Frequency modulator and fm transmission circuit using the same

Abstract

An input signal is input via a first resistor to an inverting input terminal of an operational amplifier. A second resistor is provided on a feedback path between an output terminal and the inverting input terminal of the operational amplifier. A control voltage Vcnt output from the operational amplifier is input to a VCO. A frequency divider frequency-divides an output signal Sout of the VCO. A phase comparator compares an output signal from the frequency divider with a reference clock signal and outputs a voltage according to a phase difference. A loop filter removes a high-frequency component of an output voltage Vcp of the phase comparator and outputs the voltage to a non-inverting input terminal of the operational amplifier.

1 . A frequency modulator comprising:
 an operational amplifier having an inverting input terminal to which an input signal is input via a first resistor;   a second resistor provided on a feedback path between an output terminal and the inverting input terminal of the operational amplifier;   a voltage controlled oscillator to which an output signal from the operational amplifier is input;   a frequency divider which frequency-divides an output signal from the voltage controlled oscillator;   a phase comparator which compares an output signal from the frequency divider with a reference clock signal and outputs a voltage according to a phase difference; and   a filter which removes a high-frequency component of an output voltage from the phase comparator and outputs the voltage to a non-inverting input terminal of the operational amplifier.   
     
     
         2 . The frequency modulator according to  claim 1 , wherein
 a resistance of the first resistor is set higher than a resistance of the second resistor.   
     
     
         3 . The frequency modulator according to  claim 1 , wherein
 at least one of the first and second resistor is a variable resistor.   
     
     
         4 . The frequency modulator according to  claim 1 , wherein
 the operational amplifier is a rail amplifier.   
     
     
         5 . The frequency modulator according to  claim 1 , wherein
 the filter is a passive filter.   
     
     
         6 . The frequency modulator according to  claim 1 , wherein
 the input signal is a stereo composite signal.   
     
     
         7 . The frequency modulator according to  claim 1 , wherein
 the frequency modulator is integrated on a single semiconductor substrate.   
     
     
         8 . An FM transmission circuit comprising:
 a stereo modulating unit which converts an audio signal into a stereo composite signal;   the frequency modulator according to  claim 1 , which performs frequency modulation based on the stereo composite signal output from the stereo modulating unit and outputs the modulated signal; and   an amplifier which amplifies the modulated signal output from the frequency modulator.   
     
     
         9 - 49 . (canceled)