Tips on simulating microstrip filter with irregular dimensions in HFSS

Hi,

I'm trying to simulate S-parameters of a microstrip low-pass filter using HFSS. I've played around with other designs but I'm having trouble with this one in particular. I believe it has to do with the dimensions (roughly 50mmx50umx100nm). I imagine it has something to do with the mesh not adapting properly in all dimensions. I tried changing the maximum delta S parameter which appears to change the mesh size. However, it seems like I'm getting wildly different results from changing the mesh size.

I've also tried shortening the length and width to the order of microns to make the dimensions a little more consistent. The mesh looks much better and I think the solutions are trustworthy, but slightly varying one of these dimensions reveals that the solutions are not directly scalable.

Does anyone have any suggestions as to how I can simulate reliably using the original dimensions?

Welcome 14darcia,

You could set up a "seed mesh" by assigning a "mesh operation" to your structure (e.g., the dielectric). This specifics the minimum mesh size to be used.

If your results are changing with mesh size, then I wouldn't trust any of the results.

Thanks for the suggestion,

I tried changing the maximum mesh size as you said and it seems to have helped refine the mesh, at least visually. However, I set up a parametric analysis, changing this size by a small amount, and I again got different solutions for different mesh sizes. Now, my instinct says to just decrease the mesh size until the solutions converge, but this seems extremely inefficient and will probably lead to me running out of memory. For example, if I decrease the mesh size in all directions to something suitable for the smallest dimension (nm), I would also apply a nm-scale mesh to the other dimensions (mm).

Is there a way to assign a mesh size for each individual dimension?

Is your mesh frequency in the filter passband?

I think the problem is just inherent with the device. The proposed design is way smaller than the passband wavelengths (not really a conventional transmission line), so I guess the mesh length is trying to match the wavelength rather than the length of the device, so it's coming up with all sorts of issues. I think we just need to re-think the design altogether.

However, I'm still curious if you can specify different mesh lengths in different directions. How much control does one really have over the mesh?

The adaptive mesh in regions of large field is an integral part of the FEM method.

But if you do planar work (microstrip) and want control over the mesh, why don't you switch to a planar MoM solver (ADS Momentum or Sonnet)? They give you that control, and are specialized to give accurate results for planar circuits.

I haven't looked into those yet actually. Thanks for the tip!