[SI-LIST] Re: MicroStrip capacitance

Since this sounds like a new person, I think we should specify that it isn't
just the high frequency, but the small transition time that is important and
makes Andy's comments valid. (these things usually go together but not
necessarily).

jon


-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Andrew Ingraham
Sent: Tuesday, November 23, 2004 8:14 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: MicroStrip capacitance


> after using this web calculator, i got capacitance of 1.557pf/inch, i also
> got Tpd 0.1382 ns/inch. my line length is 1.7inch.  relaying on previous
> discussions from the Si-list, i assume that just multiplying the 1.7 in
> capacitance (1.557pf/inch) would be wrong, because it is based on the
> wrong assumption that all the line will be loaded at the same time.
> what should it be, then?

Your estimate (1.556 pF/inch * 1.7 inch) is correct, for the low frequency
capacitance.

At high frequencies, you shouldn't treat it like a simple capacitance.  At
frequencies that are high compared to the length (delay) of the trace, it is
a transmission line with some characteristic impedance and you should model
it and treat it as such.

The frequencies need to be fairly high for 1.7 inches to be significant in
this respect.  It all depends on what frequencies are of interest to you.
Also, what you need the capacitance value for.

Regards,
Andy


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