I'm trying to prepare a setup to see the effect of this torodial ferrite ring.. Even though I still didn't understand what way to wind wires to obtain a common mode choke(I saw differnt ways of winding ways), assuming I achieved it, can I use the following setup to observe the choke's effect on CM interference. I plan to use a function generator to create the CM mode voltage and a scope to observe as shown below where Rg is to limit the current across return wire:

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If the model is correct, above shows the Bode plot. Lp and Ls are to model the choke made up of the wires wound around the ferrite ring.

I want to wound wires such that I can obtain a good attenuation for frequencies above 1 MegHz. Since there is nothing written about it in the datasheet I need to experimentally find out how many turns/winds I need.

Is my setup proper for such test to see the effect of the choke? If not what should it look like?

(My aim is to roughly demonstrate the effect not a professional setup)


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Above shows if I use caps to route CM currents back to the function generator instead of using a common ground.

In simulation above Scope and Scope_GND are exactly the same. So does that mean the potential difference will be zero and I will not see anything on scope regarding the attenuation? (Yes it is balanced but then I won't see any change on scope screen isnt it?)

Edit 2:

How about the following?:

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  • \$\begingroup\$ If you are trying to test a common mode choke you have to use capacitors on the output to ground like this circuit with C2 and C3 and have it balanced. Balanced means no Rg and not using the scope ground as part of the monitoring. It's more complex than you think but thanks for the other question. \$\endgroup\$ – Andy aka Dec 6 '18 at 17:07
  • \$\begingroup\$ I'd drop the Load down from 1MegOhm, to 1 Kohm. Even slightly imbalanced parasitics will be a problem. \$\endgroup\$ – analogsystemsrf Dec 7 '18 at 4:50
  • \$\begingroup\$ Please see my edit. I plan to use the capacitors but now I have some other issues. \$\endgroup\$ – cm64 Dec 7 '18 at 6:00
  • \$\begingroup\$ Sorry, is my last edit Edit 2 a correct configuration for such setup? \$\endgroup\$ – cm64 Dec 7 '18 at 6:17

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