Patent ID: 8115527

Claim:
A PLL apparatus in which a phase difference between a phase of an external reference frequency signal and a phase of a frequency signal output from a voltage-controlled oscillator via a frequency division unit is extracted in a phase comparison unit and a control voltage corresponding to the phase difference is supplied to the voltage-controlled oscillator via a control voltage output unit, the apparatus comprising: a preset signal output unit outputting a preset signal created in advance, as a substitute for the signal corresponding to the phase difference; a switching unit switching a signal that is to be supplied to the control voltage output unit between a signal of the phase comparison unit and the preset signal of the preset signal output unit; a protection circuit provided between a signal path of the reference frequency signal and a ground and having diodes that are connected in inverse parallel in order to regulate an amplitude level of the reference frequency signal; a temperature detection unit detecting an atmospheric temperature of the protection circuit; a level detection unit detecting the amplitude level of the external reference frequency signal; a storage unit storing the atmospheric temperature and an upper limit threshold value and a lower limit threshold value of the amplitude level in a related manner, in order to cope with a change in the amplitude level due to a temperature characteristic of the diodes; and a switching control unit reading the upper limit threshold value and the lower limit threshold value of the amplitude level, corresponding to a temperature detection value of the temperature detection unit, and determining whether the amplitude level detected by the level detection unit falls within a range of the threshold values to switch the switching unit to the phase comparison unit side when the amplitude level falls within the range and to switch the switching unit to the preset signal output unit side when the amplitude level does not fall within the range.