Scope, is it lying? Why and how?

[atferrari]

I came to think that my scope is lying to me.

Settings:

AC input
vertical 10 mV / div
horizontal: 10 ms/div

There is a board powered by external PSUs.

If the board IS NOT connected to the PSUs, I do not see anything in the scope.

With the board connected to the PSUs, but NOT turned on, I get, consistently, in both chanels, a signal as in the picture, no mater what points I touch with the probes even IF BOTH TO GROUND.

Impossible to get a better picture. Sorry.

The signal, evidently periodical, appears as having a frequency of 112Hz.

Mains is 50 Hz thus nothing to do with it.

 

[MikeMl]

Are the PSU switchers?
Does your scope have a third-prong-ground pin on its Line cord that connects is cabinet to power-grid ground?
Is the scope plugged into the same power outlet as the PSUs?
Do the PSUs have a ground pin on their line-cord?
Is the common terminal (negative?) on the PSUs connected to power grid ground?

Methinks you have a ground loop.

 

[atferrari]

Thanks for replying.

Are the PSU switchers?

No, simple linears.

Does your scope have a third-prong-ground pin on its Line cord that connects is cabinet to power-grid ground?

Yes, it has.

Is the scope plugged into the same power outlet as the PSUs?

Tried on the same and on a different one. Signal shown stays unchanged.

Do the PSUs have a ground pin on their line-cord?

No, they are rectifiers and filters I've put togteher.

Is the common terminal (negative?) on the PSUs connected to power grid ground?

Sorry if I understand you wrong, but both PSUs are dual so common en each one is 0V and are connected to each other and to the board. Not to the power grid ground. Should I?

I do not feel comfortable with mains as much I didn't feel well with my ex mother in law.

Methinks you have a ground loop.

Additional comment: all in this board, with both PSUs on, seemed plagued with awful noise. That moved me to check things.

Your suggestions would be of help. No doubt.

Just in case, the gound clips on the probes, are connected to common at the board.

 

[alec_t]

Looks like inductive interference pick-up on the leads/wiring. Is there a strong alternating/rotating magnetic field source nearby (e.g. fan motor inside 'scope)?

 

[atferrari]

Looks like inductive interference pick-up on the leads/wiring. Is there a strong alternating/rotating magnetic field source nearby (e.g. fan motor inside 'scope)?

Not that I know.

Just in case I turned off PC, ADSL modem and put my two mobiles away. Ah, no fan inside.

Fridge is two rooms away and that is all I have around.

But, no matter what is the cause, why even with probes shorted to ground, I get that signal?

 

[atferrari]

Further testing

Hola,

I just did another test: I disconnected the PSUs from the board, thus no signal and then connected just their ground to the board; the signal, showed up again!!

 

[atferrari]

My current setup

Run out of ideas of what to check next.

My last findings noted in the attachment (setup)

 

[jpanhalt]

What happens if all the lights in the room are off? Do you have incandescent or CFL lamps?

John

 

[atferrari]

I took the pictures with the lights off. Incandescent, the last BTW.

Doesn't the frequency means anything?

 

[jpanhalt]

Sure the frequency means something. But that may just be a calibration issue. Have you calibrated your scope or the mains recently? Just looking for reasonable sources of environmental interference. What you are seeing could have been a CFL harmonic, but apparently is not.

John

 

[MikeMl]

Do you have a third prong isolator for the scope power cord? ( In the US, it looks like this)

 

[Mikebits]

The signal only seems to be about 10mvpp, so why worry about it. I doubt it will cause you any issues.

 

[alec_t]

Is the power supply inside your 'scope a linear one or a switcher?

 

[bchris]

May I suggest you check and validate your scopes good grounding?
Maybe a wire got loose inside the scopes plug.

 

[MikeMl]

May I suggest you check and validate your scopes good grounding?
Maybe a wire got loose inside the scopes plug.

You are telling him to ground his scope, and I am telling him to un-ground it