Rohde & Schwarz FSH13 - Defining the Channel Bandwidth

R&S FSH Spectrum Analyzer Mode
Performing Spectrum Measurements
Operating Manual 1173.6275.12 - 30 64
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).
● 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.
● Selecting the detector:
It is best to use the RMS detector. This detector correctly indicates the power
irrespective of the characteristics of the signal to be measured. The whole IF
envelope is used to calculate the power for each measurement point. The IF
envelope is digitized using a sampling frequency which is at least five times the
resolution bandwidth which has been selected. Based on the sample values, the
power is calculated for each measurement point using the following formula:



N
i
iRMS
s
N
P
1
2
1
with
s
i
= linear digitized video voltage at the output of the A/D converter
N = number of A/D converter values per measurement point
P
RMS
= power represented by a measurement point
When the power has been calculated, the power units are converted into decibels
and the value is displayed as a measurement point.
In principle, the sample detector would be possible as well. Due to the limited
number of measurement points used to calculate the power in the channel, the
sample detector would yield less stable results.