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>).
Best regards,
Charles
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