[SI-LIST] TDR

Hi all,
I hit a problem in TDR measurement. A differential TDR module was =
calibrated carefully with two 'high quality' cables, and the TDR shows =
exactly 100ohm differential impedance for these two cables. However, =
after connecting with a board under test, the differential impedance of =
the cables increases up to 120ohm. I was supposed that the measured =
differential impedance of the cable should be independent to the load, =
but obviously this is not case here. So what are the reasons causing the =
measured cable differential impedance increased. How can we keep the =
differential impedance of the cables unchanged.
=20
Thanks
Yue
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