Modeling a differential microstrip transmission line

[rruss]

differential microstrip

Please forgive my ignorance, I'm very new to HFSS, and electromagnetics is not my specialty. I need to model a differential transmission line on an FR4 substrate. I've spent a lot of time reviewing tutorials and examples, but none seem to address the subtle inconsistencies I see in HFSS with respect to waveport/lumped port definition and how they relate to the solution type, etc.

When I build my model such that the driven ends of the t-line are on the boundary of the model (so that I can use a waveport), the resulting s-parameters show an open circuit from end to end.

When I encase my model in "air" and set my radiation boundaries all around the model, I can stimulate a single ended transmission line with lumped ports and see realistic results in S11, and S12 using driven modal solution type. In this case the ports are set into the model by a mm or so.

This may be a silly observation... but I can't define multi-terminal ports with more than one integration line on lumped ports at all, and I can only create them on waveports in a driven terminal solution type (not driven modal).

I'm really quite confused as to which solution type I should be using, and how to get an accurate model using waveports with 2 terminals.

If anyone has a simple model that might help me, I'd appreciate seeing a working example.

Thanks.
-Russ

 

[jallem]

grounded differential microstrip

It is kind of confusing your post since a microstrip can not
propagate differential signals, I think.

 

[fantomf4]

differential microstrip line

From your post I presume that you want to simulate a differential micriostrip.
It is unclear to me which version of HFSS you are using, but nevertheless:

Look on Edaboard under EDA Books, and download a recently posted HFSS 9.2 Full book, about 57 MB.

Looke at the chapter 8, I think is called SI (Signal Integrity), and find the " Segmented Ground" example. Follow the tutorial but DO NOT make a segmented ground ( Ground plane split in 2 parts). Just make it one uninterupted rectangle.
See the the results and try to modify the model to suit your dimensions.

Best Regards

Fantomf4

 

[jallem]

differential microstrip transmission line

First: there is not such thing "differential microstrip"
In turn exist CPS and CPW. CPS stand for Coplanar
Strip Line.It is a balance transmission line with no common
ground plane, consist of two strip of copper over a substrate.
Theoretically will not propagate common mode.
CPW is a single trace with ground on the sides and the bottom.
(the bottom ground plane can be omitted)

Second: The example 8.2 is about a non-continuos
return path and its influence on the rise time and such.
If you discard the slot in the ground plane then
become a simple microstrip.

 

[rruss]

differential transmission lines

Everyone, thanks for the information!

The LVDS example in section 8 was all the detail I needed.

-Russ