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

From: Istvan Novak - Board Design Technology ([email protected])
Date: Mon Apr 17 2000 - 11:09:19 PDT

Michael,

In stripline, if nothing else is connected to the traces, the propagation delay
is a function only of length of trace and dielectric constant, it does not
depend on the trace cross sectional dimensions or dielectric layer height. When
you add discontinuities, such as via capacitances, their effect will depend on
the ratio of via capacitance to inherent trace capacitance. The approximate
correction factor for the delay is sqrt(1+Cload/C).

If your three traces with different trace widths come with proportionally
adjusted dielectric layer height, so that the unloaded trace impedance is the
same for all three traces, the effect of vias (assuming identical via
capacitance on all three traces) will be the same. However, if the different
trace widths are used to set intentionally different trace impedances, the
effect (of identical) vias will be less on lower-impedance traces (higher
inherent trace capacitance) and will be higher on higher-impedance traces (less
inherent trace capacitance).

Istvan Novak Sun Microsystems, Inc.
[email protected] Workgroup Servers, BDT Group,
One Network Drive, Burlington, MA 01803
Phone: (781) 442 0340

From: "Greim, Michael" <[email protected]>
To: "'[email protected]'" <[email protected]>
Subject: [SI-LIST] : Question on propagation delay........
Date: Mon, 17 Apr 2000 13:14:56 -0400
MIME-Version: 1.0

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
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.

>

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