[USRP-users] Removing DC offset on USRP B200

Marcus Müller marcus.mueller at ettus.com
Sat Oct 14 08:30:59 EDT 2017


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
>
>
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