Agilent Technologies N1996A-503 User Reference page 231

In the following equations
The Speed of Light ('c') in a vacuum is a constant value of 3e+8 m/s.
Your test cable's transmission speed (relative to light) is VRel.
The Measured Distance (in meters) of the DTF measurement is determined by the
following equation:
Measured distance (in meters) = (1/4 * Number of points * c * VRel) / Freq. Span
You can see from this equation that:
To increase the measured distance:
• you can increase the number of points, or
• you can reduce the frequency span.
To reduce the measured distance:
• you can reduce the number of points, or
• you can increase the frequency span.
Resolution - the Effects of Frequency and Points It is not always obvious how frequency range
affects measured distance and resolution, and it often appears to be counter-intuitive. If
you are new to making Distance to Fault measurements, this section will help clarify what
is happening.
Resolution Distance (in meters) of the DTF measurement, that is, the shortest distance
between two faults that can still be resolved by the analyzer, is determined by the
following equation:
Resolution Distance (in meters) = Measured distance (in meters) / (1/2 * Number of
Points)
NOTE
Please be careful how you interpret this equation. Note that to increase
the resolution, you need to reduce the Resolution Distance; to reduce
the resolution, you need to increase the Resolution Distance.
You can see from this equation that:
To increase the resolution, that is, to reduce the Resolution Distance:
• you can increase the number of points, or
• you can reduce the measured distance.
To reduce the resolution, that is, to increase the Resolution Distance:
• you can reduce the number of points, or
• you can increase the measured distance.
Chapter 3
Mode
Distance To Fault
231