GND vias for return current near differential signal vias.

hi friends,

i need to simulate the effect of GND vias placed nearby to differential vias in HFSS.

The GND vias are not needed if the differential signals contain purely differential mode only. otherwise for the common mode signals that may be present, the GND vias provide the return path. Guess these GND vias will also reduce the crosstalk in differential vias.

My doubt..where to place these GND vias Based on which parameter should i find the optimum location of the GND via..

Will placing the GND via nearby to diff vias reduce the differential impedance? I want to maintain both the differential impedance and single ended impedance for the differential vias.

Any ideas on these?

Thanks