High-overtone, bulk acoustic resonator frequency stability improvements

In an effort to counter the effects of both moderate-term and long-term frequency instability associated with the (-30 PPM/°C) frequency-temperature coefficient in high overtone, bulk acoustic resonators (HBAR), AFC loop stabilization of low-noise, 640 MHz HBAR oscillators has been demonstrated. A thermoelectric (TE) cooler is used as the resonator frequency tuning element. The TE cooler approach provides a means of obtaining near-equal device heating and cooling time-response characteristics, allowing stable loop operation. Using this technique, long-term, fractional frequency stability improvement to five parts in 10⁷ has been demonstrated. Prototype resonators have been fabricated on thicker (700 ns delay) YAG crystals and evaluated in oscillators operating at both 320 MHz and 640 MHz that exhibit flicker-of-frequency noise comparable, on a Sy(f) basis, to that reported for ultra-low noise, multiple crystal oscillators operating in the 80-100 MHz range