[USRP-users] Phase offset drift over time for two x310

Ruben.Merz at swisscom.com Ruben.Merz at swisscom.com
Fri Apr 17 05:47:19 EDT 2015


Hello Folks,

Is there any update on this drift issue? I noticed “X300: Improved phase alignment across devices”
in the latest 003.008.003 changelog.
Thanks for all the work
Ruben

From: USRP-users [mailto:usrp-users-bounces at lists.ettus.com] On Behalf Of Michael West via USRP-users
Sent: Monday, March 30, 2015 7:55 PM
To: Damiano Scanferla
Cc: usrp-users
Subject: Re: [USRP-users] Phase offset drift over time for two x310

Hi Damiano,
The change Neel sent you was to reduce the oscillations, not the drift.  We are actively looking into the drift and will let you know what we find as soon as possible.
Regards,
Michael

On Mon, Mar 30, 2015 at 8:18 AM, Damiano Scanferla via USRP-users <usrp-users at lists.ettus.com<mailto:usrp-users at lists.ettus.com>> wrote:
Hello Neel,

Thanks for your answer. I have tried what you suggested but we did not see any significant improvements:
- the phase offset at 2.7 GHz drifted by 40 degrees over 3 minutes (same as before)
- the oscillations of the phase offset over short time were reduced.

Do you have a reason for this phase offset drift over time?

Damiano

On Mon, Mar 30, 2015 at 3:54 PM, Neel Pandeya <neel.pandeya at ettus.com<mailto:neel.pandeya at ettus.com>> wrote:
Hello Damiano Scanferla:

We have seen this issue before, and we believe that we have a solution that should improve the results that you're seeing. I would like to ask you to make a one-line change on line 193 of the file "uhd/host/lib/usrp/x300/x300_clock_ctrl.cpp", and then re-build and re-install UHD. I've attached a copy of the file with the change in it. It is necessary that you are running UHD 3.8.2 for this to work. I assume that you're on Linux, such as Ubuntu 14.04. Please let me know if you see better results with this change, or if you have any problems making the code change. Thanks.

--Neel


On 27 March 2015 at 05:58, Damiano Scanferla via USRP-users <usrp-users at lists.ettus.com<mailto:usrp-users at lists.ettus.com>> wrote:
Hi,
I would like to achieve phase alignment at two receivers being 2 SBX-120 daughter-boards of either the same X310 or belonging to 2 different X310s.
I can measure the phase offset between the two receivers and compensate for it. The problem is that the phase offset is not constant but it is drifting over time. Of course I can do a periodic calibration of the receivers but I read in several blogs that the phase offset should be constant so I am wondering what I am not doing properly in my setup.
SETUP
An RF signal at 800 MHz or 2700 MHz is generated with a signal generator (R&S SMJ100A) and fed to 2 daughter-boards via splitter and 2 cables of the same length. At the receiving USRPs, I just measure the phase difference between the samples received by receiver 1 and receiver 2 (phase offset). The USRPs get the 10 MHz ref and PPS from an OctoClock (both clock_source and time_source set to "external", and sync set to "unknown PPS").
I have attached the "grc file" and the slope of the phase offset over 3 minutes for the following configurations:
- 800 MHz, 2 SBX-120 in 1 X310
- 800 MHz, 1 SBX-120 in 2 different X310
- 2700 MHz, 2 SBX-120 in 1 X310
- 2700 MHz, 1 SBX-120 in 2 different X310

2 observations:
- the higher the frequency, the higher the phase drift (almost linear with frequency). So, could it be due to a phase drift of the 10 MHz reference?
- the phase drift is lower when the 2 daughter-boards belong to the same X310.
Do you know what could be the reason for this? Have you ever seen such behaviour?
Best regards,
Damiano


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