As an oscillation circuit capable of variably setting its oscillation frequency, for example, various LC type voltage control oscillators have been conventionally developed. In an LC type voltage control oscillator, the capacitance value of a capacitor is set variably by a control voltage and an oscillating state of a switching element configuring an oscillation circuit, i.e., an oscillation frequency is controlled in accordance with the capacitance value variably set.
The LC type voltage control oscillator has comparatively high frequency precision of an oscillation signal in a fixed frequency range and is widely spread as an oscillator to be provided in, for example, a communication device required to continuously vary reception frequencies and transmission frequencies.
Recently, as frequency bands used in radio communication, a very high frequency band of several gigahertz is being used. For example, there is a radio communication terminal which is used as a mobile telephone terminal and which performs radio communication in a frequency band from several hundreds of megahertz to several gigahertz with a single unit.
However, it is difficult for an LC type voltage control oscillator built in a communication device, etc., conventionally to oscillate in such a wide frequency range and if an attempt is made to expand the oscillation frequency range forcedly, there arises such a problem that the noise characteristic of the oscillation signal is deteriorated. Because of this, conventionally, for example, a plurality of LC type oscillation circuits having different oscillation frequency ranges is provided to deal with wideband communication.
Such a configuration comprising the plurality of oscillation circuits is not preferable not only because its circuit scale is large and cost is high but also because power consumption is high. Given a wideband oscillation circuit, it is only necessary to provide one circuit and because the circuit scale and cost can be reduced and at the same time, the power consumption of the communication device can also be reduced, and therefore, a high quality oscillation circuit having a wideband and excellent noise characteristic has been demanded.
In such circumstances described above, there has been proposed an oscillation circuit the oscillation frequency range of which is expanded by dividing the oscillation output of a voltage control oscillator by a plurality of dividers and mixing the outputs of the plurality of dividers. However, the circuit configuration in which the outputs of the plurality of dividers are mixed has such a problem that a signal occurs in an unwanted band at the time of mixing by a mixer, i.e., so-called spurious occurs, and this results in a problem in practical use. Further, for radio communication, a four-phase oscillation output is necessary, however, with the configuration comprising a mixer, it is difficult to obtain a four-phase output.
In order to solve the above-mentioned problem, JP2009-225438A describes a wideband voltage control oscillation circuit that combines an LC type voltage control oscillator (VCO) having a narrow band but low phase noise and an injection locked frequency divider (ILFD).
FIG. 1A to FIG. 1C are diagrams showing the configuration and operation characteristics of the wideband voltage control oscillation circuit described in JP2009-225438A, wherein FIG. 1A shows the configuration, FIG. 1B shows changes in locked frequency range of the ILFD, and FIG. 1C shows the frequency range of an oscillation signal output from the wideband voltage control oscillation circuit.
As shown in FIG. 1A, the wideband voltage control oscillation circuit described in JP2009-225438A comprises an LC type voltage control oscillator (VCO) 11, an injection locked frequency divider (ILFD) 12, and a control part 13 that controls the oscillation frequency range of the ILFD 12 by outputting a bias voltage VbD of an inverter circuit of the ILFD 12.
The LC type VCO 11 generates an oscillation signal of a frequency f0. The LC type VCO 11 is configured so that the capacitance value of a capacitor in a circuit varies in accordance with a control voltage and thereby the time constant of the oscillating operation varies. There is also a configuration in which an inductance value is varied instead of the capacitance value or a configuration in which both the capacitance value and the inductance value are varied. The LC type VCO 11 outputs an oscillation signal having a high frequency range of f0=8 GHz to 12 GHz and the oscillation signal has a comparatively excellent low noise characteristic.
The ILFD 12 comprises a plurality of inverter circuits connected in the form of a ring and is a ring oscillator type free-run oscillation circuit. The oscillating operation is regulated by the oscillation signal of the frequency f0 output from the LC type VCO 11 and a divided signal of the oscillation signal of the frequency f0 is generated. The free-run frequency of the ILFD 12 varies by controlling the bias voltage VbD of the inverter circuit, and therefore, the division ratio varies. The control circuit 13 controls the bias voltage of the inverter circuit in accordance with the frequency of the divided signal output from the ILFD 12.
FIG. 1B is a diagram showing the change in the range of the locked frequency of the ILFD 12, i.e., in the frequency range of a divided signal in accordance with the voltage signal VbD output from the control part 13 when f0=8 GHz. As shown in FIG. 1A, when the oscillation signal of f0=8 GHz is divided by 2 and a divided signal of 4 GHz is output, the bias voltage VbD of the inverter circuit is controlled so that the locked frequency of the ILFD 12 is in the range denoted by A. Similarly, when the oscillation signal of f0=8 GHz is divided by 3 and 4, the bias voltage VbD of the inverter circuit is controlled so that the locked frequencies of the ILFD 12 are in the ranges denoted by B and C, respectively.
FIG. 1C is a diagram showing the frequency range obtained by the oscillation circuit described in JP2009-225438A. As described above, the LC type VCO 11 outputs the oscillation signal of f0=8 GHz to 12 GHz and in this range, the oscillation frequency can be varied. It is possible for the ILFD 12 to divide the oscillation signal that is input so that the frequency is 1/2, 1/3, 1/4, and 1/6 the original one by controlling the bias voltage VbD of the inverter circuit. In other words, it is possible to divide the oscillation signal of the frequency f0 by 2, 3, 4, 6.
As shown in FIG. 1C, it is possible for the LC type VCO 11 to output the oscillation signal of f0=8 GHz to 12 GHz and by dividing this by 2, a divided signal of 4 GHz to 6 GHz can be obtained. Similarly, by dividing the oscillation signal by 3, a divided signal of 2.66 GHz to 4 GHz can be obtained, by dividing by 4, a divided signal of 2 GHz to 3 GHz can be obtained, and by dividing by 6, a divided signal of f0=1.33 GHz to 2 GHz can be obtained. Further, by dividing by 2 the divided signal output from the ILFD 12 by a normal division-by-2 circuit, a divided signal of 2 GHz to 3 GHz can be obtained and by repeating division by the normal division-by-2 circuit, a divided signal of a lower frequency can be obtained. A divided signal obtained by the normal division-by-2 circuit is also a low noise signal without spurious.
As described above, according to the oscillation circuit described in JP2009-225438A, it is possible to generate oscillation signals of continuous frequencies of 15 MHz to 6 GHz.