[SI-LIST] Re: Shielding Question
- From: "Ken Holman" <kholman@xxxxxxxxxxxx>
- To: <Cclewell@xxxxxxxxxxxxx>, <jain.nitin@xxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Tue, 20 Jun 2006 15:58:07 -0400
Craig,
My first recommendation is that instead of trying to measure shielding
effectiveness, measure the transfer impedance instead. Inject a
calibrated current on the outside of the shield and measure the
differential voltage induced across the pair, terminated in the
characteristic impedance of the pair. The results of shielding
effectiveness tests often depend as much on the test set up (angle of
incidence, chamber reflections, etc) as it does on the shielding quality
of the test sample. =20
But if it has to be shielding effectiveness, then if probably makes
sense to terminate the differential pair in the way it is intended to be
used. If the cable is intended for 100 ohm differential circuits, then
terminate in 100 ohms. With the appropriate probes you should be able
to measure the voltage across the termination, and from line to shield
at the same time.
Ken
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Clewell, Craig
Sent: Tuesday, June 20, 2006 12:30 PM
To: 'jain.nitin@xxxxxxxxx'; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Shielding Question
I have polled a couple of colleagues, but wanted to throw this out the
list
just to see what kind of feedback it would generate.
If you have a shielded differential pair and you need to test the
shielding
effectiveness how do you go about setting up the differential pair? I
realize in a coax scenario this would be pretty straight forward, but
for
the differential pair I'm a little confused and here is why.
You send a signal into the chamber via an antenna. You have a reference
antenna hooked up and you also have the DUT hooked up. However, how do
you connect the differential pair? Do you measure the SE of each line
individually, and ground the other leg of the diff pair on the far end,
while terminating on the near end of the quite line? Do you measure
both
channels of the diff pair at the same time as a sort of common mode
configuration?
I know that one way of doing this would be to make 2 measurements; one
with
the shield and one without, but is there a way to make the measurement
one
time using one set up?
Thanks for your insight,=20
Craig
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