Measuring the Spectra of Complex Signals R&S FSL
1300.2519.12 2.6 E-11
The delta marker moves to the maximum of the 2
n
d
harmonic. The displayed level result is
r
elative to the reference point level (= fundamental wave level).
Fig. 2-3 Measuring the level difference between the fundamental wave (= reference point
level) and the 2
nd
harmonic
The other harmonics are measured with steps 5 and 6, the center frequency being incremented or
decremented in steps of 128 MHz using the UPARROW or DNARROW key.
Measuring the Spectra of Complex Signals
Separating Signals by Selecting an Appropriate Resolution
Bandwidth
A basic feature of a spectrum analyzer is being able to separate the spectral components of a mixture
of signals. The resolution at which the individual components can be separated is determined by the
resolution bandwidth. Selecting a resolution bandwidth that is too large may make it impossible to
distinguish between spectral components, i.e. they are displayed as a single component.
An RF sinusoidal signal is displayed by means of the passband characteristic of the resolution filter
(RBW) that has been set. Its specified bandwidth is the 3 dB bandwidth of the filter.
Two signals with the same amplitude can be resolved if the resolution bandwidth is smaller than or
equal to the frequency spacing of the signal. If the resolution bandwidth is equal to the frequency
spacing, the spectrum display screen shows a level drop of 3 dB precisely in the center of the two
signals. Decreasing the resolution bandwidth makes the level drop larger, which thus makes the
individual signals clearer.
If there are large level differences between signals, the resolution is determined by selectivity as well as
by the resolution bandwidth that has been selected. The measure of selectivity used for spectrum
analyzers is the ratio of the 60 dB bandwidth to the 3 dB bandwidth (= shape factor).
For the R&S FSL, the shape factor for bandwidths is < 5, i.e. the 60 dB bandwidth of the 30 kHz filter is
< 150 kHz.
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