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

Matt Ettus matt at ettus.com
Mon Sep 9 12:52:38 EDT 2013


Cheng,

This is what I expected.  Separate GPSDOs will drift relative to each
other, by on the order of 100ns.  When used in close proximity, that 100ns
is significant, and so it is better to use a shared reference.  When used
very far apart (hundreds of meters or more), it is harder to use a shared
reference, but the 100ns error from drifting GPSDOs is less significant,
and so separate GPSDOs become useful.

So in short -- if you can use a shared reference, do that.  Only use
separate GPSDOs if it is impractical to use a shared reference because the
systems are too far apart.

Also keep in mind that GPSDOs take hours to settle down to a consistent
rate, so if you are using them, try to leave them on and with their GPS
antennas connected all the time.

Matt



On Sun, Sep 8, 2013 at 5:51 AM, Cheng Chi <ch0004hi at e.ntu.edu.sg> wrote:

> Hi Matt,
>
> Thanks for your advice. We've taken several measurements accordingly, here
> is the result:
>
> 1. Shared GPSDO:
> From the results, it seems that two signals aligned quite well with TDOA
> equals 0, but the FDOA is about 5.9Hz. We are trying to figure out why
> there's this small FDOA. We expected that the FDOA would also be 0.
>
> 2. Separate GPSDOs:
> We displayed two 10MHz signals from the two GPSDOs in an oscilloscope, and
> found that the two reference
> signals drift with respect to each other.
>
> Then we tried increasing the sampling rate to 25Msps, the TDOA results
> decreased. The TDOAs are [-88, -80, -64, -56, -48, -40, -32] in
> nanoseconds. It varied in a roughly periodic pattern. Is it caused by the
> drifting of the two reference signals we observed on the oscilloscope?
>
>
> Best regards,
> Cheng Chi
>
>
>
> On Fri, Sep 6, 2013 at 2:49 PM, Matt Ettus <matt at ettus.com> wrote:
>
>>
>> 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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>>
>
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