[time-nuts] LTE-Lite module
EWKehren at aol.com
EWKehren at aol.com
Sun Oct 19 20:23:50 UTC 2014
With all the work around if you want very good performance use a Shera. We
have super results with a Morion, Shera and ublox M7
In a message dated 10/19/2014 4:08:32 P.M. Eastern Daylight Time,
kb8tq at n1k.org writes:
> On Oct 19, 2014, at 3:35 PM, Charles Steinmetz <csteinmetz at yandex.com>
> 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.
If you take a look at some of the newer ARM MCU’s they are getting 13+
solid bits out of their ADC’s at a > 10 KHz rate. That’s more than good
enough for anything you are trying to do with this design. There’s no need to
make it any more complex.
A single gate XOR plus the eval board is just a about all you need. One
dead bug part on the eval board and the assembly process is pretty much done.
Maybe 45 minutes of work if you need to go find all the bits and pieces
around your bench. Since almost nothing in the design is running at high
speed, layout issues should not be a big deal. You could also do it on a
fragment of board like the divider from earlier in this thread.
>> 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
> 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.
The optimization process is at least 90% perspiration and preparation.
Neither of those are outside the range of what an average Joe can handle. The
other (at most) 10% is very much a “that depends” sort of thing. You can
head down all sorts of rabbit holes as you dig into this or that. For that,
the list archives have tons of information to work from.
There is *way* more in a GPSDO than what we are talking about here.
TimeNuts may or may not care much about that extra stuff, but it’s in there.
> 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
> Best regards,
> time-nuts mailing list -- time-nuts at febo.com
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The idea is not to make it as complex as you possibly could, but to make
it as simple as possible and still have it work fine. There are a lot of
shortcuts you can take with a one off unit that a commercial design would
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