[time-nuts] LTE-Lite module
csteinmetz at yandex.com
Sun Oct 19 19:35:18 UTC 2014
Bob wrote (alluding also to something Poul-Henning wrote):
>The phase comparison part of the PLL is pretty
>straightforward if you are looking at two RF
>frequencies. An XOR gate is one solution, there
>are many others. Getting something like 100 to
>200 ns full scale on the phase comparator makes
>the rest of the gizmo much easier.
All true. However...
>A 12 bit ADC on a MCU will get you to 100's of
>ps per bit. That is more resolution (it's < 1 ns) than you need for this.
Getting an ADC to sample fast and accurately
enough to provide that honest resolution is not
trivial. And if you have that, you'll almost
certainly have the resources to do the phase
comparator digitally, too, which brings many
advantages -- so I see no reason to use an analog PC.
>Custom code wise, it's a few hundred lines of C
>on a 32 bit ARM. Pre built (wizard driven)
>device init stuff will be way more than that, but you don't write any of that.
A proper digital filter that computes a new
running value at least every second will be more
complex than that, but you're right, it's not an unfathomable task.
Then comes the real work, well summarized by Bob:
>Debug, optimization and tweaking are where the
>major effort is (like 80 to 90%). That will take
>at least few months of work and require some
>test gear. Any time you plug in a significantly
>different oscillator, you will have to put in
>this part of the effort. Getting the long run
>ADEV data, making sure it's right, and then
>analyzing the result is something there is no magic shortcut around. * * *
>No it's not a "plug in a pre-made gizmo and
>forget about it" sort of thing. There is real
>work, lots of time, mental effort, working
>gear, and patience involved. You *will* get it
>wrong more often than you get it right as you go through the process.
All of this explains why the woods are not full
of state-of-the-art GPSDO controllers just
waiting for people to couple them with whatever OCXO they bought on ebay.
BTW, I mean no slight to the LTE-Light. Judging
from the JL products I've used, I expect that it
is a fine product well-designed for its
task. But that task is controlling a TCXO, not
controlling an OCXO that is stable to 10e-12 or
better at tau from 1 to 100 seconds (unless one
goes to the trouble described above).
For a general look at the magnitude of the
stability difference between a TCXO and a number
of OCXOs and other frequency standards, see
attached (if the pic doesn't make it through the
listserv, see <http://leapsecond.com/museum/manyadev.gif>).
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