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Measurements
R&S
®
FSW
144
User Manual 1173.9411.02 ─ 19
With the exception of the IS95 CDMA standards, the "Adjust Settings" function sets the
resolution bandwidth (RBW) as a function of the channel bandwidth:
"RBW ≤ 1/40 of channel bandwidth"
The maximum possible resolution bandwidth (with respect to the requirement RBW ≤
1/40) resulting from the available RBW steps (1, 3) is selected.
Video Bandwidth (VBW)
For a correct power measurement, 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).
The video bandwidth should therefore be selected at least three times the resolution
bandwidth:
"VBW ≥ 3 x RBW"
The video bandwidth for the defined channel settings can be optimized using the
"Adjust Settings" function in the "Ch Power" menu or the "General Settings" tab of the
"ACLR Setup" dialog box (see
"Optimized Settings (Adjust Settings)"
You can set the VBW manually in the "Bandwidth" configuration dialog box, see
The video bandwidth (VBW) is set as a function of the channel bandwidth (see formula
above) and the smallest possible VBW with regard to the available step size is
selected.
Detector
The RMS 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
i
RMS
s
N
P
1
2
1
where:
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.
Channel Power and Adjacent-Channel Power (ACLR) Measurement