# microstrip low pass filter design issues with sonnet

I am trying to replicate the microstrip low-pass filter in SonnetLite from the book by Jia-Sheng Hong - Microstrip Filters for RF Microwave Applications, 2nd Edition (Wiley Series in Microwave and Optical Engineering) (2011, Wiley)

However, the S12 graph shows that it is not a low pass filter at all. Could anyone advise why ?

I have attached the SonnetLite design file here.

box resonance warning analysis logs

Run 1:  Sat Jun 22 00:27:07 2019.  SL291418.107194.
Em version 15.53-Lite (32-bit Windows) on DESKTOP-DG59KC2 local.

Project:  C:\Users\kevin\Documents\sonnet\15.53-Lite\three-pole microstrip lowpass filter using open-circuited stubs\three-pole microstrip lowpass filter using open-circuited stubs.son.

Frequency:  0.001 GHZ

Sonnet Warning EG2680:
Circuit has potential box resonances.
Circuit: primary structure
The estimated box resonance frequencies are listed below.
The calculations assume lossless materials, with a box filled with
only the specified dielectric stackup materials.

4.140 GHZ  TE Mode 1,0,1
4.673 GHZ  TE Mode 1,0,2

Only lowest order modes considered.

See the chapter on package resonances in the Sonnet User's Guide.
Date: Sat Jun 22 00:27:07 2019

Sonnet Warning EG2680:
Circuit has potential box resonances.
Circuit: left box wall SOC magnetic wall standard
The estimated box resonance frequencies are listed below.
The calculations assume lossless materials, with a box filled with
only the specified dielectric stackup materials.

4.167 GHZ  TE Mode 0,1,1
4.700 GHZ  TE Mode 0,1,2

Only lowest order modes considered.

See the chapter on package resonances in the Sonnet User's Guide.
Date: Sat Jun 22 00:27:07 2019

Post-Analysis:
Errors detected: 0    Warnings detected: 2.

Analysis completed Sat Jun 22 00:27:10 2019.


• have you actually got terminals connected to your filter? In other words, are you using sonnetlite correctly? That layout is a three pole lowpass filter, there's no question it ought to behave at least qualitatively like the book says. – Neil_UK Jun 22 at 8:38
• @Neil_UK have you actually got terminals connected to your filter? <-- What do you mean ? – kevin Jun 22 at 8:44
• I don't know sonnet specifically. How does it connect to the circuit under test? Does it have 50 ohm ports, or generalized transmission lines? It says it's found box resonances, so it's doing something. But does it know that it has to push current and voltage into those copper things that you've labelled 1 and 2. Or is there a ground plane under it, or implied at the boundaries of the box. The one thing I do know about 3d solvers in general is that they can be a pig to set up. Get a simple 'hello world' circuit working first, like a length of plain transmission line. – Neil_UK Jun 22 at 8:48
• Someone helped me to solve most of the problems, and I have a working low pass filter now. But I am still very concerned about the correctness of the dielectric layer configuration. Any advise ? – kevin Jun 22 at 11:22
• You appear to have only 0.1mm of air above your conductor, but if that's real I would expect a much worse S11 from your filter. Does it ignore the thickness above and you enter the box height somewhere else? Having said that, the S11 is pretty rough, if sonnet is now set up properly I'd do another iteration on the dimensions. However, do model a simple bit of 50 ohm line from input to output, as a ground zero, make sure you get a reasonable S11. – Neil_UK Jun 22 at 13:36