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Re: [Discuss-gnuradio] BURX 26MHz clock

From: John Orlando
Subject: Re: [Discuss-gnuradio] BURX 26MHz clock
Date: Fri, 20 Aug 2010 13:30:15 -0500

On Fri, Aug 20, 2010 at 8:47 AM, David Evans <address@hidden> wrote:
> Thanks John,
> This is how I understood it, but I suppose my real question is (not being a 
> dsp
> expert), why would you want to do this?

For a system with only a single USRP2 in it, there isn't much benefit
to having phase lock between the BURX board's 26 MHz TCVCXO and the
USRP2's 100 MHz oscillator.  However, in a system with more than one
USRP2, it is possible to have the 26 MHz TCVCXO from one BURX board
feed additional BURX boards such that they all have a common
phase-locked reference clock.  This requires the appropriate
amplification/distribution of the 26 MHz signal but it can be done.

The primary benefit to using the BURX's 26 MHz TCVCXO is when using
the USRP1, since you actually get the entire system clocked from a
single clock source.  This includes the A/D converters.  In addition,
the BURX board also provides the ability to warp the TCVCXO to adjust
for clock error, and this correction translates to the sample rate as
well which is what you want for most RF receivers.

When we started the design of the BURX board, we had (mistakenly)
assumed that we would be able to clock the USRP2 from our on-board
oscillator, just like on the USRP1.  There is always the option of
un-soldering the 100 MHz reference and piping in our 26 MHz reference
clock, but this is a non-trivial modification.

> I assumed that being a 26MHz clock, this would be ideal for GSM rate signals?

Indeed, in either a USRP1 modified to accept an external clock, or a
USRP2 that has undergone the non-trivial modification mentioned above,
having the A/D converters run at a multiple of 13 MHz reduces the
computational load on the host machine.

John Orlando
CEO/System Architect
Epiq Solutions

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