close all, clear all, clc
fs = 1000;
fs2 = fs/2;
fc = [150 250];
fn = fc/fs2;
t = linspace(0, 0.05, 50);
xn = sin(200*pi*t).^2;
figure(1)
subplot(2,1,1), stem(t, xn), grid
N = 25;
B = fir1(N, fn);
[H, w] = freqz(B, 1);
yn = filter(B, 1, xn);
subplot(2,1,2), stem(t, yn), grid
figure(2)
plot(w/(2*pi)*fs, 20*log10(abs(H))), grid