[time-nuts] Which First GPSDO to buy?
kb8tq at n1k.org
Sun Dec 14 10:00:53 EST 2014
> 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.
> 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.
> 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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