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

Azelio Boriani azelio.boriani at screen.it
Wed Jan 4 17:24:49 UTC 2012


Interesting...now i wonder how they can steer the frequency. Usually in C/A
GPS receivers the oscillator is not corrected and any drift is accounted
for in software: usually you see geographic coordinates out of the
receiver. In the Motorola receiver they say the PPS has the granularity of
the oscillator (and the negative sawtooth can be used to compensate and
obtain greater precision) so no oscillator correction is necessary either:
just place the rise of the PPS at the nearest clock transition (and the
residual went in the previous sawtooh). Now to correct the frequency output
of a CW12 without adjusting the clock the only way is to change the phase
accumulator step: maybe the phase accumulator in the CW12 is greater than
32bit...

On Wed, Jan 4, 2012 at 6:13 PM, Said Jackson <saidjack at aol.com> wrote:

> Hi Ed,
>
> Since the oscillator is typically free-running in an NCO, and periodically
> corrected by phase drops to stay "on frequency" the error you are seeing
> may be caused be the offset in your crystal, combined with the limited
> digital resolution of the NCO trying to correct for this offset.
>
> If this is the case, your measured error would change with temperature.
>
> Does it do that?
>
> If yes, the NCO resolution is not fractionally corrected all the way.
> Replacing the on board Tcxo with an ocxo hand-tuned to the correct
> frequency would help, but that sort of defeats the purpose of the product..
>
> Bye,
> Said
>
> Sent from my iPad
>
> On Jan 4, 2012, at 7:05, Ed Palmer <ed_palmer at sasktel.net> wrote:
>
> > On 1/4/2012 8:49 AM, Mike S wrote:
> >> On 1/4/2012 9:29 AM, Ed Palmer wrote:
> >>
> >>> What's the point of saying that it's "steered by GPS" when it's
> >>> off-frequency. What does that even mean? Does it steer the frequency to
> >>> keep the error constant?
> >>
> >> If it's constant, maybe they just need to re-spec it as having a
> 9.99999999985 MHz output. :-)
> >>
> >
> > Ooohhhh, it's a GPS steered offset generator!  Now I get it! :-)
> >
> > Ed
> >
> >
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