**From:** amit agrawal (*amitp_agrawal@hotmail.com*)

**Date:** Fri Mar 17 2000 - 11:53:25 PST

**Next message:**Bob Perlman: "RE: [SI-LIST] : Bad IBIS models!"**Previous message:**Tom Dagostino: "RE: [SI-LIST] : Bad IBIS models!"**Maybe in reply to:**Peterson, James F (FL51): "[SI-LIST] : A basic question"**Next in thread:**Fred Balistreri: "Re: [SI-LIST] : A basic question"**Reply:**Fred Balistreri: "Re: [SI-LIST] : A basic question"

In ideal situation, the progation delay is strictly dependent on Er. The

proagation delay is given by

tau = sqrt(Er)/30 ns/cm

This is also equal to sqrt (LC)(Note: this condition is true only when C

does not include the fringe field effect)

When we use any EM tool to calculate the transmission line L and C per unit

length, the capacitance may include the fringe field effect. In that case,

the effective propagation delay is always more than the ideal propagation

delay.

tau(effective) = tau(ideal) + delta due to fringe-field

The fringe field effect can vary with the geometry of the line.

In reference to characteristic impedance, the ideal characteristic impedance

is sqrt (L/C), where C is fringe-field-free. This can also be expressed in

terms of ideal propagation delay as

Zo (ideal) = tau (ideal)/C

C depends on the geometry of the line for a given Er.

Thus, if the Er is fixed, the capacitance and impedance can be changed by

changing the geometry of the line.

The capacitance, C calculated from EM tools is not fringe-field-free, hence

the characteristic impedance is no longer ideal> we can refer it as

Zo(effective). Note that Zo(effective) will always be smaller than the

Zo(ideal).

I hope this should answer your question.

Amit Agrawal

Lightsand Communications

*>From: "Peterson, James F (FL51)" <james.f.peterson@honeywell.com>
*

*>Reply-To: si-list@silab.eng.sun.com
*

*>To: "'si-list@silab.eng.sun.com'" <si-list@silab.eng.sun.com>
*

*>Subject: [SI-LIST] : A basic question
*

*>Date: Fri, 17 Mar 2000 08:00:49 -0500
*

*>
*

*>SI List:
*

*>
*

*>I have a question about the relationship between Z and Pd (prop. delay).
*

*>The
*

*>equations used for these two are :
*

*>Z=(L/C)^(1/2)
*

*>and
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*>Pd=(LC)^(1/2)
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*>I guess my question has to do with Pd. How can we equate Pd to both the
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*>above equation and the fact that the speed of light is C/((er)1/2)? Which
*

*>says that prop. delay is just a function of the dielectric constant (or the
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*>material that light is propagating through).
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*>Can I really reduce C, without increasing L, and thus reduce the Pd of an
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*>interconnect? If I can't then the Pd=(LC)^(1/2) has problems. If I can then
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*>how does it align with light's speed being just a function of the medium
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*>it's traveling through?
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*>
*

*>regards,
*

*>Jim
*

*>
*

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**Next message:**Bob Perlman: "RE: [SI-LIST] : Bad IBIS models!"**Previous message:**Tom Dagostino: "RE: [SI-LIST] : Bad IBIS models!"**Maybe in reply to:**Peterson, James F (FL51): "[SI-LIST] : A basic question"**Next in thread:**Fred Balistreri: "Re: [SI-LIST] : A basic question"**Reply:**Fred Balistreri: "Re: [SI-LIST] : A basic question"

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