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

mleech at ripnet.com mleech at ripnet.com
Tue Jun 12 13:15:45 EDT 2012


 

On 12 Jun 2012 13:08, Knee, Peter A wrote: 

> All, 
> 
> 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. 
> 
> Thanks, 
> 
> Peter
Knee 
> 
> CODE: 
> 
> //Variables 
> 
> 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); 
> 
>
usrp->issue_stream_cmd(stream_cmd); 
> 
> //Set up buffer 
> 
>
std::vector< std::complex > buff(10000); 
> 
> //Read in short data 
>

> nSampsRead = rx_stream->recv(&buff.front(),buff.size(),md);

They're
proportional to the instantaneous voltage seen by the ADC. Power is
proportional to the square-root of the voltage. Does that make more
sense now? 

-Marcus 

 
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.ettus.com/pipermail/usrp-users_lists.ettus.com/attachments/20120612/8ecc4531/attachment-0002.html>


More information about the USRP-users mailing list