Generation of Precision Reference Clocks Utilizing Digital Techniques

Access to accurate distributed clock time is a key to successful operation of many commercial, financial, and military systems. The alignment of remote clocks and the measurement of time became important for longitude (east-west) ocean navigation in the early 18-th century. Chronometers, accurate mechanical clock movements of that bygone era, cannot maintain the accuracy required of modern timekeeping systems. In the modern era, electronic oscillators, using the piezo-electric effect exhibited by quartz crystals, meet many of our required timekeeping specifications. The vibrating crystal operating in the narrow frequency span between its series and parallel resonance modes set and control the oscillator operating frequency. The crystal oscillator (XO), operating frequency is affected by temperature and aging of the crystal. To counteract these undesired changes the XO, can be operated in a number of temperature compensated modes (TCXO). A higher performance option is an oven controlled (OCXO) mode. Ovens consume power, power not available in many applications due to power supply and heat dissipation concerns. An alternate to controlling the temperature of XO is to discipline it to GPS satellite clocks (GPSDO). All these options use the capacitance of back biased diodes (varactor diodes) to pull the parallel resonance of the crystal. A VCO cannot compensate for the temperature shift of a crystal oscillator experiencing the wide change in temperature as might be found in a Low Earth Orbit (LEO) satellite moving in and out of the Earth's shadow. The amount of available frequency pull of a crystal oscillator is smaller than the shift due to temperature variation. Our solution to this quandary is an interesting variant of the temperature compensation process. We do not alter the frequency of the oscillator but rather control the phase progression of a Direct Digital Synthesizer (DDS) with DSP based predictive additive dither and look-up sine-cosine table to convert a temperature drifting crystal input frequency to a stable fixed output frequency. The DDS can also be applied to the task of reducing levels of close-in, low frequency, phase noise which essentially eliminates the 1/f
³
and 1/f
²
spectral responses of conventional feedback oscillators. This variant will be addressed in a companion paper.