Common mode filter characteristic

[Pixelx]

Is it possible to read such common filter characteristics using LibreVna? I have several desoldered filters and I would like to find out as much information about them as possible. If so, how to connect them to the VNA?

 

[D.A.(Tony)Stewart]

I read your VNA has a lower limit of 0.1 MHz.
What spectrum are you hoping to measure?
What parameter is most important to you?
Zcm(f), Zdm, SRF or CMRR(f)

You need a 4 port VNA for those plots

 

[FvM]

For correct measurement, you need symmetrical VNA ports. A common method is to use 4-port VNA as shown in appended Wuerth datasheet and transform to 2-port. The other option is to use wide band 1:1 baluns for both ports.

 

[FvM]

The third option is to measure 4 ports with your 2-port VNA sequentially, terminating the other ports with 50 ohm.

Option 1 and 3 require a stable common ground for the single ended VNA ports, as indicates in the datasheet drawing.

 

[volker@muehlhaus]

For a rough estimate of common mode rejection, I would measure this as a 2-port with inputs 1+2 connected and outputs 3+4 connected. That's essentially common mode, isn't it?

 

[mtwieg]

I believe that's how manufacturers derive the plots in their datasheets, see here: https://product.tdk.com/en/contact/faq/emc-components-0059.html. You could presumably also measure S11 with the port 2 side grounded.

Though there may be more interesting behavior revealed by a full four-port measurement.