RE: [SI-LIST] : Upper limit of interplane capacitance

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From: Ray Anderson (raymonda@ha1mpk-mail.eng.sun.com)
Date: Fri Jun 09 2000 - 09:49:26 PDT


Barry Ma wrote:
>
> In our case measuring the frequency response of pwr/gnd planes of
.PCB, we would like also to use VNA. What I suggested is to measure S11
>with VNA by soldering a probe cable directly to two very closed Vcc
>and Gnd bonding pads on the PCB. (see the Figure in EDN magazine:
> http://www.ednmag.com/ednmag/reg/2000/03302000/pdfs/07john1.pdf ).
> Having connected the cable to Port 1 of VNA, the planecap becomes an
>unknown load being tested. The S11, thus the impedance of the
>interplane capacitance of pwr/gnd planes, would be shown in a Smith
>Chart in the VNA screen over a wide frequency range.
>

S11 measurements work, but in practice we've found that parasitic
inductance of the probe attachments to the planes caused
come inaccuracies that we've resolved by doing S12 measurements
instead.

If you haven't read it already, I'd suggest looking over my
colleague Istvan Novak's DesignCon99 paper entitled "Probes
and Setup for Measuring Powe-Plane Impedances with Vector
Network Analyzer" available at:

         http://home.att.net/~istvan.novak/papers/designcon99.pdf
         
He provides a good discussion of the pros and cons of S11 versus
S12 measurements in plance characterization.

-Ray Anderson
Sun Microsystems

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