An investigation into the frequency stability of non-oven controlled crystal oscillators

Abstract

Temperature induced frequency deviation of quartz crystal oscillators can be reduced by tuning the oscillator to compensate for the thermal effects. A varactor tuned AT cut resonator is typically used to make a controlled crystal oscillator. Compensation is then achieved by applying a voltage to the varactor. The earliest compensation circuits comprised resistors and thermistors. Over the temperature range -40° C to +85°C resistive compensation circuits have achieved frequency stabilities of approximately + Ippm. Alternative analogue circuits have achieved frequency stability of < +0.5ppm. The research described in this thesis is an investigation of resistive networks in order to improve the levels of stability that can be achieved by this method of temperature compensation for crystal oscillators. The objective was to achieve a stability of < +0.5ppm over the temperature range -40°C to +85°C. Optimisation techniques have been employed to identify a new temperature compensation circuit. This new circuit uses an amplifier with temperature sensitive gain to modify thermistor characteristics and improve the compensation over the high part of the temperature range. Two forms of this circuit are presented. Computer simulation of this circuit has shown that it is capable of achieving frequency stability of < ±0.3ppm in an oscillator operating from a 4.5 V supply over the temperature range -40°C to +85°C .