Low noise frequency modulator having variable carrier frequency
First Claim
1. A frequency modulator having a variable carrier frequency, comprising:
- a voltage controlled oscillator for frequency-modulating an oscillator input signal using an oscillation frequency of the voltage controlled oscillator as the carrier frequency, the oscillation frequency being set by a set signal;
a phase/frequency detector for detecting phase and frequency differences between an output signal of the voltage controlled oscillator and a predetermined reference signal;
a low pass filter for receiving an output of the phase/frequency detector and generating the set signal;
an audio signal amplifier for amplifying an audio signal and generating a pair of output signals that have voltage levels that change in opposing directions; and
a modulation index compensation circuit, connected to an output node of the audio signal amplifier, for changing the voltage levels of the pair of output signals and outputting the pair of output signals having the changed voltage levels to the voltage controlled oscillator as the oscillator input signal to adjust a frequency variation of the output signal of the voltage controlled oscillator according to the oscillation frequency that is set by the set signal.
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Accused Products
Abstract
A frequency modulator having variable carrier frequency is provided. A VCO frequency-modulates an oscillator input signal using an oscillation frequency set by a set signal as the carrier frequency. A phase/frequency detector outputs phase and frequency differences between a VCO output signal and a reference signal. A filter receives a phase/frequency detector output and generates the set signal. An amplifier generates a pair of output signals whose voltage levels change in opposite directions. A compensation circuit changes the voltage levels of the output signal pair and provides resulting signals to the VCO as the oscillator input signal.
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Citations
15 Claims
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1. A frequency modulator having a variable carrier frequency, comprising:
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a voltage controlled oscillator for frequency-modulating an oscillator input signal using an oscillation frequency of the voltage controlled oscillator as the carrier frequency, the oscillation frequency being set by a set signal;
a phase/frequency detector for detecting phase and frequency differences between an output signal of the voltage controlled oscillator and a predetermined reference signal;
a low pass filter for receiving an output of the phase/frequency detector and generating the set signal;
an audio signal amplifier for amplifying an audio signal and generating a pair of output signals that have voltage levels that change in opposing directions; and
a modulation index compensation circuit, connected to an output node of the audio signal amplifier, for changing the voltage levels of the pair of output signals and outputting the pair of output signals having the changed voltage levels to the voltage controlled oscillator as the oscillator input signal to adjust a frequency variation of the output signal of the voltage controlled oscillator according to the oscillation frequency that is set by the set signal. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9)
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10. In a frequency modulator that includes an audio signal amplifier for amplifying an audio signal and generating a pair of output signals that have voltage levels that change in opposing directions and a voltage controlled oscillator for frequency-modulating the pair of output signals using an oscillation frequency set by a predetermined set signal as a carrier frequency, a modulation index compensation circuit comprises:
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at least one voltage level adjustment circuit between an output node of the audio signal amplifier and a predetermined reference voltage, an operation of the at least one voltage level adjustment circuit being controlled according to a selection signal, the at least one voltage level adjustment circuit being connected in parallel with other voltage level adjustment circuits when there is two or more voltage level adjustment circuits, wherein the modulation index compensation circuit changes the voltage levels of the pair of output signals to adjust a frequency variation of an output signal of the voltage controlled oscillator according to the oscillation frequency that is set by the predetermined set signal. - View Dependent Claims (11, 12, 14, 15)
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13. A voltage controlled oscillator for frequency-modulating an input signal using an oscillation frequency of the voltage controlled oscillator as the carrier frequency, the oscillation frequency being set by a predetermined set signal, the voltage controlled oscillator comprising:
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an oscillator core for generating an output signal which changes depending on the predetermined set signal;
a capacitor for converting the output signal of the oscillator core into a current and outputting the current; and
an input stage for controlling an oscillator current drained from the oscillator core using an input signal pair and the current output from the capacitor.
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Specification