Keysight N9927-90001 User’s Guide 83
TDR (Time Domain Reflectometry) - Option 215
TDR Measurement Settings
An un-terminated cable (with NO perfect load at the end) will show faults that
appear to be beyond the end of the cable. These are NOT alias faults. These
faults appear as the signal reflects off the open at the end of the cable and
travels back down the cable toward the connection at the FieldFox.
Re-reflections are measured at the FieldFox as mirror images of the original
faults. The largest fault is the open end of the cable. To avoid confusion, set
the Stop distance shortly after that fault.
How to Prevent Undersampling
Undersampling is when the TDR measurement is inaccurate because the
frequency step size (resolution) is too large. The predominant symptom of this
is when the measured impedance is incorrect. For example, when measuring a
75Ω cable, a marker that is on a part of the trace without reflections shows
56Ω.
To prevent undersampling, set TDR Sweep to Cable. If undersampling still
occurs, set TDR Sweep to Auto. Then set Auto Max Frequency to a lower
value. Manually increase the Resolution so that the Start Frequency setting is
less than about 500 kHz.
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