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

[Matt Ettus]

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.

Matt