MATLAB: Is it possible that ploting bode diagram without transfer fuction

bode plotbodeplotfrequency responseMATLAB and Simulink Student Suitestep responsetransfer function

Hi everyone,

Is it possible that ploting bode diagram without transfer fuction?

I have some values of the Power and the Speed as inputs and some output values in excel format. However the transfer fuction of the system has not been defined.

Are there any one who knows any method or code to plotting bode diagram without transfer function?

Best Answer

Yes. Use the fft function.

input = randn(1, 10000);
output = randn(1, 10000);
time = linspace(0, 1000, 10000);
Fs = 100;                           % Sampling Frequency
Fn = Fs/2;                          % Nyquist Frequency
L = numel(time);
FTinp = fft(input)/L;
FTout = fft(output)/L;
TF = FTout ./ FTinp;                % Transfer Function
Fv = linspace(0, 1, fix(L/2)+1)*Fn; % Frequency Vector
Iv = 1:numel(Fv);                   % Index Vector
figure
subplot(2,1,1)
plot(Fv, 20*log10(abs(TF(Iv))))
title('Amplitude')
ylabel('dB')
subplot(2,1,2)
plot(Fv, angle(TF(Iv))*180/pi)
title('Phase')
ylabel('°')

Provide your own vectors for ‘input’, ‘output’, and ‘time’.

Related Solutions

MATLAB: How can i obtain the frequency response for Savitzky-Golay filtering

Try this instead:

t = linspace(0, 10);
y = sin(2*pi*t/5) + randn(size(t))/10;
sgf = sgolayfilt(y, 3, 11);
figure
plot(t, y)
hold on
plot(t, sgf, '-r')
hold off
grid
L = numel(t);
Ts = mean(diff(t));
Fs = 1/Ts;
Fn = Fs/2;
FTy = fft(y)/L;
FTsgf = fft(sgf)/L;
Fv = linspace(0, 1, fix(L/2)+1)*Fn;
Iv = 1:numel(Fv);
figure
subplot(3,1,1)
plot(Fv, abs(FTy(Iv))*2)
grid
title('Fourier Transform: y')
subplot(3,1,2)
plot(Fv, abs(FTsgf(Iv))*2)
grid
title('Fourier Transform: Savitzky-Golay Filter of y')
subplot(3,1,3)
plot(Fv, abs(FTsgf(Iv)./FTy(Iv))*2)
grid
title('Savitzky-Golay Transfer Function')

This nicely demonstratres the lowpass filter characteristic.

Experiment with other signals to get different (but likely similar) resultls.

MATLAB: Low pass filter VS high pass filter

Calculate the Fourier transform of the filter from the signals.

t = ...;                                                    % Time Vector
s = ...;                                                    % Original Signal Vector
s_filtered = ...;                                           % Filtered Signal Vector
Ts = mean(diff(t));                                         % Sampling Interval (Assumes Constant Intervals)
Fs = 1/Ts;                                                  % Sampling Frequency
Fn = Fs/2;                                                  % Nyquist Frequency
L = numel(t);                                               % Signal Length
FTs = fft(s)/L                                              % Fourier Transform of Original Signal
FTs_filt = fft(s_filtered)/L;                               % Fourier Transform of Filtered Signal
Fv = linspace(0, 1, fix(L/2)+1)*Fn;                         % Frequency Vector (One-Sided Fourier Transform)
Iv = 1:numel(Fv);                                           % Index Vector (One-Sided Fourier Transform)
H = FTs_filt ./ FTs;                                        % Complex Transfer Function
figure
subplot(2,1,1)
plot(Fv, abs(H(Iv)))
ylabel('Amplitude')
grid
subplot(2,1,2)
plot(Fv, angle(H(Iv)))
xlabel('Frequency')
ylabel('Phase')
grid
sgtitle('Bode Plot of Filter')

That should reveal the filter characteristics, as well as reasonably accurate aspects of its design (e.g. passband and stopband frequencies).

Best Answer

Related Solutions

MATLAB: How can i obtain the frequency response for Savitzky-Golay filtering

MATLAB: Low pass filter VS high pass filter

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