How use 4- port as 2-port to extract S-parameter correctly

hi

I want to simulation micro transformer in ADS (2011 and 2015).

I am confused about set Ports.

I have 4-Port transformer and . I want extract S-parameter.
i should convert 4 port to 2-port ( port (1,2) in primary coil and port(3,4) in secondary coil ) .and i think i have 3 way to do it.

1-use differential port( like group (1,2) as one port ).

2-ground port 2 and 4 (cover ground under substrate)

3- use for port (as manual) and in plot display windows by multiport equation convert 4-port to 2-port (like define SD11 as new S11 and it is equal SD11=0.5*(S11-S13-S31+S33) )

i simulate all of them and i have different result.

which one is more correct and more common in papers?

I know that in fabrication port 2 and 4 will be grounded but i don't know how to ground that port correctly.

thanks

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If you model with vias to connect to the bottom ground, the vias and their mutual coupling are included in results. In most cases, this is not desired, because that via does not exist in reality.

The better solution is to place pins on both "ends" of the coils. These can then be connected as needed, for differential operation or grounded or whatever you need.

The usual application case is to simulate differentially, but if you define the Momentum ports differentially (with explicit (+) and (-) pins) then you get results for ideal differential excitation/termination only. Common mode will "see" an open circuit at differential (+) (-) ports.

Also, regarding the re-grouping of (+)(-) explicit ground ports into other configurations: the results are 2-port data and it is not possible to re-configure this for other signal/ground combinations. This is a mistake I often see with RFIC designers, so I wrote an appnote on that topic a while ago:
http://muehlhaus.com/support/ads-application-notes/em_line_ground

To cover differential and common mode, you need 4-port data, with 4 pins -> 4 ports with common global ground.

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