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

From: Marcus D. Leech
Subject: Re: [Discuss-gnuradio] uhd_fft differences with different wire-format
Date: Mon, 05 Jan 2015 14:39:23 -0500
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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 

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 

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.


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.



Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium

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