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Why do my amplitude become wrong when I vary the frequency from int to double?

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Nina
Nina 2013 年 2 月 19 日
So I started with this code:
A = 10;
B= 17.6;
C=2;
D=11.2;
phi1 = 2;
phi2 = 5;
phi3 = 1;
phi4 = 0;
Fs = 100;
t = 0:1/Fs:1-1/Fs;
x=A*sin(2*pi*10*t+phi1)+B*cos(2*pi*5*t+phi2)+C*sin(2*pi*2*t+phi3)+D*cos(2*pi*t+phi4);
xdft = fft(x);
magnitude = 2* abs(xdft)/length(x)
phase = angle(2* max((xdft))/length(x))
And that gave me all the amplitudes exactly. However when say I change the first frequency from A*sin(2*pi*10*t+phi1) to A*sin(2*pi*10.5*t+phi1), it goes crazy and give me weird output... what am I doing wrong?
Thank you

回答 (1 件)

Wayne King
Wayne King 2013 年 2 月 19 日
It does not go "crazy", it only gets the amplitude wrong for the frequency f = 10.5 and this is problem I explained to you in the other post. 10.5 does not correspond to a DFT bin.
See
  2 件のコメント
Nina
Nina 2013 年 2 月 19 日
Thank Wayne, I guess I am still trying to understand what is going on...basically, every time I change my frequencies, I need to make sure they fall into appropriate bins?
Nina
Nina 2013 年 2 月 19 日
Also, if 10.5 does not correspond to a DFT bin, how do I deal with it?

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