[time-nuts] Which First GPSDO to buy?

Chris Albertson albertson.chris at gmail.com
Sun Dec 14 00:47:11 EST 2014

I tried to see just how simple, low cost and self contained I could make a
GPSDO.  I started with the Lars Walenius design then removed everything I
could from it.  I replace all the software with just a small loop with
about a dozen lines of code so it would be easy to understand.

My goal was to make something that could be built and tested using just
basic equipment.  The question is of course "How do you know the unit is
making a 10 MHz signal if you don't already have a 10MHz reference to
compare it to?"  Well you can assume that your 1PPS reference is accurate.
Then you count and make sure you see EXACTLY 10,000,000 oscillator cycles
per each PPS.  Count both for a few days and verify the ratio remains at
ten million to one, exactly.  I ran mine for about 8 weeks and it stays at
the desired ratio.    I know this is not a perfect test because it could
have been running at zero hertz for 30 seconds and then 20MHz for 30
seconds but I assume the OCXO is better than that.   The point is that once
you have the GPS working you DO have a  pretty good 1Hz reference.

Motorola Oncore GPS    $18
magnnetic patch antenna   6
OCXO (eBay)                   19
Arduino, mini                      3
PLL chip                             2
TTL diver chip                    1
Plug-in power cube            0
perf-board                          1

Total cost of GPSDO     $50

Actually I do have A Thunderbolt.  I place the 10MHz output of the above
unit and the TB on my dual channel scope and was able to see the phase of
the two 10MHz references was locked.  I saw the phase drift over about an
hour but then it would pull back.   But I made this very simple and it
could be better.

Actually I've added  some features to it like a 2 line by 16 character LCD
display and some status LEDs.  And I can log data to a computer via a USB
cable so it is easy to plot data and it is using my more expansive mast
mounted timing antenna.

The Arduino based design is OK for controlling an OCXO but I think it is
best used for controlling my Rubidium oscillator.  The RB is so stable I
should only update the frequency control every few hours at most.

On Sat, Dec 13, 2014 at 7:21 PM, Jim Harman <j99harman at gmail.com> wrote:
> On Sat, Dec 13, 2014 at 9:36 PM, Bob Camp <kb8tq at n1k.org> wrote:
> > The problem with “build it yourself” is that there is no way do know if
> > you got it right unless you have something to compare your design to. You
> > *will* make mistakes as you build one of these….
> I think you will have the same problem with an off-the-shelf unit if you
> don't have at least one reference for comparison. However speaking from
> experience with Lars Walenius' Arduino-based design, I can say that it is
> not hard to make a working system, even without another reference. Along
> the way you will learn a tremendous amount about how these systems work,
> plus a lot about Arduino programming.
> Lars' design will run stand-alone, but if you want it can send very useful
> logging data to a PC, much more informative than a "locked" led on a
> commercial unit.
> Total cost including processor, Adafruit GPS shield, and $25.00 ebay OCXO
> is about $100.00
> --
> --Jim Harman
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Chris Albertson
Redondo Beach, California

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