Hello Roger,
Good question. In a spectrum analyzer, resolution bandwidth is typically defined by the 3dB bandwidth. This is the width of a signal (CW) measured 3dB down from the peak on either side. This is the definition we use.
The reason the bin size is not equivalent to the RBW is due to the use of window functions. If you are not familiar with window functions see here. http://en.wikipedia.org/wiki/Window_function Most windows functions spread the spectral energy across multiple bins, which is commonly referred to as the window bandwidth, or noise bandwidth, or B. Flat-top windows have a higher bandwidth than most, usually B > 3. We use custom flat-top windows to achieve a higher amplitude accuracy, reduce spectral leakage, and maintain a high dynamic range.
You can directly calculate B using your example by the equation… 10 / 1.854 = B.
B will not be the same all the time, as we attempt to find the best B for any given setup.
Let us know if you have any other questions.
Regards,
A.J.