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Re: [Discuss-gnuradio] New external clock board for USRP

From: Alexander Chemeris
Subject: Re: [Discuss-gnuradio] New external clock board for USRP
Date: Wed, 30 Sep 2009 12:50:18 +0400

On Mon, Sep 28, 2009 at 00:53, Alexander Chemeris
<address@hidden> wrote:
> On Fri, Sep 25, 2009 at 05:20, Matt Ettus <address@hidden> wrote:
>> Alexander Chemeris wrote:
>>> We're creating a clocking unit which will be easy to use with
>>> USRP. If there will be enough interest from the community we
>>> can order some amount of PCBs and put them for sale alone or
>>> soldered and ready to use. This should be useful for everyone
>>> trying to run OpenBTS, working with GPS and everyone else
>>> who need to re-clock their USRP or have more clock precision.
>>> We've created it to run USRP at 52MHz with 0.3-0.5ppm after
>>> calibrating or 1ppm in long term without recalibrating, but actually
>>> it will support 2.84-65.83MHz, 65.91-71.82MHz, 72.5-79MHz clock
>>> ranges with about the same precision.
>> I don't know what part you are planning to use for this, but it looks like a
>> tunable oscillator, which is likely to have a lot of jitter.  To avoid
>> problems, you would need something with less than 1 picosecond of jitter.
>>  The crystal on there now has about 250 femtoseconds of jitter.
>> I would strongly suggest that you use a plain old tcxo, and pick a part
>> which is available in many different frequencies.  This would result in a
>> much cleaner output, and much less complexity.
> Thank you for your input. Our unit is based on low-noise National's VCO+PLL
> sourced from precise reference generator. Of course, all phase noises are 
> taken
> into account. I need to check with my collaborator who're actually doing this
> work to get exact numbers. I'll post them here a bit later.

Well, my workmate is very tentative in estimates and says he needs to finish
the unit design to calculate meaningful numbers. And there are still not enough
interest from people, so finishing design is somewhat low priority for us now.

Yet I have to say that our prototype works perfectly with OpenBTS - all phones
we were able to test with works as expected and calls does not drop even after
half an hour of talking. "kal" utility says our clock's jitter is
+-70Hz relative to
our closest BS clocks, which is +-0.08ppm and is well about measurement
error level.

In our prototype we use LMX2531 VCO+PLL with CW648-ND 10MHz TCXO,
but in final design this may change.

Alexander Chemeris.

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