pkg load signal
% sampling freq
fs = 256e3;
% time
t = 0:1/fs:1;
% test signal
x = 10*sin(2*pi*1e3*t) + 0.1*sin(2*pi*2e3*t) + sin(2*pi*3e3*t) + 0.5*randn(size(t));
% center and span of the fft
center = 2e3;
span = 4e3;
% complex down conversion
#loi = cos(2*pi*center*t);
#loq = sin(2*pi*center*t);
##xc = loi.*x +  i*loq.*x;
xc=hilbert(x);
% lowpass
[b,a]=butter(5,center/128e3);
#xc = filter(b,a,xc);
% decimation
xc = xc(1:256e3/span:end);
% take only 1024 samples
xc = xc(1:1024);
f = linspace(center-span/2, center+span/2, 1024);
plot(f, (20*log10(abs(fft(xc)))))
grid on