[time-nuts] GPS: ADEV or MDEV?

Magnus Danielson cfmd at bredband.net
Tue Apr 10 05:55:27 EDT 2007


From: Dr Bruce Griffiths <bruce.griffiths at xtra.co.nz>
Subject: Re: [time-nuts] GPS: ADEV or MDEV?
Date: Tue, 10 Apr 2007 17:49:03 +1200
Message-ID: <461B254F.5020208 at xtra.co.nz>

> Poul-Henning Kamp wrote:
> > In message <004e01c77b09$d095aa60$6500a8c0 at EBS8500>, "Brooks Shera" writes:
> >
> >   
> >> The impact of time averaging  to suppress white phase noise is illustrated
> >> by a new plot TVB has created and placed on his website
> >> http://www.leapsecond.com/pages/3gps/gps-adev.gif.
> >>
> >> These revealing plots show that just a few hundred-second time average has 
> >> largely removed the performance
> >> difference between the aged VP and the newer M12+ and has almost entirely
> >> removed the benefit of sawtooth correction (CNS-CNS II).  These important
> >> results are not evident in the usual ADEV plot.
> >>     
> >
> > This is simply not true.  No averaging period can ever guarantee you
> > will not get a constant offset from the sawtooth.  You may get rid
> > of the noise, but not the bias.
> >
> >   
> Poul-Henning
> 
> Not to mention the case of the GPS receiver with a pathologically stable 
> local oscillator frequency that generates lots of "hanging bridges" and 
> little sawtooth.
> Since software sawtooth correction is virtually free why not do it, when 
> such data is made available by the GPS receiver?
> 
> Hoping that the local oscillator frequency is sufficiently unstable 
> isn't good engineering, one should design to accommodate the worst case 
> possibilities.

Ovenize it to control the sawtooth frequency?

Temperature controlled oscillator! :-)

Should be fairly simple to acheive, really just a FLL.

Cheers,
Magnus




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