On 24/03/14 00:08, Charles Steinmetz wrote:
Chris wrote:
4) why use a PPS as the communications link from GPS to GPSDO?
Generally, because that is the only precision timing output you get from
a GPS unit. The models with a 10kHz output were prized by the "simple
GPSDO" crowd precisely for the fact that the phase-locked 10kHz signal
made the control loop much easier to design (or, made an analog loop
*possible* to design).
Can I find a way to move the OXCO into the guts of a GPS receiver.
I don't know. Can you? That is essentially what the Thunderbolt does,
and it brings with it the tremendous advantage that it removes the
sawtooth error -- which is why some of us think so highly of that
design. But now you are really down and dirty, designing your own GPS
receiver. Are you competent (and, if so, willing) to do that? (You are
very, very far from a "simple" GPSDO if you are considering this.)
Honestly, I never understood why all timing receivers aren't designed
that way. It seems to be such a superior way to do things that I'd have
thought the other approaches would have died out once everybody knew
about it. Even if Trimble has a patent, I can't imagine it would be
that expensive to license. But even Trimble doesn't use it on all of
their timing receivers.
Anybody know why this scheme hasn't been used more widely? Is there
something I'm missing?
For the more expensive GPS receivers, they have been using OCXO, some
even suspended OCXOs. Also, as an option you can connect an external
clock for additional stability, and for some you can steer this clock.
Many of the OEM GPS boards is just... el cheapo.
So, you need to look beyond those to find other solutions.
For a GPS receiver itself, it doesn't need a clock to be sharp on time,
as most of them is designed to handle a clock at the wrong rate. What
they however do is learn and compensate for phase and frequency errors.
Also, the performance of the clock will affect the produced results, and
αΊƒhen doing precision measures of code-phase for L1 and L2, the benefit
is certainly there.
Cheers,
Magnus
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