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

Chris Albertson albertson.chris at gmail.com
Wed Feb 24 11:30:52 EST 2016


On Wed, Feb 24, 2016 at 6:22 AM, Neil Green <ncguk at hotmail.co.uk> wrote:

>
> What would be my next step up be, hardware-wise, in terms of improving precision, stability, etc? A GPSDO? Budget is limited as far as these things go - about £150 UK/$210 US.

Given that you already have the GPS and PPS, you can build your own
GPSDO for very little cost.  The most expensive part is the crystal
and as long as you are not going for top of the line performance a
cheaper crystal will do. (You asked about the NEXT step not the LAST
step)

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.

The next step up would be something like the Thunderbolt or a much
better DIY GPSDO.






-- 

Chris Albertson
Redondo Beach, California


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