Specifications

Chapter 15 131
Concepts
Resolving Closely Spaced Signals
Concepts
For best sweep times and keeping the analyzer calibrated set the sweep
time (
Sweep, Sweep Time) to Auto, and the auto sweep time (Sweep, Auto
Sweep Time
) to Norm. Use the widest resolution bandwidth that still
permits resolution of all desired signals.
NOTE For ESA-E Series Spectrum Analyzers:
Option 1DR adds narrower resolution bandwidths, from 10 Hz to
300 Hz, in a 1-3-10 sequence and 200 Hz CISPR bandwidth. These
bandwidths are digitally implemented and have a much narrower
shape factor than the wider, analog resolution bandwidths. Also, the
auto coupled sweep times when using the digital resolution bandwidths
are much faster than analog bandwidths of the same width. For
analyzers with Option 1DR, firmware revision A.08.00 and greater, and
Option 1D5 which adds a high-stability frequency precision reference to
the analyzer, resolution bandwidths of 1 Hz and 3 Hz are also available.
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 15-1). 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 ESA and PSA have filter widths about
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 15-1 RBW Requirements for Resolving Small Signals