Hi Oliver, haven't quite thought through an understanding of what you're currently doing, but:
Since the FPGA image already comes with a convenient-to-use DUC – why not just produce a constant, and tune digitally to a non-zero frequency? That way, the CORDICs inside the duc_chain will simply generate your sine for you, presumably at a much higher resolution than your 1024 element sine table, for free. To test: use your PC to send a constant, use a uhd.tune_request_t(LO_frequency+tone_frequency, tone_frequency). If there's still a LO leakage on the output, you can manually fiddle with the IQ corrections inside the AD9361. Best regards, Marcus On 12.10.2017 02:23, Oliver Wayne via USRP-users wrote: > Hello, > > I've implemented a DDS signal on the USRP device. In radio_legacy.v, I > input get_tx_I and get_tx_Q as my I and Q channels. > always @(posedge radio_clk) begin tx[31:16] <= (run_tx) ? > get_tx_I[31:16] : tx_idle[31:16]; // I channel tx[15:0] <= (run_tx) ? > get_tx_Q[31:16] : tx_idle[15:0]; // Q channel end These are 32 bit > registers and I calculate the value of get_tx_I and get_tx_Q in > external modules. Currently I use 10 bit registers in my sine lookup > table, so something like > > sine[0] = 340; > sine[1] = 342; > ... > And I sum several of these sine terms, then set get_tx_I(Q) equal to > their sum. The problem is this has a DC offset, which means that that > the local oscillator signal also shows up in my spectrum, which I > don't want. When I try to kill the DC offset, so I write > > sine[0] = 0; > sine[1] = 1; > ... > > Then I still get a local oscillator term, but also get garbage in the > spectrum. What's the optimal way to remove a DC offset? I was not sure > whether I should explicitly write out the negative value as two's > complement in the lookup table, or use something > like > https://files.ettus.com/manual/classuhd_1_1usrp_1_1multi__usrp.html#a263ab7f0364c03e8a6e330c546769e4f > <https://files.ettus.com/manual/classuhd_1_1usrp_1_1multi__usrp.html#a263ab7f0364c03e8a6e330c546769e4f>. > > thanks > > > _______________________________________________ > USRP-users mailing list > USRP-users@lists.ettus.com > http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com
_______________________________________________ USRP-users mailing list USRP-users@lists.ettus.com http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com
