Crystal oscillator temperature compensating circuit

Abstract

A circuit for generating a temperature varying control voltage which is applied to a varactor diode to temperature compensate a crystal oscillator is disclosed. The circuit produces a substantially linear voltage versus temperature variation, including a point of inflection, in a middle temperature range, and a substantially non-linear voltage versus temperature variation in hot and cold temperature ranges. The control voltage versus temperature characteristic also undergoes a change in slope polarity in both the hot and cold temperature ranges. The temperature compensating circuit has a middle range circuit for independently creating the substantially linear variation in the middle temperature range, a cold range circuit operative below a predetermined temperature for creating the desired variation in the cold temperature range, and a hot temperature circuit operative above a predetermined temperature for creating the desired temperature variation in the hot temperature range. Each temperature range circuit includes a thermistor and a transistor which together control the operative range and the magnitude of the temperature variation contributed by each of the circuits.