LOW NOISE FREQUENCY MODULATOR HAVING VARIABLE CARRIER FREQUENCY

摘要

PURPOSE: A low noise frequency modulator having a variable carrier frequency is provided to form a frequency modulator applied to various communication methods by varying a carrier frequency. CONSTITUTION: An oscillating frequency of a voltage controlled oscillator(310) is determined by a setup signal(VSET) as an output signal of a low pass filter(350). An input signal of the voltage controlled oscillator(310) is modulated by using the determined oscillating frequency as a carrier frequency. A phase/frequency detector(340) detects a phase difference and a frequency difference between an output signal of the voltage controlled oscillator(310) and a predetermined reference signal. The low pass filter(350) receives an output of the phase/frequency detector(340) and generates the setup signal. An audio signal amplifier(360) receives and amplifies an audio signal and generates a couple of output signal having different voltage levels of opposite direction. A modulation index compensation circuit(320) provides an oscillator input signal to the voltage controlled oscillator(310) by changing the voltage levels of the output signal couple of the audio signal amplifier(360).