spectral jumps at 2^n channels (PFB Underflows)



    The spectral jumps at 2^n channels are caused by the aggressive shifting in the butterfly stages. This shifting is required because of the lband radars. This shifting can cause underflows in the fft butterflys which end up causing the spectral jumps and 2^n channel.

 The pfb overflows:

   The PFB  (polyphase filter bank) overflow errors occur in the butterfly stages of the fft. Each butterfly stage does a multiply and add. This causes  the size of the  number to grow:
    The fft computation starts with a 12 bit number stored in the uppermost 12 bits of the 18 bit fpga registers. The PSHIFT parameter tells the  fpga on which butterfly stages a downshift is needed:

Underflows in the butterflys.

The problem with the aggressive shifting is that we have seen steps  in the final spectrum. Datasets showing the steps in the spectra:.


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