Learning Forum

[hkowitt]

I am trying to simulate a 1/4 wave coplanar waveguide (CPW) notch pass filter, that is capacitively coupled to a CPW feedline, both CPW's have 50 Ohm impedence geometry. This is eventually going to be a multiplexing system, where many of these 1/4 wave resonators will act as ways to pick out each individual frequency from the feedline.

The issue I am having right now is that when I change either the size of my waveports or the frequency range that my simulation is solving over, I get different values for the resonant frequency. Neither the waveport not frequency range are physical things, and as such neither should have an effect on the resonance, but they do. To get to the frequency range, I am going to HFFSDesign-->Analysis-->Setup1 and under adaptive solutions, selecting broadband then defining a range from low to high frequency.

I have tried changing the following, and have still observed the same behavior of moving resonant frequencies when I change the frequency sweep range:
-Using Terminal solutions (center conductor as terminal and ground planes as reference for each end of the CPW feedline) vs. using a Modal solution
-Using discrete vs interpolating sweeps
-Having my conductors being a sheet vs a 10um thick body
-Changing max delta S from 2% to 0.5%
-Changing max allowed number of passes from 6 to 10 to 15
-Added Curvilinear meshing
-extended the geometry of the ground planes and transmission line

Below is an example of what one of the Meshes looks like, the capacitively coupled section of my 1/4 wave resonator is 0.2 mm long:

 

It seems like the previous results of my simulations are affecting the results of the current simulation. I did 3 different simulations across the same range, all of which gave different results. The following simulations were all done with a broadband adaptive solutions setup from 13.25 to 14.25 GHz, the same maximum number pf passes, same max delta S, all using interpolating sweeps.

sim1: No resonance, flat S11s and S12s
previous: 13.75 to 14.75 GHz, no resonance flat S11s and S12s

sim2: -18db resonance depth in S12 at 13.41 GHz
previous: 13.0 to 14.5 GHz, -1 db resonance depth in S12 at 13.41 GHz

sim3:-8db resonance depth in S12 at 13.79 GHz
previous: 13.3 to 13.55 GHz, no resonance flat S11s and S12s  

Another thing, when I do a simulation changing only the frequency sweep range (HFFSDesign-->Analysis-->Setup1-->Sweep and selecting start and end frequencies under frequency sweep), I see consistent results (negligible change by changing frequency range, about 3MHz change by scaling port size by 2x).

Based on the geometry of the 1/4 wave resonator, I expect resonance around 9.95GHz, however that is far off from what the simulations are seeing. This may be due to a difference in theory vs simulation. Let me know if there are any details I left out that could help solving this issue, any help/suggestions are appreciated!