Re: [SI-LIST] : Question on propagation delay........

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From: Mike Jenkins (jenkins@lsil.com)
Date: Mon Apr 17 2000 - 14:26:26 PDT


Michael,

Here's my two cents. By 'different widths' I assume you
mean the PCB cross section is the same. Hence, the
characteristic impedance is different for the three nets.
This would probably have profound effects on the driver
delay.

Second, if the nets are routed on internal planes, then
the propagation velocity would indeed depend only on the
dielectric. But, for external planes, wider traces would
put more of the field into the board and less in the air,
slowing the propagation.

Third, the effect of a via depends on where the return
current flows. If (as many contributors have previously
described) the change in signal layers involves a change
in current return layers, then the current return has to
find a path. If it's a significant detour, that looks
inductive, adding delay. A high frequency decap between
the two planes and near to the via minimizes this effect.

Regards,
Mike

"Greim, Michael" wrote:
>
> Hi everyone,
>
> Here is what I thought was a simple question but
> has turned into a discussion of some debate. Three
> nets are routed on a pwb as stripline structure. They
> are driven by the same type of driver and have the same
> type of receiver. There is one driver and one receiver per
> net. They contain the same number of vias in the same
> location on the nets with the same padstack. The only
> topological difference of the nets is that they are routed
> as different widths. That being said, what is the flight
> time of these topologies as a function of length. Is it
> strictly a function of the dielectric? What is the relative
> effect of varying number (say difference of no more than
> a handful) of vias on the nets.
>
> Whaddya think?
> >

-- 
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