interdigital capacitor

Hi;
I am just wandering how can I design interdigital capacitor layout?and from where
can I get the formuli of the capacitor to design it in FR4 substrate.If I know the permittivity of FR4 .Is it the starting point of the design ,I know the frequnecy which is 3.5Ghz and the value of the capacitor at this frequency?using ADS

Thanks

Hello,

Here you go...

http://www.eagleware.com/pdf/apps/00...roofMidcap.pdf
http://www.eagleware.com/pdf/apps/00...ProofLange.pdf


---manju---

you can use smart components in ADS or just simpy as you make a filter you can develop the lumped elements to distributed elements.
If you won't find the way, I can make you some pictures from it or make the capacitor.

If you have some pictures or formuli of designing this .I wil be very thankfull ,if you upload them.

I will take some pictures, if I go to the place where is ADS.

Here is some pictures.
First you need this Microstrip interdigital capacitor smart component and a substrate.(in the picture) State the values.
And then go to DesignGuide/Passive Circuits/Microstrip Control Window and then optimazation (you can see the picture), optimize it and then you got the result (in the picture). It's just max. 5 minutes.

Hi,you mean if I want to design Interdigital Capacitor for FR4 all what I need is to
change the values of MSUB?
from where I can I get the theoritical formula for designing I Capacitor?

anyway Thanks Drabos

yes, change the MSUB to FR4, and the frequency and the capacitance in the smart component to the desired values ( or the other parameters).

There is some analitically approximation, but they aren't accurate. But, you know the FR4 isn't a good material, so relative permittivity is not so accurate, so simulate with 20% variation if you want to design a cheap, mass product. Don't bother with theoretical formulas, I think they aren't useful for you.

The formula for the Interdigital Capacitor can be found on pg. 133 of "Microstrip Lines and Slotlines" by Garg, Bahl, Bhartia, and Gupta.