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Re: FFT - Defining Frequency Axis

Re: FFT - Defining Frequency Axis

Re: FFT - Defining Frequency Axis

Hello Everyone,

I'm trying to analyse a time-varying signal for helicopter main rotor pitch rod using FFT in Matlab.

I have written the code and understand the concept of FFT too but the specification of the frequency axis does not seem to work too well as I'm getting a lot of frequency leakage for my signal.

If anyone can please take a look at the following code and make suggestions, I would be grateful, particularly for defining the frequency axis in Hz.

%Define Variables
x = 24;
N = 2^x;
f_datalog = 48;
fs = f_datalog*6;

X = fft(MRPR1,N); %Perform N-Point FFT on the data
Y = abs(X); %Determine the magnitude
f = ((0:N-1)*(fs/N)); %Specify the frequency range

plot(f,Y),axis([0 60 0 3.5*10^5])
title('Magnitude Versus Frequency for MRPR1 D8C1');
xlabel('Frequency (Hz)');
%grid on

Thanks heaps,


RE: Re: FFT - Defining Frequency Axis

I doubt any problems with leakage are associated with errors in the frequency axism which by eye looks Ok to within a factor of 2.

Hanning. There's a lot more to signal analysis than 'understanding ffts'.


Greg Locock

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RE: Re: FFT - Defining Frequency Axis

if you are attempting to characterize the freq content of a time signal, especially if it is noisy, the segment of data may have a dc level that needs removing. you'll get cleaner results, windowing can help as well a averaging multiple data sets. In some cases when the spectrum is particularly noisy wavelet transforms are helpful


RE: Re: FFT - Defining Frequency Axis

Always start with a trivial synthesised signal, it's the only way.  When you're happy that the magnitude vs frequency plot is what you expect, then introduce real data.

- Steve

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