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CCS/TMS320F28335: How to read the frequency scale of spectrum from FFT result.

Chen Yan-Li
Intellectual 395 points

Part Number: TMS320F28335

Tool/software: Code Composer Studio

Hi,

I'm using Code Composer Studio(CCS) 5.3.0.00090
And I find a example project called "F2833x Real FFT with ADC Input (Real-time)" from controlSUITE
Here are the example file(s) for this project:「C:\ti\controlSUITE\libs\dsp\FPU\v120\examples_ccsv4\2833x_RFFT_ADC_RT」

My question is, how to read the ""FFT result " from TI example project.

I mean, When the FFT operation is complete, I can get the input signal's Amplitude and Phase spectrum.
I succeeded getting the spectrum, but I don't know how to read it. The information about FFT from program description is too little.

The FFT project default setting has the information about sampling rate is 48kHz and FFT_Size is 512.
So, What is the relationship between the both information and the spectrum from FFT?

After FFT calculation, I can get 256 result each Amplitude and phase, And what does these result represent's frequency ? 
For example, the Amplitude spectrum has 256 value, the x represent 256 result. 
When x is 0 is mean DC's Magnitude, When x is 1 mean "some frequency" Magnitude, ....., When x is 255 mean "some frequency".
I have 255 different Magnitude representing different frequency. That is the Amplitude spectrum I want, but I didn't know frequency scale.

When x is 1 the frequency mean 5Hz? When x is 2 the frequency mean 10Hz? ...., When x is 255 the frequency mean 1275Hz? I don't know.

So, is there any way to figure it out frequency of "some frequency"?


Thanks for your help!

Best Regards, Chen Yan-Li

  • 0
    TI__Mastermind 43730 points
    Chen,

    the library is well documented, the frequency points are dependent on sampling frequency and FFT length.

    For example, if you sample the signal at 50KHz and then do a 512 point FFT, then you get 256 frequency bins.

    Which are evenly split with 25KHz, i.e. first value corresponds to zero frequency, the next to 25Khz/256, the next after thant 25KHz/256 *2 and so forth..
  • 0 in reply to Manish Bhardwaj
    Intellectual 395 points
    Manish,

    Thank you! It's really help me a lot!

    Could you give me another help? I have some questions about changing the sampling frequency and FFT length.

    I want to use Fourier analysis to find the amplitude and phase of both 60Hz and 75Hz, frequency scale is 5 Hz.
    Project default settings sampling rate is 48KHz and 512 point FFT length cannot be used, 24KHz/256 even not integer.
    So, If I can sample the signal at 20480Hz and then do a 4096 point FFT, it's will be fine.

    Changing the sampling frequency part:
    Program description say "The signal is sampled by epwm module and sampling frequency is defined in Lab.h".
    I find the sampling frequency is defined as
    "defineADC_SAMPLE_PERIOD 3124 // 3124 = (3125-1) = 48 KHz sampling w/ 150 MHz SYSCLKOUT".
    It looks like the sampling frequency can only be set to a factor of 150M,
    Is that mean 150M divided by the sampling frequency must be an integer?
    May I set like this "7323.21875 = (7324.21875-1) = 20480Hz sampling w/ 150 MHz SYSCLKOUT"?
    Is there any problem with this setting?(Didn't show any error after compile)

    Changing the FFT length part:
    Program description say "This program shows how to compute a real FFT with 12-bit real-time ADC input and...".
    So, that mean the FFT length can up to 4096 points(12-bit), right?
    After I changed FFT_REAL_STAGES from 9 to 12(This program also defined in Lab.h),
    An error occurred "error: errors encountered during linking; "2833x_RFFT_ADC_RT.out" not built".
    Is there anything wrong with it?


    Thanks for your help!

    Best Regards, Chen Yan-Li