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 george_t...@dell.com -----Original Message----- From: Barry Ma [mailto:barry...@altavista.com] Sent: Thursday, May 18, 2000 10:50 AM To: george_t...@exchange.dell.com Cc: si-l...@silab.eng.sun.com; emc-p...@ieee.org 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, george_t...@dell.com 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 b...@anritsu.com _______________________________________________________________________ Why pay when you don't have to? Get AltaVista Free Internet Access now! http://jump.altavista.com/freeaccess4.go _______________________________________________________________________ **** To unsubscribe from si-list or si-list-digest: send e-mail to majord...@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE si-list or UNSUBSCRIBE si-list-digest, for more help, put HELP. si-list archives are accessible at http://www.qsl.net/wb6tpu ****