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R&S FSH
Spectrum Analyzer Mode
Configuring Spectrum Measurements
Operating Manual 1173.6275.12 - 30
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3.2.3 Setting Bandwidths
The bandwidth menu contains all settings to set up filter bandwidths available in the
R&S FSH. You can access it with the BW key.
3.2.3.1 Setting the Resolution Bandwidth
The resolution bandwidth in a spectrum analyzer determines the frequency resolution
for frequency domain measurements and therefore determines how well it can
separate adjacent frequencies. The results as you see them on the display depend on
the passband of a resolution filter.
The resolution bandwidth (RBW) has several effects on measurements.
● To be able to display two or more signals whose frequencies are close together
separately, you need a (resolution) filter whose bandwidth is small enough. The
frequency difference between two sinusoidal carriers can not be less than the
selected resolution bandwidth if the carriers are to be resolved, for example.
● The bandwidth of the resolution filter also affects the noise that is displayed by the
R&S FSH. The smaller the bandwidth, the less noisy the results are. The rule is,
that if you increase or decrease the bandwidth by a factor of 3, the noise goes
down or up by 5 dB. If you change the bandwidth by a factor of 10, the displayed
noise changes by 10 dB.
● The resolution bandwidth affects the speed of the measurement. If you want to
display the true spectrum, the resolution filters have to settle at all frequencies that
are of interest. Narrow bandfilters have a longer settling time compared to wide
ones. Therefore the sweep time increases the smaller the resolution bandwidth
gets. The rule is, that if you reduce the bandwidth by a factor of 3, the sweep time
goes up by a factor of 9. If you reduce the bandwidth by a factor of 10, the sweep
time goes up by a factor of 100.
The R&S FSH has resolution bandwidths from 1 Hz to 3 MHz in a 1-3-10 sequence.
Additionally, the R&S FSH provides a 200 kHz resolution bandwidth that you have to
select and enter manually.
Setting the 200 kHz resolution bandwidth
The 200 kHz bandwidth is not coupled to the span, so it will not be selected if
automatic selection of the RBW is on.
In fact, you have to enter the 200 kHz resolution bandwidth with the number keys.
When using the rotary knob or the cursor keys, the 200 kHz bandwidth will be skipped.
In the R&S FSH's default state, the resolution bandwidth is coupled to the span, i.e. if
you change the span, the R&S FSH adjusts the resolution bandwidth. Therefore, you
do not have to set the resolution bandwidth manually in many cases, because the
R&S FSH automatically sets the resolution bandwidth if you change the span.
► Press the BW key.
By default, the resolution bandwidth is coupled to the span.