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

Question

ong jia eek 2019-10-1

编辑： ong jia eek 2019-10-1

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');