Re: [Discuss-gnuradio] uhd_fft differences with different wire-format

[Marcus D. Leech]

On 01/05/2015 02:04 PM, Anderson, Douglas J. wrote:
> Hi all,
>
> I'm trying to understand the impact that changing the wire-format of a USRP has 
> on the uhd_fft script provided by GNURadio.
>
> Using UHD_003.008.001-42-g8c87a524 and GNURadio built by pybomb a few weeks ago 
> (3427a667c).
>
> On a USRP N210 with 50 Ohm load, I ran
> uhd_fft --wire-format=sc8 -s 25M -f 700M
> uhd_fft --wire-format=sc16 -s 25M -f 700M
>
> With sc16, I get a mostly flat FFT with slight roll-off toward the ends, as I'd 
> expect.
>
> With sc8, I get a dip at the center frequency that increased by about 10dB on 
> either side as it moves out from the center freq.
>
> The way I understood it is that wire-format is just the format used over the 
> ethernet cable, but that in both cases the internet representation in GNURadio 
> is unchanged (I used fc32).
>
> Could someone help me understand why the wire format affects the FFT shape in 
> such a strange way?
>
> I can attach screenshots if my description was unclear.
>
> Thanks!
> -Doug
Try specifying a stream-arg of "peak=0.1" and see if that changes your 
results.

Conceptually, there are 16-bit values (derived, on the N2xx from a 
14-bit ADC) coming out of the back of the decimation filters.  That then 
has to be
  mapped into an 8-bit value to be carried over the wire if you're 
using sc8.  That's a loss of 48dB of dynamic range.  Which means, in 
order to get
  good results, you have to tell it which part of the dynamic range 
you're interested in, and the "peak=" stream-arg is the way to do this.

See:

http://files.ettus.com/manual/structuhd_1_1stream__args__t.html#a4463f2eec2cc7ee70f84baacbb26e1ef




--
Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org