I am measuring acoustic emission signals. The a.e. frequency range is 20kHz to 1 MHz. The sampling frequency that I,set was 100kHz Samples per second. I must be able to detect a maximum of 50 kHz signal using it. But when I do fft I get a maximum of only 300Hz.

Can anyone tell me where I would have gone wrong?

  • 5
    $\begingroup$ Could you please use scientific/powers of 10 notation for your frequencies, life is too short to waste it counting zeros in numbers. $\endgroup$ – Conrad Turner Feb 15 '15 at 9:15
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    $\begingroup$ or units MHz and kHz, or grouping $\endgroup$ – Olli Niemitalo Apr 16 '15 at 12:09

How are you determining your maximum frequency?

If you sample at $f_s$ the frequency resolution (the frequency difference between frequency components) of your FFT is $f_s/N$, where $N$ is the number of samples collected. And the maximum resolvable frequency is the Nyquist frequency corresponding to the sample rate. For more information see here

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