[SI-LIST] Re: Wacky power plane impedance measurements

  • From: Ray Anderson <Raymond.Anderson@xxxxxxx>
  • To: si-list <si-list@xxxxxxxxxxxxx>
  • Date: Thu, 08 Apr 2004 09:35:23 -0700


I'm not sure if this will help or not, but see the following paper 
starting at page 22:


Istvan discusses the use of ferrites on measurement cables used to 
measure plane impedances.

One other comment, we have found it useful to connect the cables to a 
non-populated set of decap pads as opposed to connecting them directly 
to a decap on the board. If you need to remove a decap to achieve this 
it is preferable to remove the smallest value caps. Connecting to 
populated pads seems to "color" the measured impedance profile. Also try 
and keep the two cables going to the VNA ports physically close together 
(within an inch or so) on the PCB as opposed to having them attached to 
lands which are physically separated by a large distance (relative to 
your PCB).

-Ray Anderson
Sun Microsystems Inc.

Brown, Mike (Austin, TX) wrote:

>Sounds like you have signal current on the outside of the coax shield, =
>perhaps.  Try a ferrite on both coax cables, very near the board under =
>-----Original Message-----
>From: si-list-bounce@xxxxxxxxxxxxx
>[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Landrum, Chris
>Sent: Thursday, April 08, 2004 10:03 AM
>To: si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Wacky power plane impedance measurements
>Hi all,
>I am running an experiment where I am attempting to measure the power =
>impedance of a PCB. The PCB is only populated with capacitors. I am =
>using a
>spectrum analyzer to plot the freq response. I have two pieces of 50 Ohm
>shielded cable where I have cut the ends to expose the shield and the =
>conductor. I have then soldered the shield to a large cap at GND and the
>conductor to the other side of the cap at PWR. I have done the same for =
>second cable at some arbitray distance from cable #1.=20
>I have noticed that my measurement is EXTREMELY sensitive to cable =
>The results will also change when I simply touch the N-type adapter of =
>SA. I am sure there is a good connection to the PCB, as I have wire tied =
>cable to the PCB so as not to allow movement at the points of contact. =
>cables may be approx 2ft in length (matched), and I have tried shorter
>lengths with the same result. I have also tried higher quality cables.=20
>Is this result due to coupling? Has anyone performed such a measurement
>without the varying result? Perhaps someone who has can describe their

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