Problem with FM Modulation

Hi,

I am facing problem with fm modulation and demodulation.
i want to design a fm modulation with the following specs:

fm = modulating freq = 30Hz
fc = carrier freq = 10kHz
dev = frequency deviation = 480Hz
The problem is that for the carrier freq more than 1kHz, is not getting modulated properly which can be confirmed at demodulation.
Plz find below the matlab code:
-----------
fc3;
Fs*fc;
t=(0:Fs)'/Fs;
L00;
fm0;
ph=0;
devH0;
NFFT=2^nextpow2(L);
f=Fs*linspace(0,1,NFFT);

%%---- FM ----%%
x=1*cos((2*pi*fc*t)-(dev/fm)*1*sin(2*pi*fm*t+(ph*pi/180)));

[num den]=butter(1,(fc+30+30)/(Fs/2)); % -3db @ 10Hz
x1 = filtfilt(num,den,x); % leave DC

%%---- FM DEMOD ----%%

yq = hilbert(x1).*exp(-j*2*fc*t);
y = (1/(2*pi*dev))*[zeros(1,size(yq,2)); diff(unwrap(angle(yq)))*Fs];

[num den]=butter(1,25/(Fs/2),'high');
y = filtfilt(num,den,y);

[num den]=butter(1,30/(Fs/2));
y = filtfilt(num,den,y);

figure;
subplot(211)
plot(t,y)

subplot(212)
plot(f,2*abs(fft(y,NFFT)/L))
axis([0 200 0 4])
-
Kindly go through the code and help me in debugging it.
Is there any constraint in selecting modulating frequency and carrier frequency.??

Thanks and Regards,
Ashwini Start a New Thread

Basically, your code looks fine - the problem was that the FFT resolution was not sufficient to resolve the demodulated signal with higher sample rates (which you have set at 10 times the carrier). See my modified code below. I set L = 100*fc. I also added a windowing function prior to the FFT to reduce edge effects. Also, make sure you restrict your parameters so as to avoid violating Nyquest... Keep "fc + fm + dev" less then 40 percent or so of Fs or the sidebands will start to fold over around Fs/2, and the signal is no longer faithfully represented. Plots: with the "zoom on" enabled, you can take your mouse into a figure and zoom in...

Regards,
bill schintler

fc4;
Fs*fc;
t=(0:Fs)'/Fs;
L0*fc;
fm0;
ph=0;
devH0;
NFFT=2^nextpow2(L);
f=Fs*linspace(0,1,NFFT);

%%---- FM ----%%
x=1*cos((2*pi*fc*t)-(dev/fm)*1*sin(2*pi*fm*t+(ph*pi/180)));

[num den]=butter(1,(fc+30+30)/(Fs/2)); % -3db @ 10Hz
x1 = filtfilt(num,den,x); % leave DC

%%---- FM DEMOD ----%%

yq = hilbert(x1).*exp(-j*2*fc*t);
y = (1/(2*pi*dev))*[zeros(1,size(yq,2)); diff(unwrap(angle(yq)))*Fs];

[num den]=butter(1,25/(Fs/2),'high');
y = filtfilt(num,den,y);

[num den]=butter(1,30/(Fs/2));
y = filtfilt(num,den,y);

figure;
subplot(211)
plot(t,y)
grid;
zoom on;

%window
Window = hanning(length(y));

subplot(212)
plot(f,2*abs(fft(Window.*y,NFFT)/L))
grid;
zoom on;
%axis([0 200 0 4]) Start a New Thread