R&S FSH Setting the Bandwidths
1309.6275.12 4.13 E-2
Enter the resolution bandwidth you want using the
number keys and terminate the entry with the
appropriate unit (MHz, kHz or Hz), or
Change the resolution bandwidth to the value you
want using the rotary knob or the cursor keys.
Note: The 200 kHz resolution bandwidth has to be entered by means of the number keys.
When using the rotary knob or the cursor keys, the 200 kHz bandwidth will be skipped.
The box for entering the resolution bandwidth can be closed by pressing the CANCEL key.
Press the AUTO RES BW softkey.
The resolution bandwidth is coupled to the span that has been set. The AUTO RES BW softkey label is
highlighted in green to show that the coupled mode has been selected. The red circle in front of the
RBW readout disappears.
Video bandwidth
The video bandwidth smoothes the trace by reducing noise. When the filtered IF signal is envelope-
detected, an IF sine signal becomes a DC voltage in the video signal. If the sine signal is amplitude-
modulated, a signal whose frequency is the same as the AM frequency is produced in the video signal
apart from the DC voltage from the carrier. The Fig. below shows an RF signal modulated with a sine
signal and the corresponding video signal in the time domain.
AM RF signal
IF signal
Envelope signal
(video voltage)
DC voltage
from carrier
AC voltage
from modulation
Time
Time
Amplitude
Amplitude
The envelope signal contains a DC component corresponding to the carrier level and 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, the latter will be suppressed depending on its maximum frequency. If
the AM component is to be displayed faithfully, the cutoff frequency must 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.
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