About Alias Faults - Keysight Technologies C Series User Manual

DTF (Distance to Fault) Measurements
DTF Measurement Settings
Range Resolution. Indicates the accuracy of the distance to fault
measurement. For example, with range resolution of 500 mm, if the distance to
fault is 10 meters, this value could be inaccurate by +/- 500 mm or between
9.5 to 10.5 meters. This value is calculated from frequency span / resolution
(points).
Response Resolution, not displayed, indicates the distance that could be
between two faults and still show as separate faults. Learn more in "Window
Settings" on page 72.
Maximum Distance. The distance that could be viewed with the current
settings. Defined by: Vf *c*Points/(2*Bandwidth) where:
— Vf = velocity factor
— c = speed of light
— Points = resolution
— Bandwidth = frequency range
Alias-free Range (On/Off)
— On = No Alias images
— Off = Alias images may appear in the response.

About Alias Faults

An alias fault is not a true device response. An alias fault appears because of
the method used to convert frequency to time.
On the DTF Settings page (above) the c - Alias-free Range = Off setting
indicates alias images MAY appear on the screen.
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.
Keysight N9915-90015 User's Guide 73