The high frequency component of return current in a plane is concentrated
under the track.
I recall a number of 90% within 1.5 times the track-plane distance.
So intuitively one might expect less effect if the track is close to the
plane.
Say the vias are 50 drilled 16 with 10 clearance. This makes a bridge of
copper between voids of
50 - (16 + 2x10) = 14.
Best result would be obtained if the track follows the centre line of the
bridge (good argument for gridded routing).
If the above is correct, then the above geometry would support a track to
plane distance of upto 10.
Jon Keeble
**** To unsubscribe from si-list: send e-mail to [email protected]. In the BODY of message put: UNSUBSCRIBE si-list, for more help, put HELP. si-list archives are accessible at http://www.qsl.net/wb6tpu/si-list ****