VRM Filtering

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  • davmax
    Badcaps Veteran
    • Dec 2005
    • 899

    #1

    VRM Filtering

    Much has been made of the need to use ultra low ESR caps for the CPU power source.

    These caps have specified impedance at 100Khz that will be effective for power supply ripple. However what about the frequency spectrum above 100Khz that goes way up to a number Ghz. A processor operating with 400FSB and 2Ghz will generate wide band noise on the power rails. Normal digital circuit design requires that ceramic capacitors are placed as close as possible to the supply pins of each chip so that these frequencies can be bypassed. Electrolytic caps present themselves as a high impedance at these ultra high frequencies. I cannot see the use of ultra high frequency capacitors around the the CPU. Perhaps they are very small or laminated planes in the motherboard are used to remove these frequencies.

    Does anybody have a reference that indicates how complete filtering is achieved? I have tried Google searches.
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  • linuxguru
    Badcaps Legend
    • Apr 2005
    • 1564

    #2
    Re: VRM Filtering

    There are ceramics mounted on the FCPGA substrate itself on all Intel CPUs from the Copermine onwards. In addition, there are usually a bunch of ceramics next to or within the cavity of the CPU socket. The bulk of the core/FSB-frequency noise is smoothed by these ceramics - the large electrolytics are mainly charge reservoirs and VRM-frequency filters.

    Comment

    • PeteS in CA
      Badcaps Legend
      • Aug 2005
      • 3578
      • USA, Unsure of Planet

      #3
      Re: VRM Filtering

      Back in the late 90s, Sun has a program that would select a whole array of ceramic, OSCON, and lytic (tantalum was verboten) caps for a given processor model. They were contemplating designing capacitors on-chip to get capacitance close to the load transient.
      PeteS in CA

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