Differential and Common mode Impedance in HFSS

Dear all,

I have simulated a differential pair lines (GSSG). I selected driven-modal solution for a number of modes of 2. From the E-field pattern, it is clear that mode 1 is even mode, mode 2 is odd mode.

Question: The Zo in the matrix data:
Zo mode 1 ==> Zcm(common mode) or Zoe(even mode) ?
Zo mode 2 ==> Zdm(differential mode) or Zoo(odd mode) ?

For symmetric lines (slot structure), do these following condition always hold?
Zdm=2 Zoo
Zoe=2 Zcm

many thanks,
wlcsp

I have been working at this question as well and have seen the expressions you cite. I would be very cautious about saying that they always apply.


By GSSG, I assume that you are indicating side ground, trace, trace, sideground in a coplanar structure. Is there any dielectric material in the mix? You might want to look at what happens as you increase the gap to the grounds and decrease the gap between the dirven traces (and vice versa). Contrast that with both close and both relatively wide. I would expect the impecance relationships to change.

Review the notes in HFSS online help on impedance calculations. There is more there on setting up the impedance calculation and wave port.

I will be watching this thread with intterest.