function [t,st]=lpf(f,sf,B)
%this function filter an input data using a lowpass filter
%inputs: f:frequency samples
%        sf:input data spectrum samples
%        B:lowpass's bandwidth with a rectangle lowpass
%Outputs: :t:time samples
%        st:output data's time samples
df=f(2)-f(1);
T=1/df;
hf=zeros(1,length(f));
bf=[-floor(B/df):floor(B/df)]+floor(length(f)/2);
hf(bf)=1;
yf=hf.*sf;
[t,st]=F2T(f,yf);
st=real(st);