**From:** Ray Anderson (*raymonda@ha1mpk-mail.eng.sun.com*)

**Date:** Wed Jul 26 2000 - 12:18:53 PDT

**Next message:**Ravi Kiran: "[SI-LIST] : Guard band for differential signals"**Previous message:**Ron Miller: "RE: [SI-LIST] : Stub Capacitance calculation"**Maybe in reply to:**Prasanna S: "[SI-LIST] : Stub Capacitance calculation"

Just to restate what Ron said, a stub used as a capacitance

is a narrowband solution. From zero length to 1/4 lambda the

stub will look capacitive. From 1/4 lambda to 1/2 lambda it

will look inductive and so on where the sign of the reactance

changes every quarter wavelength. In microwave parlance a

quarter wave line is often termed an impedance inverter.

The input impedance of an open circuited lossless line

is Zin = -j * Zo cot(B*l) . From this you can see the

periodic change in the impedance's sign is caused by the

cotangent term as l varies.

So if your field solver tells you than your stub will exhibit

20pF/inch and you have a 1000 inch stub don't expect your

circuit to see 20,000 pF unless 1/4 lambda is > 1000 inches.

-Ray

*> From: Ron Miller <rmiller@Brocade.COM>
*

*> To: "'Mendelsohn, Joseph P (Joseph)'" <jmendelsohn@lucent.com>,
*

"'Prasanna S'" <prasan@hclt.com>, "'si-list@silab.eng.sun.com'"

<si-list@silab.eng.sun.com>

*> Subject: RE: [SI-LIST] : Stub Capacitance calculation
*

*> Date: Wed, 26 Jul 2000 11:41:53 -0700
*

*>
*

*> There is a problem with this simplistic approach, and that is that
*

the quarter wave effect

*> becomes more important than the capacitance as frequencies get into
*

the Gigahertz

*> range. If you must use SPICE then capacitance probably is the best
*

you can do, but

*> regardless of the capacitance, as the stub approaches 1/4 wavelength,
*

it approaches a

*> short in the frequency domain. This is real, not just theory.
*

*>
*

*> Ron Miller
*

*>
*

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**Next message:**Ravi Kiran: "[SI-LIST] : Guard band for differential signals"**Previous message:**Ron Miller: "RE: [SI-LIST] : Stub Capacitance calculation"**Maybe in reply to:**Prasanna S: "[SI-LIST] : Stub Capacitance calculation"

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