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. 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 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 <j99har...@gmail.com> wrote: > > On Sat, Dec 13, 2014 at 9:36 PM, Bob Camp <kb...@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 > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > 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 -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.