problem with filtering

HI, i’m working on a code that uses square wave as a message signal at 1Mhz and carrier of 5Mhz. i’m using product(coherent) demodulation technique for demodulation using butterworth filter. my message amplitude is 0.25v and it is a combination of a dc of amplitude 3.5v and periodic square at 0.25v. but i can get my square wave at the output? can u help me with this?

I_Rx=(I_mod).*(I_ch);
Fc=4e6;
Fs=32e6;
%f = [0 0.25 0.25 1]; m = [1 1 0 0];
wn=(2.*Fc)./Fs;disp(wn);
%h=butter_lpf;
[b,a]=butter(8, wn);
z=filter(b,a,I_Rx);
figure; plot(t,z);    

Here I_Rx is my product signal,I_mod is my modulated signal,I_ch is the carrier i’m multiplying for the product demodulation but i cant low pass filter it to get back my square wave? i used carrrier of 5Mhz and my message is a 1Mhz triangle of 0.25v on a 3.5v dc. so i used a low pass cut off at 4Mhz(2*Fc). my message signal is a combination of ac and dc signal.

thanks