[USRP-users] 8-bit over the wire format and noise floor

Knee, Peter A paknee at sandia.gov
Tue Jun 12 13:08:39 EDT 2012


I am currently developing a spectrum analyzer using my USRP N210.  I want to be able to display larger bandwidths so I have been experimenting with the 8-bit over the wire mode.  I'm having some issues with the ADC counts that the device is returning.  To test I am using a signal generator outputting a constant tone.

When the tone is not present, at startup, I get fairly large ADC values that quickly go to zero.  As I'm computing the FFT of these values, an all-zero sequence is causing issues with the values returned by the FFT.  There are obvious ways around this but I have a feeling it is part of a larger issue.

When the tone is present, being generated anywhere from -50 to -20 dBm, I begin seeing larger values over the wire, but they are not scaling with the increase in power output.  After the initial startup sequence, the values I'm seeing for both the I and Q channels are no larger than +/- 20 on average.

I'm attaching the basics of the C++ code I'm using to acquire the signal in 8-bit mode.  My guess is that this issue is a lack of understanding about the basics of the receiver and the ADC conversion process.  I should not that everything seems to scale accordingly when using 16-bit over the wire mode.

Any help that can be provided would be greatly appreciated.

Peter Knee


uhd::rx_metadata_t md;
uhd::rx_streamer::sptr rx_stream;

//Setup device
uhd::stream_args_t stream_args("sc16","sc8");
rx_stream = usrp->get_rx_stream(stream_args);
uhd::stream_cmd_t stream_cmd(uhd::stream_cmd_t::STREAM_MODE_START_CONTINUOUS);

//Set up buffer
std::vector< std::complex<short> > buff(10000);

//Read in short data
nSampsRead = rx_stream->recv(&buff.front(),buff.size(),md);

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