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

From: Tim Pearce
Subject: Re: [Discuss-gnuradio] New external clock board for USRP
Date: Mon, 28 Sep 2009 17:00:36 +0100

In terms of integrating more tightly with the USRP/GnuRadio

You could (maybe) just use the SPI pins and one of the digital IO lines as the enable (the ATMega8 running as a slave)

That way theres functions to communicate with the chip already built in you just need to define the various commands etc?


On Sun, Sep 27, 2009 at 9:53 PM, Alexander Chemeris <address@hidden> wrote:
Hi Matt,

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.

Alexander Chemeris.

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