[SI-LIST] : How to measure differential pattern on test coupon?

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From: Bnd (Ting.Steve@inventec.com)
Date: Tue Feb 29 2000 - 23:33:53 PST

Dear SI-List,

Some Tektronics AE told me that it's not necessary to connect differential
probes' ground to the ground pad on the test coupon (Microstrip or Stripline
structure) when measuring differential pair on the coupon . Since the TDR
will generate the same but opposite polarity signals (odd mode signals) to
each of the channel, the virtual ground will be formed between the probes.
Just keep the probes' grounds touched each other but don't have to connect
them to the ground on the board - it will make no difference if you connect
them to the ground on the board.

But some practical measurement showed that they won't be the same for most
cases. We obtained higher differential impedance from TDR reading if we
don't connect the probes' grounds to board's ground. And some colleagues
think it'll be more reasonable to connect the probes' ground to board's
ground, because it'll be closer to real case - components will reference the
board's ground. So what is the proper way to measure the differential

And the other question I'd like to ask is one regarding the return path.

What is the difference between theses two measurement for impedance of twist
pair with (1). one TDR channel - connecting probe's signal to one line and
probe's ground to the other; and (2). two TDR channels that drive odd mode
signals - connecting each of the probe's signal to one of the lines and
touch the probe's ground to each other (like the method mentioned above).
According to what I learnt about the signal's return path, I expect them
should be the same. Because in method (1), all the return signal must enter
the probe's ground and back to TDR. Then since the return current is
opposite to the TDR's signal in the twist pair, and the electric parameters
are the same for each of the line of the twist pair. They will form the odd
mode signals just like what I want to do in the method (2).

However, the real measurement showed about 6 ohm difference between them
(100 ohm - 116 ohm). The method (1) measured some lower impedance then
method (2). Where am I wrong? Does it due to the phase between the twist
pair not exact 180 degree out of phase?

Any input would be highly appreciated.

Steve Ting
CAE engineer
Inventec corp, Taiwan, ROC.

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