[time-nuts] LTE-Lite module

EWKehren at aol.com EWKehren at aol.com
Sun Oct 19 16:23:50 EDT 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
Bert Kehren
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 
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  
> Best  regards,
> Charles
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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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