Hi Keep in mind that watching the OCXO in a fixed ambient may not tell the whole story. Setting a PLL to 10,000 seconds on an OCXO and then ramping the temperature 40 or 80C is likely to create issues…..
It’s very easy to fall into the “I only have a room that swings 0.2C” loop when looking at OCXO’s or systems. The main design goal on an OCXO is performance when the temperature moves 50 to 120 C (depending on the design). Even GPSDO systems have temperature profile specifications. A common assumption is “we loose the GPS and the air-conditioning at the same time”. Another one is “this goes in a hut out in the sun”. Bob On Aug 22, 2014, at 8:39 PM, Charles Steinmetz <[email protected]> wrote: > Bob wrote: > >> The GPSTM is not as tweak friendly (no filter changes allowed) as some of >> the other GPSDO's. > > And that is a major problem. The correct filter settings for a Rb local > oscillator are very different from the settings for an OCXO, which in turn > are different from the correct settings for a TCXO. > > As a general matter, almost all of the DIY GPSDO designs I have seen use PLL > loop filter settings that are not optimal. Many are not even close (several > orders of magnitude, or more, from optimal). > > Generally speaking, the PLL loop filter cutoff should be set approximately > where the GPS xDEV curve intersects the local oscillator xDEV curve. That > puts the better device (GPS or local oscillator) "in charge" of the composite > xDEV at all tau -- the local oscillator at short and medium tau, and the GPS > at long tau. Optimal crossover tau will generally be in the range of seconds > for a TCXO, hundreds of seconds for an OCXO, and hours to tens of hours for a > Rb. > > Sometimes, there are good reasons to depart from this general rule. In > particular, if a speedy recovery from holdover is required, then one might > choose a PLL filter cutoff tau that is lower than optimal. The default > crossover tau for the Trimble Thunderbolt is chosen quite low, presumably for > this reason. See, for example, <http://www.leapsecond.com/pages/gpsdo/>, > where the GPS contributes significantly to the ADEV way down at tau = 1 > second, where the local oscillator is clearly much better than GPS and > continues to be for more than two decades. The Miller DIY GPSDO on that page > is crossed over about 3 decades lower than optimal. (The Miller GPSDO uses a > Shera DIY controller; I presume the Shera has the same crossover tau.). > > Compare this to the HP z3801A and Jackson Labs Fury on the same page. The HP > crosses over about 2 decades higher than the Thunderbolt and Miller GPSDOs, > but that is still premature by about two decades given the very high quality > of the OCXO in that particular unit. The Fury crossover is set well, but the > overall ADEV is let down by the low stability of the OCXO in that particular > unit. (Note that the crossover in commercially produced GPSDOs must > accommodate the range in production ADEVs of the local oscillators used, and > are likely set a bit lower than optimal for most of the actual OCXOs on this > account.) If the filter parameters are adjustable -- as they are in the case > of the Thunderbolt -- then a time nut can tune his or her individual sample > to get the best possible performance that particular oscillator can deliver. > > As I have mentioned before, rather than just setting the time constant low to > speed up holdover recovery, a better solution is to implement a switchable > PLL loop filter. A GPSDO designed this way uses a suitably long time > constant for normal locked operation to minimize xDEV at all frequencies, and > a faster time constant for turn-on warmup and holdover recovery. It is > rumored that the z3801 is designed this way. > > Best regards, > > Charles > > > > _______________________________________________ > 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.
