[USRP-users] packet transmission question

Ian Buckley ianb at ionconcepts.com
Thu Sep 19 11:52:30 EDT 2013


Francois,
The logic that counts time and controls the presentation of TX data to the DAC is located at the start of the USRP DSP chain after all the transport, protocol and (de)packetizatiion logic. Between there and the DAC are the CIC filter, both Halfband filters, and the CORDIC, all of which contain some pipelining in the logic. For any given interpolation ration from your GR sample rate to the DAC 100(or 64)MHz sample rate, this logic should present a constant group delay. It should change somewhat for different interpolation ratios due to changes in the CIC configuration.
-Ian

On Sep 19, 2013, at 1:53 AM, Francois Quitin <fquitin at ulb.ac.be> wrote:

> Hi all,
>  
> We have a little question for you: we use a USRP+WBX with a code derived from tx_timed_burst.cpp to create our custom transmitter, and we use another USRP that records the received data to a file.
>  
> When we look at the received signal, we observe a slight small staircase (around 30 samples long at 10MHz = 3us), as can be seen in the attached figure. It seems that our actual transmitted packet starts after this “stair”. When we decrease the sample rate to 5MHz, the “stair” is still 30 samples long (6us this time).
>  
> As we are working in a case where timing is very critical, this 3us delay is a bit of an issue (unless it’s constant in which case we can compensate for it). Any idea where this is coming from ?
>  
> Thanks!
> Francois
>  
> --
> Francois Quitin, Ph.D.
>  
> Research Fellow
> INFINITUS - Infocomm Centre of Excellence
> School of EEE, Nanyang Technological University
> 50 Nanyang Ave, S2-B4b-05
> Singapore 639798
> Phone: +65-8502-3690
> Email: fquitin at ntu.edu.sg
>  
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