R&S FSH3 Setting and Using the Measurement Functions
Further information
Setting the span:
The R&S FSH3 automatically sets the span as a function of the selected cable length and cable model.
The shorter the cable under test, the greater the selected span. If the center frequency is too high or too
low for the cable length in question, the R&S FSH3 automatically adapts it to the required span.
The R&S FSH3 calculates the span from the cable length as follows:
2048
1024
CL2
vc
1023Span
r0
⋅
⋅
⋅
⋅=
,
where
c
0
= velocity of light
v
r
= velocity factor of cable
CL = cable length
1024 = number of pixels calculated
2048 = number of pixels included in inverse Fourier transform
If, in the case of short cable lengths, the 3 GHz frequency range of the R&S FSH3 is not sufficient to set
the span obtained with the above formula, the number of pixels calculated for displaying distance to
fault is reduced accordingly.
Selecting the center frequency:
The R&S FSH3’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, the entire R&S FSH3 frequency range is used for measurements.
The center frequency is automatically set to 1.505 GHz.
Automatic setting of the center frequency can be avoided by selecting a larger cable length
(corresponding to a narrower span) or by using the zoom function.
Measurement:
The R&S FSH3 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 FSH3 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 FSH3 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 FSH3. Its uncertainty is ± 1/2 pixel or 1/2 x (length/301).
1145.5973.12 4.77 E-6
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