Specifications

34
Video filtering
Discerning signals close to the noise is not just a problem when performing
EMC tests. Spectrum analyzers display signals plus their own internal noise,
as shown in Figure 2-27. To reduce the effect of noise on the displayed signal
amplitude, we often smooth or average the display, as shown in Figure 2-28.
Spectrum analyzers include a variable video filter for this purpose. The
video filter is a low-pass filter that comes after the envelope detector and
determines the bandwidth of the video signal that will later be digitized to
yield amplitude data. The cutoff frequency of the video filter can be reduced
to the point where it becomes smaller than the bandwidth of the selected
resolution bandwidth (IF) filter. When this occurs, the video system can no
longer follow the more rapid variations of the envelope of the signal(s)
passing through the IF chain. The result is an averaging or smoothing of
the displayed signal.
Figure 2-27. Spectrum analyzers display signal plus noise
Figure 2-28. Display of figure 2-27 after full smoothing