Topband: Measuring Common Mode Chokes

Chuck Hutton charlesh3 at
Wed Dec 18 18:46:56 EST 2019

In the past, I have simply used my N2PK VNA to measure impedance of the choke by connecting the shield of the coax to the VNA ports.

Recently I've been discussing common mode chokes with others who have a different methodology.
They prefer to do a transmission test through the choke and report the "common mode rejection".
This is done by placimg a crossover cable between the VNA output and the choke. The choke output is connected in a normal fashion (center to center, shield to shield) to the VNA input.

This does not seem ideal to me.
First, the choke is being driven in differential mode rather than common mode.
Second, the measurement depends on (varying) isolation between the coax center and shield. So it's not truly common mode rejection.

Am I on thr right track?
A handful of Googles has not netted me any clear summary of test methodology for reportimg CMRR. I fimd a small number of tests reportimg impedance.


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