I generally agree, although your comment that double sided PCBs are about to disappear is rather premature. Way over half of the PCBs manufactured in the world today are SINGLE sided (look in any piece of high volume electronics). Double sided PCBs still will be used in volume for many years to come.
Many 4 layer PCBs are simply the result of lack of designer layout skill, or (in particular) a result of the use of autoplace and autoroute programs that would otherwise "ventilate" a 2-sided board with an absurd number of vias and serpentine tracks snaking willy-nilly all over the place. The "solution" to the need for actual skill in board layout is often to let the machine do it, and suffer the cost of more layers. Time to market is, it seems, more important than cost or quality. At least here in North America. Bob Wilson TIR Systems Ltd. Vancouver. -----Original Message----- From: Cortland Richmond [mailto:[email protected]] Sent: March 19, 2002 4:02 PM To: Robert Macy; ieee pstc list Subject: Re: Relative merits of various logic families in not generating RFI Probably, you won't get much choice. I've often found older, slower, quieter logic impractical or even uneconomical to use. It may be made only by one (thus off limits single-source) manufacturer, or it may be built using older, more expensive technologies and cost an arm and a leg. And it may happen one day that a manufacturer lets you know production is simply ending, and with no chance of an equivalent substitute. I am rather afraid that the best solution in these cases is to go with later, even if noisier devices -- and then design for them. Then too, if you use current devices, you may escape being blind-sided when they go to a smaller fabrication technology without letting you know. Even spting advertised an spe'd as an older device may in fact be a newer one. Who, after all, specifies devices by the fastest they go? It's always a minimum guaranteed speed. Yes, that means even MHz and KHz clock-rates with nanosecond transitions. You have to deal with it. Slew rate limiting is available, sometimes, built-in. If not, you have to add it externally. I've seen a 30 dB difference at 147 Mhz from a single 33 ohm resistor on a 1 Mhz clock. You have to be more careful with layout. You have to avoid inadvertent peaking networks - DON'T let anyone just throw HF bypasses willy-nilly on logic signals; you'd be AMAZED where the high frequencies can end up. And it means the end, really of 2-layer boards, at least as the old engineers know them. They have to be redesigned for RF, even if we weren't dealing with RF. But in the end, we get reliable boards, cheaper, that won't have to be replaced when the foundry discovers shorter-wavelength lithography. And your totem-pole short circuit? Yes, they know about that. Don't DO it. (grin) If you MUST have that kind of output, put a charge reservoir right at the device power pins, faster than a speeding junction, able to leap tall short circuits at a single bound, with enough charge to keep the transient local. But you already KNEW that! Cheers, Cortland ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: [email protected] Dave Heald: [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://ieeepstc.mindcruiser.com/ Click on "browse" and then "emc-pstc mailing list" ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: [email protected] Dave Heald: [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://ieeepstc.mindcruiser.com/ Click on "browse" and then "emc-pstc mailing list"
