Technical data

Chapter 8 131
Concepts
Resolving Closely Spaced Signals
Concepts
Resolving Small Signals Hidden by Large Signals
When dealing with the resolution of signals that are close together and
not equal in amplitude, you must consider the shape of the IF filter of
the analyzer, as well as its 3 dB bandwidth. (See “Resolving Signals of
Equal Amplitude” on page 130 for more information.) The shape of a
filter is defined by the selectivity, which is the ratio of the 60 dB
bandwidth to the 3 dB bandwidth. If a small signal is too close to a
larger signal, the smaller signal can be hidden by the skirt of the larger
signal.
To view the smaller signal, select a resolution bandwidth such that k is
less than a (see Figure 8-1). The separation between the two signals (a)
must be greater than half the filter width of the larger signal (k),
measured at the amplitude level of the smaller signal.
The digital filters in the Agilent CSA have filter widths about one-half
to one-third as wide as typical analog RBW filters. This enables you to
resolve close signals with a wider RBW (for a faster sweep time).
Figure 8-1 RBW Requirements for Resolving Small Signals