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There are many free (e.g. AppCad from Agilent) microstrip impedance calculators on the web. I've tested now several, because the first two did show a huge difference in the calculated impedance for a CPW waveguide on silicon, with exactly the same parameters (dieletric constant, geometry etc). One 50 Ohm, while the next 68 Ohm. This will probably generate huge losses.

So how can I be sure, I've chosen the right geometry of my waveguide and get correct impedance match? Should I switch to commercial simulaion software like CST microwave studio or HFSS to get the results nearest to real-world physics in this waveguides?

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  • \$\begingroup\$ Please link to the specific tools you mentioned. \$\endgroup\$ Feb 4, 2013 at 21:47
  • \$\begingroup\$ Also, a cross-section diagram of your geometry could help --- if some of your dimensions are very extreme, you could be using the tools outside the regime they're designed for. \$\endgroup\$
    – The Photon
    Feb 4, 2013 at 23:08

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I was somehow in the same situation recently but my configuration was a differential pair in a stripline mode. I tried different calculators and got different results. After trying to understand how the computation was done by the calculators, I gave up.

I looked for the most recent equations about stripline impedance and made my own calculator in a spreadsheet. Maybe the result is not the most accurate or doesn't take into account some parameters, but as an engineer in a preliminary design, I had to understand the result to be able to see some trade-off. Reading some signal integrity books teach me that the accuracy of these equations (for stripline, but I think it's about the same for CPW), is 10 to 20%.

You can find some equations on CPW impedance here : http://qucs.sourceforge.net/tech/node86.html

To get a better value, a 2D solver is required (accuracy is under 5%). Look at Polar Instruments ( http://www.polarinstruments.com ), their 2D solver is pretty simple but quite accurate. HFSS is a 3D solver, maybe too complex for a first guess of impedance.

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