We use a freeware DOS program called imped from HADCO. It calculates
Zo from geometry information. You can contact their tech centre in
Salem, NH for a copy.
We have found that HADCO can control impedance quite accurately.
We use their calculator to determine trace widths for the desired
impedance and then route with a slightly (0.001") wider track.
Then you specify to them that all X mil tracks on layer Y should be
Z Ohms. They will size the tracks as required to get your specified
impedance. They can easily size down; it's far more difficult to size up.
Not related to HADCO, just fairly satisified with their results.
-- Jim Tompkins, M.A.Sc., P.Eng. Hardware Designer Internet : email@example.com Applied Microelectronics Inc. Voice : (902) 421-1250 x252 1046 Barrington Street Fax : (902) 429-9983 Halifax, NS CANADA B3H 2R1 WWW : http://appliedmicro.ns.ca