Why ST BLE Sensor_v4.9.0 app cannot choose 4096 FFT size, only 1024 FFT size?

[JTam.1]

I want to use ST BLE Sensor_v4.9.0 app to do FFT from STEVAL-STWINKT1 MCU board with built-in vibration sensor IIS3DWB. However, I cannot select 4096 FFT size, only 1024 FFT size. Why? Please advise. By the way, the maximum sensor output data rate (Hz) is 6660 Hz only, not 26667 Hz. Why? Thanks!

[Eleon BORLINI · Accepted solution]

Hi @JTam.1​ ,

I double-checked with our experts and they confirm me that the max FFT size that can be configured is 1024 both for an implementation choice that fir a band limitation in the transmission.

Hope this answers to your question, even if it doesn't matche your desires.

-Eleon

[JTam.1]

I set Sensor Output Data Rate (ODR) 6660 Hz and FFT size 1024. The delta frequency is 6.59 Hz. How to get the 6.59 Hz by calculation? Pls advise. Thanks!

[Eleon BORLINI]

Hi @JTam.1​ ,

you have to divide the ODR value for the number of FFT point: 6660Hz / 1024. In this way you will theoretically get almost 6.5Hz, which is little different from the 6.59Hz probably due to some windowing effect.

-Eleon