[time-nuts] Which First GPSDO to buy?

Dave Daniel kc0wjn at gmail.com
Sun Dec 14 12:49:43 EST 2014

Well, thanks, everyone, for the information. I appreciate the help.

First, I am presently not up to adding another project to my long list 
of projects. I get whiplash every time I walk into the lab. Building a 
GPSDO sound like fun. Perhaps down the line.

I figured I should add some information about myself: I am an electrical 
engineer (currently employed) with a lot of digital/Verilog experience 
and a fair bit of analog experience (but less than my digital 
experience) and quite a bit of software experience, all of this from 
working for about thirty-eight years on various embedded systems. 
Currently, I shy away from writing code just because I don't enjoy it 
much and have done too much professionally. But I know that eventually I 
will need to write code in my lab. Presently, I am in the process of 
restoring some older ham radio gear, but I became sidetracked from that 
by the necessity to repair a bunch of vintage test equipment which 
effort has somehow taken on a life of it's own.

What I need right now is a frequency standard that is accurate enough to 
use as a reference for things like calibrating test gear. I also want to 
"play" with one before I build one. Just going through all of the 
educational material is a daunting task. I figured I'd combine an 
interest with GPSDOs in general with a need for an accurate enough 
"standard" (I use the term loosely here) to get some instruments 

Thanks again for all the information!


I had forgotten about the LTE-lite; I should add that to the list of 
choices. I'm tending towards either a 10 MHz version of that or the 
Lucent boxes.
On 12/14/2014 8:00 AM, Bob Camp wrote:
> Hi
>> On Dec 14, 2014, at 12:47 AM, Chris Albertson <albertson.chris at gmail.com> wrote:
>> 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.
> Except that the GPS PPS is *not* perfect, far from it. It’s only reasonably accurate over very long time spans. Over short spans the pps moves around a lot.
>> Then you count and make sure you see EXACTLY 10,000,000 oscillator cycles
>> per each PPS.
> If you do a tight lock (“EXACTLY”)  against a GPS PPS that is moving +/- 10 ns, your frequency will swing +/- 1x10^-8 every second
>> Count both for a few days and verify the ratio remains at
>> ten million to one, exactly.
> Ok, that’s looking at the long term where GPS is indeed accurate. That’s the easy part on any GPSDO design.
>>   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.
> Well, not quite so fast. You just jumped over a massive amount of work that normally gets done on a GPS. A unit that *was* swinging +/- 1x10^-8 every second would pass your test. (which is not in any way to say that your design actually does that).  It would make a lousy GPSDO for most uses. You very much *do* need to check the ADEV (or what ever) close in and tune your filter up to match your parts.
>> Cost:
>> 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
> Just a side note - A *lot* of the $19 OCXO’s I have from eBay are in very poor shape spec wise. Testing them before using them would be a very good idea.
> Bob
>> 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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