Rohde & Schwarz R&S FSP Series, Руководство по эксплуатации

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R&S FSP Getting Started
Measuring the Spectra of Complex Signals
2.6 Operating Manual 1164.4556.12 - 06
Fig. 2-2 If the RF attenuation is increased, the R&S FSP’s intrinsic intermodulation
products disappear below the noise floor.
Calculation method:
The method used by the R&S FSP to calculate the intercept point takes the average
useful signal level P
u
in dBm and calculates the intermodulation d
3
in dB as a func-
tion of the average value of the levels of the two intermodulation products. The third
order intercept (TOI) is then calculated as follows:
TOI/dBm = ½ d
3
+ P
u
Intermodulation- free dynamic range
The Intermodulation – free dynamic range , i.e. the level range in which no inter-
nal intermodulation products are generated if two-tone signals are measured, is
determined by the 3
rd order intercept point, the phase noise and the thermal noise of
the R&S FSP. At high signal levels, the range is determined by intermodulation
products. At low signal levels, intermodulation products disappear below the noise
floor, i.e. the noise floor and the phase noise of the R&S FSP determine the range.
The noise floor and the phase noise depend on the resolution bandwidth that has
been selected. At the smallest resolution bandwidth, the noise floor and phase noise
are at a minimum and so the maximum range is obtained. However, a large increase
in sweep time is required for small resolution bandwidths. It is, therefore, best to
select the largest resolution bandwidth possible to obtain the range that is required.
Since phase noise decreases as the carrier-offset increases, its influence decreases
with increasing frequency offset from the useful signals.
The following diagrams illustrate the intermodulation-free dynamic range as a func-
tion of the selected bandwidth and of the level at the input mixer (= signal level – set
RF attenuation) at different useful signal offsets.