R&S FSL Measuring the Spectra of Complex Signals
1300.2519.12 2.11 E-11
Fig. 2-7 If the RF attenuation is increased, the R&S FSL's intrinsic intermodulation products
disappear below the noise floor.
Calculation method:
The method used by the R&S FSL 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 function 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 internal 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 spectrum analyzer. 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 spectrum analyzer 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 function of the selected
bandwidth and of the level at the input mixer (= signal level – set RF attenuation) at different useful
signal offsets.
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