[time-nuts] Sawtooth correction: next or previous PPS

Bob kb8tq kb8tq at n1k.org
Mon May 21 21:19:59 EDT 2018


> On May 21, 2018, at 9:13 PM, Hal Murray <hmurray at megapathdsl.net> wrote:
> holrum at hotmail.com said:
>> One thing to look out for when messing with sawtooth messages is the
>> question of does the message come out before or after the PPS pulse...  good
>> look finding the answer in the receiver documentation... 
> Has anybody asked the manufacturers?

At least in the framework of Furuno and myself … yes. Not via email but face to face
with their head of design engineering. 

> This should be easy to see if you record the PPS offset referenced to a good 
> clock and compare that to the reported offset.  

The real simple answer is that you have four cases. Pulse before vs pulse after plus
adds or subtracts. That’s not so may that you can’t just try it and see. It turns out to 
be very obvious when you get the right one. 


> If the frequency is stable 
> and you are getting a sawtooth (rather than a bridge) then a point on a 
> corrected graph next to the jump in the sawtooth will look good if you have 
> it right or be off by a clock cycle if you have it wrong.
> holrum at hotmail.com said:
>> "After" seems to be the most common answer.  That makes hardware/delay line
>> compensation rather tricky.  ...
> The slides from Tom Clark and Rick Hambly's VLBI talk (page 29) show a before 
> setup.
>  http://www.gpstime.com/files/TOW/tow-time2015.pdf
> You said "most common".  That implies there are both types.  (or 
> documentation errors)  We should make a list of which GPS modules do it which 
> way.
> -- 
> These are my opinions.  I hate spam.
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