We spend a few hours in an EMC lab today, trying to suppress the unwanted 162MHz signal. We had to suppress it approximate 5 dB. There existed 100nF caps on the Vcc pins but the caps lead was "long". We went on decoupling (1nF and 100nF) with "short" leads (not OMF caps) on the uP Vcc pin and on other IC power pins. We reduced the 162MHz signal by 7 dB. Thereafter we went on trying to kill some loop areas, but we only won 1-2 dB. When we were kicked out, time was up ....
We would like to check the use of ferrite beads on +5V trace and the tantalum capacitor during current switching, maybe next time. Thanks for ALL ideas on this subject. Amund, Oslo/Norway -----Opprinnelig melding----- Fra: owner-emc-p...@majordomo.ieee.org [mailto:owner-emc-p...@majordomo.ieee.org]Pa vegne av Cortland Richmond Sendt: 19. april 2002 15:31 Til: richwo...@tycoint.com; ieee pstc list Emne: RE: Decoupling - capacitor values I have found removal of inductance at power pins often REDUCES emissions. When this is so, it is because designers who put it in did not insure there was sufficient local capacitance to hold up Vcc while the isolated device switches. This has happened to me often enough that when I see series inductance at a device power pin, I question the designer, and persuade him to add capacitance or remove the inductor (sometimes both - grin). Power filtering can be estimated based on data sheets and measurements on sketch boards. But designers need to do at LEAST that much, and they often do not. I consider this important enough that when I interview engineer candidates (that's when I am not one myself!) I ask them to show me how they will calculate the capacitance needed to support a digital device driving some number of loads. This is not rocket science, but many have not thought of it at all. 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" ------------------------------------------- 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"
