Hi Bob, I have built several GPSDOs based on Lars's design; comments are below
On Fri, May 8, 2020 at 7:07 PM Robert Melville <bobmelvil...@gmail.com> wrote: > Fellow time nutters: > > Any input or advice on the following project would be appreciated. > > I want to make a small, portable, batter-operated clock that will > spend most of its time docked into a charging bay with access > to a signal from a GPS receiver -- either 1 PPs or 10.00000000 MHz. > The device will have a voltage-controlled TXCO that will be disciplined > by the GPS input unless the unit is traveling. > What accuracy do you want from this clock, and how long will it be un-docked? In GPSDO parlance, this is called "holdover mode" and would be specified as X usec max drift over Y hours. > > I have seen several designs for disciplined oscillators using a uP, > such as an Arduino or a PIC. The main decision seems to be between > 1 PPs or 10 MHz as the input. 1 PPs might be preferable because > not all cheap GPS receivers seem to provide 10.000000 MHz out. > Mostly, I want accurate time but a frequency reference with > decent phase noise would be useful. > > 1. I am tending towards a published design from Lars Walenius, in part > because > it uses a familiar uP (an Arduino) but am open to other designs. > As you have probably learned from Lars' design, it compares the local 10 MHz oscillator to a 1 pps pulse from the GPS. > > 2. What advice to people have on glitch-free switching when > docking/undocking the unit from the GPS??? > This seems to be something like the de-bouncing problem for a > push-button. > Lars' design will switch smoothly from disciplined from holdover mode. Basically if the PPS pulses stop, the default is to maintain the most recent DAC setting. It also maintains 5-minute averages of the DAC value over the past 12 hours and 3 hour averages over the past 18 days, so you could easily have it switch to an average value if you want. You would have to be careful to ignore glitches on the 1 pps signal caused by docking and un-docking, but this is not difficult. Depending on how long the unit has been un-docked, when the PPS is restored there may be a significant offset between the expected pps time and the actual time. You will need to decide how to deal with this depending on your application - "jam-sync" it to the correct time, or adjust it gradually to make up for the gained or lost time. > > 3. Has anyone used the Arduino time library withOUT the Dallas RTC chip -- > i.e., some other source of time such as the from the locked oscillator? > Yes, Lars' design maintains precise time by driving one of the Arduino counters from 5 MHz derived from your 10 MHz oscillator. It does not depend on the accuracy of the Arduino's clock. You can use this along with the Time library functions to display date and time. The basic time resolution is 10 msec, driven by a timer interrupt. You may also want to use an NMEA library to decode the time reported by the GPS on its serial port. I have used the TinyGPS++ library with good results. Also you might want to consider using a 32u4 based Arduino processor because it has a hardware serial port that can be dedicated to listening to the GPS. Most recently I have been using the Adafruit Itsy Bitsy 32u4. > > 4. Can anyone share experience with conditioning the power going into the > TXCO -- to what extent can digital noise from the uP or counters > contaminate the phase noise of the TCXO? Does a separate isolating buffer > help for the "osc out" port? I have had good success in the past with > so-called "active bypassing" to deliver very clean power to an oscillator. > > Others may wish to comment on the phase noise questions, but I think the power supply sensitivity will depend a lot on the oscillator you choose. High stability oscillators generally have built-in regulators that reduce their sensitivity to the power supply. Likewise, you will often see a spec on the effect of output loading on oscillator frequency. Thanks to all for your attention to this message -- I am glad to look at any > and all possible designs. > Happy time-keeping! > _______________________________________________ > 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. > -- --Jim Harman _______________________________________________ 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.