R&S FSH
Spectrum Analyzer Mode
Performing Spectrum Measurements
Operating Manual 1309.6275.12 - 14
54
Selecting the frequency span
If the frequency span is too large in comparison to the channel bandwidth (or the
adjacent-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 filter 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.
C
Setting the resolution bandwidth:
The RBW should not be too high or too small to get both an acceptable
measurement 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 bandwidth.
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
bandwidth of 30 kHz is used. This yields correct results since the spectrum in the
neighborhood 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 FSH with the "Auto RBW"
function, the RBW is calculated as follows:
RBW
"
1/40 of channel bandwidth
The R&S FSH then selects the maximum possible resolution bandwidth resulting
from the available RBW steps (1, 3).
C
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
signal 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
resolution bandwidth.
If the VBW is automatically calculated by the R&S FSH with the "Auto VBW"
function, the VBW is calculated as follows:
VBW
'
3 x RBW
The R&S FSH then selects the smallest possible VBW with regard to the available
step size.
