I posted this to the USRP users list since we're not using GNU Radio proper for this project, but Devin's questions about spectrum sensing make me think it could be useful for GNU USRP users, too. Although we did this on the N210, it might be possible to do something similar on the USRP2.
We've customized an FPGA image for the N210 that lets us grab batches of 256K complex samples (one SRAM full) at 100 Msps with no decimation, scaling, or downconversion. It works by filling the SRAM with samples at full rate, then dribbling them out the Ethernet in the normal fashion. I posted the ISE project, Verilog code, and a built image at https://public.me.com/mepard. You'll also find some examples of visualizing the spectrum data there. I'm particularly fond of the movie. Some things you should know: * It cannot be used to transmit. Transmit support used the SRAM, which I needed to buffer the received samples. * It works with the current UHD firmware and host code. * When you issue_stream_cmd, request no more than 2^18 samples, the size of the SRAM. You can issue another stream command later to get another batch, but you can't get more than 256K contiguous samples. * The rx sample rate is ignored. For good measure, I set it to the usual maximum of 25 Msps. * Since there's no downconversion, you'll only get whatever center frequencies the daughterboard supports and you can't use LO offsets. The uhd_streamer.cpp app, also posted, is what we've used to do our testing. Enjoy. -Marc
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