I’m using a USB scope which can be used as also a network analyzer. For a network like a passive filter, to obtain Bode plots one input channel is set for reference the other as output. And the input signal is set and swept by a wave generator input. Settings are done via a software. The input channels are differential; for example for channel one Ch1+ and Ch1- are the inputs for a signal.

Below is the wiring for an RC low pass filter to use as a network analyzer and to obtain Bode plots:

enter image description here

W1 is the sweeping wave generator input; Ch1 is the reference channel; Ch2 is the output; Ground is the common ground for input channels and W1.

Now I want to obtain the common-mode response Bode plot of the following choke circuit by the same device:

enter image description here

Is my way of wiring correct below?:

enter image description here

Will I obtain common mode freq. response if I use the above measurement setup?

Second attempt:

enter image description here

  • \$\begingroup\$ Does the wave generator not have a ground/return/reference terminal? \$\endgroup\$ – The Photon Dec 19 '18 at 17:44
  • \$\begingroup\$ It has its own ground that is shown by triangle ground symbol in my drawings. \$\endgroup\$ – pnatk Dec 19 '18 at 17:45
  • \$\begingroup\$ Thats the only ground in the setup. \$\endgroup\$ – pnatk Dec 19 '18 at 17:48
  • \$\begingroup\$ No. That will measure the differential voltage only. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Dec 19 '18 at 17:54
  • \$\begingroup\$ Are you going to compute a scalar or vector result of Zcm for the choke? \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Dec 19 '18 at 18:02

This is what I suggest.


simulate this circuit – Schematic created using CircuitLab

CM Choke use flux addition and DM uses flux cancellation.

Ideally the DM only sees the wire resistance. Such as a clamshell ferrites or beads around wire pair has no effect on DM impedance.

The wire balanced impedance also matters after the caps or imbalance Z. A High input impedance gets little or no benefit from raising impedance of CM choke alone unless shunt cap is after. The lower the impedance of the load, the more DM CMRR.

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  • \$\begingroup\$ Thanks would it be too much if I need your help to use this drawing instead i.stack.imgur.com/NqETJ.jpg ? I need to be sure about it to relate to your diagrams symbols. \$\endgroup\$ – pnatk Dec 19 '18 at 18:59
  • \$\begingroup\$ Is the location of the Y-capacitor "C1" correct? \$\endgroup\$ – pnatk Dec 20 '18 at 9:34
  • \$\begingroup\$ I think C1 C2 are stray capacitances. But can you also add the Y capacitors after the choke? \$\endgroup\$ – pnatk Dec 20 '18 at 10:06

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