Let's assume we have a test set-up involving an oscilloscope and a unit we are testing. we measure the voltage between two points using a standard scope lead and see significant noise, I now take out a clip on ferrite wrap 2 or 3 turns around this ferrite and close it. The noise on my scope is visably reduced. What have I done?
We should note here that the scope is plugged into the wall and so connected to mains earth. The scope return lead is tied to earth via the scope and the UUT may otherwise be floating or tied to earth by a different route locally (same building).
My personal understanding is I have effectively added a common-mode choke in series with the signal and so I am filtering out at least some of the common-mode noise.
There is at least one user on this site who has a different opinion on this.
The last thing I want to do is start a flame war but I am sure in this case that we are both able to discuss this calmly and both keen to learn if we have misinterpreted things.
So does wrapping a scope lead around a ferrite reduce noise because it cuts down common-mode noise or some other method?
Answer
What you are describing is a current balun which serves to add common mode impedance and thus limit the high frequency ground loop sometimes formed by longitudinal mode current flowing in the screen of the scope lead.
I have a couple made up on ETD cores wound with micro coax, occasionally very handy.
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