RE: [SI-LIST] : Return path for stripline, two ground planes or o ne power and one ground?

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From: jrbarnes@lexmark.com
Date: Wed Apr 12 2000 - 05:33:16 PDT


>Tom - you are correct, there is a small amount of current from a
>transmission line that charges a capacitive load even at a drop along a
>transmission line. Depending on what that capacitive load is attached
>to (power and/or ground) and the plane that is carrying transmission
>line return current, we may need decoupling capacitance to resolve
>return currents.
> <snip>
>In the case of a terminating resistor, the charge is large because DC
>current continues to flow in the transmission line and terminating
>resistor for a long time. We might be integrating I*dt forever. Then
>we have to pay close attention to where the return current comes from.
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Larry,
In support of your point, just last night I was reading the Pentium(R) III
Xeon(TM) Processor Power Distribution Guidelines, Intel Application Note
AP-912, dated March 1999. (Available at
ftp://download.intel.com/design/pentiumiii/xeon/applnots/24509501.pdf )

Section 7.4, on terminating the AGTL+ signals, says:
"Intel recommends using a single VTT regulator and distributing VTT on a single
dedicated plane. Intel also
recommends one 0.1 uF capacitor for each termination resistor package for high
frequency decoupling."

                                         John Barnes Advisory Engineer
                                         Lexmark International

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