Frequency modulation circuit, transmission circuit and communication device

1. A frequency modulation circuit for frequency-modulating an input signal and outputting a resultant signal, the frequency modulation circuit comprising:

an input terminal to which the input signal is inputted;

a VCO for frequency-modulating the input signal by controlling an oscillatory frequency of the VCO in accordance with the input signal, and outputting a resultant signal as a frequency-modulated signal;

a feedback loop for feeding, back to the VCO, the frequency-modulated signal outputted from the VCO; and

an open loop for connecting the input terminal and the VCO, wherein

the VCO includes at least a first variable capacitor having a predetermined capacitance change rate, and a second variable capacitor having a greater capacitance change rate than that of the first variable capacitor,

the open loop includes a switch for switching a connection path between the input terminal and the VCO, and

when the frequency modulation circuit is applied in a narrowband modulation method, the switch switches the connection path of the open loop such that the input terminal and the first variable capacitor are connected, and when the frequency modulation circuit is applied in a wideband modulation method, the switch switches the connection path of the open loop such that the input terminal and the second variable capacitor are connected.

2. The frequency modulation circuit according to claim 1 , wherein

the feedback loop includes:

a loop filter for suppressing high-frequency components of the input signal and outputting the input signal to the VCO;

a feedback section for feeding back the frequency-modulated signal outputted from the VCO; and

a comparing section for comparing the fed back frequency-modulated signal with the input signal, synchronizing a frequency of the input signal with a frequency of the frequency-modulated signal, and outputting the synchronized input signal to the loop filter.

3. The frequency modulation circuit according to claim 1 , further comprising a first computing section, connected between the input terminal and the feedback loop, for adding or subtracting a predetermined fixed number to or from the input signal, thereby controlling a center frequency of the input signal, wherein

the feedback loop includes:

a loop filter for suppressing high-frequency components of the input signal and outputting the input signal to the VCO;

an FDC for converting a frequency of the frequency-modulated signal, which is outputted from the VCO, to a digital value in accordance with a predetermined rule; and

a second computing section for adding or subtracting the digital value, which results from conversion performed by the FDC, to or from the input signal, and outputting a resultant signal to the loop filter.

4. The frequency modulation circuit according to claim 1 , wherein

a modulation signal and a reference signal are each inputted as the input signal, and

the feedback loop includes:

a loop filter for suppressing high-frequency components of the input signal, and outputting the input signal to the VCO;

a frequency divider for frequency-dividing, using the modulation signal, the frequency-modulated signal outputted from the VCO, and outputting the frequency-modulated signal; and

a phase comparator for comparing a phase of the reference signal with a phase of the frequency-modulated signal having been frequency-divided by the frequency divider, and outputting a phase difference therebetween to the loop filter as a difference signal.

5. The frequency modulation circuit according to claim 1 , wherein

the open loop:

performs analogue signal processing on the input signal; and

further includes, between the switch and the second variable capacitor, an adder for adding a predetermined DC voltage to the input signal.

6. The frequency modulation circuit according to claim 1 , wherein

the open loop:

performs digital signal processing on the input signal; and

further includes, between the switch and the second variable capacitor, an adder for adding a fixed number to the input signal.

7. A transmission circuit for generating and outputting a transmission signal based on input data, the transmission circuit comprising:

a signal generation section for generating, based on the input data, an amplitude signal and a phase signal;

a regulator for outputting a signal controlled in accordance with the amplitude signal;

a frequency modulation section for frequency-modulating the phase signal and outputting a resultant signal as a frequency-modulated signal; and

an amplitude modulation section for amplitude-modulating the frequency-modulated signal by using the signal outputted from the regulator, and outputting a resultant signal, which has been frequency-modulated and amplitude-modulated, as a transmission signal, wherein

the frequency modulation section is the frequency modulation circuit according to claim 1 .

8. A communication device comprising:

a transmission circuit for generating a transmission signal; and

an antenna for outputting the transmission signal generated by the transmission circuit, wherein

the transmission circuit is the transmission circuit according to claim 7 .

9. The communication device according to claim 8 , further comprising:

a reception circuit for processing a reception signal received from the antenna; and

an antenna duplexer for outputting the transmission signal generated by the transmission circuit to the antenna, and outputting the reception signal received from the antenna to the reception circuit.