# [SI-LIST] Re: Effective impedance of a uniformly loaded bus

• From: steve weir <weirsi@xxxxxxxxxx>
• To: Edi.Fraiman@xxxxxxxxxxxxx, <si-list@xxxxxxxxxxxxx>
• Date: Mon, 28 Nov 2005 14:14:29 -0800

```Edi, was Dr. Johnson's explanation sufficient?

My message was thatat you clearly understand the C' formula.  But, when it
comes to line impedance, discontinuities are what disturb signal quality,
not the line length.

But, well past the era of TTL, I think this is mostly academic.  For any
modern rise time, work the coefficients and I think you will find that
manipulating the segment lengths will not make the difference between a
design that is viable and one that is not.

Steve.
At 06:09 PM 11/28/2005 +0200, Edi Fraiman wrote:
>Steve,
>
>
>Edi=20
>
> > -----Original Message-----
> > From: steve weir [mailto:weirsi@xxxxxxxxxx]
> > Sent: Monday, November 28, 2005 4:49 PM
> > To: Edi Fraiman; si-list@xxxxxxxxxxxxx
> > Subject: Re: [SI-LIST] Effective impedance of a uniformly loaded bus
> >=20
> > Edi, your understanding of C' is fine, but your conclusion is not
>quite
> > right.  Think in terms of impedance continuity.
> >=20
> > Steve.
> >=20
> > At 09:01 AM 11/28/2005 +0200, Edi Fraiman wrote:
> > >Hello experts,
> > >
> > >I have question regarding formula C'=3D3D Cline + NCload/length and =
>Z'
>=3D3D
> > >(L/C')^1/2 ("A Handbook of Black Magic" Paragraph 4.4.3.1).
> > >
> > >If I design trace with several loads than according to the formula I
>can
> > >conclude next statement:
> > >Longest trace cause of reducing in total capacitance C' and increase
>the
> > >Z'.
> > >
> > >So, long trace help to driver drive loads (impedance is larger).
> > >I fill confuse, everyone knows that long trace is not good for SI.
> > >=3D20
> > >Could you please clear me this issue?
> > >
> > >  =3D20
> > >
> > >Best Regards,
> > >
> > >Edi Fraiman
> > >------------------------------------------------------------------
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> >=20
>
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```