Tektronix 11801C User Manual page 351

11801C User Manual
Notice that the upper channel is asserting a negative TDR step. This is
equivalent to differential TDR. Note that in both traces, the step appears as
though the cable is shorted at the center of the cable (since the two steps
meet and cancel to zero volts).
When the TDR steps on the two channels are opposite (one positive and
one negative), you can now define a trace that represents the true differen
tial signal by touching the DefTra icon and touching Mainframe 1, - ,
Mainframe 2, Enter Desc.
When the TDR steps on the two channels are the same polarity (both posi
tive or negative), you can define a trace that represents the true common
mode signal by touching the DefTra icon and touching Mainframe 1, +,
Mainframe 2, Enter Desc.
This example has been designed to show how the instrument acquires
common mode and differential TDR traces, and how they relate to each
other.
Differential TDR is quickly set up by using the Diff TDR Preset selector on
the Sampling Head Fnc's pop up menu. This preset automates the setup
for differential TDR by:
H Turning on both channels of the SD 24
H Turning on both step generators
H Inverting the polarity on one step generator
H Autosetting both traces
H Turning on Baseline Correction
H Changing the vertical scaling from volts to rho
TDT Measurements
You can make forward and reverse Time Domain Transmission (TDT) mea
surements using the SD 24. To perform a TDT measurement, connect one
sampling head channel to the input of the device under test and the other
sampling head channel to the output of the device under test. You can then
alternately enable the step generators on both channels and sample the
transmitted signal on the other channel to perform forward and reverse TDT
measurements.
TDR
3 201