[USRP-users] packet transmission question

fquitin fquitin at ulb.ac.be
Thu Sep 19 20:28:35 EDT 2013

Hi Ian,

Thanks a lot for the info. So, for a given sample rate (and a given 
daughterboard model), this delay will be constant?


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>_______________________________________________
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