My ethernet 10/100 physical (KS8721) chip tracks to the RJ45 integrated
magnetics connector should have had a differential impedance of 100
ohms according to the manufactures spec, ( 5 thou tracks and 5 thou
gap). However the board was laid with 10 thou tracks 5 thou gap giving
70 ohms differential impedance (ground plane 5 thou below). The tracks
are about 25mm long and the board seems to work ok but I am about to do
a few changes and wondered if its worth changing, 5 thou tracks are
very thin and I have noted that the manufacturers development board
also uses 10 thou tracks.

sjones@scannex.co.uk wrote:
No problem - it's so short compared to the wavelength that
it can be regarded as a short stub and the mismatch is not
too serious.

The maximum base frequency is 62.5 MHz, giving nearly 5
meters wavelength in free space and about 3 meters on your
board trace.

HTH

--

Tauno Voipio
tauno voipio (at) iki fi