[time-nuts] 1 pps correction

[shalimr9 at gmail.com]

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.


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