# RE: [SI-LIST] : Rise Time Degradation

From: Larry Miller ([email protected])
Date: Tue Apr 10 2001 - 20:55:57 PDT

In the frequency domain you multiply the frequency responses of the cascaded
units. In the time domain this corresponds to complex convolution. This is,
I believe the source of the RSS formulae. In Dr Howard Johnson's book
(Digital Black Magic) there is discussion and an Appendix dealing with the
various rise time waveform assumptions that these rule-of-thumb formulas
use: raised cosine, gaussian, etc. If I remember correctly the choice made
an insignificant difference, within reason.

Larry Miller

-----Original Message-----
From: [email protected] [mailto:[email protected]]
Sent: Tuesday, April 10, 2001 1:44 PM
To: [email protected]
Subject: [SI-LIST] : Rise Time Degradation

Hello All.

I am involved in some discussions (arguments) over an EIA test spec for rise

time degradation (RTD) of connectors/interconnects. The spec in question is

Basically, this spec uses a "square root of the sum of the squares" type of
calculation, where, first, the RTD of a test fixture is measured. The
connector or whatever DUT is then inserted into the fixture, and the RTD of
this combination is measured. The RTD of the DUT is then calculated from
these two measurements using the sum of squares method.

Does anyone know where this method originated? I have seen a few references

that refer to it as a rule of thumb type calculation for cascading RTD's of
various devices. I have also seen a basic mathematical justification for it

in relation to oscilloscope bandwidths/risetimes. But in that case, it was
assumed that the devices being cascaded where R-C type networks. And even
then, I believe they said it was an approximation.

Not that I want to question the technical reasoning of the EIA or anything,
but does anyone out there have an opinion on how accurate this method might
be when applied to interconnects? Or might someone possibly even have a
mathmatical/physical justification for this method?

Thanks for any help anyone might offer.

Julian Ferry
Samtec, Inc.

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