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Re: [Discuss-gnuradio] Possible bug in the vrt branch firmware?

From: Marcus D. Leech
Subject: Re: [Discuss-gnuradio] Possible bug in the vrt branch firmware?
Date: Tue, 09 Mar 2010 16:27:04 -0500
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On 03/09/2010 04:58 AM, John Gilmore wrote:
> If our library is providing a standard call to set the timestamps of
> returned samples, shouldn't the standard or default way to do it
> result in those timestamps being accurate wallclock UTC realtime,
> rather than "counting up from zero" or from a random number?  If by
> default our streams of samples came back with accurate nanosecond
> timestamps, that would be a big plus in the long run.  You could later
> sync up signals from receivers all around the world; recordings would
> contain the time when the signal was received; etc.
> Any computer on the Internet can easily sync using NTP to within about
> 10 ms or so, to set the high order bits.  And anyone with a PPS clock
> hooked to their USRP would get real cesium-linked timestamps.
>       John
I'm working on a "paper tiger" design for a specialized
direct-conversion receiver for radio astronomy
  that would include a GPS receiver, and a 64-bit nanosecond counter in
it, the idea being that each
  sample block would include an accurate timestamp.  My plan is to make
the wire format compatible
  with the USRP2--which I haven't actually looked at yet.

GPS modules can be purchased very cheaply these days, and integrated
into various hardware platforms,
  including receivers and digitizers, for use as a phase reference.

My receiver will use 1PPS to phase-lock both LO frequency, and the
sample clocks, and also to provide
  "head of block" nano-second timestamps.

I've been working with GPS clocks quite a bit in my "real(tm)" job. 
Phase agreement between GPS units,
  even connected to the same antenna, isn't as spectacular as you'd
like.  Most of them can get to within
  40nsec phase-agreement on the 1PPS outputs.   So that means that for
doing things like VLBI, you'll
  still need to "futz" with the data to get fringes to come out, but
your futzing "window" will be smaller :-)


Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium

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