Rohde & Schwarz R&S FSL3 - Intermodulation Measurements

R&S FSL Measuring the Spectra of Complex Signals
1300.2519.12 2.7 E-11
The higher spectral resolution with smaller bandwidths is won by longer sweep times for the same
s
pan. The sweep time has to allow the resolution filters to settle during a sweep at all signal levels and
frequencies to be displayed. It is given by the following formula.
2
Span/RBWkSWT •= (4)
SWT = max. sweep time for correct measurement
k = factor depending on type of resolution filter
= 1 for digital IF filters
Span = frequency display range
RBW = resolution bandwidth
If the resolution bandwidth is reduced by a factor of 3, the sweep time is increased by a factor of 9.
Note: The impact of the video bandwidth on the sweep time is not taken into account in (4). For the
formula to be applied, the video bandwidth must be  3 x the resolution bandwidth.
FFT filters can be used for resolution bandwidths up to 30 kHz. Like digital filters, they have a shape
factor of less than 5 up to 30 kHz. For FFT filters, however, the sweep time is given by the following
formula:
SWT = k
 span/RBW (5)
If the resolution bandwidth is reduced by a factor of 3, the sweep time is increased by a factor of 3 only.
Intermodulation Measurements
If several signals are applied to a transmission two–port device with nonlinear characteristic,
intermodulation products appear at its output by the sums and differences of the signals. The nonlinear
characteristic produces harmonics of the useful signals which intermodulate at the characteristic. The
intermodulation products of lower order have a special effect since their level is largest and they are
near the useful signals. The intermodulation product of third order causes the highest interference. It is
the intermodulation product generated from one of the useful signals and the 2nd harmonic of the
second useful signal in case of two–tone modulation.
The frequencies of the intermodulation products are above and below the useful signals. Fig. 2-4 shows
intermodulation products P
I1
and P
I2
generated by the two useful signals P
U1
and P
U2
.
Fig. 2-4 Intermodulation products P
U1
and P
U2