Agilent Technologies FieldFox N9912A, User Manual

Page: 38 / 209

38 N9912A FieldFox User’s Guide

Then choose from:
o
M (meters)
o
Feet
Calculated DTF values
Press
Next Page
on the DTF Settings Table to view the following calculated
Values noted on the FieldFox screen with c - <setting> :
Start Frequency – Start frequency that is used to calculate DTF.
Stop Frequency – Stop frequency that is used to calculate DTF.
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 37.
Maximum Distance . The distance that could be viewed with the current settings.
Defined by: Vf*c*Points/(2*Bandwidth) where:
o
Vf = velocity factor
o
c = speed of light
o
Points = resolution
o
Bandwidth = frequency range
Alias-free Range (On/Off)
o
On = No Alias images
o
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.