1 Reply Latest reply on Sep 27, 2015 10:50 PM by resh

    arm_cfft returns improper fft

    scott.carlson

      Hi all,

         

      So I've been working on this small project here and there for awhile and I started taking a more in depth look at some of the results I was getting, and I realized that the arm_cfft_q15.c wasn't returning what it should be.

         

       

         

      If I had

         

       

         

              q15_t ADC_samples[2048]

         

              q15_t mag_of_fft [1024]

         

       

         

      and ADC_samples where samples of a 1kHz wave sampled at 10Khz you would expect the resulting peaks to be at 1/5 pi and 9/5 pi.

         

      when I do

         

           

         

              arm_cfft_q15(&arm_cfft_SR_length1024, ADC_samples, 0,1);

         

       

         

      which basically says compute the 1024 pt fft of ADC_samples, with the inverse fft bit false and the bit reversal bit true.

         

      and

         

       

         

              arm_cmplx_mag_q15(ADC_samples,mag_of_fft,1024);

         

       

         

      which says compute the magnitude of the 1024 pt fft in ADC_samples

         

       

         

      I get peaks at 2/5pi and at 8/5 pi?

         

       

         

      I have confirmed my results in matlab as well.  I have checked my ADC sampling, and just about everything I can think of.

         

      I have also tried the example, but unfortunately they don't tell you the sampling rate and with the white noise they added in to their values it becomes pointless.

         

       

         

      what am I doing wrong?  I can't imagine arm has got it wrong.  as my use is only for the relativity of results to each other and I can always just increase my ADC's clock, this isn't slowing me down, but it is very concerning that I may be mis-interpreting how one of these functions handles things.

         

       

         

      Any help is greatly appreciated,

         

      scarlson