FFT x-axis from samples to frequency

hi @ all,

somehow I don't get proper frequency, if I convert the samples to frequency.

The Data which are stored in the output vector were sampled with 320 MHz.

FS = 32000000; %Sample rate

NFFT = length(output); % Output= vector with samples

Y = fft(output,NFFT);

F = FS*(0:NFFT/2-1); % Convert x-axis samples to Frequency

magnitudeY = abs(Y); % Magnitude

dB_mag=mag2db(magnitudeY);

subplot(2,2,3);plot(F(1:NFFT/2),dB_mag(1:NFFT/2));title('Magnitude');

ylabel('Magnitude(dB)');

Answer 1

[Star Strider]

Try this instead —

Y = fft(output,NFFT)/numel(output);  
F = (FS/2)*(0:NFFT/2-1);   % Convert x-axis samples to Frequency 

Since you are calculating a one-sided fft, the highest uniquely identifiable frequency is the Nyquist frequency, one-half of the sampling frequency. It is also necessary to normalise the fft result by the length of the array being processed, then multiply that result by 2 (that I did not do here) to approximate the original signal amplitudes.

.