[time-nuts] 1 pps correction

Brooke Clarke brooke at pacific.net
Sat Mar 31 18:33:01 UTC 2012


Hi Chris:

I've done it both ways.
Made up a hardware corrector that used a time delay IC driven by a PIC that corrected the 1 PPS.  This is great for 
conditioning the 1 PPS into an SRS PSR10 oscillator.
http://www.prc68.com/I/PRS10.shtml

Also wrote some Lab View code that applied the sawtooth correction to the readings from the SR620 TI counter where one 
of the inputs was the uncorrected 1 PPS.  This works great and saves the expense and complication of the hardware solution.
http://www.prc68.com/I/TandFTE.shtml#SR620

But for the general first case the hardware solution with the delay line is the only way.  I think there may be an 
exception specifically for the PRS10 in that it may accept a sawtooth correction on the RS-232 input (I seem to remember 
that).

Have Fun,

Brooke Clarke
http://www.PRC68.com
http://www.end2partygovernment.com/Brooke4Congress.html


Chris Albertson 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



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