Re: [SI-LIST] : Trace Capacitance

Harry Selfridge (harrys@encoreengineering.com)
Mon, 21 Jun 1999 10:25:37 -0700

The answer is that microstrip capacitance is an inverse logarithmic
function of trace width, thickness, and trace height above ground. It is
not just a linear relationship with surface area. In a capacitor, the
dielectric thickness and plate thickness is almost always much, much, less
than the width and length of the "plate". In a microstrip, width is much
less than length, may be equal to or less than dielectric thickness, etc.
Terms which disappear in the capacitance equation for something
manufactured as a capacitor become significant for a microstrip.

For the appropriate equations, see "High Speed Digital Design" by Howard W.
Johnson and Martin Graham.

At 08:59 AM 6/21/99 -0700, you wrote:
>I hope someone can explain what is happening with the following capacitance
>measurement. A friend asks if capacitance is directly proportional to
>surface area, why doesn't his measurement scale linearly. On a pcb made
>specifically for trace characteristics measurement, he measures
>1) 10 pf for a 10 cm long trace that is 5 mils (.12 mm) wide
>2) 25 pf for a 10 cm long trace that is 20 mils (.50 mm) wide
>3) Both traces are far apart from each other and from other components.
>4) Both traces are over a solid ground plane.
>5) Equipment is HP 4285A Precision LCR meter set at 1 MHz
>
>Thanks,
>
>Farrokh Mottahedin
>Quantum Corporation
>(408) 324-7934
>farrokh.mottahedin@quantum.com
>
>
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