Capacitors can continue to function quite well above their self resonance. You should not be worrying about choosing a cap based on self resonance per se.
However, it is standard practice to parallel a 0,1 uF "bulk" bypass cap with a much smaller NP0 or C0G type (say 470 pF or 1 nF), especially in RF work. (I'm not sure why you chose the rather "odd" value of 820 pF) The reason has little to do with self resonance directly. The main reason is that the ESR of the larger cap begins to rise to unacceptable levels as frequency rises (chiefly due to its self inductance). The low-value NP0 cap has much lower self inductance, hence it continues to function as a true capacitor at higher frequencies. As for as specifically using an SMT cap is concerned, you can do this if you want, but it isn't a requirement. The frequency you are concerned with (the low hundreds of MHz) is still pretty low frequency. Normal leaded devices will work just fine provided you ensure the board layout is done properly (which you need to do anyway), that the cap is inserted straight into the PCB, and that the leads are not "formed" so it stands above the PCB. Bob Wilson TIR Systems Ltd. Vancouver. -----Original Message----- From: [email protected] [mailto:[email protected]] Sent: April 17, 2002 1:50 PM To: [email protected] 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: [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"
