[time-nuts] ARM boards for low-cost GPSDOs

Chris Albertson albertson.chris at gmail.com
Fri Apr 11 00:11:25 UTC 2014

I'm thinking about good reasons to build a GPSDO using something as big and
powerful as a 32-bit ARM processor.    I think the reason is that you are
not really building a GPSDO but some other device that just happens to have
a GPSDO inside of it.

For example you want to build a laser range finder and you need to measure
time of flight delay.  You'd need a very good clock and while you are at it
why not discipline the clock with GPS.      I could think of some radio
experiments where I would want pairs of receivers with their local
oscillators running in phase but many miles apart, so I'd build a GPSDO
into the radio.    The ARM would support running the GPSDO, the bigger
application and also remote access over the network or Internet.

Today people mostly will build a stand alone GPSDO in a dedicated box and
then connect the 10MHZ output to whatever is needed but now as we have
seen, you can build a GPSDO completely in software, if your project already
has a computer then you can run a GPSDO inside an interrupt handler as a
background task.  Adding GPSDO functionality to an existing product is
almost trivially simple, just $2 in parts and some software if you already
have a CPU and OCXO as part of your system.

Placing the GPSDO inside the product means the gpsdo can run at a frequency
that is more useful and needs no conversion.   So you can have the GPS
control a 23 Mhz crystal if that is what your laser rangefinder needs.  Now
that the cost of a GPSDO has fallen to $3 you can build them into
"everything".  It no longer needs to be a shared device.

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