The MDO4000 specifications say that the minimum RBW (resolution bandwidth) is 20Hz, but I could only select 50Hz or 100Hz, Why?

The MDO4000 specifications say that the minimum RBW (resolution bandwidth) is 20Hz, but I could only select 50Hz or 100Hz, Why?

The minimum RBW is determined by at least three settings: span, RBW Mode and DFT Window.  Only certain MDO configurations support a minimum 20 Hz RBW. 

For example, when using center frequencies from 2 MHz to 6 GHz the following settings will determine the minimum RBW.

·       Span is set to 2 MHz. (A larger Span will increase minimum RBW.)

·       RBW Mode is Manual and the minimum RBW is set.

·       Last, the minimum RBW is determined by DFT Window type.

o      Rectangular, Hamming and Hanning windows minimum RBW is 20 Hz.

o      Kaiser and Blackman-Harris windows minimum RBW is 30 Hz.

o      Flat-top window minimum RBW is 50 Hz.