Patent ID: 6211748
Filing Date: 2001-04-03
Classification: H03C

Abstract:
A frequency modulation circuit, comprising:a first VCO;a frequency divider circuit for dividing an oscillation signal generated by said first VCO and for outputting a divided signal;an oscillation circuit for outputting an oscillation signal having a reference frequency;a phase comparison circuit for comparing phases of said divided signal output from said frequency divider circuit and said oscillation signal output from said oscillation circuit and for outputting a phase comparison output; anda low-pass filter for taking a direct-current voltage, having a level that corresponds to a phase difference between said divided signal and said oscillation signal, out of the phase comparison output of phase comparison circuit, and for outputting said direct-current voltage, whereinsaid direct-current voltage output from said low-pass filter is supplied to said first VCO as a control signal for its oscillation frequency, so that a PLL is composed;said oscillation circuit is composed of a second VCO;a first audio signal is supplied to said second VCO and an oscillation frequency thereof is controlled thereby;a sub-carrier signal is modulated by a second audio signal, so that a modulated signal is formed having a frequency band outside a frequency band of said first audio signal; andsaid modulated signal is added to said direct-current voltage output from said low-pass filter, and an obtained result is supplied to said first VCO.