Every GPS receiver calculates its clock offset (phase error, in time-nut terms) as part of its four-dimensional position fix. It can apply almost the same math to calculate its clock rate (frequency error) as part of a four-dimensional velocity fix.
In "position hold" mode, the Thunderbolt calculates its timing errors (both phase and frequency) using similar one-dimensional algorithms. The accuracy of these measurements is determined by all the factors that affect GPS, but the precision (resolution) of these measurements is effectively infinite, limited only by IEEE double-precision floating-point math. Almost every other GPSDO uses a hardware time-to-digital converter (TDC, interpolator, etc) to compare the OCXO timebase to the PPS output of a separate GPS receiver. The PPS/TDC scheme has four disadvantages relative to the Trimble scheme: A) The resolution of the phase error measurement is limited by the TDC hardware. For example, the HP Z38xx units appear to have 10ns resolution. B) The accuracy of the phase error measurement may be degraded by analog effects in the PPS connection. C) The phase error measurement must be compensated by a software "sawtooth correction" for best accuracy, because the PPS output is quantized by the receiver clock. D) Frequency error cannot be measured directly, but must be derived from successive phase measurements. The derivative process introduces noise, so the derived frequency error must be heavily filtered. Unfortunately, the Trimble scheme is only available to GPSDO builders who have access to the internal architecture of their GPS receiver. Historically, among the major players, only Trimble and perhaps Zyfer did. Even mighty HP did not. Fortunately, SwiftNav is now selling a (mostly) open-source GPS receiver. Only the FPGA correlator chip is closed-source, and that can be ignored for timing purposes. One would need to build a VCXO-based synthesizer to create the SwiftNav receiver clock frequency from a good OCXO, add a DAC to control the OCXO, and do some software work to add timing functions to the receiver firmware. Adding WAAS corrections to this hypothetical open-source GPSDO could make it noticeably more accurate than a Thunderbolt. Cheers! --Stu _______________________________________________ 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.