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

[Poul-Henning Kamp]

In message <20180522011345.4db43406...@ip-64-139-1-69.sjc.megapath.net>, Hal Mu
rray writes:
>
>hol...@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?

It is trivial to measure with a HP5370: Capture a series where you
measure the 1PPS against a good 10MHz and record the serial datastream.

Then offline apply the negative sawtooth and plot the result.

If you get ugly spikes at the turning points of the "hanging bridges"
you're doing it wrong.

For the UT Oncore etc. it is predictive of the next 1PPS flank.

-- 
Poul-Henning Kamp   | UNIX since Zilog Zeus 3.20
p...@freebsd.org | TCP/IP since RFC 956
FreeBSD committer   | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
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Re: [time-nuts] Sawtooth correction: next or previous PPS

[Bob kb8tq]

Hi

> On May 21, 2018, at 9:13 PM, Hal Murray  wrote:
> 
> 
> hol...@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. 

Bob


> 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.
> 
> 
> hol...@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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[time-nuts] Sawtooth correction: next or previous PPS

[Hal Murray]

hol...@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?

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


hol...@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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