Re: [SI-LIST] : Definition of Hi-Speed (UC)

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From: Pat Zabinski (zabinski.patrick@mayo.edu)
Date: Mon Nov 01 1999 - 05:26:20 PST


Good question.

My personal use of "high speed" is where single lumped-element
models of parasitics can no longer be used. This will happen
when the edge-rate is some small fraction (?) of the time of flight
(per Abe's recent posting). In a mostly-pure sense, I disagree
with using frequency (e.g., xx MHz) as the sole-means of defining
"high speed", as the edge-rate more relevant.

To give an idea of what this practically means, we have designed
digital boards (multichip modules) less than 2 inches in
dimension, and "high speed" effects were not noticed
until edge rates got below 300-400 psec. In a larger system,
the signals propogate up to eight feet, and "high speed"
effects can be seen at edge rates as slow as 4-5 nsec.
Cleary, the actual values for "high speed" in these two cases
are a good order of magnitude different, and one simple
"xx MHz" cannot be used.

If you're stuck with defining high speed in terms of frequency,
I'd suggest using some convoluted mess like "where the
highest frequency component of the edge rate ....".

Have fun, and I'm interested to see what responses you get.

Pat

> Pardon me for asking this stupid question because I am at a loss of how
> to explain hi-speed to my boss. He thinks that hi-speed is as simple and
> straightforward as resistance = V/I and nothing else. Hi-speed should be
> some circuits that need to operate at xxMHz or more. Anything less than
> xxMhz is not hi-speed.
>
> I would appreciate anyone of you experts out there who can enlighten me
> in a simple and easy to understand definition so that my boss can
> understand.
>
> Regards.

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