[SI-LIST] Re: Decoupling at 800Mhz
- From: "DAVID CUTHBERT" <telegrapher9@xxxxxxxxx>
- To: "Subramanian R" <subramanian1683@xxxxxxxxx>
- Date: Fri, 23 May 2008 08:12:03 -0600
Mani,
at 800 MHz any capacitor over 39 pF will look inductive. Although it is
inductive, the impedance of the 1 uF will be as low as, say a 1 nF. The
impedance at 800 MHz is over +j5 ohms. At 800 MHz the real decoupling will
come from the power plane. It will present a low impedance as well as tie
together the decoupling capacitors.
The simple and inexpensive method for modeling the power plane and
decoupling is SPICE. The free LT SPICE works as well as any other. If you
want a recipe that will work: Lay down power and ground layers spaced 4 mils
apart. Use at least one 1 uF cap for every 100 mA.
Dave Cuthbert
On Thu, May 22, 2008 at 9:06 AM, Subramanian R <subramanian1683@xxxxxxxxx>
wrote:
> Hi All,
> We are planning to use a processor running at 800 MHz.
> I was just reading up on the decoupling methods.
>
> The Processor PCB guidelines recommend using 1uF 0402 pack for high
> frequency decoupling. But at 800 MHz, will not the cap/package inductance
> dominate.
>
> The guideline doesn't say anything about plane decoupling also.
>
> Am I missing something? What are the best approaches for decoupling?
>
> Any suggestions will be very useful.
>
> Thank you in advance,
>
> Regards
> Mani
>
>
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