Resolving Closely Spaced Signals; Resolving Signals Of Equal Amplitude; Resolving Small Signals Hidden By Large Signals - Keysight X Series Measurement Manual

Concepts

Resolving Closely Spaced Signals

Resolving Closely Spaced Signals

Resolving signals of equal amplitude

Two equal-amplitude input signals that are close in frequency can appear as a single
signal trace on the analyzer display. Responding to a single-frequency signal, a
swept-tuned analyzer traces out the shape of the selected internal IF (intermediate
frequency) filter (typically referred to as the resolution bandwidth or RBW filter). As
you change the filter bandwidth, you change the width of the displayed response. If a
wide filter is used and two equal-amplitude input signals are close enough in
frequency, then the two signals will appear as one signal. If a narrow enough filter is
used, the two input signals can be discriminated and appear as separate peaks. Thus,
signal resolution is determined by the IF filters inside the analyzer.
The bandwidth of the IF filter tells us how close together equal amplitude signals can
be and still be distinguished from each other. The resolution bandwidth function
selects an IF filter setting for a measurement. Typically, resolution bandwidth is
defined as the 3 dB bandwidth of the filter. However, resolution bandwidth may also
be defined as the 6 dB or impulse bandwidth of the filter.
Generally, to resolve two signals of equal amplitude, the resolution bandwidth must
be less than or equal to the frequency separation of the two signals. If the bandwidth
is equal to the separation and the video bandwidth is less than the resolution
bandwidth, a dip of approximately 3 dB is seen between the peaks of the two equal
signals, and it is clear that more than one signal is present.
For Signal Analyzers in swept mode, sweep time is automatically set to a value that
is inversely proportional to the square of the resolution bandwidth (1/BW
the analyzer measurement calibrated. So, if the resolution bandwidth is reduced by a
factor of 10, the sweep time is increased by a factor of 100 when sweep time and
bandwidth settings are coupled. For the shortest measurement times, use the widest
resolution bandwidth that still permits discrimination of all desired signals. Sweep
time is also a function of which detector is in use, peak and normal detectors sweep
as fast or more quickly than sample or average detectors. The analyzer allows RBW
selections up to 8 MHz in 1, 3, 10 steps and it has the flexibility to fine tune RBWs in
increments of 10% for a total of 160 RBW settings.
For best sweep times and keeping the analyzer calibrated set the sweep time
(Sweep/Control, Sweep Time) to Auto, and the sweep type (Sweep/Control,
Sweep Setup, Sweep Type) to Auto. Use the widest resolution bandwidth and the
narrowest span that still permits resolution of all desired signals.

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
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.
"Resolving signals of equal amplitude" on page 192
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