[H_1, w] = freqz([1 2 1], [1]);
subplot(2, 1, 1);
plot(w, abs(H_1));
xlabel('Angular frequency, \omega');
ylabel('H_1 Magnitude');
subplot(2, 1, 2);
plot(w, angle(H_1));
xlabel('Angular frequency, \omega');
ylabel('H_1 Phase');

%

[H_2, w] = freqz([1], [1 0.9]);
subplot(2, 1, 1);
plot(w, abs(H_2));
xlabel('Angular frequency, \omega');
ylabel('H_2 Magnitude');
subplot(2, 1, 2);
plot(w, angle(H_2));
xlabel('Angular frequency, \omega');
ylabel('H_2 Phase');