A low cost high stability microcontroller compensated crystal oscillator
N. SCOTT DENO, C.L. Hahnlen, D.L. Landis, Peter G. Chin, J.K. Switalski · 2002
A Microcontroller Compensated Crystal Oscillator (MCXO) is described that incorporates varactor compensation techniques and high precision analog-to-digital and digital-to-analog converters. The analog circuit design and microcontroller hardware are similar to that previously reported, preserving the low cost, small size, and low power consumption. Design enhancements are reported which improve stability and spurious noise characteristics and add a "run-time" continuous calibration feature. Temperature stability is improved through firmware enhancements that better utilize the capabilities of the MCXO, resulting in measured performance of /spl plusmn/0.1 ppm from minus 40 to plus 85/spl deg/C. Improvements in the decoupling between digital and analog circuits reduced spurious noise to below -85 dbC and phase noise to -130 dbC at 100 Hz. A component level thermal analysis identifies those areas of the design that are most sensitive to thermal gradients. Analytical results are presented which indicate that component level temperature gradients, and differences in component thermal time constants, are dominant sources of "run-time" frequency error. Thermal packaging and calibration temperature protocol improvements are described which reduce differences in thermal gradients between calibration and run-time operating modes. Finally, modifications to the microcontroller firmware are described that support compensation for aging. This aging compensation can be continuous or periodic, where a typical implementation would re-calibrate automatically whenever a reference standard input signal is applied to the MCXO.