[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.
rx_stream = usrp->get_rx_stream(stream_args);
//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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