[SI-LIST] Re: Wacky power plane impedance measurements
- From: Doug Smith <doug@xxxxxxxxxx>
- To: chris_landrum@xxxxxxxxxx
- Date: Fri, 09 Apr 2004 10:09:41 -0700
Hi Chris and the group,
Assuming tight connections, when results are a function of cable
placement it means you are developing a voltage across the shield
connection to the board. This voltage, either from your signal current
or noise current, causes an error to be added to the desired signal.
The error is position dependent, because the common mode impedance
looking down the shield changes with placement and thus the common
mode current flowing is affected. As the current changes, so does the
voltage across the ground connection (sounds like you are using a
pigtail).
This is a general description. If you send me pictures and a good
description of exactly how the test is being performed, I can give you
the exact variation on the general description above.
It all means though that you need a better shield connection at the
board such as SMA with as much of a 360 degree connection to the board
ground as possible. With proper shield connections, placement of the
cable is not an issue. Ferrites will help suppress the common mode
current but will not help with impedance errors due to the shield
connections unless you can cancel them out in the measurement.
Doug
Landrum, Chris wrote:
> Hi all,
> I am running an experiment where I am attempting to measure the power plane
> 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 center
> 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 the
> second cable at some arbitray distance from cable #1.
> I have noticed that my measurement is EXTREMELY sensitive to cable movement.
> The results will also change when I simply touch the N-type adapter of the
> SA. I am sure there is a good connection to the PCB, as I have wire tied the
> cable to the PCB so as not to allow movement at the points of contact. The
> 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.
>
> Is this result due to coupling? Has anyone performed such a measurement
> without the varying result? Perhaps someone who has can describe their
> set-up.
>
> Thanks.
> Chris
>
>
>
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