The relationship between frequency resolution and display resolution
For spectrum measurements, your ability to resolve two closely-spaced
components—that is, the analyzer’s ability to display each component as a separate
frequency point—may be limited by the display resolution. However, the maximum
frequency resolution obtainable is actually determined by the resolution bandwidth
you’ve selected. As you select increasingly narrower spans, the display resolution
improves until the point where you reach the maximum resolution available with
the current resolution bandwidth setting.
Resolution limited by current resolution bandwidth setting:
If the resolution bandwidth is insufficient to reveal two closely-spaced
components, the detector simply sees these two components as a single frequency
and displays them on as a single frequency point. The amplitude of the two
components is combined, though it varies from sweep to sweep as the phase
relationship between the two components changes.
Resolution limited by current display resolution:
If the display resolution is insufficient to reveal two closely-spaced
components—but the current resolution bandwidth setting does provide enough
resolution to distinguish two separate components—the detector uses an algorithm
(peak, normal, or sample) to select a frequency component and displays it as a
single frequency point. However, were you to narrow the span enough or increase
the number of frequency points—keeping the resolution bandwidth constant—you
would be able to view two discrete frequency components as the display resolution
improved.
Resolution bandwidth limitations
The standard swept analyzer uses the last in a series of analog IF filters to
determine resolution bandwidth. In contrast, the 89400 series analyzers use digital
IF and and DSP algorithm to determine resolution bandwidth. For this reason, the
narrowest practical resolution bandwidth in Scalar mode is constrained by the
length of time you are willing to wait for a measurement to be performed.
The widest resolution bandwidth in Scalar mode is 300 kHz for combinations of
span, resolution bandwidth, and maximum number of frequency points which
require that the local oscillator be stepped to more than one frequency. But for
measurements which do not require that the oscillator be stepped the widest
resolution bandwidth is determined by the resolution bandwidth to span ratio:
RBW
max
= .3 × span
Fundamental Measurement Interactions
19 - 11
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