Barry Thanks for correcting me. Practical solutions have changed from the time I made many measurements on PCBs with discrete components. I'll be interested to read the article you mentioned since I wasn't aware a single SMA cap could do a better job.
Regards, Ralph Cameron ----- Original Message ----- From: "Barry Ma" <[email protected]> To: <[email protected]> Cc: <[email protected]> Sent: Friday, May 19, 2000 11:33 AM Subject: Re: Charge moving from decoupling capacitors > 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]
