[USRP-users] packet transmission question

Ian Buckley ianb at ionconcepts.com
Thu Sep 19 20:36:54 EDT 2013


All the logic I mentioned is digital and on the base USRP system, so independent of Daughter board choice and constant for any given sample rate. Clearly there is some propagation delay of the signal in the analogue domain from DAC to antenna, but I've no wisdom to offer that helps quantify that.
-Ian



On Sep 19, 2013, at 5:28 PM, fquitin <fquitin at ulb.ac.be> wrote:

> Hi Ian,
> 
> Thanks a lot for the info. So, for a given sample rate (and a given daughterboard model), this delay will be constant?
> 
> Thanks,
> Francois
> 
> 
> On 19.09.2013 17:52, Ian Buckley wrote:
>> 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
>>> <rx_packet.png>_______________________________________________
>>> USRP-users mailing list
>>> USRP-users at lists.ettus.com
>>> http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com [1]
>> Links:
>> ------
>> [1] http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com





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