# RE: [SI-LIST] : Critical Length

Peterson, James F ([email protected])
Wed, 31 Mar 1999 07:46:52 -0500

D.C.
saw the error and assumed it was just a simple decimal point slip.
honest mistake - still good input.
jim

-----Original Message-----
From: D. C. Sessions [mailto:[email protected]]
Sent: Tuesday, March 30, 1999 5:49 PM
To: [email protected]
Subject: Re: [SI-LIST] : Critical Length

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
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