Re: [Discuss-gnuradio] [USRP-users] sample time alignment in GRC

[Lapointe, Benjamin - 1008 - MITLL]

Nicholas,

 

Thanks for your message, I implemented your changes to my top_block.py file, but was unable to get the two data streams from the X310s to start sampling at the same time.  I verified with the print statements that python was doing the time conversions and math correctly. 

 

I used a WX Scope Sinc and analyzed recorded data in MATLAB to look at time offsets in the two data streams.  I also set the 1 second waits to 2 second waits.  I generally saw time offsets in the range of 5 to 25 samples (1 to 5 usec with 5MHz output), and one occurrence of ~200 sample offset between the two sample streams.  Nicholas, how did you verify sample time alignment?

 

I attached my top_block python file, in case anyone has time to look at it. 

Any other ideas/comments?

 

Thanks!

-Ben

 

From: Linnenkamp, Nicholas [mailto:address@hidden
Sent: Tuesday, June 17, 2014 1:54 PM
To: Robert Kossler; Lapointe, Benjamin - 1008 - MITLL
Cc: address@hidden
Subject: RE: [USRP-users] sample time alignment in GRC

 

Ben/Rob,

 

I addressed the issue of getting time aligned samples going for USRP devices sometime back.  I had similar issues until I did a deep dive and worked out some of the problems.  There is a bit more to the process than just setting sync and reference sources.

 

http://lists.ettus.com/pipermail/usrp-users_lists.ettus.com/2014-April/009277.html

 

That contains code for getting gnu-radio to perform the required initialization for the devices so that they sample at the same time.  Perhaps someone from the gnuradio camp can figure out how to do it automagically.

 

Nicholas

 

From: USRP-users [mailto:address@hidden] On Behalf Of Robert Kossler via USRP-users
Sent: Tuesday, June 17, 2014 10:28 AM
To: Lapointe, Benjamin - 1008 - MITLL
Cc: address@hidden; address@hidden
Subject: Re: [USRP-users] sample time alignment in GRC

 

Hi Ben,

I am having a similar (but not identical issue).  I have a single X310 for which I am trying to make sure both channels are time aligned.  I have tried both the internal PPS signal and an external PPS signal.  I noticed channel-to-channel time delays of 1, 2, or 3 "clock" cycles (at clock rate, not sample rate) which varied from run to run.   My measurements were done with a modified "rx_samples_to_file" program and Matlab processing.  I have not yet duplicated using GRC.

Rob

 

 

On Tue, Jun 17, 2014 at 9:58 AM, Lapointe, Benjamin - 1008 - MITLL via USRP-users <address@hidden> wrote:

Hi All,

 

I am still having trouble time aligning sample streams from two USRP X310 devices.  In GRC I noticed a random time offset from run to run in the two data streams using a WX GUI Scope Sink.  Looking at recorded data in MATLAB I also see a random time offset from run to run in the two data streams (8, 18, and 24 sample offset).  I verified that the two data streams that I am inputting into the X310 devices are time aligned using a physical scope. 

 

My GRC setup:

 

USRP Source 1 (with internal GPSDO-MINI)

-          Sync = unknown PPS

-          Mb0: Clock Source = Default

-          Mb0: Time Source = Default

USRP Source 2

-          Sync = unknown PPS

-          Mb0: Clock Source = External

-          Mb0: Time Source = External

 

For looking at the data streams I have USRP Source -> Complex to Mag -> WX GUI Scope Sink.

For recording the data streams I have USRP Source -> Head (5K) -> File Sink (Unbuffered: OFF)

 

Ref Out SMA of USRP 1 is connected to Ref In SMA of USRP 2 with a 6” SMA cable.

PPS Trig Out SMA of USRP 1 is connected to PPS Trig In SMA of USRP 2 with a 6” SMA cable.

RF input to USRP devices is a pulsed RF signal, to make it easier to look at time offset.

GPS on USRP 1 is locked; however, I work with tall buildings completely surrounding me and so I don’t know the strength of the GPS lock.    

I have an OctoClock-G on order to distribute 10 MHz Ref and 1 PPS signals, but until then..

 

Does anyone have any other ideas for getting time-aligned samples from run to run in GRC, or what I am doing wrong? I would expect at most a minimal constant time offset between data streams if the 10 MHz Ref and 1 PPS signals are locked.

 

Thanks!

-ben

 

From: Marcus Leech [mailto:address@hidden]
Sent: Friday, June 13, 2014 2:04 PM
To: Lapointe, Benjamin - 1008 - MITLL
Cc: address@hidden
Subject: Re: [Discuss-gnuradio] sample time alignment in GRC

 

Make sure that you specify that the 2nd X310 uses external clock and 1PPS, and all of them should use time synch of

  "unknown PPS".

 

Also, there has been a bug in the scope sink (dunno if fixed) where samples are *not* time-aligned in the scope sink.  The except

  is that a complex-pair will be time-aligned internally, but not necessarily to other streams being displayed.

 

 

 

on Jun 13, 2014, Lapointe, Benjamin - 1008 - MITLL <address@hidden> wrote:

Hi,

 

I have two USRP X310 devices that I am trying to time align in GNU Radio Companion.  One X310 has a GPSDO that is sending 10 MHz reference and 1 PPS signals to the other one. The GPS is locked.  Ideally I would have matched length cables for 10 MHz reference and 1 PPS, but I think my setup is close enough. (Input signal from sig gen = pulsed 10.005 MHz, input is split with matched length cables, USRP output sampling rate = 5M, USRP center frequency = 10M.) 

 

I am using WX GUI Scope Sink to look at the magnitudes of each stream from the USRP devices.  I expect to see no/minimal delay between the two signal streams, but I am seeing delays of 24, 13, 9, 0, 3, 6, 25, 24 samples from run to run between the two signal streams.  The period of the signal is 50 samples, so the maximum delay difference is 25 samples.  Am I missing something in my configuration?  Since I am using a 10 MHz reference and 1 PPS signals, I expect time alignment between the two sample streams.  Is there a GRC block for forcing time alignment? 

 

Thanks!

-Ben

 

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time_align_test_no_delay.py
Description: time_align_test_no_delay.py