R&S FSQ Signal Analyzer - Rohde & Schwarz

R&S FSQ Getting Started
rel. noise level /dB
14
12
10
8
6
4
2
0
-2
Measuring Signals in the Vicinity of Noise
-70 -60 -50 -40 -30 -20 -10
Reference level /dBm
Fig. 2.7 Change in displayed noise as a function of the selected reference level at bandwidths of
10 kHz and 300 kHz (-30 dBm reference level)
Impact of the resolution bandwidth
The sensitivity of a Signal Analyzer also directly depends on the selected bandwidth.
The highest sensitivity is obtained at the smallest bandwidth (for the R&S FSQ: 10
Hz, for FFT filtering: 1 Hz). If the bandwidth is increased, the reduction in sensitivity
is proportional to the change in bandwidth. The R&S FSQ has bandwidth settings in
2, 3, 5, 10 sequence. Increasing the bandwidth by a factor of 3 increases the displayed
noise by approx. 5 dB (4.77 dB precisely). If the bandwidth is increased by a
factor of 10, the displayed noise increases by a factor of 10, i.e. 10 dB. Because of
the way the resolution filters are designed, the sensitivity of Signal Analyzers often
depends on the selected resolution bandwidth. In data sheets, the displayed average
noise level is often indicated for the smallest available bandwidth. The extra
sensitivity obtained if the bandwidth is reduced may therefore deviate from the values
indicated above. The following table illustrates typical deviations from the noise
figure for a resolution bandwidth of 10 kHz which is used as a reference value (= 0
dB).
Noise figure
offset /dB
3
2
1
0
-1
RBW = 10 kHz
RBW = 300 kHz
digital RBW analog RBW
0,01 0,1 1 10 100 1000 10000
RBW /kHz
Fig. 2.8 Change in R&S FSQ noise figure at various bandwidths. The reference bandwidth is 10
kHz
2.10 Operating Manual 1313.9681.12 - 01