Allow Maximum frequency in FFT to be lower than the sampling frequency

I have data that is sampled at 262144Hz and I want to take its FFT by allowing only 64000Hz to be the maximum allowed frequency in the FFT. How do I do that?

[Anubhav Dubey]

I would like to add that, I need to do this to obtain FFT with higher prescision

Answer 1

[Star Strider]

The maximum frequency for a sampled signal is the Nyquist frequency, half the sampling frequency.

Here:

Fs = 262144;
Fn = Fs/2

evaluating to:

Fn =
      131072

You can use or display any part of the spectrum between 0 Hz and the Nyquist frequency. Use the appropriate subscripts or xlim arguments to do that.

[Anubhav Dubey]

I would like to add that, I am looking for increasing the prescion of the FFT I obtain. Can you help me with that?

[Star Strider]

To increase the frequency resolution, zero-pad the end of the time-domain signal. (This is usually extended as powers-of-2 for efficiency, however any length will work.) See the fft documentation section on Gaussian Pulse for an extended discussion.