RE: [SI-LIST] : Source termination of transmission line

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From: abe riazi (ariazi@serverworks.com)
Date: Mon Apr 23 2001 - 18:59:21 PDT


Dave Instone Wrote:

"how long does the transmission line
have to be to be recognised ,by the signal, as a transmission line?"

Hi Dave:

The Critical Length (Lc) offers useful guideline for answering above question. It is given by:

Lc = k(Tr)/(Tpd)

Where, Tr is rise time and Tpd is signal propagation delay.
A good value for coefficient k is 1/6; and assuming FR-4 substrate typical delays for outer and inner layers include:

Tpd = 150 ps/in (Microstrip),
Tpd = 180 ps/in (Stripline)

 After critical line length is calculated, the trace segments smaller than Lc can be classified as lumped and those exceeding Lc as distributed.

Best Regards,

Abe Riazi,
ServerWorks

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