[SI-LIST] Re: Shielding Question

Craig,

In short since you are using a 2 port VNA you need to make a
differential signal to feed the differential cable. There are lots of
ways to do it.

You may try getting a demo board for a fully differential operational
amplifier like the TI THS4511. Connect the input of the demo board to
port one and do a S21 calibration to the non-inverting output of the
amp. Have the inverting output terminated in 50 ohms. Be careful of the
DC current in the terminators. Pick you favorite antenna and connect it
to port 2. Check the setup by terminating both outputs and confirm you
have good isolation.

If you want to have fun with it, you can compare various cables. I made
a 200 ohm transmission line to use as a reference. By connecting the 200
ohm line to port one, you can do a S21 calibration with the line and
antenna in place and take out the antenna factors. The 200 ohm
transmission line has a 180 degree splitter (Balun) that converts from a
50 ohm single ended signal to a 200 ohm differential output. The line
itself is 200 ohms. The result is then referenced to a physical line.
This helps the managers understand what you are doing. There is also a
M/A Com 180 degree hybrid that can be used.

The point is to make yourself a differential signal out of the single
ended VNA output and feed the cable. The new VNAs have the ability to do
this with two measurements, but there is too much math in it for me. I
like more direct measurements. I would rather measure the Balun and
calibrate it out. I like to use a real differential signal to make a
differential measurement.

Best regards,

John



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-----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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