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R&S FSH
Spectrum Analyzer Mode
Configuring Spectrum Measurements
Operating Manual 1173.6275.12 - 30
104
If the bandwidth of the video filter is less than the frequency of the AC component, it is
suppressed depending on its maximum frequency. If the AM component should be
displayed truly, the cutoff frequency of the filter has to be greater than the modulation
frequency.
If there is noise on the sine signal, the modulation signal can be thought of as noise. If
the video bandwidth is reduced, the high-frequency noise components above the cutoff
frequency of the video filter will be rejected. The smaller the video bandwidth, the
smaller the noise amplitude at the video filter output.
The R&S FSH provides video bandwidths from 1 Hz to 3 MHz in a 1-3-10 sequence. In
its default state, the video bandwidth is coupled to the resolution bandwidth and is the
same as the resolution bandwidth. If you change the resolution bandwidth, the
R&S FSH adjusts the video bandwidth accordingly.
The effects of the video bandwidth on measurements are as follows.
● if you are performing measurements on modulated signals, the video bandwidth
must be sufficiently large so that significant modulation components are not
rejected (
≥ RBW)
● if you want to keep signals free of noise, you should select the smallest video
bandwidth possible (
≤ 0.1 x RBW)
● if you are performing measurements on pulsed signals, the video bandwidth should
be at least three times greater than the resolution bandwidth so that the pulse
edges are not distorted
Like the resolution bandwidth, the video bandwidth has an effect on sweep speed.
Before each measurement, the video filter has to settle.
► Press the BW key.
► Press the "Manual VBW" softkey.
The R&S FSH opens an input field to define the video bandwidth.
► Enter the video bandwidth you need.
The R&S FSH uses the video bandwidth you have entered for the measurement.
If the video bandwidth is no longer coupled to the resolution bandwidth, the
R&S FSH puts a red dot at the "VBW" hardware setting (
).
► Press the "Auto VBW" softkey to again couple the video bandwidth to the RBW.