AUDIO FM TRANSMITTER

摘要

[Problem to be Solved];To provide an audio FM transmitter that achieves high-precision frequency control as well as compact size and low cost by enabling FM modulation using a fractional-N type PLL circuit.;[Solution];An audio FM transmitter 1 that analog FM-modulates an audio signal using a PLL modulator 8 and transmits the FM-modulated audio signal includes: the PLL modulator 8; and a controller 11 for outputting a frequency divider setting signal to the PLL modulator 8. The PLL modulator 8 includes: a voltage-controlled crystal oscillator 8a for performing outputting according to a reference signal caused by the oscillation of a resonator; a phase comparator 8b for converting into a voltage the phase difference between the signal output from the voltage-controlled crystal oscillator 8a and a phase comparison signal input from a fractional-N type frequency divider 8e and outputting the voltage as a phase difference signal; a loop filter 8c; a voltage-controlled oscillator 8d; and the fractional-N type frequency divider 8e. The PLL modulator 8 is configured as a digital PLL modulation means that can control the oscillation frequency according to the frequency divider setting signal. The voltage-controlled crystal oscillator 8a outputs an FM-modulated signal by analog FM-modulating the oscillation frequency of the reference signal with the audio signal.

主权项

1. An audio FM transmitter that analog FM-modulates an audio signal using a PLL modulation means and transmits the FM-modulated audio signal, comprising:
the PLL modulation means; and a control means for outputting a frequency divider setting signal to the PLL modulation means, the PLL modulation means comprising: a first voltage-controlled oscillation means for performing outputting according to a reference signal caused by the oscillation of a resonator; a phase comparison means for converting into a voltage the phase difference between the signal output from the first voltage-controlled oscillation means and a phase comparison signal input from a fractional-N type frequency divider and outputting the voltage as a phase difference signal; a low-pass filtering means for removing a high-frequency component from the phase difference signal output from the phase comparison means; a second voltage-controlled oscillation means for controlling an oscillation frequency according to the phase difference signal output from the low-pass filtering means as a control voltage; and the fractional-N type frequency divider for frequency-dividing an branched output from the second voltage-controlled oscillation means, wherein the PLL modulation means is configured as a digital PLL modulation means that can control the oscillation frequency according to the frequency divider setting signal, and wherein the first voltage-controlled oscillation means outputs an FM-modulated signal by analog FM-modulating the oscillation frequency of the reference signal with a modulating signal in accordance with the audio signal.