Nikhil*, sorry! On Fri, Feb 4, 2022 at 9:08 AM Morag Brown <[email protected]> wrote:
> Hey Nikhail, > > The FFT is a Hermitian function, which means that it has the property: > > [image: Screenshot from 2022-02-04 08-34-24.png] > > This principle is used in the real wideband FFT to compute 2 real FFTs > using one complex FFT core - this > <http://www.hyperdynelabs.com/dspdude/papers/COMPUTING%20THE%20FFT%20OF%20TWO%20REAL%20SIGNALS%20USING%20A%20SINGLE%20FFT.pdf> > paper > explains it well. For a detailed explanation on how the CASPER FFT works > specifically, Ryan Monroe's paper on Improving the Performance and Resource > Utilization of the CASPER FFT and Polyphase FIlterbank goes into quite a > lot of detail. > > You only get 32 output channels because the CASPER wideband FFT discards > all negative frequency components as they're just a mirror of positive > frequency components and aren't needed. > > Morag > > On Fri, Feb 4, 2022 at 7:10 AM Nikhil Mahajan <[email protected]> > wrote: > >> Dear CASPERites, >> >> I am a graduate student at the University of Toronto (working with Marten >> van Kerkwijk) and I have some raw baseband data collected using PUPPI >> (Arecibo) - and I am on a quest to invert the polyphase filter bank. I have >> 32 channels of complex-baseband that I would very much like to combine into >> a single 100 MHz bandwidth stream. >> >> To do this, I would need to understand some of the specifics of the >> filter bank pipeline (so that I can successfully invert each step). This is >> my current understanding of what happened to the data I have: >> >> 1. Real-valued data sampled at 200 MS/s arrives at the Casper BEE2 board. >> 2. This goes through a real-input PFB implementation such as >> `pfb_fir_real` and using a 12-tap, 64-branch polyphase filter (I have the >> filter coefficients that were used here). This step outputs 64 streams of >> real-valued data. >> 3. Then, for the DFT step of the filterbank, the 64 real-valued streams >> are passed through the `fft_wideband_real` block to get 32 channels of >> complex-valued data. >> 4. This is then saved to disk. >> >> (I hope someone familiar with PUPPI can correct me here if I am wrong >> about any of the above) >> >> Step 3 is the step I am confused about. `fft_wideband_real` does not >> appear to be a conventional real-input N-point FFT implementation (Else I >> would have N/2 + 1 channels instead of just N/2). Some documentation on >> this block says that it "computes the real-sampled Fast Fourier >> Transform using the standard Hermitian conjugation trick". What is this >> standard Hermitian conjugation trick? I am totally unfamiliar with this. >> Would I be wrong in guessing it uses some sort of trick to convert 64 real >> numbers to 32 complex numbers and then applies a regular ol' complex-valued >> FFT on them? >> >> Thank you so much! I appreciate any and all guidance this mailing list >> can provide. >> >> Cheers, >> Nikhil Mahajan >> >> -- >> You received this message because you are subscribed to the Google Groups >> "[email protected]" group. >> To unsubscribe from this group and stop receiving emails from it, send an >> email to [email protected]. >> To view this discussion on the web visit >> https://groups.google.com/a/lists.berkeley.edu/d/msgid/casper/CAA39X0%2B3z051d%2B%2BAB0D%3D1LHNHdOSDJp6JEYzzRXatj-ij11_qA%40mail.gmail.com >> <https://groups.google.com/a/lists.berkeley.edu/d/msgid/casper/CAA39X0%2B3z051d%2B%2BAB0D%3D1LHNHdOSDJp6JEYzzRXatj-ij11_qA%40mail.gmail.com?utm_medium=email&utm_source=footer> >> . >> > -- You received this message because you are subscribed to the Google Groups "[email protected]" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web visit https://groups.google.com/a/lists.berkeley.edu/d/msgid/casper/CAGH-0Tdawu4HVd%2BSiakUJLn4gBOZeP1KidP0qqJ4sjW_bRXMNw%40mail.gmail.com.
