus is the RFI can help you design (audio) gear that is unaffected and learn that it's a constant source of "grit" in otherwise vulnerable devices.

[dietert1]

Yes, we also have a large photovoltaic generator with switched mode power converters on our roof and an antenna tower about 100 m away. Times are changing and lot's of older home electronics isn't "radiation hard". Recently i found some youtube videos on the issue, where they show how to trace down the noise using a spectrum analyzer. It's in German, but youtube will help you find similar videos. At least he shows how to prepare a test setup with a self made magnetic sensor.


Regards, Dieter

[peteb2]

And for the record.... i've just found Dave's vid on this nightmare BS & steps to track down COMMON MODE NOISE which is what i am fighting with.

So far, even with every single appliance or device in my home either powered off & if it can be, unplugged there's still plenty of hash being displayed by my new scope just like in Dave's vid. Even my old analog Tek shows similar issues are going on.... Somewhere there's gotta be another 'something' that's drawing current & creating the noise or it's coming in on the Mains from the neighbour's property.....

Clearly more work needed....

[jmelson]

So far, even with every single appliance or device in my home either powered off & if it can be, unplugged there's still plenty of hash being displayed by my new scope just like in Dave's vid.

Well, the problem is you didn't turn off everything within a 500 m radius, or powered off the same transformer.  There are so many poorly-built, or just plain ILLEGAL devices out there that it is now out of control.  Even the test equipment, ITSELF, is often bad.  Make up a sense coil (a number of turns of magnet wire, 20 mm diameter) and connect to scope probe and ground, and sweep around the lab.  You will likely find the LCD scope screen is a MAJOR RFI source, anything with LEDs is a source, fluorescent lights can be massive radiators, LED lamps can be either very quiet or very bad.  With a search coil and maybe a couple cm metal plate on your scope probe, you can search for the sources and change out what can be eliminated.  If it is the scope screen, that is hard to deal with.

Watch out for all computer gear, keyboards and mice can be sources, too.

Jon