# How to code a spectrum frequency that allow me do scaling frequency?

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ong jia eek on 1 Oct 2019
Edited: ong jia eek on 1 Oct 2019
This is the code for FM in matlab
I used fft to produce frequency domain graph but my lecturer wanted a frequency spectrum .
fc=input('Enter the carrier signal freq in hz,fc=');
fm=input('Enter the modulating signal freq in hz,fm =');
m=input('Modulation index,m= ');
tm=(1/fm)*2;
t=0:1/(fc*1000):tm;
c=cos(2*pi*fc*t);%carrier signal
M=sin(2*pi*fm*t);% modulating signal
subplot(3,1,1);plot(t,c);
ylabel('amplitude');xlabel('time index');title('Carrier signal');
subplot(3,1,2);plot(t,M);
ylabel('amplitude');xlabel('time index');title('Modulating signal');
y=cos(2*pi*fc*t-(m.*cos(2*pi*fm*t)));
subplot(3,1,3);plot(t,y);
ylabel('amplitude');xlabel('time index');title('Frequency Modulated signal');
fs=1000;
z=fft(y);
nfft = length(y);
f = (0:1/nfft:1-1/nfft)*fs; % define frequency-domain
figure; % figure should be written before subplot to open new figure
subplot(2,1,1); % subplot(2,1,4) will give error beacause for a 2x1 vector valid indeces are 1&2, 4 is wrong
plot(f,z); % t (time-domain) is replace with f (frequency-domain)
ylabel('amplitude');xlabel('frequency domain');title('Frequency Modulated signal');

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