Hmmm..... I've read a couple of other replies on this...good suggestions. I have also read your re-replies which suggest that you have a two layer "planeless" board.
I understand that you are trying to get an unwanted 156Mhz off of the 5V line. A couple of thoughts come to mind. First 156Mhz would need quite a long radiator to radiate efficiently. 156Mhz is approximately a 2m wavelength. One tenth of this wavelength is still 20cm. Why does this matter? Well, I like the decoupling cap idea. Another suggestion might be to add a large bulk decoupling cap such as a 10uF tantalum (you may have to experiment with the value) right at the chip. Then cut the 5V trace (on the other side of this cap) and put a good, strong ferrite (I like PREM SPG-104's myself) in the 5V trace. The bulk cap will serve as a local bulk storage device, the ferrite will help to isolate the harmonics to the area immediately around the processor. If you can keep the harmonics within a small area, perhaps they won't radiate very effectively and....maybe your product can live for another few months. Here's hoping that you find a solution. Chris Maxwell | Design Engineer - Optical Division email chris.maxw...@nettest.com | dir +1 315 266 5128 | fax +1 315 797 8024 NetTest | 6 Rhoads Drive, Utica, NY 13502 | USA web www.nettest.com | tel +1 315 797 4449 | > -----Original Message----- > From: am...@westin-emission.no [SMTP:am...@westin-emission.no] > Sent: Wednesday, April 17, 2002 4:50 PM > To: emc-p...@majordomo.ieee.org > Subject: Decoupling - capacitor values > > > A microprocessor is driven by a 12MHz clock. The 5V Vcc-pin is > decoupled by > a 100nF capacitor with a few mm leads. We can observe an unwanted > 156MHz > signal on the 5V line, maybe 13th harmonic of 12MHz. We will try to > suppress/decouple this 156MHz signal. > > Suggestion: > Insert a SMD ceramic capacitor of value 820pF in parallel with the > existing > 100nF. The reason for the low value 820pF is because the capacitor > self-resonance frequency is approximate 180MHz, and I believe it is > important to choose a Cap value with a resonance frequency higher than > the > frequency we would like to decouple. > > Does it make sense? > > Best regards > Amund Westin, Oslo/Norway > > > > ------------------------------------------- > 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" ------------------------------------------- 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"
