[time-nuts] Thunderbolt vs Z3801A
magnus at rubidium.dyndns.org
Sun Jul 20 21:25:17 EDT 2008
Didier Juges wrote:
> In my unqualified opinion, Trimble did something very smart with the
> 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
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.
> 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
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.
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