Rohde & Schwarz R&S FSP Series, Operating Manual

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R&S FSP Getting Started
Measuring Signals in the Vicinity of Noise
2.8 Operating Manual 1164.4556.12 - 06
2.3 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 measure-
ment is influenced by the intrinsic noise of the R&S FSP.
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
R&S FSP’s RF attenuation can be set in 10 dB steps up to 70 dB (5 dB steps up to
75 dB with option
Electronic Attenuator R&S FSP-B25 ). Each additional 10 dB step
reduces the R&S FSP’s sensitivity by 10 dB, i.e. the displayed noise is increased by
10 dB.
Impact of the reference level setting
If the reference level is changed, the R&S FSP changes the gain on the last IF so
that the voltage at the logarithmic amplifier and the A/D converter is always the
same for signal levels corresponding to the reference level. This ensures that the
dynamic range of the log amp or the A/D converter is fully utilized. Therefore, the
total gain of the signal path is low at high reference levels and the noise figure of the
IF amplifier makes a substantial contribution to the total noise figure of the
R&S FSP. The figure below shows the change in the displayed noise depending on
the set reference level at 10 kHz and 300 kHz resolution bandwidth. With digital
bandwidths (
≤
100 kHz) the noise increases sharply at high reference levels because
of the dynamic range of the A/D converter.
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
FSP.