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origin of angles and gain amplitude relationship between FFT

jose pedrero
Prodigy 10 points 
Hi to all , 

I have some doubts about the FFT using ADC values .

- I want to know , when the FFT is calculated and returned vector magnitude and phase , the relationship between the amplitude of the input signal and the 
amplitude of the output signal vector magnitude.

- And my other question is , in the phase vector, which is the angles origin ? because I have problems in the calculation of total harmonic distortion and reactive power , active power and power factor . 

I set the ADC mode dual sample and hold , to acquire voltage and current simultaneously , and when assigned to the tension is the reference , the calcul is not  correct, because the phase that brings me to the position of the fundamental frequency  is  -1.64 rad and should be 0 or 2pi. 

The FFT  takes as input 64 values ​​, the fundamental frequency of the input signal is 50Hz and the sampling frequency that is 3K2 Hz . 

Thank you. hope to answer
  • 0
    TI__Mastermind 20850 points

    Hi Jose, 

    Which DSP library are you using-the fixed-point or FPU library? For a 50Hz fundamental, you should see a peak in the (50*64/300 ~ 11) 11th bin. Since the frequency spacing, i.e. Fs/N =  300/64 = 4.6875 Hz/bin you wont get an exact match for 50Hz. You will see a peak in the 11th bin, and probably leakage in the 10th bin as well. 

    I believe we normalize the FFT by dividing by N, so you should see the amplitude preserved, but again you will see leakage for this specific case.

    Im not really sure i understand about the phase part. you expect it to be 0 or 2pi at the fundamental, i.e. purely imaginary in the 11th bin?