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The Spectrum Application
R&S
®
FPC
107
User Manual 1178.4130.02 ─ 07
14.5.1
Resolution Filter
The resolution filter (or IF filter) is a 3 dB bandpass filter located in the IF path of the
analyzer. Its bandwidth determines how well individual spectral components can be
separated from each other in frequency domain measurements.
The resolution bandwidth (RBW) has several effects on measurements.
●
Selectivity
Measuring and displaying two or more spectral components whose frequencies are
close together requires a resolution filter bandwidth (RBW) small enough to resolve
these components. Otherwise they will be displayed as a single component.
Two signals with the same amplitude can be resolved if the resolution bandwidth is
smaller than or equal to the frequency spacing of the signal.
The highest selectivity is obtained at the smallest bandwidth. If the bandwidth is
increased, the reduction in selectivity is proportional to the change in bandwidth.
●
Noise
The bandwidth of the resolution filter affects the noise that is displayed by the
R&S
FPC. The smaller the bandwidth, the less noise is visible.
Increasing the bandwidth by a factor of 3 increases the displayed noise by approx.
5
dB (4.77
dB precisely). Increasing the bandwidth by a factor of 10, increases the
displayed noise by a factor of 10 (10
dB).
●
Measurement speed
The resolution bandwidth affects the speed of the measurement. If you want to dis-
play the true spectrum, the resolution filters have to settle at all frequencies that
are of interest. Narrow filters have a longer settling time compared to wide ones.
Therefore the
increases the smaller the resolution bandwidth gets.
Reducing the bandwidth by a factor of 3, increases the measurement time by a fac-
tor of 9. Reducing the bandwidth by a factor of 10, increases the measurement
time by a factor of 100.
Setting the resolution bandwidth
1. Press the "BW" key to open the bandwidth menu.
2. Select one of the "RBW: <x>" menu items.
● "RBW: Manual"
Opens an input field to select one of the available bandwidths.
If you enter a bandwidth that the R&S
FPC does not support, the R&S
FPC
rounds to the next available bandwidth. Refer to the data sheet for more infor-
mation on available resolution bandwidths
Note that you have to select the 200
kHz bandwidth manually with the number
keys. This bandwidth will neither be selected when "RBW: Auto" is on, nor will
it be selected when you use the rotary knob or cursor keys.
A manually selected RBW is indicated by a blue dot in front of the "RBW" label
in the status bar.
Figure 14-10: Indicator of manual resolution bandwidth
● "RBW: Auto"
Bandwidth Configuration