Amund, I have encountered similar problems with similarly designed boards. In one interesting case, I was able to get 30 dB reduction in radiated EMI by adding a series resistor to a (fairly short) low-frequency clock line. Due to use of modern devices, its rise and fall times were less than a nanosecond. The resistor limits current in the trace, reducing also the power required of the clock driver, and slows the slew rate. Improving power distribution also made a large difference; power and ground planes were added to the board and also improved signal integrity.
But I prefer to first attack problems such as yours with better decoupling. One all-nighter long ago resulted in my soldering SMD capacitors directly across pins on a device package, 1208 for the pins farther from grounds, and 805's for ones closer to grounds. This made a considerable difference but was greeted by my boss with, "YOU DID WHAT?!" It DID open him up to the possibility of less drastic measures previously ruled out. The suggestions you've been given about power routing are on target. If you distribute power and its return as a transmission line themselves, you can at least control where noise on it goes, reducing THAT loop size, and filters on this line may be expected to work predictably. However, do remember that once you put noise on silicon, it gets onto everything else. You must satisfy the need for current at each device: enough capacitance, able to supply current quickly enough, at enough power and ground pin pairs. If you allow device Vcc/ground to be periodically modulated by device switching current, the noise produced will be present on every trace leaving the device and you will not make it go away. That's why I first look for good decoupling when doing schematic and layout reviews. THEN I look for routing missteps and inadvertent loops. Others may prefer to approach things in a different order; that's just my preference. But it works for me. Good luck! Cortland Richmond ------------------------------------------- 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: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: davehe...@attbi.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org 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"
