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Sonnet simulation of CPW transmission line with GSG pads

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Yong Wang

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Dear all,
I am currently simulating a CPW tline with GSG pads. The schematic is shown below. The substrate is p-Si with 1Ohm.cm resistivity. The SiO2 is 3um thick, and on top of that is 300nm Au as the conductor.
The simulated Z0 of the pads (w/ taper) is 45 Ohm, while that of the tline itself is 65 Ohm. And Z0 of the whole structure (pad, taper, tline) is 45 Ohm, no matter how long the tline is. Intuitively speaking, if the tline is much longer than the pad/taper, then the pad/taper would have no effect on the Z0 of the line, is it? Even if I made the tline with very funny structure, the overall Z0 is still 45 Ohm...
Please help me out.
Thanks! tline.PNGtline.PNG
 

Yong Wang said:
Dear all,
I am currently simulating a CPW tline with GSG pads. The schematic is shown below. The substrate is p-Si with 1Ohm.cm resistivity. The SiO2 is 3um thick, and on top of that is 300nm Au as the conductor.
The simulated Z0 of the pads (w/ taper) is 45 Ohm, while that of the tline itself is 65 Ohm. And Z0 of the whole structure (pad, taper, tline) is 45 Ohm, no matter how long the tline is. Intuitively speaking, if the tline is much longer than the pad/taper, then the pad/taper would have no effect on the Z0 of the line, is it? Even if I made the tline with very funny structure, the overall Z0 is still 45 Ohm...
Please help me out.
Thanks!View attachment 93714View attachment 93714
Click to expand...

I think I found the reason myself. It seems that the build-in Z0 function in Sonnet is not accurate when discontinuity appears in transmission line, like the case with both taper and tline. After importing the s2p file into Matlab, the extracted Z0 seems correct :)
 

Hello, I need some help with GSG pad design . We all want to design it as a 50 ohm impedance line but once the line is tapered obviously the impedance is changed. How can one determine that whats the impedance in the taper section and after that because the ports at the input would give the impedance of the structure before any discontinuity?
 

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