Analog ALC crystal oscillators for high-temperature applications

A fundamental mode and a third-harmonic mode integrated high-performance automatic level controlled (ALC) crystal oscillators for high-temperature applications (up to 250 degrees C), are described in this paper, These oscillators were specially designed for a pressure measurement system in high-temperature environments, which output signal is the difference between both generated frequencies. Frequency variations smaller than 0.0001 ppm/s for each oscillator and a frequency drift of about 2.5 ppm/year of the frequency difference are the measured performance concerning, respectively, the short-term (1 s) and long-term frequency stability of these integrated high-performance crystal oscillators over the 30 degrees C-225 degrees C temperature range. Other important characteristics are the very stable and constant oscillation levels (similar to 1.1 Vpp), the small second-harmonic distortion (-60 dB), and the phase noise (-95 dB at 50 kHz shift). The characteristics of these oscillators make them also suitable for many other measurement systems (time, temperature, and other physical and chemical quantities), especially if they are constrained to operate under severe temperature conditions.