[time-nuts] Looking for GPSDO for home use

Chris Albertson albertson.chris at gmail.com
Wed Apr 16 05:21:26 EDT 2014

On Tue, Apr 15, 2014 at 10:09 PM, David J Taylor <
david-taylor at blueyonder.co.uk> wrote:
> []
> Even 1 PPS output seems like a workable starting point, but at the
expense of a different and/or more difficult path to get to a 10 MHz
reference signal I seek.
> Any advance or pointer to source (reasonable cost, whatever that means!)
would be appreciated.

Any GPS receiver with 1PPS is OK.    All PPS signals are interchangeable.
 It is just a 5 volt 1Hz square wave.  The raising edge of the wave is
right at the "tick" of a new second.

The GPSDO is simple too.  It counts the cycles of the 10MHz oscillator from
one PPS raising edge to the next and it should get exactly 10,000,000
cycles.   If more or less are counted the software moves the voltage on
OXCO's control pin up or down.

Controllers can be more complex, but this much will get you started.  The
simplest next step is to count for 10 seconds and get to 0.1 Hz then add an
interpolator and get to milli Hz

If you are going to buy and set up a GPS receiver. The hardest part is the
antenna.  It is best if it can see the entire sky, horizon to horizon and
if it is not near any reflecting surfaces.  It is best if the antenna is
mounted on a mast on the tallest building but a modern GPS will work if the
antenna is playing on the desk near a window.  You can connect a computer
and get software to plot data from inside the GPS but really all you need
to go is apply power and get the PPS.

The old Motorola "Oncore" series of GPS is reliable and low cost.   The
"UT" has a PPS one sigma error of about 50 nanoseconds which is "good
enough"  they sell for under $20.   The current new state of the art
version is about $60 or $35 used.
Here is an example
Be SURE to buy the "timing" version.  There are non-timing or navigation
versions.  Make sure it says "timing" in the description.

GPS receivers spew out tons of data but you can ignore it all.  All you
need is the PPS signal.


Chris Albertson
Redondo Beach, California

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