Best Sensitivity Using Minimum RBW

So if we change the resolution bandwidth by a factor of 10, the displayed
noise level changes by 10 dB, as shown in Figure 5-2. For continuous wave
(CW) signals, we get best signal-to-noise ratio, or best sensitivity, using the
minimum resolution bandwidth available in our spectrum analyzer 2.

Figure 5-2. Displayed noise level changes as 10 log (BW2 / BW1)

A spectrum analyzer displays signal plus noise, and a low signal-to-noise ratio
makes the signal difficult to distinguish. We noted previously that the video
filter can be used to reduce the amplitude fluctuations of noisy signals while
at the same time having no effect on constant signals. Figure 5-3 shows how
the video filter can improve our ability to discern low-level signals. It should
be noted that the video filter does not affect the average noise level and so
does not, by this definition, affect the sensitivity of an analyzer.

In summary, we get best sensitivity for narrowband signals by selecting the
minimum resolution bandwidth and minimum input attenuation. These settings
give us best signal-to-noise ratio. We can also select minimum video bandwidth
to help us see a signal at or close to the noise level 3. Of course, selecting
narrow resolution and video bandwidths does lengthen the sweep time.

Figure 5-3. Video filtering makes low-level signals more discernable

2. Broadband, pulsed signals can exhibit the opposite
behavior, where the SNR increases as the bandwidth
gets larger.
3. For the effect of noise on accuracy, see Dynamic
range versus measurement uncertainty in
Chapter 6.