Patent ID: 7656208

Claim:
A digitally controlled PLL oscillation circuit comprising: a voltage controlled oscillator: a frequency divider that divides an output from the voltage controlled oscillator; a reference oscillation circuit that generates a reference signal; an A/D converter that performs analog-to-digital conversion of an output from the frequency divider based on the reference signal; a phase comparator that compares a phase of an output from the A/D converter with a phase of the reference signal; a digital filter that filters an output from the phase comparator: a D/A converter that performs digital-to-analog conversion of an output from the digital filter based on the reference signal; an analog filter that smoothes an output of the D/A converter and outputs as a control signal for the voltage controlled oscillator; and a first narrow-band crystal filter provided between the reference oscillation circuit and the A/D converter so that the reference signal which is used in the A/D converter to convert an analog signal from the frequency divider to a digital signal to be supplied to the A/D converter from the reference oscillation circuit passes through the first narrow-band crystal filter; a first amplifier is being provided between the first narrow-band crystal filter and the A/D converter to amplify a signal from the first narrow-band crystal filter.