[time-nuts] TimeLab

Poul-Henning Kamp phk at phk.freebsd.dk
Mon Oct 10 02:22:28 EDT 2016

In message <0f6c1eb7-18cb-06e3-48dd-6cd618f19575 at rubidium.dyndns.org>, Magnus D
anielson writes:

>This is why the two-counter setup is so messy, you have to have software 
>that will sync up and query them alternatively.

It is not that bad messy.

Counter A  Start=DUT, Stop=REF

Counter B  Start=DUT, Stop=REF + half period [1]

Now you know that at least one counter will always measure a DUT flank.

The crucial detail is that you also know that the counters will not
spit out their result at the same time, so timestamping the
measurements on the computer will definitively sort them into

I always run separate programs/scripts for each counter outputting
into separate files, but that's a matter of temperatment.

You end up with a reliable sequence with three possible scenarios:

	{AB}* fine...

	{AB}*B{AB}*  lost one from A

	{AB}*A{AB}*  lost one from B

And it is trivial to insert markers for the missing measurements.

In addition you end up with two entirely separate series of
measurements which you can compare for sanity, and if they look
good, you combine them and reduce your noise by sqrt(2)


[1] PPSDIV gives nice symmetrical PPS, just trigger opposite flank.

[2] Obviously: Do not use a computer running a weather model for this

Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk at 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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