phase plot fft (strange)values

Question

KE BR 2020-8-9

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/577252-phase-plot-fft-strange-values

回答： Sulaymon Eshkabilov 2020-8-9

i Have the following code;

Fs = 500;
t = 0:1/Fs:10-1/Fs;                   
s = cos(2*pi*15*t - 30) + cos(2*pi*40*t +60);     
                                               
 NFFT=length(s);                
 ft =(1/NFFT)*fft(s);                    
 freq = Fs*(0:(NFFT/2))/NFFT;
 Am=abs(ft);
 tol = 1e-6;
 ft(abs(ft) < tol) = 0;
 Ph = angle(ft);
    
 Am = Am*2;
 Am = Am';
 Am = Am(1:end/2,:);
 freq = freq';
 freq = freq(1:end-1,:);
 Ph=Ph';
 Ph = Ph(1:end/2,:);
 plot(freq,Ph*(180/pi));
 

How do i interpretend the phase of this signal? shouldnt the phase plot give me -30° at 15Hz and 60° at 40Hz? How does it come that the fft gives 81.13° and -162.3°?

Kind regards