I want to say thank you to Dr.Joel for previous answers.You wrote so much to explain these questions to me.But I still have questions that I can't figure out.The first thing is how to determine sample points according to window shape.When we do FFT,there is a equation:N=Fs/RBW,which is written on a DSP book I read.But the sample points changes when I change window shape.Does it mean the frequency resolution will change after windowing?What's the relation between window and sample points.Is there a function to calculate the sample points according to sample rate,window shape and RBW?The second question is,the "number of LOs" seems not equal to span*N/68MHz when I use it.I tested some frequency span and image reject selection and made a list.How does this happen?

Another question is how dose the system change VBW in PNA-X.This SA function is based on FFT,and the traditional SA is based on sweep,so I think there should be some difference when we change VBW.Is there a digital low pass filter which determine the VBW?