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

From: Matt Ettus
Subject: Re: [Discuss-gnuradio] New external clock board for USRP
Date: Thu, 24 Sep 2009 18:20:20 -0700
User-agent: Thunderbird (X11/20090825)

Alexander Chemeris wrote:
Hi all,

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.


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