Shortfalls of Sample Detection

While the sample detection mode does a good job of indicating the randomness
of noise, it is not a good mode for analyzing sinusoidal signals. If we were
to look at a 100 MHz comb on an Agilent ESA E4407B, we might set it to
span from 0 to 26.5 GHz. Even with 1, 001 display points, each display point
represents a span (bucket) of 26.5 MHz. This is far wider than the maximum
5 MHz resolution bandwidth.

As a result, the true amplitude of a comb tooth is shown only if its mixing
product happens to fall at the center of the IF when the sample is taken.
Figure 2-22a shows a 5 GHz span with a 1 MHz bandwidth using sample
detection. The comb teeth should be relatively equal in amplitude as shown
in Figure 2-22b (using peak detection). Therefore, sample detection does not
catch all the signals, nor does it necessarily reflect the true peak values of the
displayed signals. When resolution bandwidth is narrower than the sample
interval (i. e., the bucket width), the sample mode can give erroneous results.

Figure 2-22a. A 5 GHz span of a 100 MHz comb in the sample display mode

Figure 2-22b. The actual comb over a 500 MHz span using peak
( positive) detection