Positive Peak & Average Detection Mode

If we set the analyzer to positive peak detection mode, we notice two things:
First, if VBW > RBW, then changing the resolution bandwidth does not make
much difference in the peak-to-peak fluctuations of the noise. Second, if
VBW < RBW, then changing the video bandwidth seems to affect the noise
level. The fluctuations do not change much because the analyzer is displaying
only the peak values of the noise. However, the noise level appears to change
with video bandwidth because the averaging (smoothing) changes, thereby
changing the peak values of the smoothed noise envelope. See Figure 2-30a.
When we select average detection, we see the average noise level remains
constant. See Figure 2-30b.

Because the video filter has its own response time, the sweep time increases
approximately inversely with video bandwidth when the VBW is less than
the resolution bandwidth. The sweep time can therefore be described by
this equation:

The analyzer sets the sweep time automatically to account for video
bandwidth as well as span and resolution bandwidth.

Figure 2-30a. Positive peak detection mode; reducing video bandwidth
lowers peak noise but not average noise

Figure 2-30b. Average detection mode; noise level remains constant, regardless of VBW-to-RBW ratios ( 3: 1, 1: 10, and 1: 100)

Trace Averaging
Digital displays offer another choice for smoothing the display: trace
averaging. This is a completely different process than that performed using
the average detector. In this case, averaging is accomplished over two or
more sweeps on a point-by-point basis. At each display point, the new
value is averaged in with the previously averaged data: