[time-nuts] GPS DO Alternatives

Dennis Ferguson dennis.c.ferguson at gmail.com
Sun Dec 9 22:43:47 UTC 2012

On 9 Dec, 2012, at 02:14 , Hal Murray <hmurray at megapathdsl.net> wrote:
> Which brings up another worm for the can.  How are you going to get a PPS 
> from the OCXO?  That's going to be hard without some soldering.  My straw man 
> would be to use the OCXO to clock whatever uP you end up using and generate 
> the PPS with a counter/timer.

I think I'd do that too, and pick a processor with enough of the right
peripherals to avoid having to fart around with an external phase comparison
or interrupt timestamping.

The TI Stellaris LaunchPad, mentioned earlier, is pretty cheap, the
processor can generate its internal clocks directly from a 10 MHz
reference input, and it has a set of 32 bit timer/counters that can either
capture a timestamp for an input edge event or generate a timed outgoing
pulse, both with a resolution of 12.5 ns.  The capture could be used to
time the GPS PPS (in hardware, no external comparator, no interrupt
timestamps) while I think the PWM generation mode could be used to generate
a PPS output synchronous with the 10 MHz oscillator.  The only thing that
is missing for a GPSDO is an internal DAC (it has a bunch of ADCs) so the
10 MHz oscillator would need a digital frequency adjustment for the
processor to be sufficient for the job without any external peripherals.
The processor core is a fully functional ARM, including floating point,
which might make it easier to, say, do a Kalman filter implementation
that wasn't write-only.

The thing is, I still don't find this all that compelling.  Personally
I think it might be better to maybe aim higher on the software value
chain, find a processor board with the same on-chip processor peripherals
as above along with an on-chip Ethernet MAC and enough external peripherals
to run a real operating system and use that to build a combination GPSDO
and very accurate NTP/PTP server.  You generally can't just buy one of
those on eBay (at least they are much rarer than a GPSDO alone), and it
is a lot easier to do something which is attractive both in price and in
quality compared to what you can buy.

Dennis Ferguson

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