I second this request. All of the quantitative data that I have seen basically says that for a given dielectric, the inductance is a function of the package size and shape regardless of the capacitance. And yet the rule of thumb continues to be mixing capacitor values to handle a range of frequencies. Is that advice wrong (outdated, perhaps?) or is there more to it than the datasheets show? -Alan
On Mon, Oct 19, 2009 at 3:40 PM, Larry Doolittle <ldool...@recycle.lbl.gov> wrote: > Neil - > > On Mon, Oct 19, 2009 at 01:20:23PM -0700, Neil Hendin wrote: >> If you look at the RF S-Parameters of the capacitor at frequencies >> above the self resonance, they look inductive, not capacitive. > > I'm not sure what the precise definition is for the S-Parameters > of a two-terminal device, but the conclusion is accurate. > >> Good RF decoupling standard practice is to use a smaller cap >> (e.g. 20pF in parallel with some larger ones such as 1000pF >> _and_ 0.1uF or larger as needed) to get a good broad band >> capacitive reactance across frequency). > > I have yet to see a 20pF or 1000pF cap with less parasitic inductance > than a decent (e.g., X5R) 0402 cap up in the uF range. Say, in > particular, > TaiyoYuden JMK105BJ225MV-F 2.2uF 0402 6.3V X5R 0.1560 in 100's > If you're going to occupy board area with a cap and its connection > to the power nets, can anyone explain why I should choose anything > other than the largest value available in that size and voltage? > > OK, I suppose if you're building cell phones and selling them by > the hundreds of thousands, the nickel you could save by using > a lower value would add up. But for me, the cost of assembly and > documentation probably exceeds the cost of the component itself. > > - Larry > > > _______________________________________________ > geda-user mailing list > geda-user@moria.seul.org > http://www.seul.org/cgi-bin/mailman/listinfo/geda-user > _______________________________________________ geda-user mailing list geda-user@moria.seul.org http://www.seul.org/cgi-bin/mailman/listinfo/geda-user