Currently I am working on one project that includes 24 Bit external MCP3919 and MCP3914 ADCs and Tiva C series 32 Bit MCU. I am trying to calculate harmonic values of input signal.
I am using CMSIS DSP Library in Code Composer Studio. FFT functions are working great I have no problem with implementation of these functions. I tested them with internal generated sin signal and modulated it. Results are expected.
- ADC_Output : 3255 Hz
- FFT_Size : 64 point
- Window_Size : 2*FFT_Size
I have a problem that when I read real world signal from signal generator, my FFT bins are changing with time in n*50Hz signals. When I increase the input signal to 50.9 Hz there is no problem. It is same with 150Hz, 250Hz, 350Hz. It seems like there is fixed frequency error related with 50Hz.
For example : FFT functions work great with 50.9 Hz signal instead of 50Hz (1*0.9Hz) 152.7 Hz signal instead of 150 Hz ( 3*0.9Hz) 254.5 Hz signal instead of 250 Hz (5*0.9Hz)
**Fig1 : 50.0 Hz and 250.0Hz**
**Fig2 : 50.9 Hz and 254.5Hz**
As a result, with frequency changing FFT bins are true. I need your experiences with this issue.
Thanks in advance