How to know about FFT bin frequencies

Hi guys Im not used to computing fft on matlab and I have some doubts so if you could tell me if im right or wrong... My signal is h : N = 16384 Sampling frequency = 65536Hz

if I do : fft(h,65536) I should have a frequency step of 1Hz, right???? Like first bin stands for 0Hz, second for 1 Hz...

I would like to compute a FFT in the frequency range [0 - 6000 Hz] but with a 1 Hz frequency step. Could you help me please??

I would be very thankful for all answers and explainations ! Bye

Answer 1

[Star Strider]

The easiest way to find out is to do the experiment:

Fs = 65536;                                     % Sampling Frequency (Hz)
Ts = 1/Fs;                                      % Sampling Interval (sec)
Fn = Fs/2;                                      % Nyquist Frequency (Hz)
L  = 16384;                                     % Signal Length (samples)
t = [0:(L-1)]*Ts;                               % Time Vector (sec)
s = cos(2*pi*100*t);                            % Signal (100 Hz)

FTs = fft(s,Fs)/L;                              % Experiment #1
Fv = linspace(0, 1, fix(Fs/2)+1)*Fn;
Iv = 1:length(Fv);

[PeakFTs,Idx] = max(abs(FTs(Iv))*2);            % Find MAximum and Index

figure(1)
plot(Fv, abs(FTs(Iv))*2)
axis([0  500    ylim])
grid
text(Fv(Idx)+10, PeakFTs, sprintf('Peak at %.3f Hz, Index %d', Fv(Idx), Idx), 'HorizontalAlignment','left', 'VerticalAlignment','top')

Answer 2

[Adam]

A 65536-pt FFT would, in your case give 1Hz bins for a sample frequency of 65536 Hz.

If you only want the FFT from 0 to 6000 then just clip the result of the FFT to

y = fft( h, 65536 );
y = y(1:6001);

and this should give you the frequencies from 0 to 6000 at 1Hz intervals.

Answer 3

[johnmay]

Thank you a lot guys!!!

Answer 4

[himanshu tripathi]

Answer 5

[himanshu tripathi]

Please have a look at the code and plot of 1kHz frequency (file attached here).

There is a problem, why the reversed V-shaped plot is obtained in the frequency domain.

Note: Sample 1kHz signal data is attached.

And why Fv = linspace(0,1,fix(LF/2)+1)*Fn;

is used in the fft analysis and what is the concept of the nyquist frequency.