Specifications

130 Chapter 15
Concepts
Resolving Closely Spaced Signals
Concepts
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 ESA spectrum analyzers when the resolution bandwidth is
≥ 1kHz
and for PSA spectrum analyzers in swept mode, to keep the analyzer
measurement calibrated, sweep time is automatically set to a value
that is inversely proportional to the square of the resolution bandwidth
(1/BW
2
). 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 detector sweeps as fast or more quickly than sample or
average detectors. The ESA allows you to select from 1 kHz to 3 MHz
resolution bandwidths in a 1, 3, 10 sequence and select a 5 MHz
resolution bandwidth. The PSA allows RBW selections up to 8 MHz in
the same steps as ESA and it has the flexibility to fine tune RBWs in
increments of 10% for a total of 160 RBW settings. The ESA and PSA
have CISPR bandwidths (200 Hz, 9 kHz and 120 kHz at
−6 dB) for
maximum measurement flexibility. The PSA also has MIL EMI
bandwidths of 10 Hz, 100 Hz, 1 kHz, 10 kHz, 100 kHz and 1 MHz.