Ralph, Please be cautious about what you said below: "the reason why most power buses on PCBs use several values of decoupling is to ensure that a wide range of frequencies are covered."
Several V curves shown in the figure of impedance vs. frequency, which we are all familiar with, would easily convince us the above statement. But those V curves only show the absolute value of impedance around self-resonance frequency of various capacitors. If considering related phase relations, the total impedance of several values of decaps would become not as simple as we expected - having low impedance over wider frequecy range. You may refer to an article by Paul, C. R.: "Effectiveness of multiple decoupling capacitors," IEEE EMC Vol. 34, p. 130, May 1992." In my practice of using SMA caps, only the largest value of capacitance available for given SMA size is selected, if I have a PCB with 10 mil or less plane spacing. Regards, Barry Ma [email protected] ------------ On Thu, 18 May 2000, "Ralph Cameron" wrote: > As I read in an article related to bypassing for good decoupling ( in 1971) > one can select from a number of EIA values and by cutting the lead lengths > correctly ( e.g. from 1/2"- less than 1/4" ) the series reonant frequency > will drop by a considerable amount so - yes, the reason why most power buses > on PCBs use several values of decoupling is to ensure that a wide range of > requencies are covered. Perhaps, with surface mount caps, that is easier to > predict because they are essentially leadless. > > I once cured a very severe case of an FM receiver responding to the 7th > harmonic of a 14Mhz transmitter because an untuned mixer was used. Placing > a 100pf cap with 1/4" leads right across the mixer IC completely cured the > problem without degrading mixer sensitivity. > > Ralph Cameron > EMC Consultant for Suppression of Consumer Electronics > (After Sale) _______________________________________________________________________ Why pay when you don't have to? Get AltaVista Free Internet Access now! http://jump.altavista.com/freeaccess4.go _______________________________________________________________________ ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: [email protected] Michael Garretson: [email protected] For policy questions, send mail to: Richard Nute: [email protected]
