R&S FSL Measuring Signals in the Vicinity of Noise
1300.2519.12 2.13 E-11
Note: If the intermodulation products of a DUT with a very high dynamic range are to be measured
and the resolution bandwidth to be used is therefore very small, it is best to measure the levels
of the useful signals and those of the intermodulation products separately using a small span.
The measurement time will be reduced– in particular if the offset of the useful signals is large.
To find signals reliably when frequency span is small, it is best to synchronize the signal
sources and the R&S FSL.
Measuring Signals in the Vicinity of Noise
The minimum signal level a spectrum analyzer can measure is limited by its intrinsic noise. Small
signals can be swamped by noise and therefore cannot be measured. For signals that are just above
the intrinsic noise, the accuracy of the level measurement is influenced by the intrinsic noise of the
spectrum analyzer.
The displayed noise level of a spectrum analyzer depends on its noise figure, the selected RF
attenuation, the selected reference level, the selected resolution and video bandwidth and the detector.
The effect of the different parameters is explained in the following.
Impact of the RF attenuation setting
The sensitivity of a spectrum analyzer is directly influenced by the selected RF attenuation. The highest
sensitivity is obtained at a RF attenuation of 0 dB. The attenuation can be set in 10 dB steps up to 70
dB. Each additional 10 dB step reduces the sensitivity by 10 dB, i.e. the displayed noise is increased by
10 dB.
Impact of the resolution bandwidth
The sensitivity of a spectrum analyzer also directly depends on the selected bandwidth. The highest
sensitivity is obtained at the smallest bandwidth (for the R&S FSL: 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 FSL has bandwidth settings in 1, 3, 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.
Impact of the video bandwidth
The displayed noise of a spectrum analyzer is also influenced by the selected video bandwidth. If the
video bandwidth is considerably smaller than the resolution bandwidth, noise spikes are suppressed,
i.e. the trace becomes much smoother. The level of a sinewave signal is not influenced by the video
bandwidth. A sinewave signal can therefore be freed from noise by using a video bandwidth that is
small compared with the resolution bandwidth, and thus be measured more accurately.
Impact of the detector
Noise is evaluated differently by the different detectors. The noise display is therefore influenced by the
choice of detector. Sinewave signals are weighted in the same way by all detectors, i.e. the level
display for a sinewave RF signal does not depend on the selected detector, provided that the signal–to–
noise ratio is high enough. The measurement accuracy for signals in the vicinity of intrinsic spectrum
analyzer noise is also influenced by the detector which has been selected. For details on the detectors
of the R&S FSL refer to chapter "Instrument Functions", section "Detector overview" or the Online Help.
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