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How can I extend the range of the fft result graph of x-axis?

Hi Devzone!

I'm using the fft function to check the frequency of sound from mic sensor.

All seems good but, only up to 2500hz is shown and the frequency above it is shifted to the left and when it reaches to 0, it is shifted to the right.

When I used fpu-fft, and generated sin function, the graph can show the value up to 20000hz.

How can I change the limit of x-axis?

The whole code is below,

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/**
* Copyright (c) 2014 - 2019, Nordic Semiconductor ASA
*
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without modification,
* are permitted provided that the following conditions are met:
*
* 1. Redistributions of source code must retain the above copyright notice, this
* list of conditions and the following disclaimer.
*
* 2. Redistributions in binary form, except as embedded into a Nordic
* Semiconductor ASA integrated circuit in a product or a software update for
* such product, must reproduce the above copyright notice, this list of
* conditions and the following disclaimer in the documentation and/or other
* materials provided with the distribution.
*
* 3. Neither the name of Nordic Semiconductor ASA nor the names of its
* contributors may be used to endorse or promote products derived from this
* software without specific prior written permission.
*
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