[time-nuts] Next step up from basic GPS/PPS timekeeping

Neil Green ncguk at hotmail.co.uk
Thu Feb 25 18:08:08 UTC 2016


On 24 Feb 2016, at 4:30 pm, Chris Albertson <albertson.chris at gmail.com> wrote:

> A simple GPSDO is, well, simple.  All you do is compare the phase of
> the PPS to the phase of the 10MHz crystal and then adjust the
> frequency of the crystal to keep the phase difference constant.   Can
> be done all in software by a $2 micro controller chip.  By Time Nut
> standards the performance will not be top notch but compared to your
> NTP server it could literally be 10,000 times better   A while back I
> tried to build the cheapest and simplest GPSDO possible and except for
> the price of the crystal and GPS receiver I spent about $5.
> Performan ce was such that when I put the cheap GPSDO  10MHz output
> and my Thunderbolt both on a dual trace scope I could see some
> movement in phase over hours long time but over days in stays locked
> 
> First divide the 10MHz crystal to say about 1Mhz using a TTL divider.
> Some (most) micro processor chips have a way that an external signal
> can trap the current value of a counter.  Let both the PPS and the
> 1Mhz do this and look at the difference in counts.  Adjust the crystal
> so the difference remains constant.   Don't worry about details, that
> is for latter make it as simple as possible.  Improvement is for
> version #2.

This appeals to me, but I think it’s currently beyond my level of knowledge. However, I’ve decided to build this:

http://www.hackersbench.com/Projects/1Hz/

which has an easy to follow diagram. Parts have cost less than £5. I’m not expecting great things from it by any means, but it’s something I can get my head around and should improve my understanding (a little). 


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