[USRP-users] How good can USRPs with internal GPSDOs being synchronized?

Matt Ettus matt at ettus.com
Fri Sep 6 02:49:55 EDT 2013


Cheng,

How far apart are the two USRPs?  If they are close by, then you would be
better off sharing a reference rather than giving them separate GPSDOs.
 Can you have them share a reference?

Matt


On Thu, Sep 5, 2013 at 12:10 AM, Cheng Chi <ch0004hi at e.ntu.edu.sg> wrote:

> Hi Matt,
>
> The time difference is the TDOA we estimated by cross-correlating the two
> signals. For our setup, we expected that the TDOA would be less than 100ns,
> but constantly got a TDOA between +0.2 and -0.2 us.
>
> So the code to align LOs in the frontend would have no effect for DBSRX2.
> Do you have any suggestions on how to deal with this phase difference? We
> use single tone signal to measure the phase difference between the two
> signals, it seems that the phase difference is not constant.
>
> Best regards,
> Cheng Chi
>
>
>
>
> On Thu, Sep 5, 2013 at 12:47 PM, Matt Ettus <matt at ettus.com> wrote:
>
>>
>> Chi,
>>
>> Are you measuring time difference by measuring the phase difference
>> between the two signals?  That is different than time alignment.  You can
>> check for time alignment by comparing the outputs of the two GPSDOs on an
>> oscilloscope.  They should be closer than 100ns RMS.
>>
>> The DBSRX2 will have an arbitrary phase difference between 2 boards
>> because they have fractional-N synthesizers without phase alignment
>> circuitry.  The SBX includes phase alignment capability which works with
>> the code you have there for alignment.
>>
>> Matt
>>
>>
>> On Wed, Sep 4, 2013 at 4:50 AM, Chi <chengchibt at gmail.com> wrote:
>>
>>> Hi all,
>>>
>>> I am trying to synchronise two USRP N210 with the goal of receiving
>>> time-aligned samples.
>>> I've modified the rx_samples_to_file.cpp code according to this page:
>>> http://files.ettus.com/uhd_docs/manual/html/sync.html
>>>
>>> It seems that the signals at the USRPs have between +0.2 and -0.2 us
>>> time difference.
>>> This time difference seems to be random, as it changes every time I run
>>> a new experiment.
>>>
>>> Does anyone know why the samples receive at these two USRPs are still
>>> not aligned?
>>> I thought the time difference should be less than 100ns. What's the
>>> right way to minimize the
>>> time difference??
>>>
>>> Best regards,
>>> Cheng Chi
>>>
>>>
>>> ------------------------------------------------------------------------------------------------
>>> Here is the setup and the code snippet that I've add to the example
>>> rx_samples_to_file.cpp.
>>>
>>> - USRP N210 with internal GPSDO module, daughtherboard is DBSRX2
>>> - Signal generator outputs a common signal and connects to the two USRPs
>>> through equal length cables
>>>
>>> Transmitter part: Common signal is FM signal with 1MHz bandwidth,
>>> carrier frequency is 1.6GHz.
>>> Receiver part: Sampling rate is 5Msps
>>>
>>>
>>> 1. For Common Reference Signals
>>> {{{
>>> usrp->set_clock_source("gpsdo");
>>> usrp->set_time_source("gpsdo");
>>> }}}
>>>
>>> 2. For synchronising the Device Time
>>> Because each N210 has an internal GPDSO module, the device times are
>>> automatically synchronised
>>>
>>> 3. For Synchronising Channel Phase
>>>
>>> To align CORDIC:
>>> {{{
>>>     uhd::stream_cmd_t stream_cmd((num_requested_samples == 0)?
>>>         uhd::stream_cmd_t::STREAM_MODE_START_CONTINUOUS:
>>>         uhd::stream_cmd_t::STREAM_MODE_NUM_SAMPS_AND_DONE
>>>     );
>>>     stream_cmd.num_samps = num_requested_samples;
>>>     stream_cmd.stream_now = false;
>>>     stream_cmd.time_spec = uhd::time_spec_t(time_to_receive);
>>>     usrp->issue_stream_cmd(stream_cmd);
>>> }}}
>>>
>>> To align LOs in the frontend:
>>> {{{
>>>     uhd::time_spec_t cmd_time = usrp->get_time_now() +
>>> uhd::time_spec_t(0.1);
>>>     usrp->set_command_time(cmd_time);
>>>     usrp->set_rx_freq(1.6e9);
>>>     usrp->clear_command_time();
>>>
>>> }}}
>>> For DBSRX2, does this method for aligning LO work?
>>>
>>>
>>> _______________________________________________
>>> USRP-users mailing list
>>> USRP-users at lists.ettus.com
>>> http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com
>>>
>>>
>>
>
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