calculate normalised frequency of the below plot

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Venkatkumar M 2019-8-12

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https://ww2.mathworks.cn/matlabcentral/answers/475813-calculate-normalised-frequency-of-the-below-plot

评论： Star Strider 2019-8-13

采纳的回答： Star Strider

How to calulate frequency from above graph?

convert normalised frequency(0.03) into frequncy(hz)?

how to do this?

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Star Strider 2019-8-12

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此回答的直接链接

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‘convert normalised frequency(0.03) into frequncy(hz)?’

Multiply the normalised frequency by the Nyquist frequency.

The sampling frequency is 1/(sampling interval). The Nyquist frequency is one-half of the sampling frequency.

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Star Strider 2019-8-12

在 MATLAB Online 中打开

Try this:

fnm2 = 'op.txt';
D2 = load(fnm2);
ts = 1:size(D2,1);                          % Time Units In Seconds
L = numel(ts);                              % Time Units In Seconds
Ts = ts(2)-ts(1);                           % Sampling Tiume (Assumes Uniform Sampling)
Fs = 1/Ts;                                  % Sampling Frequency
Fn = Fs/2;                                  % Nyquist Frequency
FTD2 = fft(D2)/L;
Fv = linspace(0, 1, fix(L/2)+1)*Fn;         % Frequency Vector
Iv = 1:numel(Fv);                           % Index Vector
figure
subplot(2,1,1)
plot(Fv, abs(FTD2(Iv,:))*2)
grid
xlabel('Frequency (Hz)')
ylabel('Amplitude')
title('Time Units: s')
subplot(2,1,2)
plot(Fv*1E+9, abs(FTD2(Iv,:))*2)
grid
xlabel('Frequency (Hz)')
ylabel('Amplitude')
title('Time Units: ns')

Using the ‘ts’ vector as the time vector, the Fourrier transform is straightforward to calculate and plot. To use nanoseconds as the time unit, it is only necessary to multiply ‘Fv’ by

for the plot, since that is the only thing that changes.

All the data are plotted, however the data for the third vector obscures the first two. You may want to plot them separately:

figure
subplot(3,1,1)
plot(Fv*1E+9, abs(FTD2(Iv,1))*2)
grid
xlabel('Frequency (Hz)')
ylabel('Amplitude')
title('Time Units: ns')
subplot(3,1,2)
plot(Fv*1E+9, abs(FTD2(Iv,2))*2)
grid
xlabel('Frequency (Hz)')
ylabel('Amplitude')
title('Time Units: ns')
subplot(3,1,3)
plot(Fv*1E+9, abs(FTD2(Iv,3))*2)
grid
xlabel('Frequency (Hz)')
ylabel('Amplitude')
title('Time Units: ns')

Experiment to get the result you want.

Venkatkumar M 2019-8-13

since fs =1 it shows error and in case if i change it the fs=1000 figure plot displays

Star Strider 2019-8-13

在 MATLAB Online 中打开

You need to understand the argument list order for the pwelch function.

Try this:

[pxx,f] = pwelch(D2,[],[],[],Fs);
figure
subplot(3,1,1)
plot(f, pxx(:,1))
grid
xlabel('Frequency (Hz)')
ylabel('PSD')
title('Column 1')
subplot(3,1,2)
plot(f, pxx(:,2))
grid
xlabel('Frequency (Hz)')
ylabel('PSD')
title('Column 2')
subplot(3,1,3)
plot(f, pxx(:,3))
grid
xlabel('Frequency (Hz)')
ylabel('PSD')
title('Column 3')