Simple GPS Disciplined Reference

Posted on Feb 6, 2014

This design came about as a result of efforts to design a high stability reference which locked to a 10MHz source. With some minor hardware adaptation and completely new firmware, it proved possible to design an excellent GPS Disciplined Oscillator (GPSDO) using the same circuit board. The design gently locks a high performance 10MHz OCXO to the G

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PS or other 1pps reference, and is capable of providing an accurate reference, with high stability at the level of a few parts in 1012. The unit operates without requiring any intervention or control. A serial link provides telemetry and a user-set real-time clock. Controls are provided for initial calibration. The OCXO sine wave output can drive 50 Ohm loads (1V p-p). There is also an optional regenerated CMOS 1pps output which can be used to operate other devices, and it would be easy to swap this for a CMOS 10MHz output instead. The clock and reference continue to operate when GPS is lost, and although the reference frequency will then very slowly change due to OCXO ageing, it is held digitally at the current operating point until GPS returns. The clock will be accurate to parts of a second per year (provided power is not lost), even if GPS is occasionally or frequently lost. The unit is not difficult to build, although performance will be best if a double-sided PCB with extensive ground plane is used. There are six ICs and one transistor. One of the ICs in the photo above is not used. The prototype used surface mount components extensively, mostly to save space. There are no components on the back of the board except the LEDs, which are fitted when the unit is placed in the box. The circuit board shown above was designed to fit into the lid of a standard Hammond 1590WP1 diecast box on 10mm spacers. The high stability...

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