Design and performance of ultraprecise 2.5-mc quartz crystal units

A 2.5-mc crystal unit has been developed for use in a new, extremely stable frequency standard oscillator. A well-balanced design was achieved by using a 30-mm-diameter, plano-convex, polished quartz plate, coated with gold and operated on its fifth overtone. The quartz plate is mounted on its quiescent edge in an evacuated bulb, and achieves a Q of five to six million, representative of the Q of the quartz itself. The temperature coefficient, current coefficient, frequency adjustment tolerance and frequency aging of the crystal unit are all consistent with a frequency stability in the order of one part in 10
¹⁰
. It was necessary to develop polishing methods that would not disturb the crystal structure of the quartz plate and new methods of orienting the crystollographic axes to achieve better temperature coefficient control. New methods of mounting the quartz plate were found that avoid strain and reduce the effects of shock and vibration. The new crystal unit makes possible oscillators characterized by excellent frequency stability, small and uniform aging and straightforward design. For periods up to one month, the frequency stability of such standards compares favorably with that of atomic frequency standards.