Is my measurement setup correct for the common mode frequency response?
This is what I suggest.







simulate this circuit &ndash; 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.