Boss just sent around something he got from a consultant on doing "proper" EMI design (which I've been doing for years already, I thought until consultant came up with this):
"Eliminate separate Vcc planes. This ancient practice is long overdue for an overhaul. Years ago, the leaded capacitors were not able to provide a good enough short at VHF and above, so the reasoning was that the parallel plates of Vcc and ground made a good UHF capacitor. The problem with this is twofold: it takes away one or more ground planes, and more importantly doesn’t allow the designer to control where the noise current goes. Noise follows the path of least impedance, which may be anywhere on the PCB after you punch holes in the Vcc plane for vias and to route traces that have no other room to go. The best way to control noise is to use a separate trace for Vcc, and apply series and shunt elements to control the noise currents." There is no attribution as to were that advice comes from. The frequencies in question are 400 MHz to 3 GHz. To me running Vcc traces all over the board is the surest way to raise inductance etc., and seems wrong to me. Want to know what you thought of this consultants advice? Doesn't cover what happens in multi-rail systems either. _______________________________________________ geda-user mailing list [email protected] http://www.seul.org/cgi-bin/mailman/listinfo/geda-user
