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

From: Fred Balistreri ([email protected])
Date: Fri Mar 17 2000 - 12:30:57 PST

In the equation sqrt LC, the C is assumed to be total C, fringe included.
If the methods used do not compute fringe then the methods are at
fault not the equation.

Best Regards,

amit agrawal wrote:

> 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).
>
>
> 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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```--
Fred Balistreri
[email protected]
http://www.apsimtech.com
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