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

[Andy Walls]

> From:         Lapointe, Benjamin - 1008 - MITLL
> Date:         Thu, 19 Jun 2014 16:39:55 +0000
> 
> 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?

FWIW,

I wrote a custom block that monitors the X310 USRP w/GPSDO time at the
PPS and resets the X310 time, if they are off by more than 1
microsecond.

I often get my debug output looking like this:

        uhd_foo_impl::run(): Setting USRP time to 1401798161 seconds on next PPS
        uhd_foo_impl::run(): USRP time was 1401798159.993764020  seconds on 
last PPS
        
        uhd_foo_impl::run(): Setting USRP time to 1401802468 seconds on next PPS
        uhd_foo_impl::run(): USRP time was 1401802467.000001295  seconds on 
last PPS

The first pair of lines always happens right at block start up.  The UHD
driver does an initial time setting from the GPSDO, but I'm guessing it
must do some hardware magic with the X310 to cause it to lose 3 to 6
milliseconds.

The second pair of lines occasionally happens, usually after the first
power on of the X310.  I do not know the cause.  This particular example
is odd, in that the jump happened 72 minutes later.  It usually happens
sooner.


BTW, GPS units can throw in leap seconds unannounced in two cases:

1. When GPS ground stations add or remove a leap second to the GPS-UTC
offset at the end of June or December.

2. When your GPS has an old GPS-UTC offset in NVRAM at power on, and
then gets the correct GPS-UTC offset from the almanac which it downloads
from the GPS satellites up to 12.5 minutes after power up.

Regards,
Andy


> Thanks!
> 
> -Ben