Re: [SI-LIST] : Critical Length

Larry McMillan ([email protected])
Tue, 30 Mar 1999 14:34:24 -0800

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.


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

If the speed of light in a vacuum (c) is the upper limit of velocity,
then 84.667 ps/in is the lower limit for propagation delay, and it is
only realizable in free space. All other media will have larger
(longer) values of propagation delay. If you arrive at a value for
propagation delay that is less than that of the free space value, then
you can be very certain that something is wrong in your calculations.

Larry McMillan
Team Leader, Physical Interface Group
Western Digital Corporation

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