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arm_cfft returns improper fft | Cypress Semiconductor

arm_cfft returns improper fft

Summary: 1 Reply, Latest post by SmartPSoC on 27 Sep 2015 10:50 PM PDT
Verified Answers: 0
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scarlson's picture
61 posts

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.





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,




SmartPSoC's picture
Cypress Employee
68 posts

Hello Scarlson,

Did you resolve the issue? If not, please create a case with the project attached.

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