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AWR1243: How to understand IF position in the positive/negative frequency domain?

Adam Yao94020
Genius 10910 points
Part Number: AWR1243

Hi champion,
   Could you help to explain how the IF frequency f_IF deterimind by the received signal frequency f_R and LO frequency f_LO?
   Is f_IF = f_R - f_LO, or f_IF = f_LO - f_R?
   If the slope of the chirp is positive, f_R < f_LO, if the slope of the chirp is negative, f_R > f_LO. How can DFE map IF of both cases into positive frequency domain?

Thanks,
Adam

  • 0
    TI__Guru 50240 points
    Hi Adam,

    I'm not so sure about this, I will have to talk to AWR team and have an answer for you sometime on Monday.


    Cheers,
    Akash
  • 0 in reply to Akash Gondalia
    TI__Expert 4720 points
    Hi Adam,

    The formula 'f_IF = f_LO - f_R' is correct. If the slope of the chirp is negative, f_R > f_LO, then the IF frequencies of targets are negative. In the complex2X mode, the DFE configuration doesn't change between positive and negative slopes. In the complex 1X mode, if the slope is negative, the frequencies from 0 to f_IFmax are filtered out. If the slope is positive, frequencies from -f_IFmax to 0 are filtered out.

    Regards
    Anil
  • 0 in reply to Anil Mani
    TI__Genius 10910 points
    Hi Anil,
    Thanks a lot for your reply.
    An further question is for negitive slope case, do have have further steps besides filtering frequencies from 0 to F_IFmax. Since the near object locates in negative frequency near DC, and far object locates in higher negative frequency. If we do FFT on IF signals, the zero bin corresponds to the far object, and higher bin corresponds to the near object. Is this correct? If yes, we need a software step to handle the mapping between range bin indxe and object distance.

    Thanks,
    Adam
  • 0 in reply to Adam Yao94020
    TI__Expert 4720 points

    Hi Adam, 

    You are correct. If the slope is negative, you need to reverse the spectrum to view it correctly . When reversing the spectrum, use the following matlab equation, 

    chirp_data = chirp_data([1 end:-1:2]); 

    Essentially, the 'bin zero' remains where it is, and the remaining bins are reversed about 'bin zero'.

    Regards
    Anil

  • 0 in reply to Anil Mani
    TI__Genius 10910 points
    Hi Anil,
    Thanks a lot for your detailed explanation. I am clear now.

    Best regards,
    Adam