User manual

Spectrum Analyzer Mode
R&S
®
Spectrum Rider FPH
171User Manual 1321.1011.02 ─ 05.00
Automatic adjustment of the sweep time
In its default mode, the R&S Spectrum Rider automatically adjusts the sweep time as
soon as you change the resolution bandwidth. This adjustment makes sure that the
settling time required for the selected resolution filter is properly taken into account.
The maximum allowed sweep time is 1000 s. For narrow resolution filters this value
would be exceeded for large spans. In order to avoid this situation, the R&S Spectrum
Rider adjusts the span automatically as soon as the maximum sweep time is reached.
6.2.4.2 Setting the Video Bandwidth
The video bandwidth (VBW) basically smoothes the trace by reducing the noise and
therefore making power levels easier to see.
The noise reduction is a result of the video filter. This lowpass filter defines the video
bandwidth and filters the higher frequency parts of the voltage from the signal. Video
voltage is the (DC) voltage that results from the IF signal passing through the envelope
detector which removes the IF components and outputs the envelope only. This output
is also known as the video signal.
The figure below shows that process on an AM modulated signal in the time domain.
In case of an AM modulated signal, the envelope (or video) signal contains a DC com-
ponent that corresponds to the level of the carrier. The video signal also contains an
AC component whose frequency is the same as the AM frequency.
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 dis-
played truly, the cutoff frequency of the filter has to be greater than the modulation fre-
quency.
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 is rejected. The smaller the video bandwidth, the smaller
the noise amplitude at the video filter output.
The R&S Spectrum Rider 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 band-
Configuring Spectrum Measurements