[time-nuts] LTE-Lite module
kb8tq at n1k.org
Sun Oct 19 16:08:09 EDT 2014
> On Oct 19, 2014, at 3:35 PM, Charles Steinmetz <csteinmetz at yandex.com> wrote:
> 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 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.
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 <http://leapsecond.com/museum/manyadev.gif>).
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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 never use.
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