[time-nuts] TimeLab

Hal Murray hmurray at megapathdsl.net
Sun Oct 9 18:00:44 EDT 2016

tvb at LeapSecond.com said:
> 1) My *work-around* is to adjust the REF 1PPS by tens of microseconds, or
> even 500 ms. That avoids running into sign changes and skipped samples when
> a TIC gets near zero. This works really well for stable clocks where 500 ms
> drift is next to impossible. 

How well does using the other edge of the pulse work?

I'd guess it would work well for narrow pulses from GPSDOs but might be 
complicated if you are looking at a typical low cost GPS unit.

I have a scope with one probe on the PPS from a TBolt and another on the PPS 
from a Sparkfun Venus board.  The start of the Venus PPS jumps around by 
50-200 ns p-p relative to the TBolt, but the trailing edge is solid relative 
to the leading edge - at least at the ns level.  The infinite persistence 
band is about 1 ns wide.  That's about the same as the leading edge, and 
pushing things on a Rigol DS1102E.  After a longer wait, it's now 2 ns wide.  
I'll guess that's the crystal changing temperature.  I don't see a crystal on 
the board so I assume it's inside the big chip.  Or maybe the crystal in my 
scope warmed up.

The Sparkfun/Venus PPS is 4 ms wide.


Another option is to use a scope.  :)

I haven't done it, but I'm pretty sure I could grab the data and rearm the 
scope within a second.  It's not too hard to scan the data and find a level 
crossing point.  I'll write some sample code if anybody is interested.

These are my opinions.  I hate spam.

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