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

[Marcus Müller]

Hi Doug,

you're right, the wire format is only used between device and host
computer, and shouldn't change the values you're seeing, so I'd actually
be curious about that screenshot. I have a suspicion that what you're
seeing might be a raised noise floor due to quantization noise becoming
increasingly relevant at 8bits, and the dip you're seeing might be the
successfull DC offset correction. If my suspicion is correct, the 0Hz
magnitude would be largely unchanged, whereas the average level around
it would be raised; is that true?

However, since this is more of a USRP related problem than a GNU Radio
topic, it might be wise to post your reply to the USRP-users mailing
list[1].

Greetings,
Marcus

[1] http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com
On 01/05/2015 08:04 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
>
>
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