Specifications

Setting and Using the Measurement Functions R&S FSH
1145.5973.12 4.114 E-10
The trace on the right shows the fault location at the end of the cable clearly wider than the left one
which was measured with optimum span. The reason is the reduced number of points in the calculation
with reduced span. The points in reduced span are calculated using the following formula:
1024
AUTOSPAN
SPAN
N ⋅= ,
where
N = number of measurement points
SPAN = manually set span
AUTOSPAN = span used by the R&S FSH in automatic setting of the span
In the above measurement example, the resolution is thus 202 points for a cable length of 25 m, i.e. the
distance is approx. 12.4 cm between the two measurement points.
Selecting the center frequency:
The R&S FSH’s center frequency should be as close to the cable under test’s operating frequency as
possible (for example the transmission frequency of the antenna connected to the cable). Cable
attenuation increases with increasing frequency. This means that both the incident wave and the
reflected wave from the end of the cable or at any faults is attenuated more at higher frequencies. This
restricts the dynamic range at higher center frequencies. Therefore, never select a center frequency
that is higher than necessary.
In the case of short cable lengths and automatic setting of the span, the R&S FSH uses its entire
frequency range for the measurement. It automatically uses 1.505 GHz as the center frequency.
After the span is reduced, the R&S FSH can be set to the desired center frequency.
Measurement:
The R&S FSH performs a sweep over 1024 test points to measure the sum signal of forward and
reflected waveform. It transforms the sum signal in the frequency domain into the time domain by
means of the inverse FFT (IFFT). The IFFT has a length of 2048 points. The data set is zero-padded to
2048 points and evaluated by means of a Hamming window before performing the IFFT. The R&S FSH
corrects the result of the IFFT by using the correction values from calibration.
It then calculates the IFFT result into length units from the cable parameters, light velocity and
frequency range. In addition, the R&S FSH considers the attenuation of the cable to be measured in
order to display the discontinuities with correct level.
Length measurement accuracy:
The length measurement accuracy is primarily determined by the deviation of the cable data of the
cable model from the real cable data. Depending on the cable, the data may exhibit a tolerance of up to
10 %. This deviation directly affects the measurement error. A second factor of influence is the display
resolution of the R&S FSH. Its uncertainty is
± 1/2 pixel or 1/2 x (length/301).