A conservative requirement for resolving 2 sinusoids (in noisy conditions) with a spacing of Hz is to choose a window length long enough so that their main lobes are clearly discernible. For example, we may require that their main lobes meet at the first zero crossings.
To obtain the separation shown above, we must have , where is the main lobe width in Hz, and is the sinusoidal frequency separation in Hz.
For the rectangular window, can be expressed as
Hence we need: