Patent ID: 7884657

Claim:
An oscillation frequency control circuit comprising: a voltage-controlled oscillator; a divider configured to divide an output from the voltage-controlled oscillator; a phase comparator configured to compare phases of an external reference signal and an output from the divider and output a phase difference signal; a loop filter configured to smooth out an output from the phase comparator and output the smoothed output; a detection circuit configured to detect the external reference signal; a pulse generation circuit configured to generate pulses upon receipt of pulse generation information and output the generated pulses to the loop filter; a memory configured to store control voltage information and the pulse generation information corresponding to the control voltage information; a control voltage variable circuit configured to perform adjustment of changing a fixed voltage; a select switch configured to turn on/off a connection between the phase comparator and the loop filter; and a control unit configured to turn on the select switch when a level of the external reference signal detected by the detection circuit is within an appropriate range while turning off the select switch when the level is outside the appropriate range, and in the case of selecting a control voltage from the control voltage variable circuit, to output a command for selecting the control voltage to the pulse generation circuit, and in the case of not selecting the control voltage from the control voltage variable circuit, to output the pulse generation information stored in the memory to the pulse generation circuit, wherein the select switch turns off the connection between the phase comparator and the loop filter and selects the control voltage to output the control voltage to the loop filter upon receipt of a control signal for selecting the control voltage input from the control voltage variable circuit, turns on the connection between the phase comparator and the loop filter upon receipt of a control signal for selecting the external reference signal, and turns off the connection between the phase comparator and the loop filter upon receipt of a control signal for selecting the pulses from the pulse generation circuit, and wherein the control unit outputs the control signal for selecting the external reference signal to turn on the select switch when the level of the external reference signal detected by the detection circuit is within the appropriate range, and outputs any one of the control signal set in advance for selecting the control voltage from the control voltage variable circuit and the control signal set in advance for selecting the pulses from the pulse generation circuit to the select switch when the level of the external reference signal is outside the appropriate range in the state in which the select switch is turned on.