On Sun, Aug 16, 2015 at 3:31 PM, Tom Van Baak <[email protected]> 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 time. 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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