[time-nuts] Navsync CW-12 GPS Board - I don't believe it!!

Azelio Boriani azelio.boriani at screen.it
Wed Jan 4 10:16:00 UTC 2012


You are right. We use the CW12 (timing version) and it behaves as you
depicted. Of course there is also no holdover on its 10MHz output. Only the
FTS125 (and similar) have first a fixed OCXO to drive the GPS receiver and
then an OCXO to be aligned with the 10MHz to have a real 10MHz with
holdover BUT, curiously enough, PPS comes always from the GPS receiver.
Never tested the 10MHz being lower... now I'll try from the CW12 and from
the FTS125.

On Wed, Jan 4, 2012 at 9:50 AM, <SAIDJACK at aol.com> wrote:

> These Conner Winfield (Navsync) parts are not GPS Disciplined Oscillators,
> many folks are initially fooled by this.
>
> They use digital phase hopping techniques to generate something close to
> 10MHz. You can see the 10MHz 100ns phase jump around on a scope.
>
> I believe their FTS250 and 125 modules use this output to drive a VCXO
> cleanup oscillator to try to remove the phase jumps, phase noise, and
>  spurs.
>
> From their literature:
>
> "This ... module has an onboard, programmable NCO oscillator that outputs a
>  synthesized frequency up to 30MHz steered by a GPS receiver"
>
> So it's a noisy digitally steered NCO, not a clean phase locked  analog
> oscillator.
>
> The frequency error can be up to +/-100ppb according to their literature
> while the unit is trying to align itself to GPS (that's a massive drift of
> 100ns per second!)
>
> I believe the uBlox 6T frequency output is generated in a similar  manner.
>
> Many folks try these GPS NCO's because they are cheaper than real GPSDO's
> and seem to work similarly, then find out they don't perform as well...
>
>
>
>
>
>
>
> In a message dated 1/4/2012 00:32:33 Pacific Standard Time,
> hmurray at megapathdsl.net writes:
>
> > I  compared the OCXO output to the 10 MHz output of both a Z3801A and a
> >  Tbolt and discovered that the CW-12's 10 MHz output is about 1.5e-11
> (i.e.
> > 1.5e-4 Hz) low in frequency.  I emailed Navsync and they  replied:
>
> > "The CW12 Motorola Binary and NMEA versions both do not  phase align the
> > frequency output so the long term drift that Ed is  seeing is expected."
>
> Is there a timing version that does the right  thing?
>
> > Incredible.  It's 'steered by the GPS receiver' -  that's a direct quote
> > from the data sheet and the user manual - but  it's not quite on
> frequency
> > and that's fine with them.
>
> Is it consistently off that much, or does it wander around the right
> answer?
> (perhaps because there is a small dead band in the phase  detector)
>
>
> --
> These are my opinions, not necessarily my  employer's.  I hate  spam.
>
>
>
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