I’d like to thank Jim and Lars both publicly for that design - I quite shamelessly stole the phase detector for my own GPSDO gizmos. Jim at one point suggested adding a JFET to make the pulse with to voltage curve more linear, but that probably doesn’t make all that much difference. On my 5660 controller, there is no DAC - you send digital commands to affect the EFC. For my OCXO I use an 18 bit DAC compressed somewhat into the most useful part of the XO’s EFC range. My project page on hackaday is at https://hackaday.io/project/6872-gps-disciplined-xcxo
> On Sep 7, 2018, at 12:25 PM, Lars Walenius <lars.walen...@hotmail.com> wrote: > > I heard Jim mentioned my GPSDO. I would not go to the time nuts archive as > that describes a far from ready software design. The hardware is almost > identical. Last year I published a new design on EEVBlog: > > https://www.eevblog.com/forum/projects/lars-diy-gpsdo-with-arduino-and-1ns-resolution-tic/?all > > This is a, as I think, a much more robust design and much more tested. By > just setting the”gain” it will make a stable 1Hz digital PLL. As the DAC can > be set and you have a counter with 1ns resolution you don’t need any extra > instruments to do this. The nice thing is that the controller is very > flexible and can handle all kinds of 10MHz oscillators as XO, TCXO, OXCO and > rubidium. The useful control range of the oscillators is 0.001-6ppm. As only > a 1PPS with 3.3-5V amplitude is needed a wide variation of GPS modules can be > used. The time constant can be set from 4 to 32000 seconds and stored in > EEPROM. > > In the 21 pages of instruction I describe the design and also give > performance data as both a GPSDO and a time interval counter. As Jim says my > GPSDO gives useful information how you can do your own GPSDO. On the first > pages I have some discussion of the GPS modules like NEO7M that can be used > and also about different oscillators. Note I think the oscillators are the > first you should think of as they set the most important specs for the GPSDO. > See also the user manual for Stanford FS740 that contains a lot of > information about a modern GPSDO. Here the link to Stanford FS740 seems to be > broken in my instruction. Today this worked: > https://www.thinksrs.com/downloads/pdfs/manuals/FS740m.pdf See for example > pages 25-35 for a good description of the GPSDO part. In the end of the > manual you also find complete schematics! > > On pages 8-9 in my instruction you find a very short description of my > software and it is a standard PI-loop with a low pass filter before just as > in the FS740. Paul-Henning has a very simple introduction to the PI-loop that > I like http://phk.freebsd.dk/time/20141018.html . I would also recommend > learning from the GPSDO simulator by Tom Van Baak > http://www.leapsecond.com/pages/gpsdo-sim/ it also uses a PI-loop + low > pass filter and his code tells you a lot. > > From Tom’s GPSDO simulator you can also learn how the time resolution and the > DAC resolution work with real time data. Magnus Danielsson in March 2014 > published the result of some his simulations with different time and DAC > resolutions on time nuts. My takeaway was that a time resolution below 5-10ns > was ok and DAC resolutions in the ballpark of the oscillator ADEV was fine, > but have a look on his data. > > On page 15 in my instruction you find a list of basic links that I have found > useful. Some links seems broken so for example for Stanford’s you have to > find again on their homepage. > > On pages 16-17 you find help commands and description of serial information. > By the way Mark Sims excellent LH can read this and use the time, DAC and > temperature data that are spit out every second > http://www.eevblog.com/forum/metrology/lady-heather-v6-beta-for-windows-exe/?all > . This together with the GPS module of choice serial output data should > make this very useful. I am sorry I haven’t used this as I am very weak of > chemo and haven’t done anything to my GPSDO for a long time. > > Page 18 shows setup to read for example DAC data into Timelab. > > On page 19 you find some ADEV results of two of my prototype GPSDO’s. One > with a TCXO DOT050V and one with a OCXO 131(that by purpose uses a bad DAC > resolution of 1.2E-11 but still gets ADEV’s in the 5E-12 range. > > Last two pages show data with the controller used as a time interval counter. > By measuring the TIC with a PICDIV PD26 it is possible to enter a few > linearization parameters in EEPROM and get ADEV’s in the 8E-10 range at 1sec. > > Even if I don’t recommend reading specific about my GPSDO on time nuts I > really recommend to search and read the time nuts archive. It is a lot of > useful information and a special thank to Bob kb8tq who contributes with so > much valuable information. After I published the GPSDO on time nuts I also > got good help from Jim H with the hardware interrupt in the ATmega328. With > the hardware interrupt the 200ns resolution on timer 1 works well with a > jitter of just 40ns p-p. TvB also gave the idea to set the timer1 to an > factor that suits the input frequency that in this case is 5Mhz so I choose > 50000(-1) (instead of 65536-1). This gives overflow at 10ms and is easy to > handle. I could probably thank a lot more people for good advises. > > Lars > > Ps. Note that Lars GPSDO is more a 10MHz frequency reference, not a time > reference and by that I mean it has no jitter free PPS out. This can be added > by external hardware. > > > > > > ________________________________ > Från: time-nuts <time-nuts-boun...@lists.febo.com> för Jim Harman > <j99har...@gmail.com> > Skickat: Tuesday, September 4, 2018 6:28:00 PM > Till: Discussion of precise time and frequency measurement > Ämne: Re: [time-nuts] PLL/GPSDO/etc learning resources for mere mortals > >> The challenge seems to me to be > > how to temperature stabilize the entire DAC chain without putting it all in > an oven > > Hi Forrest, > > Generally you don't have to temperature stabilize the DAC because it is > inside the phase-locked control loop and the loop will adjust for > temperature dependencies, just as it adjusts for drift and temperature > dependencies in the oscillator. You will want your phase detector to be > reasonably temperature stable however. > > In the time-nuts archives you will find posts from Lars Walenius and myself > with schematics and code for Arduino-based GPSDOs which you might want to > study. > _______________________________________________ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. > <Minimum working schematic for GPSDO controller > 170629a.jpg>_______________________________________________ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
