electrically small structure
I'm currently trying to simulate a simple lossy Metal-Insulator-Semiconductor interconnect line structures with HFSS. I use this simple structure for EM simulator setup verification because I have an analytical solution for the per-unit-lenght elements. ADS Momentum with single-ports captures all details in the per-unit-length elements for this structure accurately.
The straightforward approach for simulation of this microstrip type of structure using HFSS would be to use waveports to find the 2D FEM solution for Zo and Gamma at the ports and from this extract the per-unit-length elements. In my HFSS setup I make sure the waveports covers the full faces of my structure. The boundaries of the waveports are made sufficient large (more than 2xTsub) so any interaction with the boundaries can be neglected. An airbox encloses my simulation structure. The height above the substrate is around 2xTsub which is less than the recommended lambda/4, but my structure is electrically small so this should'nt matter ? Furthermore I use low-order solution basic functions which is recommended for electrical small structures.
My problem is that the per-unit-lenght elements that HFSS calculates are very wrong compared with my expectations. It looks like the waveports can not deal with losses even though the HFSS technical note claims it can. If I look in the paper behind the theory implemented for the waveports in HFSS (Jin-Fa Lee, Din-Kow Sun, and Zoltan J. Cendes, ?Full-Wave Analysis of Dielectric Waveguides Using Tangential Vector Finite Elements,? IEEE Transactions on Microwave Theory and Techniques, vol. 39, No 8, August 1991.) I observe that a e^{-jbeta z} dependence for the fields are assumed (so no losses are included in the deviation). This is about the only thing in this rather complicated paper I understand. I'm therefore not sure about the extention of the theory to the lossy case.
So to wrap things up, my basic question is this:
Can waveports in HFSS handle lossy structures ???
Hope someone out there can answer me or perhaps have similar experiences.
Best regards,
tkj
I think HFSS can handle lossy structure. It does not make sense that FEM has above limitation.
I guess you can upload your model setup or picture, so that people on this board can help you.
Yes, the waveport in HFSS can have lossy dielectric and/or good (non-perfect) conductor.
my guess for your problem:
1. 2xTsub is hardly "large" enough, check convergence w.r.t. all the sizes that you set to be around 2xTsub.
2. make sure you understand the unit of the HFSS answer correctly.
Hi Loucy,
Thanks for your reply.
I have been looking a bit more into my problem with waveports and I think I have some idea about my problems:
1. Simulating with different air boxes (up to 8xTsub) does not make a big difference. Another HFSS setup using lumped ports and a surrounding ground ring accurately predicts these losses, so no air-box related problems are observed.
2. Port accuracy have to be increased by using more triangles (up to the limit of 5000). The larger the waveports the more triangles I have to use for meaningfull results.
3. It is the series losses due to the conductive substrate (eddy current losses) which are the problem, shunt losses are accurately accounted for by waveports.
4. Having done a bit of thinking I believe that a 2D solution are not able to calculate these losses at all. If this is the case, I see a potential accuracy problem with adapting the 2D waveport mesh to the 3D volume mesh, and perhaps also with de-embedding ?
Best regards,
tkj