[time-nuts] I've designed a GPSDO, but how "good" is it?

Jim Harman j99harman at gmail.com
Sun Aug 16 18:15:56 EDT 2015

On Sun, Aug 16, 2015 at 3:31 PM, Tom Van Baak <tvb at leapsecond.com> wrote:

> I was hoping someone would someday use a cheap Sparkfun / Parallax /
> Adafruit GPS to make a low-cost GPSDO.

I have made such a device using the Adafruit module and an Arduino
processor. The design is based on the one posted here by Lars Walenius last
year. My design constraints were basically:
-- no surface mount components that I have to solder myself
-- no custom PC boards
-- no built-in display, runs stand-alone or with a PC as a monitor

My oscillator is a 10 MHz C-Mac STP-2322 from the auction site.

Like Nick, I do not have an independent reference to compare it to, but it
is set up with a laptop as a monitor so I can plot the TIC output in real

I have attached a screenshot that shows the TIC variation over 24
hours.This is with a filter time constant of 2048 seconds. The black line
at the center is the raw TIC value, sampled every 86 seconds, and the red
line is a smoothed version of that. The vertical scale is approx. 100 ns
per division (range is 300-700). As you can see, TIC variation is typically
about +/- 40 ns. The longer term variations are very repeatable from day to
day, probably due to the periodicity of the GPS constellation.

I am using Adafruit's external antenna located in my attic with 10m of
extra cable. As often observed here, the variation is much worse if I have
a poor antenna location. It would probably be better if my antenna was up
high and outdoors.

The blue line at the bottom is the value being sent to the DAC and the
green line at the top is the ambient temperature

--Jim Harman
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