Hi > On Dec 14, 2014, at 12:47 AM, Chris Albertson <[email protected]> > 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 <[email protected]> wrote: >> >> On Sat, Dec 13, 2014 at 9:36 PM, Bob Camp <[email protected]> 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 >> _______________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. >> > > > -- > > Chris Albertson > Redondo Beach, California > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
