Multiplication of FFT by a Phase Filter

Hi, ultimately I am trying to remove high frequency components from a signal. I don’t want to use the TDHist Smooth function but rather want to make my own script to smoothen. I’ve been trying this with first a sine function and then my signal (D.txt).

I successfully input the data and do an FFT. Then I want to remove high frequency components. I think I am supposed to filter out the F(w) where w is high. How can I do this even though I don’t know what the range of frequencies are?
DoRCFilterNew.C (9.75 KB)
D.txt (10.8 KB)