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. Thats why decaps work on low frequency portion. Lets set 100 MHz and below for decaps to cover. The wavelength at 100 MHz is 3 meters. A quarter of it is 75 cm. Its 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] _______________________________________________________________________ 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]
