# Analytical parasitic estimation - RF Cafe Forums

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Camelot

Post subject: Analytical parasitic estimation Posted: Thu May 10, 2007 9:26 am

Captain

Joined: Fri Jan 19, 2007 10:36 am

Posts: 7

Hello,

I hope it is not a too much weird question…

We all know that if you have two metal layer with a distant d and thickness th1, th2, we could calculate the capacitance for area unit with C = (Er*Eo)/d. My question is, in order to get merely an estimation, does someone know if exists such as similar formula for calculating the capacitance for perimeter unit between the edge th1 of the first layer with the surface (with infinite extension) of the second?

Thank you very much!

Camelot

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nubbage

Post subject: Posted: Fri May 11, 2007 10:33 am

General

Joined: Fri Feb 17, 2006 12:07 pm

Posts: 218

Location: London UK

Hi camelot

I suspect that the structure you are enquiring about is the structure that actually makes the formula value of C = the measured value of C.

The classic formula assumes the electric field is perpendicular to the plane of the plates. Without the device you seem to be mentioning, the field at the edge of both plates actually curves outwards before returning to the plates. In consequence the formula in that case does not predict an accurate answer corresponding to what you will measure with a bridge.

The extra plate is called a fringing plate (I think) and is always used for constructing a standard reference capacitor. It has the effect of restoring parallelism to the E field between the plates, a situation for which the formula is valid.

Posted  11/12/2012