Learning Forum

[Zorah]

I'm designing an RF switch in HFSS as a part of my thesis. My professor suggested I try to recreate some of the work done in the past by researchers to gain a better understanding of the working of switches. For this purpose I have tried to recreate a working model of Dr. Pals design. It is his design and I take no credit for it whatsoever.

When the switch is in the ON or OFF configuration I do not get the expected S parameters. In the ON state my S11 is near 0dB and S21 near -60dB indicating that nearly all power is being reflected and none being passed on to the output port. If I plot the E and J fields it shows that my switch is inactive and not working. I have checked and rechecked my geometry, excitations and boundary conditions but can't seem to figure out what's wrong. Can anyone help me out in figuring what I might be doing wrong?

[Sorceress Gia]

Can you please post some screenshots of your model?

[Zorah]

Hi Peter,

Thank you for getting back to me. Attached are some of the screenshots and an explanation of what im trying to do.

Geometry: I'm currently in the driven modal solution mode and the dimensions in micrometers. The figure below shows the geometry's top view. It consists of a 5G SOIMUMPS switch composed of a coplanar waveguide. The green comb-like inner structure are the actuation electrodes whereas the dark blue electrodes on either side are the signal electrodes. The actuation electrodes are touching the signal electrodes in this ON state completing the signal path and conducting the signal. The yellow external structures on either side, are the grounds of the coplanar waveguide. All structures are made of 25um Si in accordance with the SOIMUMPS process on top of 400um Si substrate and 1um Silicon dioxide layer. On top of the ground, signal lines and inner actuation electrodes a 0.65um layer of gold has been deposited for conducting the signal.

 

Boundary Condition: I have made an airbox around the device and selected all its faces and assigned them the Radiation boundary.

Excitations: I have made two rectangles on the cross-section of the device on the face of the airbox and assigned them wave port excitations as shown below.

The integration line is defined from the top right-hand edge of the signal line to the top left-hand edge of the adjacent ground. I understood that the integration line is to be defined across the region of the highest potential difference hence I drew it from the signal line(highest voltage) to the adjacent ground(0V) on the right-hand side. However, I'm not certain how to encompass both grounds(left and right) into this. The number of modes is kept to 1. I have repeated this for the rectangle on the output port.

Solution Setup: I have added a solution setup and frequency sweep as follows:

Do let me know if you need any more information regarding the simulation in order to solve the issue. 

I look forward to your response.

Zorah Ahmad.

 

[Sorceress Gia]

Can you upload an archived copy of your model?

[Zorah]

Here you go Peter. 

 

[Sorceress Gia]

The first thing I noticed is that you're using HFSS v15 - which is over 5 years old. Do you have access to a later version? Academic pricing has been significantly reduced.

Otherwise I ran your model in the curent version 2019 R3 and the results look good:

 

I did use the new Broadband Adaptive Mesher - but your results at 6GHz should be nearly the same. - If not, plot the mesh on all the surfaces and look to see if perhaps some features are not being captured properly...

 

EDIT: Apologies for the low resolution image upload, it's a limitation of the forum - My results were S11 = -14.9dB and S21 = -0.4dB @ 6GHz

[Zorah]

I understand.Thank you for your help Peter.