FFT bins size and averaging

Started by pointblanke 4 years ago3 replieslatest reply 4 years ago258 views


I have a signal coming from an ADC that I oversample with an fs=80kHz and then I subsequently average 8 samples to get a rate of 10ks/s.

I feed this averaged samples to my fft with N=1024.

Would the bin size remain fs/N = 80k/1024 = 78.125Hz?

Or would the averaging affect the bin size? i.e. 10k/1024 = 9.765625Hz


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Reply by SlartibartfastJuly 26, 2019

Do you reduce the sample rate when you average?   In other words, when you average eight samples, is it a sliding average (i.e., a FIR filter) where you output one sample for each input sample, or do you decimate (reduce) the sample rate at the same time?

The FFT bin spacing will be proportional to the input *sample* rate, so the answer to the above question will provide the answer to your question.

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Reply by Tim77July 26, 2019

If the input to the FFT is in fact 10 k/s then the 80k/s is being decimated. Given that a sample rate of 10k/s supports the bandwidth of the analog signal averaging won’t buy you anything so you can simplify.

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Reply by pointblankeJuly 26, 2019

thanks for the reply.

No, it's not a sliding average. Every 8 samples I output one sample. then I get 8 new samples and I average those to get another output sample. and so on. so I am decimating.