On Mon, Aug 26, 2024 at 10:24 AM Marcus D. Leech <patchvonbr...@gmail.com> wrote:
> On 26/08/2024 10:21, Rob Kossler via USRP-users wrote: > > Hi Olo, > On one point regarding an FFT length of 8192, there is likely an issue > with using the Ettus FFT block. In the past (I haven't checked recently), > this block was limited to a maximum FFT size of 1024 because the entire FFT > had to fit in one packet where the maximum packet payload was about 2000 > samples. It is possible to use larger FFTs, but this requires some custom > code that divorces the FFT size from the packet size. > Rob > > My understanding is that in recent RFNoC, the limit has been raised to > 2048: > > > https://files.ettus.com/manual/classuhd_1_1rfnoc_1_1fft__block__control.html > > The xci file <https://github.com/EttusResearch/uhd/blob/master/fpga/usrp3/lib/ip/axi_fft/axi_fft.xci> still shows a transform length of 1024. Also, I think that the X300 MTU size is 10 which implies 2^10=1024 x 64bit is the max payload. This implies 2048 32-bit words in the payload. But, because of a few bytes of header, it is not possible to use an FFT of length 2048 unless the FFT length is divorced from the packet length. Rob
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