# Re: [SI-LIST] : Critical Length

D. C. Sessions ([email protected])
Wed, 31 Mar 1999 09:04:02 -0700

"Peterson, James F (FL51)" wrote:
>
> D.C.
> I believe most of the engineers that read your insightful e-mail
> saw the error and assumed it was just a simple decimal point slip.
> honest mistake - still good input.

I'd like to think so, but as it happens one of my personal hot
buttons is the (sometimes it seems damn near universal) refusal
to 'fess up to mistakes. Under the circumstances, I'd feel
pretty hypocritical not admitting the slip as publically as I

> Larry McMillan wrote:
> >
> > Abe Riazi wrote:
> > Delay = ( C )( Zo) = ( 2.9 pF/in)( 28 Ohms ) = 81.2 ps/in
> >
> > D. C. Sessions wrote:
> > Your numbers are off. 800ps/in is the speed of light in vacuum.
> > Actual PWB speeds range from about 160ps/in for microstrips to
> > almost 200ps/in for striplines.
>
> Now one *might* think that the (correct) pwb numbers should have
> given me a clue, wouldn't one?
>
> > --------------------------------------
> >
> > By my calculations, I believe you are both in error. I have for the
> > speed of light in free space (vacuum):
> >
> > c = 300,000,000 meters/second
> > = (300,000,000 m/s)(1 inch/0.0254 meter) = 11811023622.05 in/s
> > = (11811023622.05 in/s)(1s/1,000,000,000,000ps) = 0.0118 in/ps
> >
> > free space propagation delay = 1/c = 1/(0.0118 in/ps) = 84.667 ps/in
>
> Ya got me. Pure stupid decimal-point stuff.
> (You'da thought I'd have learned me lesson back
> when I was using slipsticks. Been wrong, too.)
>
> --
> D. C. Sessions
> [email protected]
>
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D. C. Sessions
[email protected]

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