Digital control type oscillation circuit of portable telephone, crystal

Abstract

In a portable telephone which uses a CPU, a memory, a temperature sensor, a D/A converter, and an A/d converter as control elements, and which has an oscillation circuit including a crystal resonator and a variable-capacitance diode, temperature compensation of the output frequency is made by using these existing control elements. The memory is previously stored with control information for correcting an output frequency drift of the portable telephone caused by a temperature change. The temperature of the oscillation circuit is detected with the temperature sensor and converted into a digital value in the A/D converter. The CPU reads control information corresponding to the detected temperature from the memory and applies it to the variable-capacitance diode of the oscillation circuit through the D/A converter, thereby maintaining the output frequency at a constant level. Also disclosed is a method of efficiently calculating an oscillation frequency of a crystal resonator with a reduced number of points of measurement for a temperature characteristic of the crystal resonator. A method of accurately carrying out the above-described temperature compensation is also disclosed.