How to solve aliasing affect in signal?

clear all;
clc;

fs = 8000; % sampling frequency
f1 = 1000; % frequency 1
f2 = 6000; % frequency 1

dt = 1/fs;
T = 0.2; % time
t=(0:1/fs:T)';	

y1 = cos(2*pi*f1*t);
y2 = cos(2*pi*f2*t);

y= y1+y2;

lowpass(y,3000,fs);

Hi,

I have simulated a signal 'y' to study aliasing effect, where the the frequency component f2=6000 in 'y' appears as 2000 Hz in spectral map. Though I applied an anti aliasing filter of lowpass with cutoff frequency of 3k Hz, still I see the aliaisng effect showing 2000Hz. Couls some one help me to solve this?

a

Answer 1

[Jonas]

the error occurs already in the creation of the two signals. how should the lowpass know how exactly you produce your signals and do that what you want. the creation of y2 brings just a signal which unknown way of creation (from the filter's view). Thats why

fs = 8000; % sampling frequency
f1 = 1000; % frequency 1
dt = 1/fs;
T = 0.2; % time
t=(0:1/fs:T)';
y1 = cos(2*pi*f1*t);
lowpass(y1,3000,fs);


f2 = 7000; % frequency 2
y2 = cos(2*pi*f2*t);
figure;
lowpass(y2,3000,fs);

results into the same frequency plot.