A Low-Power Miniature Thermoelectric Controlled Crystal Oscillator (TECXO)

Abstract

This thesis aims to advance the field of high-stability crystal oscillators through the novel application of thin-film thermoelectric temperature control technology. A low-power miniature thermoelectric controlled crystal oscillator (TECXO) has been designed, manufactured, controlled and qualified for the first time. The TECXO employs a thin-film thermoelectric device to either remove heat from or add heat to a crystal oscillator in order to tightly control the temperature, and by consequence, the output frequency of the crystal oscillator. A dedicated off-board control and measurement system has been developed for the TECXO, and a proportional-integral controller has been implemented on this system to regulate the internal temperature of the TECXO. A tuning method for this controller has been developed to provide a balance between warm-up time, steady-state stability and ambient temperature change rejection. A novel non-linear compensation algorithm for thermoelectric modules has been proposed and validated as part of the temperature control system, providing a significant improvement to control performance. The temperature controller has been demonstrated to provide temperature stability of ±0.05°C under ramp ambient temperature conditions up to 2°C/min, warm-up time of less than 20s, and regulation of the TECXO's internal temperature up to 30°C below ambient. A novel temperature compensation process has been proposed and validated as improving the frequency stability of the prototype TECXO from ±151ppb to ±13.7ppb over a temperature range of -20 to 70°C. This process significantly reduces the negative effect of static thermal gradients within the TECXO on frequency stability over temperature. The performance of the TECXO has been qualified: power consumption is 76mW at an ambient temperature of 25°C and 2.7mW at an ambient temperature of 50°C, frequency stability is better than ±10ppb over an ambient temperature range of 0 to 85°C, the short term noise floor is 0.15ppb at a measurement time of 10s, and the frequency warm-up time is less than 20s following power-on. As a result of the qualification process and comparison to existing crystal oscillator products, the TECXO has been shown to provide dramatic performance advantage in power consumption, ambient operating temperature range and warm-up time over traditional temperature controlled crystal oscillators.