On FPGAs, ~million channel spectrometers are typically built by doing a 1024ch PFB, followed by a matrix transpose (to collect 1024 samples of a single channel together) and another FFT. Mostly, you'll find you run out of memory if you try to do a million-channel FFT directly on an FPGA.
There are implications for this 2-D approach, though. If you use a critically-sampled filterbank in the first stage, your channels cross-over at -6dB, and so if you input a signal near this intersection point, you'll get some fraction of the power in each of the two adjacent channels. After you then do a second FFT on these two channels, you get a spectrum that has some power in one channel, and also power in a channel 1024 channels later. So it now looks like you've got two CW signals going in, 1024 channels apart, even if you only actually had one CW tone. There was a million channel spectrometer built this way for a ROACH1 tutorial at the 2009 CASPER workshop. That used to live on the CASPER SVN server, but is no longer maintained. On a ROACH2, with about 1GHz bandwidth, I'd expect you might possibly achieve something closer to 16M channels, if you wanted (4k by 4k). No doubt this won't work in modern Simulink toolflows, but it might be a useful reference... try here: https://casper.ssl.berkeley.edu/svn/trunk/projects/roach_mspec/ Jason Manley DSP Manager SKA-SA Cell: +27 82 662 7726 Work: +27 21 506 7300 > On 10 Mar 2020, at 08:23, James Smith <[email protected]> wrote: > > Hello Molly, > > We have done something broadly similar using ROACH2 and katADC, but only 1024 > channels: > https://github.com/ska-sa/AVNRoachGateware/tree/master/WideBandSpectrometer > > What frequency are you looking at? The only thing I'm thinking is that you > might struggle to get the design to meet timing if you're clocking the board > too fast. > > Shout if you get stuck. > > Regards, > James > > > On Mon, Mar 9, 2020 at 8:34 PM Molly Smith <[email protected]> wrote: > Hello, > > I'm new to CASPER and am trying to implement a million channel spectrometer > on ROACH2 with the katADC. Does anyone know of any useful resources or has > done something similar? > > I have experience using Vivado for newer FPGA development and am comfortable > with Verilog & VHDL. I'm not familiar with the Simulink/ISE workflow and have > been relying on the tutorials to come up to speed. > > I'm currently trying to modify Tutorial 3 (wideband spectrometer) to use the > katADC instead. > > Thanks, > Molly > > -- > 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/ad7c8afb-5775-401a-b53f-e769ba5d53e1%40lists.berkeley.edu. > > -- > 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/CAG67D35z5XjBi4mu1s8e7EqCKnnV3NDf7r4UUT%2BQHr9LfF6EkQ%40mail.gmail.com. -- 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/8C7473F8-95E1-4A6D-A2E5-E7A4A314EE3D%40ska.ac.za.
