A Microcontroller Compensated Crystal Oscillator (MCXO) isdescribed that incorporates varactor compensation techniques and highprecision analog-to-digital and digital-to-analog converters. The analogcircuit design and microcontroller hardware are similar to thatpreviously reported, preserving the low cost, small size, and low powerconsumption. Design enhancements are reported which improve stabilityand spurious noise characteristics and add a “run-time”continuous calibration feature. Temperature stability is improvedthrough firmware enhancements that better utilize the capabilities ofthe MCXO, resulting in measured performance of ±0.1 ppm fromminus 40 to plus 85° C. Improvements in the decoupling betweendigital and analog circuits reduced spurious noise to below -85 dbC andphase noise to -130 dbC at 100 Hz. A component level thermal analysisidentifies those areas of the design that are most sensitive to thermalgradients. Analytical results are presented which indicate thatcomponent level temperature gradients, and differences in componentthermal time constants, are dominant sources of “run-time”frequency error. Thermal packaging and calibration temperature protocolimprovements are described which reduce differences in thermal gradientsbetween calibration and run-time operating modes. Finally, modificationsto the microcontroller firmware are described that support compensationfor aging. This aging compensation can be continuous or periodic, wherea typical implementation would re-calibrate automatically whenever areference standard input signal is applied to the MCXO
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