[SI-LIST] Re: Length of Discontinuity

This is quite easy to model. We use this as an example for
our in-house training on Linear Tech Spice (SWCAD III). The
engineers look at their board stack-up, determine trace=20
impedance, and use an idealized transmission line in segments
whose electrical length and impedance match that of the routed
trace. They can then look at the signal received and the signal
reflected (driver and receiver). Adding stubs is straight-forward.
The stubs should be terminated in a load that approximates the receiver.

                                        Curt

Curt McNamara. P.E.=20
Senior Electrical Engineer=20
Logic Product Development=20
411 Washington Ave. N Suite 101=20
Minneapolis, MN 55401=20
Tel 612-436-5178=20
Fax 612-672-0443=20
curtm@xxxxxxxxxxx=20
www.logicpd.com=20
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-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Andrew Seddon
Sent: Tuesday, January 17, 2006 9:13 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Length of Discontinuity


Hello,

I was wondering if there are any rules of thumb or formulas as to how
the length of an impedance discontinuity will effect a signal, related
to it's rise time?

For example I heard a general rule would be, keep the electrical length
under 10% of the signal rise time. So about 0.6inches of FR4 with a 1ns
rise time?

Is there any literature as to how the placement/size of the
discontinuity affects the signal?

Cheers

Andrew




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