> That looks like a classical underdamped PLL response. You usually see this > sort of hump in a phase-noise plot when your PLL is about to break into > oscillation. :) > > Either someone at HP was careless with the loop-filter constants and came > close to running out of phase margin, the Z3801A is not configured properly, > or (who knows?) something fundamental about the disciplining process changed > when selective availability was turned off. > > It looks like the disciplining loop bandwidth should be reduced, and/or the > phase margin should be increased. It's at least an order of magnitude too > wide. > > -- john, KE5FX
S/A is one factor; remember that all the HP SmartClocks were manufactured back when S/A was on. So the optimal loop constant today could safely be a bit longer from the optimal loop constant back then. I assume S/A won't ever be turned on again? For time-nuts newcomers, see: http://www.leapsecond.com/pages/saoff/ or http://www.google.com/search?hl=en&safe=active&q=gps+selective+availability Another factor is the OCXO itself. I assume HP was conservative and picked a firmware loop constant to cover the spec for the 10811 OCXO inside the GPSDO. If you happen to get a 10811 that is better than usual, as #4 was, then the filter appears today to be less than optimally tuned. One of the reasons I did that Z3801a oscillator experiment http://www.leapsecond.com/pages/z3801a-osc/ was to show that not all OCXO are the same, but all meet spec. A final factor might be the environment that the GPSDO is designed for. The SmartClocks were designed for cell sites which perhaps have their own set of extreme weather conditions, To keep look you want to err on the side of a short time constant rather than assuming scummy days. This brings up an issue. When amateurs make GPSDO they almost always can do better than a manufacturer because they can tweak the loop-filter to exactly match their particular GPS receiver, the type of antenna, it's location, the level of environmental extremes, and the measured quality of their LO. A GPSDO manufacturer, on the other hand, must create a product that meets their published specs, regardless of actual temperature, location, or variation in the OCXO that they purchase from a third party. The Trimble Thunderbolt and Jackson Fury allow one to tweak the filter parameters. In this way, an amateur user can tweak the loop parameters. To do this reliably, though, usually requires a good local frequency reference that is more stable than the GPSDO itself for tau up to about 1 hour; an Rb or Cs, perhaps. /tvb _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.