Quartz Crystal Units for Temperature Compensated Crystal Oscillators

Abstract

Quartz crystal units in the frequency range of 2.48 MHz to 5.0 MHz were designed and fabricated for use in Temperature Compensated Crystal Oscillators (TCXO's). Their dynamic and static behaviour of frequency and resistance with respect to temperature changes from–45°C to +85°C were studied. The geometry of the resonator namely, the resonator diameter, piano convex contour, electrode thickness and size were properly selected to make them free from activity dips and frequency discontinuities in the desired temperature range. The resonator quality factor [Q] was estimated with different amount of etching in NH₄ HF₂ solution and an optimum level of etch was determined. Quality factors around 0.6 million could easily be achieved at 3.5 MHz in all the production batches. Crystals processed and encapsulated in HC-6/U cold weld enclosure with < 10⁻⁶Tor vacuum showed an ageing rate of few parts in 10⁻⁷ per month. At 3.2 MHz and 3.5 MHz several thousands of these crystal units were successfully manufactured in the last few years. Production statistics for the resonator parameters like Q, L and R are given for a batch of 100 crystals.