RE: [SI-LIST] : Critical Parameter: rise-time or slew-rate?

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From: Ingraham, Andrew (Andrew.Ingraham@compaq.com)
Date: Tue Nov 14 2000 - 14:06:17 PST


For me, the intuitive answer is to mimic risetime ... unless the system you
are looking at is nonlinear and you want to look at that nonlinear behavior.

For pulses going down board traces, your system is linear. If you are
looking at what happens when those pulses hit a CMOS receiver at the end of
the trace, and looking specifically at what happens within that receiver,
then slew rate *might* be your best bet.

A pulse with a constant risetime has the same exact frequency spectrum, no
matter what its amplitude. As you vary the pulse amplitude, the entire
spectrum magnitude goes up and down, but its shape (i.e., percentage of
harmonics) stays constant. Whatever results you observe for a 0.5V TDR
pulse, can be directly applied to ANY amplitude pulse, just by scaling the
whole thing up or down. Or perhaps by not scaling it at all (if you are
looking at waveshapes or impedances).

Not so when you vary the amplitude of a pulse with constant slew rate.

> The non-intuitive answer is that rise-time is the important
> characteristic,
> regardless of slew-rate. This leaves the rules-of-thumb intact, but
> violates our gut sense that a signal switching from 0 to 5V in 1nS is much
> more "aggressive" (and sensitive to smaller discontinuities on a
> transmission line, for instance) than one switching from 0 to 1V in 1nS.
 
The 5V/1ns edge has both stronger harmonics and stronger DC (and low
frequency) components. They are in exact proportion. Yes, the 5V/1ns edge
will show bigger wiggles (measured in volts) due to discontinuities, but
when these wiggles are measured in percentage of the waveform step, they are
exactly the same in both cases.

Put another way, if you adjusted both waveforms to full-screen, and laid
them on top of one another, one could not be distinguished from the other.
(As long as the system is linear.)

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