Patent ID: 8044722
Filing Date: 2011-10-25
Classification: H03L

Abstract:
1. An oscillation frequency control circuit, comprising: a voltage-controlled oscillator; a divider that divides an output from the voltage-controlled oscillator; a phase comparator that compares phases of an external reference signal and of the output from the divider to output a phase difference signal; a loop filter that smoothes the output from the phase comparator to output; a detector circuit that detects an input level of the external reference signal; a temperature sensor that detects temperature in the vicinity of the detector circuit; a memory that stores a voltage value for determining an adequate range and outside of the adequate range in accordance with temperature characteristic for detected voltage of the external reference signal; a fixed voltage supplying circuit that supplies fixed voltage; a selection switch that connects the phase comparator to the loop filter in an external reference synchronization mode or connects the fixed voltage supplying circuit to the loop filter in a fixed voltage mode; and a controlling unit that outputs a control signal to switch the selection switch to the external reference synchronization mode when the detected voltage of an external reference signal level detected by the detector circuit is within the adequate range in accordance with the temperature characteristics of the temperature detected by the temperature sensor, and outputs the control signal to switch the selection switch to the fixed voltage mode when the detected voltage is outside the adequate range in accordance with the temperature characteristics of the temperature detected by the temperature sensor.