Generate Real Signal With Random Phase and Specified Modulus at 10 Harmonics

Question

Jeff 2011-8-23

0
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https://ww2.mathworks.cn/matlabcentral/answers/14378-generate-real-signal-with-random-phase-and-specified-modulus-at-10-harmonics

Hi all,

I want to create a 1 sec duration real signal with 10 harmonics of a specified modulus and randomized phase. Here's what I've done:

phase = [2*rand(1)*pi - pi, ... , 2*rand(1)*pi - pi]; % ... is 8 repetitions

modulus = [1000, ... , 1000]; % ... is 8 repetitions

real = sqrt(modulus.^2./([1,1,1,1,1,1,1,1,1,1]+tan(phase).^2)); % substitute and calculate real part first

imag = sqrt(modulus.^2 - real.^2); % solve for imag part

sigfft = zeros(44100,1);

for i = 1:10

    sigfft(1+200*i) = real(i) + j*imag(i); % j = sqrt(-1) previously assigned
    sigfft(44100+1-200*i) = real(i) + j*imag(i);

end

sig = ifft(sigfft);

Thus, I assign a random phase in the interval [-pi,pi), and find the imaginary and real parts which correspond to the assigned random phase and the specified modulus of 1000. Then I code these into the 10 harmonic bins of 200Hz, 400Hz, ... , 2000Hz. However, when I execute sig = ifft(sigfft), Matlab returns a complex signal. I want a real signal, what's going on here?