On Monday 19 October 2009, Bob Paddock wrote: >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.
What's he/she smoking, it must be great stuff and I want a sample. >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. And as a C.E.T. with 60 years of electronics experience, troubleshooting to the part level, I sure as heck would not want my name attached to such advice. >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. It is. But it is sometimes helpful to make those layers a bit like a star topology to help steer the noise properly and keep it from going willy-nilly anyplace it wants to go. >Want to know what you thought of this consultants advice? Rubbish. Whats worse is that your boss probably _paid_ for that advice. >Doesn't cover what happens in multi-rail systems either. -- Cheers, Gene "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) The NRA is offering FREE Associate memberships to anyone who wants them. <https://www.nrahq.org/nrabonus/accept-membership.asp> The law will never make men free; it is men who have got to make the law free. -- Henry David Thoreau _______________________________________________ geda-user mailing list [email protected] http://www.seul.org/cgi-bin/mailman/listinfo/geda-user
