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Two questions about spectrum analyze with FFT method

Question asked by Macree on Feb 15, 2017
Latest reply on Feb 19, 2017 by Macree

Hi,everyone.In my previous questions Dr.Joel explained a lot but these two questions still confuse me.First,how to decide number of repeated FFTs according to RBW/VBW ratio.Dr.Joel mentioned that for a RBW/VBW ratio of 10,there are 5 FFTs averaged before the result is processed.If I change the ratio,how dose the number of FFT average change?A function to calculate that is better.And the second is still about window function.If I use a window before FFT,to maintain the RBW,increasing record size is necessary.Dr.Joel mentioned that the gaussian filter,with 90dB sidelobes is about 1.87 longer than the no window case.I use the SA option in PNA-X and I found things are different.With gaussian filter,record size equals the record size under no window case multiply by about 1.986.And with flattop,kaiser and blckman filter,the number is 3.764,1.964,1.623.I don't know why these numbers,how did they been calculated.Is there some functions can solve this?Another small question:the kaiser window is a adjustable window,the parameter β is the key.I want to know what dose β equal in SA option of PNA-X.Thanks a lot.