**From:** Istvan Novak - Board Design Technology (*[email protected]*)

**Date:** Mon Apr 17 2000 - 11:09:19 PDT

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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

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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**Next message:**Lynne Green: "[SI-LIST] : App Notes & Papers"**Previous message:**Scott McMorrow: "Re: [SI-LIST] : IBIS for SSTL_2"**Maybe in reply to:**Greim, Michael: "[SI-LIST] : Question on propagation delay........"**Next in thread:**Steve Corey: "Re: [SI-LIST] : Question on propagation delay........"**Reply:**Steve Corey: "Re: [SI-LIST] : Question on propagation delay........"

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