Omicron LF common-mode choke

[jmw]

Dave did a teardown of the Omicron injection transformer and bunch of us built our own imitations, but has anyone peeked inside their B-LCM common mode choke https://www.omicron-lab.com/products/vector-network-analysis/accessories/b-lcm/? They have charts showing 40 dB common mode attenuation at 1 Hz. What kind of core material is going to be effective at that frequency?

[slugrustle]

The footnote for the "B-LCM typical common mode rejection" graph reads: "S21 measurement (Bode 100 output signal is fed to inner and outer conductor of B-LCM input connector,
B-LCM output signal is connected to input of Bode 100 and terminated with 50Ω)"

Which I take to correspond to the attached drawing.  If the combined impedance of the "ground" side of the common mode choke and associated wiring is ~505mΩ or less at 1Hz, this gets you an attenuation of 40dB against a 50Ω source all by itself, even ignoring both the impedance in the "signal" side of the common mode choke and the 50Ω termination on the bode analyzer input.

I agree with you that the 40dB attenuation figure definitely does not seem right, and it's probably down to test setup.  The common mode inductance is specified as 75mH at 1kHz, and that's probably a more useful figure here.