Spectrum Analyzer Mode
R&S
®
Cable Rider ZPH
180
User Manual 1321.0950.02 ─ 07
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Setting the span
The frequency span must at least cover the carriers and the adjacent channels
plus a measurement margin of about 10 % to get valid results.
If the span is automatically calculated by the R&S Cable Rider ZPH with the "Auto
Span" function, the span is calculated as follows:
–
(No. of transmission channels - 1) x transmission channel s 2 x trans-
mission channel ban measurement margin
with the measurement margin being approx. 10 % of the value obtained by adding
the channel spacing and the channel bandwidth.
Selecting the frequency span
If the frequency span is too large in comparison to the channel bandwidth (or the adja-
cent-channel bandwidths) being examined, only a few points on the trace are available
per channel. This reduces the accuracy of the waveform calculation for the channel fil-
ter used, which has a negative effect on the measurement accuracy. It is therefore
strongly recommended that the formulas mentioned be taken into consideration when
selecting the frequency span.
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Setting the resolution bandwidth:
The RBW should not be too high or too small to get both an acceptable measure-
ment speed and a suppression of spectral components outside the channels. As a
rule of thumb, it is recommended to set it to about 1 % to 4 % of the channel band-
width.
You can select a larger resolution bandwidth if the spectrum within the channel to
be measured and around it has a flat characteristic. In the standard setting, e.g. for
standard cdmaOne at an adjacent channel bandwidth of 30 kHz, a resolution band-
width of 30 kHz is used. This yields correct results since the spectrum in the neigh-
borhood of the adjacent channels normally has a constant level.
For standard NADC/IS136 this would not be not possible, for example, since the
spectrum of the transmit signal penetrates into the adjacent channels and a too
large resolution bandwidth causes a too low selection of the channel filter. The
adjacent-channel power would thus be measured too high.
If the RBW is automatically calculated by the R&S Cable Rider ZPH with the "Auto
RBW" function, the RBW is calculated as follows:
–
RBW
≥
1 / 40 of channel bandwidth
The R&S Cable Rider ZPH then selects the maximum possible resolution band-
width resulting from the available RBW steps (1, 3).
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Setting the video bandwidth
To obtain correct power measurements, the video signal must not be limited in
bandwidth. A restricted bandwidth of the logarithmic video signal would cause sig-
nal averaging and thus result in a too low indication of the power (-2.51 dB at very
low video bandwidths). Therefore, the VBW should be at least three times the reso-
lution bandwidth.
If the VBW is automatically calculated by the R&S Cable Rider ZPH with the "Auto
VBW" function, the VBW is calculated as follows:
–
VBW
≥
3 x RBW
Performing Spectrum Measurements
