Display resolution and frequency span
FFT analyzers have a finite record length, usually stated in number of “points,”
“lines,” or “bins.” We will use the term “frequency points.” Most FFT analyzers use
the same number of frequency points regardless of frequency span. However, with
the 89400 series analyzers you may select a variable number of frequency points.
The 89400 series analyzers have a default resolution of 401 frequency points but
you may select from 51 to 3201 points of display resolution. However, for a given
number of frequency points, narrower spans have finer frequency resolution. This
is because the same number of frequency points represents a smaller range of
frequencies.
For each frequency span, the analyzer assigns a discrete frequency value to each
frequency point:
Display resolution =
Frequency span
Number of frequency points−1
The analyzer then uses the specified start frequency to calculate nominal frequency
values for each of the remaining points.
To better understand the concept of display resolution, move the main marker from
display point to display point. Notice how the marker jumps to each point—you
cannot put the marker between points. As you move the marker, also notice how
the marker readout steps through a series of discrete frequencies that corresponds
to each display point.
Display resolution is different from frequency resolution. See “The relationship
between Frequency Resolution and Display Resolution” later in this chapter.
Hint For a given display resolution and window selection, the best measurement
resolution is achieved with the bandwidth mode set to arb and bandwidth coupling
set to auto.
Fundamental Measurement Interactions
19 - 5
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