It's definitly of interest to me :) I'm considering buying a FA-SY 1 myself, or possibly doing a clock gen board from scratch (to have multiple clock input for GSM / GPS / ...).

On Aug 24, 2009 9:03pm, Joshua Lackey <jl@thre.at> wrote:
> I recently purchased an FA-SY 1 from Funkamateur that I'm actually quite
>
> pleased with.  One nice feature is that it warms up the synthesizer to
>
> reduce temperature drift.
>
>
>
> I used the method you mention below, that of synchronizing to a local
>
> BTS, to calibrate it.  The factory calibration was roughly 6kHz off of
>
> my local tower.  (~7ppm.)  After calibration, I am normally
> the tower.  (.03ppm.)  I say normally because last night I was +/- 3Hz
>
> off the tower frequency!
>
>
>
> I wrote an implementation of a rather good paper ("Robust Frequency Burst
>
> Detection Algorithm for GSM/GPRS," G. Narendra Varma, Usha Sahu, G.
>
> Prabhu Charan) which uses an adaptive filter to identify the FCH burst.
>
> Once I have the burst, I calculate the offset.  My current
>
> implementation actually averages 80 offsets to get the results I give
>
> above.  (I take 100 offsets and throw away the highest and lowest 10.)
>
>
>
> I'd be glad to post my calibration program if there is any interest.
>
>
>
>
>
> Quoting Harald Welte (laforge@gnumonks.org):
>
> > Hi David,
>
> >
>
> > On Tue, Aug 18, 2009 at 08:25:34PM -0700, David Burgess wrote:
>
> >
>
> > > First, we tried to fix this problem by putting a 26 MHz TCXO on the
>
> > > USRP.  The problem with that is that the USRP firmware fails for
>
> > > clock rates below 48 MHz.  So we looked for a 52 MHz TCXO bout could
>
> > > not find any available in small lots with reasonable lead times.  So
>
> > > then we got someone to build us a clock module PCB with a 26 MHz TCXO
>
> > > and a clock doubler.
>
> > >
>
> > > We got our first sample PCB a few days ago.  The doubler adds a
>
> > > little jitter to the clock, but not enough to break GSM.
>
> >
>
> > I'm quite surprised to read this.  What accuracy is the TCXO specified
>
> > for?  GSM spec says you have to be within 0.03 ppm accuracy, and I have
>
> > never seen any TCXO at that accuracy.  Only OCXO, which are obviously very
>
> > expensive.
>
> >
>
> > And even all the OCXO based GSM BTS clocks that I've seen can and need to be
>
> > calibrated.
>
> >
>
> > The question is also: over which temperature range and which aging do you
>
> > want to be that precise? Some (not even all) OCXO's are specified to have
>
> > this precsion over 10 years.
>
> >
>
> > I have a device with internal OCXO that can measure the difference in ppb
>
> > between its internal OCXO oscillator and the clock of all the BTS's in its
>
> > vicinity.  I have never seen any of those BTS exceed 27 ppb difference from
>
> > that reference - mostly actualy within 10ppb difference.
>
> >
>
> > All those measurements where done in the GSM900 band in Germany. I have not yet
>
> > tried in GSM1800.
>
> >
>
> > Have you ever thought of calibrating the OpenBTS clock against the carrier
>
> > clock derived from other BTS?  I think that would provide a very high accuracy
>
> > when you need it (i.e. when phones are receiving other GSM networks).  Once you
>
> > don't receive any other BTS, you just use the latest PLL calibration value.  It
>
> > is no longer too critical, since you can safely assume the phones also don't
>
> > receive the carrier of other BTS and thus have to search over a wider range to
>
> > get a carrier lock.
>
> >
>
> > Regards,
>
> > --
>
> > - Harald Welte laforge@gnumonks.org>           http://laforge.gnumonks.org/
>
> > ============================================================================
>
> > "Privacy in residential applications is a desirable marketing option."
>
> >                                                   (ETSI EN 300 175-7 Ch. A6)
>
> >
>
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