Signal generator and oscilloscope earth clip issue

Hello everyone.
I just realized something strange playing around with a simple voltage doubler.
The doubler works fine. I use a signal generator as input. A simple DMM measures the DC output.
The thing is: just cliping a scope's probe clip to the circuit can make it go crazy.

Attached, two scope captures.
First, with scope's probe earth clip is connected to signal generator's negative terminal. Ok.
Second, with earth clip swapped (clip to positive output, probe tip to signal generator negative terminal).
That disturbs the operation so badly!

The siggen is not earthed (it is powered from a two prong power cable) so the BNC barrels are not earthed, and so, I see no reason for this to happend... The DMM can measure any two points in the circuit with no problem whatsoever. There must be something importanted I'm missing. Any idea?
Thank you!

More info: if I power off the siggen, it still happends (the second scope capture looks very similar). I have to disconnect the siggen probe from the instrument for it to stop happening (of course, if this didn't work, we would be talking about magic ).

When the 'scope input and ground clip are backwards then the circuit is picking up the 50Hz of your electricity that is all around you. Try turning around the two prong AC plug.

audioguru said:

When the 'scope input and ground clip are backwards then the circuit is picking up the 50Hz of your electricity that is all around you. Try turning around the two prong AC plug.

Click to expand...

Thank you. I'll try when I get home. But that sound strange... look at the voltages measured: 90 V peak! I've seen circuits picking up noise, but 90 V peak looks like a powerful noise! One of my DMM measures a high DC voltage too (around 50 V), so it is not something related to my scope reading.

Elerion said:

The siggen is not earthed (it is powered from a two prong power cable) so the BNC barrels are not earthed, and so, I see no reason for this to happend... The DMM can measure any two points in the circuit with no problem whatsoever. There must be something importanted I'm missing. Any idea?

Click to expand...

I have no idea where you are located or how your mains power is distributed. That said here in N. America, USA and Canada the mains Neutral is tied to Earth Ground at the mains entry point.


You may want to measure between your signal generator BNC output shell and mains neutral on your plug but here the neutral and ground are tied at the power entry point.

Next if using a 10X probe you may want to adjust your probe compensation of your scope probe.

Ron

audioguru said:

Try turning around the two prong AC plug.

Click to expand...

The same thing happends.

Reloadron said:

That said here in N. America, USA and Canada the mains Neutral is tied to Earth Ground at the mains entry point.

Click to expand...

Here in Europe it is the same.

Reloadron said:

Next if using a 10X probe you may want to adjust your probe compensation of your scope probe.

Click to expand...

10x or 1x probe makes no difference. Anyway, the compensation is spot on.

Again, looking at the high voltage measured, this is a bit of concern to me. Could this be a fault in the signal generator?
If I attach the scope directly to the siggen, nothing is wrong either polarity. The issue shows up with the voltage doubler...
The signal just seems more stable if I attach the earth to the signal generator negative terminal.