[USRP-users] Using a reference clock on USRP E310/312

Derek Kozel derek.kozel at ettus.com
Wed Jul 26 12:23:09 EDT 2017


Hello,

Unfortunately the phase offset between channels on separate E31xs is not
repeatable between tunes or runs. In order to use multiple E31xs for
beamforming or other applications needing known phase offsets it is
necessary to supply an external reference signal and use that to measure
the offsets each time they are tuned. This is a limitation of the AD9361
and the USRP's design.

The two RX channels on a single E31x are driven by a single LO, so their
phase offset will be stable without any external references. The same is
true for the TX pair of channels.

The E31x will discipline the internal frequency reference when an external
1PPS signal is supplied. The 1PPS can also be used to set the FPGA's
timestamp very accurately so sample streams from multiple USRPs can be
aligned in time.


After you set the time source for both USRPs then the frequencies will be
aligned.
usrp->set_time_source("external");

You will have to then use the 1PPS signal to set the timestamps. I
recommend reading the manual page on Synchronization, I hope it will
clarify these steps.
http://files.ettus.com/manual/page_sync.html

For MIMO setups with more than two channels the N2x0 and X3x0 USRPs are
recommended. Properly configured these do not require external signals to
be supplied for calibrating out the phase on every tune.
https://kb.ettus.com/Synchronization_and_MIMO_Capability_with_USRP_Devices

Regards,
Derek


On Wed, Jul 26, 2017 at 3:43 PM, Cho, Daniel J (332C) <
daniel.j.cho at jpl.nasa.gov> wrote:

> Hello –
>
>
>
> Yes I am trying to achieve phase synchronization.  So once I set the time
> source using the function call if (not time_source.empty()) usrp->
> set_time_source(time_source);, will time and frequency for the two USRPs
> be aligned?  Also, is it true that the USRP E310/E312 cannot use a 10 MHz
> ref clock or can I just set the time source using the 10MHz reference
> instead of the 1PPs?
>
>
>
> Thanks
>
> *From:* Derek Kozel [mailto:derek.kozel at ettus.com]
> *Sent:* Wednesday, July 26, 2017 6:56 AM
> *To:* Cho, Daniel J (332C) <daniel.j.cho at jpl.nasa.gov>
> *Cc:* usrp-users at lists.ettus.com
> *Subject:* Re: [USRP-users] Using a reference clock on USRP E310/312
>
>
>
> Hello,
>
> To get time and frequency alignment on a pair of E31xs you can supply the
> 1PPS to both units. The example programs do not all support setting the
> time source.
> http://files.ettus.com/manual/page_usrp_e3x0.html#e3x0_hw_sync
>
> Here is an example which supports setting the time source, you can modify
> other examples based on this.
> https://github.com/EttusResearch/uhd/blob/maint/
> host/examples/test_pps_input.cpp#L68
>
> Are you trying to achieve phase synchronization?
>
> Regards,
>
> Derek
>
>
>
> On Tue, Jul 25, 2017 at 11:40 PM, Cho, Daniel J (332C) via USRP-users <
> usrp-users at lists.ettus.com> wrote:
>
> Hello –
>
>
>
> I have 2 USRPs (both E310/E312) which I want to sync up using an external
> clock.  I read that the embedded series cannot take in a 10MHz reference
> clock but can take in a 1PPS.  I generated a 1 PPS signal using a signal
> generator and using a power splitter, I sent the 1PPS signal to both USRPs
> via the sync port on the USRPs.  When I try to run an example programs
> (rx_samples_to_file on one USRP and tx_samples_from_file on the other USRP)
> using the ref argument, it says that the ref argument is not supported.
>  How can I sync up the two USRPs?
>
>
>
> Thanks
>
>
> _______________________________________________
> 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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