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R&S FSH
Distance-to-Fault Mode (R&S FSH-K41)
Configuring Cable and Antenna Tests
Operating Manual 1309.6275.12 - 14
159
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Press the "Start Freq" or the "Stop Freq" softkey.
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Enter the start frequency and stop frequency you need.
The R&S FSH adjusts the frequency range according to your settings.
Note that the distance from start to stop frequency has to be equal to the span.
The R&S FSH adjusts the values if you enter a frequency range that is not the
same as the span.
5.2.2.2 Selecting the Cable Length
The R&S FSH uses the cable length to determine the span for the measurement. The
longer the cable under test, the smaller the span. Together with the cable model, the
cable length is also the basis for the calculation of the cable attenuation. The cable
attenuation in turn is necessary for a correct calculation of the reflection magnitude at
the cable fault. The cable length also defines the scale of the horizontal axis. In the
default configuration, the R&S FSH shows the whole cable length.
If the cable length you have set is shorter than the actual cable length, the R&S FSH
does not display the faults of the whole cable. A reflection at the end of the cable will
not be shown. However, deliberately entering a cable length that is too short is a good
way to increase measurement accuracy for a fault that is near to the measurement
plane. If the entered cable length is greater than the actual length, the results for
lengths beyond the actual cable length are obsolete because they are caused by
multiple reflections.
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Press the MEAS key.
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Press the "Cable Length" softkey.
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Enter the length of the cable.
The range is from 3 m to 1500 m (about 10 ft to 5000 ft.). If you do not know the exact
length of the cable, define a length that is about 20 to 50 % larger than the best
estimate.
The maximum cable length that is suitable for measurements depends on the cable
attenuation. Since the test signal must be routed twice through the cable, the signal
reflected at the cable end arrives with twice the cable attenuation in attenuated form at
the input of the power divider. Dynamic range decreases with increasing cable length.
If the cable attenuation exceeds 10 dB, the R&S FSH displays a warning indicating that
the cable attenuation is too high. It also indicates the maximum recommended cable
length to get accurate results.