[time-nuts] Thunderbolt vs Z3801A

Didier Juges didier at cox.net
Sun Jul 20 20:56:40 EDT 2008

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

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


> -----Original Message-----
> From: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] On Behalf Of Alberto di Bene
> Sent: Sunday, July 20, 2008 5:32 PM
> To: Time Nuts
> Subject: [time-nuts] Thunderbolt vs Z3801A
> Somebody had to pose this question... given the respective 
> sizes of the Trimble Thunderbolt and the HP Z3801A, the 
> simplicity of the first and the complexity of the second, it 
> comes natural to ask ourselves, what advantages does the HP 
> unit have to justify the differences ? Is the Z3801A really 
> so better than the TB, or simply did Trimble do a much better design ?
> Alberto  I2PHD
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