Balun Differential Matching

hi

i have made a balun with single ended input and differential output. My questions are:
a) when and why we should AC ground the center tapped terminal?
b) i have made the matching at input which was single ended but how will i made it for the differential output. i figured out the values of the component for differential side as shown in figure (attachment), but how i will select these values for two differential lines?

thanks

Why are you using a s5p component to represent the balun?

What's the idea behind matching both unbalanced and balanced side of the balun? Changing the termination of one port will also change the impedance seen at the other port.

A wide band balun can be expected to basically match real impedances at both sides, e.g. 50:200 ohm as assumed in your simulation setup. It's unlikely that the matching can be improved (if necessary at all) over a wide frequency range with simple LC matching elements.

Or do you intend small band matching?

What type of Balun you have designed? It has Input/Output impedances rather than 50/200 Ohm ?
More info needed..

Thanks FvM and BigBoss for your kind reply

Actually i used s5p file as it has the EM simulated results of the balun. my balun ports setup is as follows:

I am interested only in narrow band matching with center frequency of 120 GHz

Thanks

You can do the matching for single ended side with well known method.
For differential side, do the matching for each branch as Complex Input Impedance of Differential Side/2 --->25 Ohm
Then combine the grounds of these two matching circuit.