[time-nuts] 1 pps correction

[Said Jackson]

Chris,

You are describing the differences and advantages of software correction versus hardware correction.

The delay line will also have a temperature dependent drift besides the added noise, but it does give you a real time 1pps.

The software correction requires a good time interval counter. But then it works very well.

On our CSAC GPSDO boards for example with 20ps resolution interval counter the user can display the corrected time capture of the 1pps second to second, and the numbers vary less than 100ps typically (some of which is noise from the reference oscillator) showing how well the 1pps is corrected internally by the software.

There is also a cns paper (link was sent to this list some weeks ago) that shows the software solution being slightly better on an m12 receiver than the delay line version.

Bye,
Said





On Mar 31, 2012, at 9:55, Chris Albertson <albertson.chris at gmail.com> wrote:

> On Sat, Mar 31, 2012 at 7:00 AM, Azelio Boriani
> <azelio.boriani at screen.it> wrote:
>> And here:
>> www.cnssys.com/files/PTTI/PTTI_2006.pdf
>> 
>> Anyway, using a Dallas/Maxim DS1023-100 delay line (and a microprocessor,
>> of course) you read the @@Hn data from the iLotus M12M and apply the
>> correction to the delay line. Of course the delay line cannot anticipate
>> the PPS, so that you have to set a "zero" at the center of the delay line
>> total span. This fixed delay can, in turn, be nulled out using the M12M
>> capability to displace its PPS with respect to the UTC. Please, read the
>> M12M user manual and the DS1023-100 data sheet
> 
> 
> It's clear that a fixed offset combined with a variable delay can work
> in theory but how much noise (jitter) is added by the delay line?  The
> uncorrected M12M is so god that I wonder of the delay line correction
> is worse than the problem it seek to address.
> 
> Maybe a better way is to use the method of NTP.  It uses the PPS to
> snapshot a counter and stores the value in RAM.  It also tracks the
> sawtooth functionsand allies the corrected to the value stored in RAM.
>  So you get the correction but no extra noise for a delay line.   But
> then in most real-world NTP systems the counter has only uSec
> resolution so the sawtooth is mostly moot.  But the idea perfectly if
> you have a better counter.   But only for the one system
> -- 
> 
> Chris Albertson
> Redondo Beach, California
> 
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