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width is not infinitesimal. Measure the width of the spectrum line at 3dB below
the peak, and that width is RBW.
Apparently, RBW is relevant to SPAN, FFT points and window shape. The
smaller the SPAN, the more FFT point, the smaller the RBW, the better the
resolution. If the FFT points and window shape are set, RBW is only relevant to
SPAN theoretically. In other words, there is a function mapping SPAN and
RBW and SPAN is the variable. This fact brings inconvenience to real-world
operation, thus KC908 adopts the minimum RBW of current SPAN by default.
Also, it's allowed to manually adjust in a limited range. Effectively, this is
adjusting FFT points and window shape.
With a relatively large value in SPAN setting, the resolution of the screen
would be inferior to RBW. Each pixel on the horizontal axis represents several
pixels that could have been distinguished from one another. In the worst
scenario, one pixel would even represent a thousand adjcent frequencies. At
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