×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Are you an
Engineering professional?
Join Eng-Tips Forums!
• Talk With Other Members
• Be Notified Of Responses
• Keyword Search
Favorite Forums
• Automated Signatures
• Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

#### Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

# Re: FFT - Defining Frequency Axis

## Re: FFT - Defining Frequency Axis

(OP)
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)');
ylabel('Magnitude');
%grid on

Thanks heaps,
SpaceEngineer

### 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'.

Cheers

Greg Locock

New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies  http://eng-tips.com/market.cfm?

### 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

#### Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

#### Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Close Box

# Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

• Talk To Other Members
• Notification Of Responses To Questions
• Favorite Forums One Click Access
• Keyword Search Of All Posts, And More...

Register now while it's still free!