Reducing the acceleration sensitivity of AT-strip quartz crystal oscillators

Abstract

Miniature AT-strip quartz crystal resonators are capable of providing excellent frequency stability with good immunity to environmental influences. In particular, the fractional frequency change under acceleration is typically about 2×10⁻⁹ g⁻¹. While more than good enough for most applications, for those that require low phase noise under vibration this may not be adequate. In this case, the sensitivity to acceleration must be below 1×10⁻⁹ g⁻¹ and for some applications near to if not below 1×10⁻¹⁰ g⁻¹. While the method of using two crystals oriented so that their acceleration sensitivity vectors are anti-parallel to produce a composite resonator with reduced sensitivity has been known for many years, difficulties in characterizing and orienting the crystals has limited the implementation of this technique. We adopt this method using AT-strip crystals designed and manufactured to have acceleration sensitivities below 1×10⁻⁹ g⁻¹ that are sufficiently repeatable so that two crystals can be paired and easily oriented in an oscillator with a resultant acceleration sensitivity below 1×10⁻¹⁰ g⁻¹. The results of implementing this method in the production of 10 MHz to 40 MHz oscillators are summarized.