# Frequency resolution - resolution BW relation for different window functions

I am working on FFT based spectrum analyser and I am not sure how to proceed calculating the RBW.

According to the document (page 20) in the link, RBW described as $$k \frac{f_s}N$$ where $$f_s$$ sampling frequency, $$N$$ FFT size and $$k$$ is window coefficent.

Since $$\frac{f_s}N$$ is simply frequency resolution, is there a way to calculate $$k$$ coefficent for different window functions?

I tried to check the book in the references but I couldn't find the information.