How to calculate bin frequency after FFT?

JTam.1 · ‎2021-05-04

The ODR of vibration sensor iis3dwb is 26667 Hz. If I set 2048 of FFT size, is the below calculation correct? Or we need to use the bandwidth 6 kHz to calculate the bin frequency. Please advise. Thanks!

0: 0 * 26667 / 2048 = 0.0 Hz

1: 1 * 26667 / 2048 = 13.02 Hz

2: 2 * 26667 / 2048 = 26.04 Hz

3: 3 * 26667 / 2048 = 39.06 Hz

4: ...

5: ...

1024: 1024 * 26667 / 2048 = 13333.5 Hz

niccolò · ‎2021-05-05

Hi @Javier Muñoz and @JTam.1 ,

your first comment was correct on the bin calculation, but you are right, it depends also on the actual data rate at which you poll the data.

I think that what @Eleon BORLINI wrote in his answer means that after 6 kHz the data is attenuated.

that is just an approximation, the real system could be more complicated, but in most scenarios, the approximation is good enough.

Niccolò

JTam.1 · ‎2021-05-10

If I use the full capability of iis3dwb and the FFT size has 2048, the table would be:

0: 0 * 6000 / 2048 = 0.0 Hz

1: 1 * 6000 / 2048 = 2.93 Hz

2: 2 * 6000 / 2048 = 5.86 Hz

3: 3 * 6000 / 2048 = 8.79 Hz

4: ...

5: ...

2048: 2048* 6000 / 2048 = 6 kHz

Is it correct? Please advise. Thanks!

niccolò · ‎2021-05-11

I would say that, if you are working with 26kHz ODR the right calculation is the one from @Javier Muñoz :

0: 0 * 13333,5/ 2048 = 0.0 Hz

1: 1 * 13333,5/ 2048 = 7.51 Hz

2: 2 * 13333,5/ 2048 = 13.02 Hz

3: 3 * 13333,5/ 2048 = 19.53 Hz

4: ...

5: ...

2048: 2048* 13333,5/ 2048 = 13333,5 Hz

Niccolò

JTam.1 · ‎2021-05-11

There is a Low Pass Filter (LPF2) of 6.3kHz in iis3dwb. So the output will cut off at 6.3k Hz as follow. Please advise. Thanks!

0: 0 * 13333,5/ 2048 = 0.0 Hz

1: 1 * 13333,5/ 2048 = 6.51 Hz

2: 2 * 13333,5/ 2048 = 13.02 Hz

3: 3 * 13333,5/ 2048 = 19.53 Hz

4: ...

5: ...

968: 968* 13333,5/ 2048 = 6.3 kHz

niccolò · ‎2021-05-11

Hi @JTam.1 ,

the LP filter will attenuate frequencies above 6kHz, but you still need to take them into account

Niccolò