1. Field of the Invention
The present invention relates to a voltage controlled oscillator used for generating a local oscillation signal for a radio communication device, and a PLL circuit and a radio communication device each including the same.
2. Description of the Background Art
A voltage controlled oscillator (VCO) is widely used as a device that generates a local oscillation signal for a radio communication device. FIG. 7 shows an exemplary configuration of a conventional voltage controlled oscillator. The conventional voltage controlled oscillator includes inductors 604a and 604b, variable capacitance elements 605a and 605b, oscillation transistors 603a and 603b, and a current source 601. It is noted that in FIG. 7, a bias circuit and the like are omitted.
The inductors 604a and 604b and the variable capacitance elements 605a and 605b constitute a parallel resonant circuit. The capacitance values of the variable capacitance elements 605a and 605b depend on the difference in voltage between both ends thereof. In other words, the capacitance values of the variable capacitance elements 605a and 605b vary in accordance with a control voltage Vt applied from an external circuit to a frequency control terminal 602, resulting in that the resonance frequency of the parallel resonant circuit varies. A set of the variable capacitance elements 605a and 605b are called a direct-coupling type variable capacitance circuit. Because the oscillation frequency of the conventional voltage controlled oscillator oscillates near the resonance frequency of the parallel resonant circuit, the oscillation frequency can be controlled so as to be a desired frequency by adjusting the control voltage Vt. The oscillation transistors 603a and 603b are provided for generating a negative resistance and canceling a loss, which occurs due to a parasitic resistance component of the parallel resonant circuit, to meet an oscillation condition.
Here, a relation between the control voltage and the oscillation frequency of the voltage controlled oscillator substantially depends on characteristics of the variable capacitance elements. Thus, the capacitance of a variable capacitance element to be used is desired to gradually vary throughout a wide range of the control voltage Vt. This is synonymous with that an oscillation frequency is desired to linearly vary throughout a wide range of the control voltage Vt.
In the case where a PLL (phase-locked loop) circuit is configured by using a conventional voltage controlled oscillator, the transient response characteristic and the noise band characteristic of the PLL circuit depend on a frequency sensitivity (the ratio of variation of the oscillation frequency with respect to the control voltage Vt). The reason why the gradual variation of the capacitance of the variable capacitance element is desired is that if the frequency sensitivity changes depending on the oscillation frequency (the oscillation frequency varies non-linearly), the characteristic of the PLL circuit also changes depending on the oscillation frequency. Further, there is a problem that in a high region of the frequency sensitivity with respect to the control voltage Vt, the oscillation frequency varies even due to slight noise applied to the frequency control terminal 602, resulting in deterioration of a phase noise characteristic.
However, if a special process is introduced for forming the variable capacitance elements 605a and 605b when the aforementioned conventional voltage controlled oscillator is realized on a semiconductor substrate, this leads to an increase in cost. Thus, it is actually difficult to use a variable capacitance element having high linearity. FIG. 8A is a symbol representing an Inversion type MOS transistor that is an example of a variable capacitance element and that utilizes a gate capacitance, which is widely used in a CMOS process, between a gate terminal and a terminal connected to a drain terminal and a source terminal. FIG. 8B shows a variation of the gate capacitance when a reference voltage Vref is applied to the gate terminal of the MOS transistor and a control voltage Vt is applied to the drain terminal and the source terminal of the MOS transistor.
Because the capacitance value rapidly varies near a threshold voltage (a voltage Vth in FIG. 8B) in a variable capacitance element that utilizes the gate capacitance of a generally-used MOS transistor as shown in FIG. 8B, the oscillation frequency also rapidly varies near the threshold voltage. This results in a problem that the transient response characteristic and the noise band characteristic of a PLL circuit including a conventional voltage controlled oscillator change drastically depending on the oscillation frequency.
In order to solve this problem, a below-described circuit has been already proposed.
FIG. 9 is a view of a conventional voltage controlled oscillator using a method that improves the linearity of a variable capacitance element (e.g. see U.S. Pat. No. 6,995,626 (Patent Document 1)).
As shown in FIG. 9, the conventional voltage controlled oscillator includes inductors 604a and 604b, variable capacitance elements 605a, 605b, 606a, 606b, 607a, and 607b, DC-cut capacitance elements 608a, 608b, 609a, 609b, 610a, and 610b for blocking a direct current component, high frequency interruption resistors 611a, 611b, 612a, 612b, 613a, and 613b, oscillation transistors 603a and 603b, and a current source 601. In FIG. 9, the same components as those in FIG. 7 are designated by the same reference characters, and the description thereof will not be provided.
The variable capacitance elements 605a and 605b and the DC-cut capacitance elements 603a and 608b constitute a variable capacitance circuit A. The variable capacitance elements 606a and 606b and the DC-cut capacitance elements 609a and 609b constitute a variable capacitance circuit B. The variable capacitance elements 607a and 607b and the DC-cut capacitance elements 610a and 610b constitute a variable capacitance circuit C. The variable capacitance elements 605a, 605b, 606a, 606b, 607a, and 607b are Inversion type MOS transistors each of which utilizes a gate capacitance, which is used in a CMOS process, between a gate terminal and a terminal connected to a drain terminal and a source terminal. In the variable capacitance circuits A to C, the capacitance values of the variable capacitance elements vary depending on reference voltages Vref1 to Vref3 inputted to connection points between the variable capacitance elements and the DC-cut capacitance elements and a control voltage Vt applied to a frequency control terminal 602. As a result, the resonance frequency of a parallel resonant circuit varies. The variable capacitance circuits A to C are called C-coupling type variable capacitance circuits.
Here, if each of the differences among the reference voltages Vref1 to Vref3 is a voltage Vd, the characteristics of the variable capacitance circuits A to C with respect to the control voltage Vt are shifted from each other by Vd. Patent Document 1 shows an example where Vd=160 mV (FIG. 10A). The capacitance of the parallel resonant circuit is the total of the capacitances of the variable capacitance circuits A to C. Thus, the total capacitance has a characteristic as indicated by a chain line in FIG. 10B, and variation of the capacitance with respect to the control voltage Vt can be gradual.
The oscillation frequency fo of the voltage controlled oscillator is represented by the following formula [1] where L denotes the inductance of the inductor of the parallel resonant circuit, Cv denotes the total of the capacitance values of the variable capacitance circuits A to C, and C denotes the capacitance value of a parasitic capacitance generated at a negative resistance circuit and the like.
                              f          0                =                  1                      2            ⁢            π            ⁢                                          L                ×                                  (                                                            C                      V                                        +                    C                                    )                                                                                        [        1        ]            
When the formula [1] is solved with respect to the total capacitance value Cv of the variable capacitance circuits A to C, the formula [1] becomes the following formula [2].
                              C          V                =                              1                          4              ⁢                              π                2                            ⁢                              f                0                2                            ×              L                                -          C                                    [        2        ]            
The inductance L and the capacitance value C of the parasitic capacitance are constant. Thus, in order to cause the oscillation frequency fo to vary linearly with respect to the control voltage Vt, the total capacitance value Cv of the variable capacitance circuits A to C is desired to be proportional to 1/(fo2), not caused to vary linearly.
Further, it is known that the variation range of the capacitance value of a C-coupling type variable capacitance circuit is narrower than that of a direct-coupling type variable capacitance circuit, in other words, the capacitance variation ratio is smaller.
For example, a case is considered where the variation range of the capacitance value in the direct-coupling type variable capacitance circuit is from CH to CL when the control voltage Vt is changed from High to Low (see FIG. 8B), the capacitance value of the variable capacitance element of the c-coupling type variable capacitance circuit is represented by Cx, and the capacitance value of the DC-cut capacitance element thereof is set to CH. In this case, the composite capacitance value Ctotal of the C-coupling type variable capacitance circuit is represented by the following formula [3].
                              C          total                =                              CH            ×            Cx                                CH            +            Cx                                              [        3        ]            
In the formula [3], when the control voltage Vt is at High (Cx=CH), the composite capacitance value Ctotal becomes CH/2. On the other hand, when the control voltage Vt is at Low (Cx=CL), the composite capacitance value Ctotal becomes CL (∵CH>>CL). In other words, the upper limit of the variation range of the capacitance value of the C-coupling type variable capacitance circuit is smaller than that of the direct-coupling type variable capacitance circuit by “CH/2”.
Thus, in the conventional improvement method described above, the capacitance can be caused to vary gradually with respect to the control voltage Vt, thereby allowing the capacitance variation in a wide range of the control voltage Vt, namely, improvement in the linearity of the oscillation frequency. However, there is a problem that because many C-coupling type variable capacitance circuits are connected in parallel, the capacitance variation ratio becomes small, and hence the variation range of the oscillation frequency of the voltage controlled oscillator becomes narrow.