[time-nuts] LTE-Lite module

Chris Albertson albertson.chris at gmail.com
Sun Oct 19 22:14:29 UTC 2014

At the low end of the spectrum, I tried to make the simplest possible
GPSDO what would still work.  Assuming you have a GPS with  1PPS
output, an OCXO and a small DC power supply I was able to get the
entire parts for the controller, board, hookup wire and all for under
$5.  I purposely took the lowest cost solution at each decision point
just to see what you'd end up with.   Part were from eBay.

The result is not bad. but I don't have a really good way to test it.
I'm using a Thunderbolt for the 1PPS and a pretty decent OXCO part.
Why build a low-end GPSDO when yo have a Thunderbolt?  It's and
experiment.   The way I test is to place the sine output from the TB
and from my GPSDO both on a dual channel scope and adjust it so the
two sine waves are superimposed.   Then I wait for them not to be
superimposed.  What I see is that over 1/2 hour or so they get
slightly out of phase but then drift back in phase,  This happens
cyclically.   It is because of the VERY simply controller.  I tried to
minimize lines of C++ code.  It's running about 16 lines of code, more
or less.      Using my counter I think the GPSDO is good to  1E-10.

Rather than using a $15 ARM MCU board I used a $3 AVR board and used
100% 16-bit integer math in a very simple control loop.  There is one
external chip because the little AVR could not deal with the 10MHz
signal from the OCXO so I used a divider chip.  I use two 8-bit DACs
to control the EFC on the OCXO.  One is curse adjustment, one fine.
Added with a resister network and an RC filter with almost a 1 second
time constant.

If you can spend $35 you can build a very sophisticated controller
that logs internal diagnostic data to a computer over USB and displays
it's internal status on a graphic LCD panel.   Well, actually my
controller has an LCD status display and logs data to a PC.  But with
those parts plugged in the cost is closer to $10.

On Sun, Oct 19, 2014 at 2:13 PM, Bob Camp <kb8tq at n1k.org> wrote:
> Hi
>> On Oct 19, 2014, at 5:00 PM, Jim Lux <jimlux at earthlink.net> wrote:
>> On 10/19/14, 1:08 PM, Bob Camp wrote:
>>> Hi
>>>> 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.
>> I'm using the Freescale Kinetix K20 parts, which have 16 bit differential input ADCs, and built in averaging.  The raw ADC can sample at about 400kHz.
>> You can easily get 14 bit performance from these at tens of kHz rates.
>> I need I/Q, so I sample two inputs at 50 kHz (read one, then the other) without averaging (so they're about 2.5 microseconds apart), and then decimate them through a 2 stage CIC and a 13 tap FIR filter down to 200 Hz.  This takes about 60% of the processor running at 48MHz.
> I’m using parts from the same family, but not doing the whole DDS thing. Single input and control loop - the part sleeps about 98% of the time. The demo boards (Freedom boards) are all below $15 and free if you go to one of their (often free) classes.
> Bob
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Chris Albertson
Redondo Beach, California

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