[SI-LIST] Re: Wacky power plane impedance measurements
- From: Larry Smith <Larry.Smith@xxxxxxx>
- To: chris_landrum@xxxxxxxxxx
- Date: Thu, 08 Apr 2004 09:39:48 -0700
Chris - We do this type of measurement all the time with good success.
I have never had a large sensitivity to cable movement. Sounds like
a loose connection.
Try disconnecting your 50 Ohm coax probes from the board and soldering
them directly together to make a continuous 50 Ohm transmission line
from port 1 to port 2 (with a small discontinuity where the cables are
joined). In this configuration, perform a "through response"
calibration. After calibration, you should see a nice flat line at 0
db with no sensitivity to cable movement.
Next, short the inner conductors to the outer conductors right at the
solder joints. This is similar to your low impedance DUT because your
PDS should be nearly a short circuit (mOhms). The short in the DUT
position should give you the noise floor at low frequencies and
eventually take on the slope of an inductance (probably around 100pH)
at frequencies in the MHz range. Again, there should be very little
sensitivity to cable movement. If there is sensitivity, you are
dealing with a loose connection somewhere.
Finally, solder your 50 Ohm probes back onto your board. Make sure
that you have the outer conductors soldered to the plane with the
largest area (probably ground) and the inner conductors soldered to
the smaller plane (probably power). I have seen some erratic behavior
when the inner conductor is soldered to a large surface and the inner
conductor is soldered to a massive node (I have accidently hooked the
probes up backwards..).
Your measurement technique sounds good. I suspect you have a problem
with your instrumentation.
regards,
Larry Smith
Sun Microsystems
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
>
>
>
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
> or to administer your membership from a web page, go to:
> //www.freelists.org/webpage/si-list
>
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
> List FAQ wiki page is located at:
> http://si-list.org/wiki/wiki.pl
>
> List technical documents are available at:
> http://www.si-list.org
>
> List archives are viewable at:
> //www.freelists.org/archives/si-list
> or at our remote archives:
> http://groups.yahoo.com/group/si-list/messages
> Old (prior to June 6, 2001) list archives are viewable at:
> http://www.qsl.net/wb6tpu
>
>
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List FAQ wiki page is located at:
http://si-list.org/wiki/wiki.pl
List technical documents are available at:
http://www.si-list.org
List archives are viewable at:
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts: