I'm working on a GPSDO but with different goals. I want mine to be 1) very low cost, under $50 for everything if I can 2) No PCB required. 3) very easy to replicate by a first time builder 4) Easy to understand. The parts count is very low, no "exotic" parts and the software written very clearly so the code reads like the tutorial for a beginner.
I started with Lars' Arduino based design and I've making slight mods. But shipping from China takes a month and I'm waiting on parts. I expect only 1E-11 level performance The next one I build I want to be different. I don't need yet another copy of an old design. 1) Can I combine two oscillator technologies to get the best of both? Perhaps phase lock an OCXO to a Rb and then discipline the Rb's frequency. 2) is there some good way to control the temperature of the entire assembly? Perhaps dunk the entire thing into a container of transformer oil? Or use thermal epoxy to connect all the critical parts to ONE common heat sink and then keep that at constant temperature. First I need some way to measure temperature very accurately. 3) are 10MHz crystals the most stable ones? I bet there is a sweat spot frequency that is better. Would 50MHz or 5Mhz be better? 4) why use a PPS as the communications link from GPS to GPSDO? Can I find a way to move the OXCO into the guts of a GPS receiver. That may mean I have to build a SDR based GPS receiver. But first the simple one. I'll post progress reports and photos On Sun, Mar 23, 2014 at 11:32 AM, <[email protected]> wrote: > Hi > There are many issues when it comes to a GPSO. But what has to be first > discussed what is it one wants to accomplish. Last year when we worked on the > latest Shera GPSDO we always got better than 1E-11 with a unit lying on > the bench with no enclosure or thermal management. > Chasing elusive 1 E-13 and better, allow me to make a couple of comments. > In order to get there, the total system has to be under review. Since I > know nothing about writing programs I leave that to smarter people but be > clear software and code will not do it by it self. The most critical part is > the thermal management of the OCXO or Rb and if analog control is used the > DAC.and if used its output amp. We are controlling the back plate of the > M100 and FRK to within 0.01 C and the front 0.1 C. The DAC board and the > temperature controller are on the front, Voltage regulators on the back. > After extensive testing the LTC1655 is our preferred choice. Take a close > look specs are great for this application and most important solderable. > There are better DAC's out there but very expensive and I am not able to > solder. 18 bits would be nicer but 16 bits are for Rb's usable. The DAC part > has to have its own ground plane because ground loops can create noise and > voltage changes it has to be tied as close and separate to the OCXO or Rb. No > opto Isolation necessary as long as the controller and DAC are in the same > box, sharing the same system ground. Input to the DAC can handle wide > ground variations. Took me years to find that out. > Absolute must how ever is opto isolation between GPS, controller and PC, > again found out the hard way. > The other part I like to touch on is the GPS input section. I am not a > time nut but a frequency nut, but there has been so much talk in the past and > more recently about sawtooth. I am disappointed but not surprised that no > one has stepped up and offered a solution. The site has deteriorated to a lot > of talk very little action. Right now there are still affordable and > solderable DS1023's out there. Combined with a 12F629 or 12F1840 a sawtooth > correction is possible for much less than $20. I bought last year four DIP > and > 10 in SOIC DS's average price below $ 5! Even though I had a very bad > experience once with a Dutch so called time nut I am willing to make the > following offer. For the right person I make a board, PIC and DS1023 > available. > Maybe I just missed it but I do not think that there is something out there > readily available. > Bert Kehren > > > In a message dated 3/23/2014 9:02:59 A.M. Eastern Daylight Time, > [email protected] writes: > > Hi > > The real answer is always "that depends". > > 1) How much does the sensitivity of your OCXO change with a change in EFC? > 1.4:1, 2:1, 4:1 .... (slope sensitivity not % linearity) > > 2) How quiet is your DAC compared to your OCXO? > > 3) How quiet is your reference compared to your OCXO? > > 4) How much do the DAC, reference, op-amps, resistors, capacitors, ... drift > with time? > > 5) How much test time is enough? (hours, days, weeks ,.....) > > 6) How good is the survey on your GPS this time? > > 7) How much does your room temperature impact your OCXO when you do this > or that? > > 8) Is your room temperature representative of the real world? (is mine > like yours?) > > 9) Do you intend this gizmo to work over a temperature range? Did you test > that range? > > 10) Are you trying for best frequency or best time? Is your definition of > time "GPS local time"? > > 11) Are there voltage drops on your real board? Do they change with > anything? (or everything ?) > > 12) Does your controller generate spurs inside the control loop and > modulate the output with them when tuned to an offset of > x.xxxxxxxxxxxxxxxxxx Hz? > > 13) How do things respond to load changes or supply voltage changes? > > 14) Are the parts (OCXO, reference, dac, op amps ...) responses to > temperature, load, supply, tip, tune, linear / immediate or do they have > artifacts > that extend out over longer time periods? > > This is by no means a complete list. A lot of common GPS issues are > notably absent. However, I've seen designs fail or fall short for problems > related to every item on that list. Can you put this all in a model - sure. > Did > you put all this in the model .. .. > > Bob > > > On Mar 23, 2014, at 5:06 AM, Bill Hawkins <[email protected]> wrote: > >> An idea is struggling to take shape in my fevered brain. I'd like to >> check some foundation assumptions. >> >> 1. The difficulty with disciplining a local oscillator to a GPS signal >> is due to variations in the received GPS signal and the LO. >> >> 2. The variations occur slowly, as crystal aging, and quickly - perhaps >> sawtooth or crystal crack propagation - and maybe something in between. >> >> 3. The gain of the system, in degrees of phase angle at 10 MHz (or >> higher) per microvolt of control signal, is fairly constant in a >> controlled environment. >> >> 4. The power supply for the device providing the control signal cannot >> be regulated to the accuracy required of the system, and so is a source >> of variance. (Does anyone put the voltage reference device in the oven >> with the crystal?) >> >> 5. The principle source of environmental variation is temperature. >> Humidity and barometric pressure are not significant. This may not be >> true of the received GPS signal due to atmospheric variations. >> >> 6. A digital computational device is available to calculate the control >> signal from various measurements and previous values. >> >> 7. There are no supernatural forces at work, such as the experimenter >> mentally influencing the results. :-) >> >> That's a start . . . >> >> Thanks for any replies. >> >> Bill Hawkins >> >> >> >> >> _______________________________________________ >> 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. > _______________________________________________ > 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.
