Hi Charles, I've changed the subject line, so I hope this isn't perceived as a thread-jack.
It's always dangerous to post anything substantive to this list, but I think it's about time. Here's a timelab file of my GPSDO, which I've been working on for quite some time with help from Dan K and others. It is just about ready to release. My unit is a frequency standard, not a time standard. IOW, I do not have a disciplined PPS output. What I do have is 10Mhz, 5MHz, and 1MHz outputs. The file is here, which is on my personal domain. It is compressed using gzip. http://www.evoria.net/AE6RV/GFS-6A/225.80.15.32.tim.gz Bob -------------------------------------------- On Mon, 4/4/16, Charles Steinmetz <[email protected]> wrote: Subject: Re: [time-nuts] Advise on building a DIY GPSDO? To: "Discussion of precise time and frequency measurement" <[email protected]> Date: Monday, April 4, 2016, 8:56 PM Don wrote: >5. Design a voltage tracking / filter to match the OCXO >control requirements. >6. And, ...ta-dah; >7. You have a 10MHz, bench frequency standard that will >rival all others Not very likely. The whole point of a GPSDO is for the frequency to be controlled by the more stable source (OCXO or GPS) at all integration times (tau). But the OCXO will typically be more stable than the GPS for tau less than several hundred seconds (see graph below -- black line is GPS, brown line is a typical OCXO). So, the PLL needs to have a time constant of hundreds of seconds. Such a PLL filter cannot practicably be designed in the analog domain, so one needs to design a digital filter with appropriate time constant and damping. Because of the very long time constant, it is almost necessary for the filter to have more than one, switchable time constants to avoid extremely long lock times. Very few home builders are capable of designing a proper digital filter suitable for this application (the counter-based loops of most published DIY GPSDO designs are not proper digital filters). So, no -- it is very unlikely that a home-built GPSDO will "rival all comers," whether the builder designs his or her own circuit or uses one of the many published circuits. Best regards, Charles Graph below. Note that a properly designed GPSDO would show stability that follows the OCXO (brown line) at low tau, and the GPS (black line) above the point where they intersect -- here, about 350 seconds. Note that a loop filter with proper damping will NOT exhibit a "hump" near the crossover (many GPSDOs do exhibit a pronounced hump, betraying that their loop filters are not properly designed). -----Inline Attachment Follows----- _______________________________________________ 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.
