Minimal-GPSDO Files

                             Project folder
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Why choose this GPSDO
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With so many home grown GPSDO designs to choose from, why choose this one?

It has several features I believe are unique:

1. the discipline algorithm allows the oscillator to 'cruise' for many minutes without a change to the control voltage. This takes advantage of the short term stability of the oscillator. It has several benefits:
    * It is immune to the inherent jitter of the 1PPS (1 pulse per second) output of most GPS modules
    * It is also immune to short excursions of the 1PPS that happen with low cost GPS units with less than ideal antenna
    * By comparing the local oscillator with the incoming 1PPS, a fair estimate of the oscillator's short term frequency can be made. This cannot be done with most designs as they alter the control voltage continuously. All GPSDO designs will show good figures when averaged over time, for some uses (for example a down converter) short term stability is more important.

2. The dithered PWM (pulse width modulator) produces a control voltage variable in less than 1µV steps. Many GPSDO designs use the equivalent of a 16 bit DAC, resulting in 75µV steps. With a low cost oscillators with pulling of 10Hz/V a 75uV step causes a frequency change of .75mHz which places an absolute lower limit on the accuracy of the oscillator.

3. The control algorithm is unusual and progressive. It will accept small deviations of the 1PPS while the oscillator is 'cruising' but acts quickly to correct larger deviations. In practice it is phase locked to the 1PPS after a short time and tenacious in maintaining the lock.

Some other features:

* Minimal components to get a working GPSDO

* one LED indicates status. It is easy to see if the system is running, if there is an error, if it locked to the 1PPS, if it is running within specification. No need to peer at a small screen. Cannot mistake a power light for a lock light as there is only one.

* a comprehensive user facility via a 9600 baud serial interface. This can be configured for pass through of the NMEA data or control and logging of the GPSDO.

* the software has a calibration phase that eliminates the need for any user tuning. It is designed to handle an arbitrary 10MHz oscillator as long as it can be pulled ±0.25Hz.   

Folders in this repository:
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LatestReleases
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This contains the latest version of the software (source and compiled). Past versions
are not retained as the current version either improves functionality or fixes bugs 
(or both).

It will include instructions to update the software via the serial uploader.

The latest release was created 2025-03-21.

Construction
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Files relating to development and current design of the circuit.

Documentation
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Various files about the design and operation of the GPSDO