Keysight N9912A User Manual page 32

DTF Start and Stop Distance
In DTF measurements, you set the physical length of cable or other device to be
tested. The FieldFox calculates the frequency range of the measurement from this
distance. The longer the cable to be tested, the lower the frequencies that are used.
You can also set the frequencies manually using the Frequency Mode [Bandpass]
setting.
How to set Start and Stop Distance
 With a DTF measurement present, press Freq/Dist.
 Then choose from the following:
Frequency Mode
All DTF measurements are made with frequency settings and, using Inverse Fourier
Transform (IFT), the time and distance to faults are calculated.
NOTE The start and stop frequencies for the measurement are always annotated on the
screen below the start and stop distances.
How to set Frequency Mode
With a DTF measurement present,
 Press Meas Setup 4
 Then
 Choose one of the following:
Typically, you will set the frequency range of the measurement to the passband of
the filter. However, you may also want to test the ability of the filter to reject
unwanted frequencies. In this case, set the frequency range to include those
frequencies which the filter may not be adequately rejecting.
When the DTF frequencies are set manually, they may not be the optimum
frequencies for measuring the distance to fault. The distance may no longer be
alias-free. Learn more about alias-free range on page 37.
How to manually set Frequencies in Bandpass Mode
 Press Freq/Dist
32
then
More DTF Lin
o
format.
Enter a value using the numeric keypad, the ▲|▼ arrows, or
Start Distance
o
the rotary knob, then Enter. By default, the Start Distance is set to 0 Meters.
This means that the measurement will display faults starting at the point at
which calibration standards are connected.
Enter a value between the start distance and 5 km (or 16,404
Stop Distance
o
ft.) using the numeric keypad, the ▲|▼ arrows, or the rotary knob, then Enter.
Freq Mode
The frequency range of a DTF measurement is set
Lowpass Mode
o
automatically based on the Start and Stop Distances. Use Lowpass mode when
the DUT is a cable ONLY.
(Default setting) The frequency range of a DTF measurement
Bandpass Mode
o
is set manually. Use Bandpass mode when the DUT contains a diplexer or other
filtering device which does not pass some frequencies.
Faults are displayed on the Y-axis in linear (unitless)
FieldFox User's Guide