Keysight FieldFox B Series User Manual page 74

DTF (Distance to Fault) Measurements
DTF Measurement Settings
Shorter stop distances (less than 10 meters) and a higher resolution (1001
points) will be more likely to result in Alias-free Range = Off.
When the Alias-free Range = Off, the following procedure will help to
determine if a response is true or an alias response:
1. Put a marker on the response in question and note the distance to the
fault.
2. Change the start or stop distance.
A true fault response will not move in distance. That is, if a true fault is present
at 10.3 meters, changing the stop distance from 15 m to 20 m will not move
the fault; the fault will remain at 10.3 meters. However, an alias response will
appear to move.
An unterminated 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.
74 Keysight N9938-90006 User's Guide