Keysight N9912A User Manual page 37

Alias-free Range (On/Off)
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 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.
DTF (Distance to Fault) Measurements
Points = resolution
o
Bandwidth = frequency range
o
On = No Alias images
o
Off = Alias images may appear in the response.
o
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