[time-nuts] 1 pps correction

Azelio Boriani azelio.boriani at screen.it
Sun Apr 1 21:26:16 UTC 2012


Agreed. The TBolt is the best.

On Sun, Apr 1, 2012 at 9:10 PM, <shalimr9 at gmail.com> wrote:

> The need for sawtooth correction comes from the fact that standalone GPS
> receivers use a standard clock oscillator that is basically fixed frequency
> (save for temperature and other fluctuations) and their only option is to
> align the desired PPS to the closest clock pulse.
>
> Please note that the Thunderbolt does not need sawtooth correction because
> it uses the OCXO as its clock, and the GPS sofware can adjust its frequency
> of course. It is a very elegant and effective solution that was available
> to them because they were building an integrated GPSDO from the outset, so
> not only they avoided the problem altogether, but they also saved an
> oscillator in the process.
>
> It seems to me that if standalone GPS timing receivers used a VCXO instead
> of a fixed frequency clock, the cost delta would not be that significant,
> and they too could avoid the need for sawtooth correction.
>
> Didier KO4BB
>
> Sent from my BlackBerry Wireless thingy while I do other things...
>
> -----Original Message-----
> From: Hal Murray <hmurray at megapathdsl.net>
> Sender: time-nuts-bounces at febo.com
> Date: Sat, 31 Mar 2012 14:44:26
> To: Discussion of precise time and frequency measurement<
> time-nuts at febo.com>
> Reply-To: Discussion of precise time and frequency measurement
>        <time-nuts at febo.com>
> Subject: Re: [time-nuts] 1 pps correction
>
>
> martyn at ptsyst.com said:
> > I’ve seen that the peak to peak jitter is reduced from something like
> 27 ns
> > to < 10 ns.
>
> > Is this a reduction of just the jitter, or is the actual accuracy to UTC
> > also improved by this amount.
>
> Have you read the hanging-bridges paper?
>  Tom Clark and Rick Hambly: Timing for VLBI
>  http://gpstime.com/files/tow-time2009.pdf
> I think that is the key to understanding this area.
>
> If you could average over many sawtooth cycles, you should get an accurate
> answer.
>
> The problem is that you don't get to pick how many cycles fit into your
> averaging time.  The sawtooth pattern is the beat between two frequencies.
> One of them is drifting with time/temperature.  If you are unlucky, the
> beat
> frequency can be very very low.
>
> The sawtooth correction lets you correct on a cycle-by-cycle basis.  You
> don't need to average over many samples.
>
>
> --
> These are my opinions, not necessarily my employer's.  I hate spam.
>
>
>
>
>
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