On Thu, May 26, 2022 at 10:43 AM <luca.vi...@argotecgroup.com> wrote:
> Hi all! > > > I am starting to look through some of the FPGA code of the USRP X300 in > order to understand which is the DDC chain configuration in the default > image. > > > I have understood that in the DDC chain there is 1 CIC filter + 3 Halfband > filters. Since I want to characterize the DDC chain I have the following > questions: > > 1. > > How are they used? I suppose that the halfband filters are used based > on the decimation factor we need (max. 1024) > 2. > > Which is the order of the CIC filter? > 3. > > How many taps each halfband filters have? Which are the taps? > > Taking a look at the ddc.v file is the best insight. The cic_decimate has an N which is the order it's given: https://github.com/EttusResearch/uhd/blob/5333d3d12ffc21229ec4203a9ea1c7f68d82e57f/fpga/usrp3/lib/rfnoc/ddc.v#L283 Here it's listed as 4. In general, the CIC is used for bulk decimation up to 2, 4, or 8x oversampled - ideally 8x. The hbdec1 is the first, hbdec2 is the next, and hbdec3 is the last. Their coefficients can be found here: https://github.com/EttusResearch/uhd/blob/5333d3d12ffc21229ec4203a9ea1c7f68d82e57f/fpga/usrp3/top/x300/coregen_dsp/hbdec1.mif https://github.com/EttusResearch/uhd/blob/5333d3d12ffc21229ec4203a9ea1c7f68d82e57f/fpga/usrp3/top/x300/coregen_dsp/hbdec2.mif https://github.com/EttusResearch/uhd/blob/5333d3d12ffc21229ec4203a9ea1c7f68d82e57f/fpga/usrp3/top/x300/coregen_dsp/hbdec3.mif If the desired decimation rate is divisible by 8, then all 3 halfbands are used. If not, a division by 4 is checked, and 2 are used. If not, a division by 2 is checked and 1 is used. If not, only the CIC is used. If you want to test your model against the HDL itself, a testbench is located here: https://github.com/EttusResearch/uhd/blob/5333d3d12ffc21229ec4203a9ea1c7f68d82e57f/fpga/usrp3/lib/rfnoc/blocks/rfnoc_block_ddc/rfnoc_block_ddc_tb.sv But it's probably easier and much faster to pass samples through the actual RFNoC block in a custom graph that just exercises the DDC. Brian
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