clear;                        
t = 0:.001:1;  % Define 10 sin(2 pi t) with high frequency variations.
x = 10*sin(2*pi*t) + 2*sin(2*pi*200*t) + 1*cos(2*pi*209*t);

fftx = fft(x,2002);     % Padded fft.
fftx = fftshift(fftx);

figure(1)
subplot(2,1,1);
plot(t,x);
subplot(2,1,2);
plot(abs(fftx));

freqfilter = zeros(1,2002);    % Low pass filter.
freqfilter(1,700:1300) = 1;
newfftx = freqfilter.*fftx;

newx = ifftshift(newfftx);
newx = ifft(newx,2002);    % Padded inverse fft.

figure(2)
subplot(2,1,1);
plot(abs(newfftx));
subplot(2,1,2);
plot(t,real(newx(1,1:1001)));