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Coplanar Waveguide Marchand Balun

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2 years 2 months ago #575 by raschu
raschu created the topic: Coplanar Waveguide Marchand Balun
I was playing around in Sonnet Lite to learn more about RF and I simulated an simple cpw marchand balun with one or more coupled on the balanced and unbalanced port. I found US patent 6819199 on google patents has a good image or two of the balun, except the patent adds capacitors on both ends of the unbalanced port. I just used the standard cpw line and space widths and it worked fine. Two balanced lines and one unbalanced gives 100/50 ohms, one and one does not. No ground is needed, and if there are multiple unbalanced lines need to be connected at the middle to keep the currents symmetric.

I have a question because something seems weird about my simulation results. I read that a marchand balun has an unequal group delay to create a phase shift on the balanced port. My results do not show that. They end up very close to equal, and can even cross. The phase is still 180 degrees. Is this a correct way to look at the results, or is this a simulation error?

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3 months 3 weeks ago #914 by Microwaves101
Microwaves101 replied the topic: Coplanar Waveguide Marchand Balun
Sorry this message seems to have sat here unanswered for a long time.

Group delay of anything that provides a flat phase shift in an interesting topic. Sometimes it adds confusion as it does not appear to make sense. In a rat-race used as a 180 divider, you will see a clear difference in the outputs, the 180 leg is always longer at center frequency. But the two delays intersect at band edges, and the shorter leg has more delay than the longer leg outside the band sweet spot!

In general, I would not put a lot of worry into analyzing group delay of a balun structure. But I will try to look at this, we are going to make a Marchand Balun page shortly.

Regards
Steve

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