A 0.45 ppm and low phase noise analog crystal oscillator using a four order temperature compensation algorithm

Based on the analysis of the limitations of traditional compensation methods for analog temperature compensated crystal oscillators (ATCXOs), a new compensation algorithm and compensation network for ATCXOs are proposed in this paper. According to the frequency-temperature characteristics of AT-cut quartz crystals, the gain-temperature characteristics of voltage control crystal oscillators (VCXOs) are firstly derived and approximated by cubic functions. To improve the gain-temperature characteristics of ATCXOs, a quartic compensation algorithm and network are derived using curve fitting technique. A proposed design is implemented using 0.40 µm BCD process. Chip measurement results show that the frequency-temperature stability is within ±0.45 ppm over a temperature range of −40 °C to 85 °C. The phase noise levels are −87 dBc/Hz, −113 dBc/Hz, −137 dBc/Hz, and −152 dBc/Hz at 10 Hz, 100 Hz, 1 kHz, and 10 kHz frequency offset of 26 MHz AT-cut crystal, respectively. Compared with existing ATCXO designs in the literature, our proposed ATCXO chip improves the frequency-temperature stability by at least 55%, while maintaining a similarly low level of phase noise.