how to calculate central frequency from fft of wave function

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kavya saxena
kavya saxena 2012-5-24
hi all, is there any difference between central freq and maximum freq of a wave function. how i can calculate central frequency of this function using fft in matlab y=0.5(1-cos(2*pi*f*t)*sin(2*pi*f*t)

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回答（2 个）

Wayne King
Wayne King 2012-5-24
Are you trying to show us an AM signal? If so, are the two f's really the same frequency, or did you mean something like this:
t = 0:1e-4:1-1e-4;
f0 = 50;
fc = 1e3;
y=0.5*(1-cos(2*pi*f0*t)).*sin(2*pi*fc*t);
ydft = fft(y);
ydft = ydft(1:length(y)/2+1);
freq = 0:1e4/length(y):1e4/2;
plot(freq,abs(ydft))
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kavya saxena
kavya saxena 2012-5-24
these two are same frequency. let it wiil be 20Khz then i did
f=20e3;
fs=f*100;
ts=1/fs;
n=3;
t=0:ts:n/f;
y=0.5*(1-cos(2*pi*f*t)).*sin(2*pi*f*t);
ydft = fft(y);
now how i get central freq?

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Wayne King
Wayne King 2012-5-24
It appears to me that just rewriting that using trigonometric identities you see that it equals
1/2*sin(2*pi*f*t)-1/4*sin(2*pi*2*f*t)
So you get a line component at 20 kHz and one at 40 kHz with 1/2 the amplitude of the component at 20 kHz.
freq = 0:fs/length(y):fs/2;
ydft = fft(y);
ydft = ydft(1:ceil(length(y)/2));
plot(freq,abs(ydft))
So I'm guessing you call the center frequency 30 kHz.

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