On 12/05/2012 08:03 AM, Fabio Eboli wrote:
I'm seriously thinking to attempt a gpsdo.
It's mainly to learn something new.
For some reason I collected some Rb oscillators,
and I'd like to have a 10MHz absolute reference,
so I will try to discipline one of the Rb, and
later maybe an OCXO.
The project will proceed slowly and there is
some probability (small, but not null) that
it will be abandoned, because of time problems
of the author (could be a paradox?).
The platform I will try to use is the STM32F103
microcontroller
Coincidentally, my previous time-nut project was built around the same
chip. I built a simple GPSDO using a STM32F103C with a bit of support
circuitry, using the timer in "input capture" mode to timestamp pulses
and act as a coarse time-to-digital converter. I got a simple PLL
control algorithm working but haven't yet refined it so it tracks rather
poorly. My intent was to adopt some of the self-tuning attributes of
NTPns, which I will likely revisit for the next project.
Some more details about what was on the board:
- A NC7WZ14 CMOS inverter to square up the sine wave from the OCXO,
which then feeds...
- A PIC12F1501 as a programmable divider, using TVB's picDIV code
lightly modified to work on that particular chip
- The STM32F103 itself, which compares pulses from the divider to pulses
from the GPS receiver and makes adjustments via...
- A slow 16-bit DAC constructed from a PWM output on the STM32, a
two-pole RC filter, a buffer op-amp, and a third RC pole. This drives
the OCXO's frequency control. The PWM is also tweaked over 16
consecutive periods to add 4 more bits of precision, a sort of crude
pulse-density modulation.
- There's also an op-amp to buffer the 10MHz sine wave for 50 ohm
output, and a digital buffer for a 50 ohm PPS output from the divider
Here are the design documents, if you're curious:
http://hg.partiallystapled.com/circuits/serafine/raw-file/d75ab09ca163/out/production.PDF
The precise parts of course are not important, it's just an example of
things I chose to get the job done. The general shape of it is the same
as many, if not all, other GPSDOs out there. I'm reasonably happy with
the hardware as a GPSDO experimentation platform (but not looking to
sell anything at this time).
The current project, as I've mentioned before, is a self-contained
GPS-to-NTP server based on STM32F107, which has built-in ethernet but is
otherwise very similar to the F103. The finished board won't be nearly
precise enough to compete with a "real" GPSDO as it is based on a small
on-board VCTCXO but should shore up the algorithms enough for me to
revisit the GPSDO again.
-- m. tharp
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