Dear Didier, Didier Juges wrote: > In my unqualified opinion, Trimble did something very smart with the > Thunderbolt. > > Most previous GPSDOs use a stand alone GPS receiver, with its own CPU clock, > to generate the 1PPS signal, to which a separate OCXO is servoed via PLL and > occasionaly smart software. > > The basic problem I see with this is that because the GPS receiver is > clocked with a (20, 25, 40 MHz, or anything in between, pick your choice) > crystal of generally dubious quality, the PPS signal is aligned with one > edge of that CPU clock, and therefore the PPS signal error will fluctuate > over that 1/f period. The GPS receiver may know by how much the PPS is off > (and some tell you so), but it cannot align the PPS between two successive > CPU clock edges, it has to choose one, so the PPS is adjusted with quantum > leaps of 1/f, typically 20 to 50 nS. > > Because Trimble integrated the GPS receiver and the OCXO, they use the OCXO > as a clock to run the processor, therefore that source of error is not > there.
First of all, so called sawtooth correction overcome part of this problem, but only to the degree by which the sawtooth correction reports corrections (often in steps of 1 ns) where as the Thunderbolt seems to report in 10 ps steps during locked conditions. There is another aspect to the use of a good OCXO rather than standard TCXOs and that is that the lower noise allows for a better "position" in space and time as there is less noise in the input signal. Especially at lower frequencies/longer times. Locking the LO to the GPS allows for a different kind of clock error handling than normal OEM GPS recievers typically use. > The ultimate performance is not better than a perfectly optimized > "conventional" GPSDO + OCXO, but excellent performance can be achieved at > lower cost, and without having to deal with certain tradeoffs. Certainly. > Now, I am sure there are other, more subtle details that explain why the > Z3801A is generally superior to the Thunderbolt, but that I cannot > explain... It is strange considering the 100 ps resolution TI counter in the Z3801A with the TCXO-GPS beating and high sawtooth error of the Oncore VP. Locking the OCXO up and using the GPS receiver as a phase comparator to GPS time is a more direct approach than the acquward PPS method. You always gain alittle by doing it through the LO. Cheers, Magnus _______________________________________________ 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.
