# Re: [SI-LIST] : A basic question

From: amit agrawal ([email protected])
Date: Fri Mar 17 2000 - 11:53:25 PST

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)" <[email protected]>
>To: "'[email protected]'" <[email protected]>
>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
>Pd=(LC)^(1/2)
>I guess my question has to do with Pd. How can we equate Pd to both the
>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
>material that light is propagating through).
>Can I really reduce C, without increasing L, and thus reduce the Pd of an
>interconnect? If I can't then the Pd=(LC)^(1/2) has problems. If I can then
>how does it align with light's speed being just a function of the medium
>it's traveling through?
>
>regards,
>Jim
>
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