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POFrequency response using FFT in MATLAB
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copied!<p>Here is the scenario: using a spectrum analyzer i have the input values and the output values. the number of samples is <code>32000</code> and the sampling rate is <code>2000</code> samples/sec, and the input is a sine wave of <code>50 hz</code>, the input is current and the output is pressure in psi.</p> <p>How do i calculate the frequency response from this data using MATLAB, using the FFT function in MATLAB.</p> <p>i was able to generate a sine wave, that gives out the the magnitude and phase angles, here is the code that i used:</p> <pre><code>%FFT Analysis to calculate the frequency response for the raw data %The FFT allows you to efficiently estimate component frequencies in data from a discrete set of values sampled at a fixed rate % Sampling frequency(Hz) Fs = 2000; % Time vector of 16 second t = 0:1/Fs:16-1; % Create a sine wave of 50 Hz. x = sin(2*pi*t*50); % Use next highest power of 2 greater than or equal to length(x) to calculate FFT. nfft = pow2(nextpow2(length(x))) % Take fft, padding with zeros so that length(fftx) is equal to nfft fftx = fft(x,nfft); % Calculate the number of unique points NumUniquePts = ceil((nfft+1)/2); % FFT is symmetric, throw away second half fftx = fftx(1:NumUniquePts); % Take the magnitude of fft of x and scale the fft so that it is not a function of the length of x mx = abs(fftx)/length(x); % Take the square of the magnitude of fft of x. mx = mx.^2; % Since we dropped half the FFT, we multiply mx by 2 to keep the same energy. % The DC component and Nyquist component, if it exists, are unique and should not be multiplied by 2. if rem(nfft, 2) % odd nfft excludes Nyquist point mx(2:end) = mx(2:end)*2; else mx(2:end -1) = mx(2:end -1)*2; end % This is an evenly spaced frequency vector with NumUniquePts points. f = (0:NumUniquePts-1)*Fs/nfft; % Generate the plot, title and labels. subplot(211),plot(f,mx); title('Power Spectrum of a 50Hz Sine Wave'); xlabel('Frequency (Hz)'); ylabel('Power'); % returns the phase angles, in radians, for each element of complex array fftx phase = unwrap(angle(fftx)); PHA = phase*180/pi; subplot(212),plot(f,PHA),title('frequency response'); xlabel('Frequency (Hz)') ylabel('Phase (Degrees)') grid on </code></pre> <p>i took the frequency response from the phase plot at <code>90</code> degree phase angle, is this the right way to calculate the frequency response?</p> <p>how do i compare this response to the values that is obtained from the analyzer? this is a cross check to see if the analyzer logic makes sense or not. </p>

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