National Instruments NI Spectral Measurements Toolkit User Manual page 23

Larger RBW (103.5 kHz)
Smaller FFT size, requiring less computation
time
Cannot distinguish between the two tones in
the spectrum
An RBW of 103.5 kHz is not sufficient to distinguish between two tones that
Note
are 100 kHz apart.
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Figure 9 shows the spectrum of a multitone signal calculated using
two RBW values. The multitone signal consists of two tones, at frequencies
1.0 MHz and 1.1 MHz, separated by 100 kHz. Table 1 shows the trade-offs
of using two different RBWs.
Table 1. Larger versus Smaller RBW
Smaller RBW (9.94 kHz)
Larger FFT size, requiring more computation
time
Can distinguish between the two tones in
the spectrum
Figure 9. RBW Example
23 NI Spectral Measurements Toolkit User Guide