# 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