*From*: Ray Anderson <Raymond.Anderson@xxxxxxx>*To*: si-list@xxxxxxxxxxxxx*Date*: Wed, 13 Aug 2003 09:24:26 -0700 (PDT)

Vishram Pandit wrote: >I have seen some comments on the SI list that the capacitors are ineffective >say above 500MHz. Is this really true?? I have seen improvement in EMI at >600MHz-800MHz if I tune the > >* value of the capacitors > >* ESR / ESL of the capacitors > >* location of the capacitors > >Any comments/advice will be appreciated. > >Thanks, > >Vishram > I think you will find that discrete decoupling capacitors placed on a PCB are largely ineffective at 500 MHz (actually above about 100 MHz or so) for SI purposes. However, as you have discovered, they can be useful for dealing with EMI issues up to 500 MHz (and sometimes beyond). The problem with using decaps for SI purposes above about 100 MHz is that there is usually enough parasitic inductance in the chip package (and the package mounting) that any attempt at bypassing on the PCB never shows much if any effect at the silicon. Depending on the exact design of the package and the mounting arrangement (pad design, vias, board thickness, etc.) the useful frequency for the SI decaps may even be considerably less than 100 MHz.To provide effective bypassing at the higher frequencies you need to consider decaps in the package or on the silicon. For EMI purposes, you can select decaps that will resonate with their mounting inductance to provide low impedances at specific problematic frequencies. You can select physical locations on the board that are "hot-spots" to place your decaps for maximum effect. The distributed capacitance provided by the power planes is also very effective in dealing with EMI issues at high frequencies , but as mentioned above, being present at the PCB it doesn't do much for SI pds problems on the chip. There are lots of other things to consider, but these are some of the main ones. -Ray Anderson Sun Microsystems ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: http://www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List archives are viewable at: http://www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu

**Follow-Ups**:**[SI-LIST] Re: UltraCAD ESR and Bypass Capacitor Caculator***From:*Dr. Sainath Nimmagadda

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