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) ----- Original Message ----- From: "Jim Bacher" <[email protected]> To: <[email protected]> Cc: <[email protected]> Sent: Thursday, May 18, 2000 4:24 PM Subject: Re:RE: Charge moving from decoupling capacitors > > Forwarded for George. > > ____________________Reply Separator____________________ > Subject: RE: Charge moving from decoupling capacitors > Author: [email protected] > Date: 5/18/00 2:30 PM > > Barry, > > Thanks for the comments. Here are my comments: > > Ok, you put caps at a certain distance away from the IC because you only > want them to work at 100 MHz. But that distance turns out to be the 1/4 > wave distance at 400 MHz, and you placed enough caps at the 1/4 wave > distance to cause board resonance. Now what? Do you tell the caps not to > work at 400 MHz because it's not their frequency? > > > For your 2nd comment: > > I used the words "loosely define" for that reason. If you are interested in > high frequency decoupling and instantaneous current, you really want to have > all your charges moving in phase. At 1/4 wavelength, the charges are 90 > degrees out of phase, so they will not do much for your instantaneous > current. 1/8 wavelength is what I consider to be acceptable. You can > certainly pick a different number. > > Regards, > > George Tang > [email protected] > > > -----Original Message----- > From: Barry Ma [mailto:[email protected]] > Sent: Thursday, May 18, 2000 10:50 AM > To: [email protected] > Cc: [email protected]; [email protected] > Subject: RE: Charge moving from decoupling capacitors > > > George, > > Thanks for your long input. I'd like to make some comments below. > ------------- > On Wed, 17 May 2000, [email protected] wrote: > > > Large parallel plates behave as transmission lines. A quarter wavelength > > transmission line with a short at the end has infinite impedance, so > > capacitors placed 1/4 wavelength away are bad. > > That's why decaps work on low frequency portion. Let's set 100 MHz and below > for decaps to cover. The wavelength at 100 MHz is 3 meters. A quarter of it > is 75 cm. It's long enough to ordinary PCB size. (The cap is directly > connected to pwr/gnd planes.) > > > > This means that we can loosely define the largest usable board area > capacitance as 1/8 > > wavelength radius of copper surrounding the IC power pin. Charges stored > on the planes > > further than 1/8 wavelength away are not very usable due to the time > delay. > > At 500MHz in FR4, 1/8 wavelength is 1.5 inches. Is such a board capacitor > > good enough for your IC? > > George, I beg for differentials. How did you jump from "capacitors placed > 1/4 wavelength away are bad" to "the largest usable board area capacitance > as 1/8 wavelength radius"? > > Can I use the same token to infer from "caps placed one wavelength away are > good" to "the largest usable board area capacitance is within 1/2 wavelength > radius"? And so, and so on. > > Regards, > Barry Ma > [email protected] > > > > ------------------------------------------- > 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] > > > ------------------------------------------- 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]
