A voltage-controlled oscillator includes (i) a first variable-capacity element, (ii) a resonance circuit whose resonance frequency changes in accordance with a control voltage applied to the first variable-capacity element, (iii) a second variable-capacity element connected in parallel with the first variable-capacity element, (iv) resonance frequency range switching means which switches the variation range of the resonance frequency of the resonance circuit by switching the capacity of the second variable-capacity element, and (v) a resonance frequency correction circuit which corrects the resonance frequency in such a manner as to prevent the ratio between resonance frequencies before and after the switching of the variation range from depending on the control voltage.

This Nonprovisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No. 358268/2005 filed in Japan on Dec. 12, 2005, the entire contents if which are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to a voltage-controlled oscillator, and particularly to a voltage-controlled oscillator which can switch and control the variation range of oscillating frequencies.

BACKGROUND OF THE INVENTION

In recent years voltage-controlled oscillators (VCOs), which can control oscillating frequencies by applying a control voltage, have widely been used for, for example, local oscillators of wireless transmitter-receivers. Such a voltage-controlled oscillator is required to be highly stable, have low phase noise, generate low noise, or the like. However, since the range of oscillating frequencies of a wireless transmitter-receiver is wide, the aforesaid requirements cannot be satisfied by a single voltage-controlled oscillator. Taking into account of this, a plurality of voltage-controlled oscillators with different variation ranges of oscillating frequencies are provided on one integrated circuit, in order to cover a required oscillating frequency range.

In case where a plurality of voltage-controlled oscillators are provided on an integrated circuit, the chip size increases on account of an area for those oscillators, so that the cost is high. Taking account of this problem, there is a voltage-controlled oscillator which can cover a required oscillating frequency range as if different voltage-controlled oscillators are provided. In this voltage-controlled oscillator, variation ranges of oscillating frequencies are switched by switching the inductance of a resonance circuit.

FIG. 5is a circuit diagram which outlines a voltage-controlled oscillator500with the inductance switching capability. This voltage-controlled oscillator500is disclosed in Japanese Laid-Open Patent Application No. 2003-229718 (published on Aug. 15, 2003; corresponding to U.S. Pat. No. 6,954,111). As illustrated inFIG. 5, the voltage-controlled oscillator500includes a pair of variable-capacity elements C501whose capacity control terminals are connected to a control voltage input terminal501. To this control voltage input terminal501, a control voltage is applied from the outside. With this, the capacities of the variable-capacity elements C501are changed, so that the resonance frequency of a resonance circuit including inductors L501and variable-capacity elements C50-1is controlled. Moreover, the voltage-controlled oscillator500includes switches SW501by which a power source502is connected to either terminals of the inductors L501or intermediate parts of the inductors L501. In this voltage-controlled oscillator500, the inductance of the resonance circuit is varied by switching the switches SW501, with the result that the variation range of oscillating frequencies is switched.

FIG. 6is a circuit diagram which outlines another voltage-controlled oscillator600having the inductance switching capability. This oscillator600is disclosed in Japanese Laid-Open Patent Application No. 2002-151953 (published on May 24, 2002). As illustrated inFIG. 6, the voltage-controlled oscillator600includes a pair of variable-capacity elements C601whose capacity control terminals are connected to a control voltage input terminal601. A control voltage is applied to the control voltage input terminal601from the outside, so that the capacities of the variable-capacity elements C601are varied. In doing so, the resonance frequency of the resonance circuit including inductors L601and the variable-capacity elements C601is controlled. Furthermore, as shown inFIG. 6, each main inductor L601is provided with an auxiliary inductor L602which is magnetically coupled with the main inductor L601. On this account, in the voltage-controlled oscillator600, the inductance of the resonance circuit is changed by switching on/off the switches SW601which are connected to the closed circuit including the auxiliary inductors L602, so that the variation range of the oscillating frequencies is switched.

However, the conventional voltage-controlled oscillator in which the variation range of the oscillating frequencies is switched by switching the inductance of the resonance circuit has the following problem.

That is, in the voltage-controlled oscillator500, the switches SW501and the inductors L501are connected in series. On this account, the currents passing through the switches SW501and the inductors L501are susceptible to a parasitic capacity and noise, and hence the phase noise characteristic is deteriorated and noise is increased.

In the meanwhile, in the voltage-controlled oscillator600, the auxiliary inductors L602by which mutual magnetic induction with the main inductors L601is generated are required, in addition to the main inductors L601. For this reason, the voltage-controlled oscillator600requires an area where the inductors L601and L602are provided, and hence the chip size is large. To solve this problem, there has been a proposal to provide two or more wiring layers where those inductors are formed. In this case, however, it is necessary to provide, between the neighboring wiring layers, a ground layer (termed ground shield) made of polysilicon or the like. This increases manufacturing costs.

In consideration of the above, there has been a voltage-controlled oscillator in which the variation range of the oscillating frequencies is switched by switching the capacity of the resonance circuit.FIG. 7is a circuit diagram which outlines a voltage-controlled oscillator700having a capability of switching the capacity of a resonance circuit. As shown in this figure, the voltage-controlled oscillator700includes a pair of variable-capacity elements C701whose capacity control terminals are connected to a control voltage input terminal701. As the control voltage input terminal701receives a control voltage, the capacities of the variable-capacity elements C701are changed, so that the resonance frequency of the resonance circuit including inductors L701and the variable-capacity elements C701is controlled. Furthermore, the voltage-controlled oscillator700is also provided with another pair of variable-capacity elements C702. The capacity control terminal of each of the variable-capacity elements C702is connected to either the GND or the power source, via the switch SW701. With this arrangement, the capacities of the variable-capacity elements C702are switched by a switch SW701, so that the variation range of the oscillating frequencies is switched.

The voltage-controlled oscillator700does not require an inductor for switching the variation range of the oscillating frequencies. For this reason, the voltage-controlled oscillator has a good phase noise characteristic can be easily downsized.

The voltage-controlled oscillator, in which the variation range of the oscillating frequencies is switched by switching the capacity of the resonance circuit, has a good phase noise characteristic and can be easily downsized, but has a problem such that the ratio between the oscillating frequencies before and after the switching of the variation range of the resonance frequency is not constant, because of the dependency on the control voltage. When such a voltage-controlled oscillator is used as a local oscillator of a transmitter or a receiver, the transmitter or the receiver requires complicated circuitry, and hence is costly.

The above-mentioned problem is specifically described as below, taking the voltage-controlled oscillator700shown inFIG. 7as an example.

The resonance circuit of the voltage-controlled oscillator700is constituted by inductors L701, variable-capacity elements C701which are connected in parallel with the inductors L701, and other variable-capacity elements C702. The total capacity C of the resonance circuit is therefore equal to CV+CX, which is the sum total of (i) the capacity CVof the variable-capacity elements C701, which is determined by a control voltage Vc applied to a control voltage input terminal701and (ii) the capacity CXof the variable-capacity elements C702.

As described above, the capacity CXof the variable-capacity elements C702is switchable using the switch SW701. That is, in case where the capacity control terminal of the variable-capacity elements C702is grounded, the capacity of the variable-capacity elements C702is CA. On the other hand, in case where the capacity control terminal of the variable-capacity elements C702is connected to the power source, the capacity of the variable-capacity elements C702is CB(CA>CB).

The oscillating frequency of the voltage-controller oscillator, i.e. the resonance frequency f of the resonance circuit is determined as below, by the total capacity C of the resonance circuit and the inductance L of the resonance circuit:

Therefore, when the capacity of the variable-capacity elements C702is CA, the resonance frequency fAof the voltage-controlled oscillator700is determined as below:

In the meanwhile, when the capacity of the variable-capacity element is CB, the resonance frequency fBof the voltage-controlled oscillator700is determined as below:

In the equations above, indicated by L is the inductance of the inductors L701.

When the variation range of the resonance frequency is switched while the control voltage Vc is kept constant, the ratio between the resonance frequencies fA/fBbefore and after the switching is represented as below.

In this manner, the ratio between the resonance frequencies before and after the switching of the resonance frequency range depends on the capacity CV, i.e. the control voltage Vc. The ratio is therefore inconstant.

The present invention was done to solve the above-described problem. The objective of the present invention is to provide a voltage-controlled oscillator (i) which has a good phase noise characteristic, (ii) which can be easily downsized, and (iii) whose ratio of oscillating frequencies before and after the switching of the variation range of a resonance frequency does not depend on a control voltage.

SUMMARY OF THE INVENTION

The objective of the present invention is to provide a voltage-controlled oscillator which has a good phase noise characteristic, which can be easily downsized, and whose ratio between oscillating frequencies before and after the switching of the variation range of the resonance frequency which does not depend on a control voltage.

To achieve the objective above, the voltage-controlled oscillator of the present invention includes: a resonance circuit which includes a first variable-capacity element, a resonance frequency of the resonance circuit being changed in accordance with a control voltage applied to the first variable-capacity element; resonance frequency range switching section which includes a second variable-capacity element connected in parallel with the first variable-capacity element, the resonance frequency range switching section switching a variation range of the resonance frequency of the resonance circuit by switching capacity of the second variable-capacity element; and resonance frequency correcting means which corrects the resonance frequency in such a manner as to prevent a ratio between resonance frequencies before and after switching the variation range of the resonance frequency from depending on the control voltage.

According to this arrangement, the oscillating frequency of the voltage control circuit varies in accordance with the resonance frequency of the resonance circuit, and hence the oscillating frequency of the voltage control circuit can be controlled by the control voltage. Also, the variation range of the oscillating frequency can be switched by the resonance frequency range switching section.

Moreover, according to the arrangement above, the variation range of the resonance frequency is switched by switching the capacity of the second variable-capacity element. In other words, the switching of the variation range is achieved without series connection to an inductor. A good phase noise characteristic of the voltage-controlled oscillator is therefore obtained. Furthermore, since the variation range of the resonance frequency is switched without auxiliary inductance, it is possible to downsize the voltage-controlled oscillator.

In addition to the above, on account of the resonance frequency correction section, it is possible to provide a voltage-controlled oscillator in which the ratio of resonance frequencies before and after the switching of the variation range of the resonance frequency which does not depend on the control voltage.

DESCRIPTION OF THE EMBODIMENTS

The following will describe an embodiment of the present invention in reference toFIGS. 1-4.

First, a voltage-controlled oscillator100of the embodiment of the present invention will be described in reference toFIG. 1.FIG. 1is a circuit block diagram which outlines the voltage-controlled oscillator100. The voltage-controlled oscillator100basically includes an amplifier circuit100a, a resonance circuit100b, and a resonance frequency correction circuit100c, so that oscillation is performed by feeding the output of the amplifier100aback to the amplifier circuit100avia the resonance circuit100b.

The amplifier circuit100aof the voltage control circuit100includes a pair of transistors Tr101aand Tr101b. These transistors Tr101aand Tr101bconstitute a differential pair. As shown inFIG. 1, the emitters of the transistors Tr101aand Tr101bare grounded via a shared current source I101. The collector of the transistor Tr101ais connected to the base of the transistor Tr101b, via a fixed capacity element C104a. Similarly, the collector of the transistor Tr101bis connected to the base of the transistor Tr101a, via a fixed capacity element C104b.

The collectors of the transistors Tr101aand Tr101bof the amplifier100aare connected to the resonance circuit100b. As shown inFIG. 1, the resonance circuit100bof the voltage-controlled oscillator100is an LC resonance circuit including inductors and variable-capacity elements. More specifically, the resonance circuit100bincludes inductors L101aand L101b, variable-capacity elements C101aand C101bconnected in parallel with the inductors L101aand L101b, and variable-capacity elements C102aand C102b. The inductors of the resonance circuit100binclude a pair of inductors L101aand L101bconnected in series with one another. The middle point between the inductors L101aand L101breceives a power source voltage VDD.

The variable-capacity elements C101aand C101bare used for changing the resonance frequency of the resonance circuit100b, by means of a control voltage. A capacity control terminal C101cof the variable-capacity elements C101aand101bconnected to one another is connected to the control voltage input terminal101. With this, the capacity control terminal C101ccan receive the control voltage Vc from the outside. Therefore, the resonance frequency of the resonance circuit100bcan be controlled by changing the capacity CVof the variable-capacity elements C101aand C101bby applying the control voltage Vc to the control voltage input terminal101. That is, the resonance circuit100bincludes the variable-capacity elements C101aand C101b, and functions as a resonance circuit in which the resonance frequency changes in accordance with the control voltage applied to the variable-capacity elements C101aand C101b.

The variable-capacity elements C101aand C102bare used for switching the variation range of the resonance frequency (i.e. resonance frequency range). A capacity control terminals C102cof the variable-capacity elements C102aand C102bconnected to one another is connected to a switch SW101. The switch SW101is a single-pole double-throw switch, and has two terminals in addition to the terminal connected to the capacity control terminal C102c. One of these two terminal is grounded, whereas the other one is connected to a power source102. The switch SW101can therefore determine whether the capacity control terminal C102cof the variable-capacity elements C102aand C102bis grounded or connected to the power source102. On this account, in accordance with the state of the switch SW101, the capacity CXof the variable-capacity elements C102aand C102bis switched in a binary manner. That is, the switch SW101is switched so that either a ground potential or a power source voltage VDD is supplied to the capacity control terminal C102c. As a result of this, the capacity of the variable-capacity elements C102aand C102bis CAwhen the capacity control terminal C102cis grounded, whereas the capacity of the variable-capacity elements C102aand C102bis CB(CA>CB) when the capacity control terminal C102cis connected to the power source102. In this manner, the variable-capacity elements C102aand C102band the switch SW101function as means for switching the variation range of the resonance frequency of the resonance circuit100b(i.e. functioning as a resonance frequency range switch section).

The capacity C of the resonance circuit100bis CV+CX, i.e. the sum total of (i) the capacity CVof the variable-capacity elements C101aand C101b, which capacity is determined by the control voltage Vc and (ii) the capacity CXof the variable-capacity elements C102aand C102b.

When the capacity control terminal C102cof the variable-capacity elements C102aand C102bis grounded, i.e. when the capacity CXof the variable-capacity elements C102aand C102bis CA, the resonance frequency fAof the resonance circuit100bis determined as follows:

In the equation above, indicated by L is the inductance of the inductors L101aand L101b. In case where the control voltage Vc is changed while the capacity control terminal C102cof the variable-capacity elements C102aand C102bis grounded, the resonance frequency of the resonance circuit100bcontinuously changes within a particular variation range, in accordance with the above-described equation.

On the other hand, when the capacity control terminal C102cof the variable-capacity elements C102aand C102bis connected to the power source102, i.e. when the capacity CXof the variable-capacity elements is CB, the resonance frequency fBof the resonance circuit100bis determined as below:

In case where the control voltage Vc is changed while the capacity control terminal C102cof the variable-capacity elements C102aand C102bis connected to the power source102, the resonance frequency of the resonance circuit100bcontinuously changes within a variation range different from the above, in accordance with the above-described equation.

When the resonance frequency range is switched while the control voltage Vc applied to the control voltage input terminal101is kept constant, the ratio fA/fBbetween the resonance frequencies before and after the switching of the resonance frequency range is determined as follows:

Since the capacity CVof the variable-capacity elements C101aand C101bis determined in dependence upon the control voltage Vc, the ratio fA/fBbetween the resonance frequencies before and after the switching of the resonance frequency range depends on the control voltage Vc, so as not be constant.

However, the voltage-controlled oscillator100characteristically includes the resonance frequency correction circuit100c. The resonance frequency correction circuit100ccorrects the resonance frequency of the resonance circuit100bin such a manner as to prevent the ratio of the oscillating frequencies before and after the switching of the resonance frequency variation range from depending on the control voltage Vc. That is, the resonance frequency correction circuit100cfunctions as means for correcting the resonance frequency so as to prevent the ratio of the oscillating frequencies before and after the switching of the resonance frequency variation range from depending on the control voltage Vc (i.e. functioning as a resonance frequency correction section).

When the capacity control terminal C102cof the variable-capacity elements C102aand C102bis grounded, the resonance frequency fA′ of the voltage-controlled oscillator100including the resonance frequency correction circuit100cis determined as follows, on account of the resonance frequency correction circuit100c:

When the capacity control terminal C102cof the variable-capacity elements C102aand C102bis connected to the power source102, the resonance frequency fB′ of the voltage-controlled oscillator100including the resonance frequency correction circuit100cis determined as follows:

In the equation above, indicated by FAand FBare correction terms for the resonance frequency, by the resonance frequency correction circuit100c. Also, the above-described equation is arranged to satisfy the following equation:

fA′fB′=
a constant value independent of the control voltage Vc

That is, because of the resonance frequency correction circuit100c, the ratio between the resonance frequencies of the voltage-controlled oscillator100including the resonance frequency correction circuit100c, i.e. the ratio fA′/fB′ of the oscillating frequencies is constant before and after the switching of the variation range of the resonance frequency, independently of the control voltage Vc.

Now, in reference toFIGS. 2(a)-2(c), the following will describe specific circuitry of the resonance frequency correction circuit100c.FIGS. 2(a)-2(c) are circuit diagrams of the voltage-controlled oscillator100shown inFIG. 1, and show specific circuitry of the resonance frequency correction circuit100cwhich is illustrated as a circuit block inFIG. 1.

As shown inFIG. 2(a), the resonance frequency correction circuit100cincludes a pair of variable-capacity elements C103aand C103bconnected in parallel with the aforesaid variable-capacity elements C102aand C102b. A capacity control terminal C103cof the variable-capacity elements C103aand C103bconnected to one another is connected to a switch SW102. The switch SW102is a single-pole double-throw switch, and has two terminals in addition to the terminal connected to the capacity control terminal C103c. One of these two terminal is connected to a control voltage input terminal101, whereas the other one is grounded via a voltage source V101. The switch SW102can therefore determine whether the capacity control terminal C103cof the variable-capacity elements C103aand C103bis connected to the control voltage input terminal101or grounded via the voltage source V101. On this account, the switch SW102is switched so that the capacity control terminal C103cof the variable-capacity elements C103aand C103breceives either a control voltage Vc or an output voltage V0of the voltage source V101. In this manner, the capacity of the variable-capacity elements C103aand C103bis changed in a binary manner.

Before and after the switching of the resonance frequency range, the switches SW101and SW102operates together in such a manner as to cause the ratio between the resonance frequencies in the voltage control circuit100including the resonance frequency correction circuit100c, i.e. the ratio fA′/fB′ of the oscillating frequencies to be constant independently of the control voltage Vc. More specifically, when the capacity control terminal C102cof the variable-capacity elements C102aand C102bis grounded by the switch SW101, the switch SW102connects the capacity control terminal C103cof the variable-capacity elements C103aand C103bwith the control voltage input terminal101. On the other hand, when the capacity control terminal C102cof the variable-capacity elements C102aand C102bis connected to the power source102by the switch SW101, the switch SW102causes the capacity control terminal C103cof the variable-capacity elements C103aand C103bto be grounded via the voltage source V101.

FIG. 2(b) illustrates the voltage-controlled oscillator100in which the capacity control terminal C102cof the variable-capacity elements C102aand C102bis grounded by the switch SW101whereas the capacity control terminal C103of the variable-capacity elements C103aand C103bis connected to the control voltage input terminal101by the switch SW102.

In case where the voltage-controlled oscillator100is arranged as shown inFIG. 2(b), the capacity of the variable-capacity elements C102aand C102bis CAas described above. In the meanwhile, the variable-capacity elements C103aand103bare connected in parallel with the variable-capacity elements C101aand C101b, and receive the same control voltage Vc as the variable-capacity elements C101aand C101b. The capacity CYof the variable-capacity elements C103aand C103bis therefore in proportion to the capacity CVof the variable-capacity elements C101aand C101b. In other words, the following equation is given:
CY=a·CV

In this equation, indicated by a is a positive constant (a>0).

Therefore, when the voltage-controlled oscillator100is arranged as shown inFIG. 2(b), the total capacity CA′ of the voltage-controlled oscillator100is given by the following equation:
CA′=CA+CV+a·CV

Therefore, when the voltage-controlled oscillator100is arranged as shown inFIG. 2(b), the resonance frequency fA′ of the voltage-controlled oscillator100is given by the following equation:

FIG. 2(c) shows the voltage-controlled oscillator100in which the capacity control terminal C102cof the variable-capacity elements C102aand C102bis connected to the power source102by the switch SW101whereas the capacity control terminal C103cof the variable-capacity elements C103aand C103bis, by the switch SW102, grounded via the voltage source V101.

In case where the voltage-controlled oscillator100is arranged as shown inFIG. 2(c), the capacity of the variable-capacity elements C102aand C102bis CBas described above. On the other hand, the capacity of the variable-capacity elements C103aand C103bis a constant value Co which is determined in line with the output voltage V0of the voltage source V101.

Therefore, when the voltage-controlled oscillator100is arranged as shown inFIG. 2(b), the total capacity CB′ of the voltage-controlled oscillator100is given by the following equation:
CB′=CB+CV+Co

Therefore, when the voltage-controlled oscillator100is arranged as shown inFIG. 2(b), the resonance frequency fB′ of the voltage-controlled oscillator100is given by the following equation:

On this account, the ratio fA′/fB′ between the resonance frequencies before and after the switching of the resonance frequency range is represented by the following equation:

The output voltage Vo of the voltage source V101is set so that the constant a and the capacity Co satisfy the following equation:

As a result, the ratio fA′/fB′ between the resonance frequencies before and after the switching of the variation range of the resonance frequency is given as follows:

In this manner, the ratio is a constant value independent of the control voltage Vc.

Now, the following will describe a switch which can be suitably used as the switch SW101or SW102, in reference toFIGS. 3(a)-3(c).

FIG. 3(a) is a circuit diagram showing a switching element30constituting the switch SW101or SW102. As shown inFIG. 3(a), the switching element30includes an NMOS transistor31and a PMOS transistor32. The switching element30is switched, i.e. terminals33and34are connected/disconnected, by a control signal supplied from a control signal input terminal35.

FIG. 3(b) shows a single-pole double-throw switch made up of switching elements30aand30b, which are equivalent to the above-described switching element. By the single-pole double-throw switch shown inFIG. 3(b), terminals36and37are connected/disconnected by a control signal supplied to a control signal input terminal35a, whereas terminals36and38are connected/disconnected by a control signal supplied to a control signal input terminal35b. On this account, the terminal36is connected to either the terminal37or the terminal38, by applying, to the control signal input terminals35aand35b, control signals which are inverse to one another. The single-pole double-throw switch shown inFIG. 3(b) can therefore constitute the switch SW101or SW102.

FIG. 3(c) is a variant example of the single-pole double-throw switch shown inFIG. 3(b). Since an inverter39is provided, the single-pole double-throw switch inFIG. 3(c) can connect the terminal36with either the terminal37or the terminal38, only by a single control signal. In the single-pole double-throw switch inFIG. 3(c), a control signal inputted to a single control signal input terminal35cis supplied to a switching element30a, whereas the control signal inverted by the inverter39is supplied to a switching element30b. In other words, the single-pole double-throw switch shown inFIG. 3(c) is controlled by the control signal supplied to the control signal input terminal35cand the inversion signal which is generated by inverting the control signal by the inverter.

In case where the single-pole double-throw switch ofFIG. 3(b) or3(c) is used as the switch SW101or SW102of the voltage-controlled oscillator100, the control signal for controlling the switch may be a digital signal. In particular, when the single-pole double-throw switch ofFIG. 3(c) is used as the switch SW101or SW102, the switching is achieved by a single control signal. This makes it possible to simplify and downsize the circuitry for the switching.

The above-described voltage-controlled oscillator100can be suitably used as a local oscillator of a transmitter, receiver, or a transmitter-receiver.FIG. 4is a circuit block diagram which outlines a transmitter-receiver400including, as a local oscillator, the voltage-controlled oscillator100.

As shown inFIG. 4, the transmitter-receiver400includes members constituting a receiving circuit, and hence functions as a receiver. The members constituting the receiving circuit are a LNA (low noise amplifier)403, a down mixer404, a variable amplifier405, a BPF (band pass filter)406, an amplifier407, and a demodulator408. The transmitter-receiver400further includes members constituting a transmitting circuit, and hence functions as a transmitter. The members constituting the transmitting circuit are a modulator410, a BPF (band pass filter)411, an up mixer412, and a power amplifier413. The transmitter-receiver400further includes an antenna401and a switch402which switches the connection of the antenna401to either the transmitting circuit or the receiving circuit. The transmitter-receiver400also includes a down mixer404and a local oscillator409connected to the up mixer412.

The antenna401receives an RF signal, and the RF signal is supplied to the LNA403via the switch402. The LNA403amplifies the RF signal supplied from the antenna401, and sends the signal to the down mixer404. The down mixer404mixes the supplied RF signal with the output of the local oscillator409, so that down-conversion is carried out. As a result, an IF signal is obtained. The IF signal is amplified by the variable amplifier405, and an unnecessary frequency of the IF signal is cut off by the BPF406. The signal from the BPF is further amplified by the amplifier407. The amplified signal is supplied to the demodulator408.

The IF signal is outputted from the demodulator410, and an unnecessary frequency of the IF signal is cut off by the BPF411. The IF signal is then supplied to the up mixer412. The up mixer412mixes the supplied IF signal with the output of the local oscillator409, so that up-conversion is carried out. As a result, an RF signal is obtained. This RF signal is amplified by the power amplifier413, and then supplied to the antenna401via the switch402. The antenna401outputs the RF signal.

As the local oscillator409of the transmitter-receiver400, it is possible to use the voltage-controlled oscillator of the present invention, e.g. the voltage-controlled oscillator100. In the voltage-controlled oscillator of the present invention, the ratio between the oscillating frequencies before and after the switching of the variation range of the oscillating frequencies does not depend on the control voltage. It is therefore possible to simplify the circuit design of a transmitter, receiver, or a transmitter-receiver, which includes, as a local oscillator, the voltage-controlled oscillator of the present invention. Furthermore, the voltage-controlled oscillator of the present invention does not require plural inductors, even if a wide oscillating frequency range is achieved. It is therefore possible to provide a local oscillator which is small and inexpensive as compared to conventional local oscillators. In other words, in case where the voltage-controlled oscillator of the present invention is used as a local oscillator, a small and inexpensive transmitter, receiver, or transmitter-receiver is realized.

As described above, a typical voltage-controlled oscillator of the present invention includes: a first variable-capacity element; a resonance circuit whose resonance frequency varies in accordance with a control voltage applied to the first variable-capacity element; a second variable-capacity element connected in parallel with the first variable-capacity element; resonance frequency range switching means (e.g. a resonance frequency range switching circuit) which switches the variation range of the resonance frequency of the resonance circuit by switching the capacity of the second variable-capacity element; and resonance frequency correcting means (e.g. a resonance frequency correction circuit) which corrects the resonance frequency in such a manner as to prevent the ratio between resonance frequencies before and after the switching of the variation range of the resonance frequency from depending on the control voltage.

According to this arrangement, the oscillating frequency of the voltage control circuit varies in accordance with the resonance frequency of the resonance circuit, and hence the oscillating frequency of the voltage control circuit can be controlled by the control voltage. Also, the variation range of the oscillating frequency can be switched by the resonance frequency range switching section.

Moreover, according to the arrangement above, the variation range of the resonance frequency is switched by switching the capacity of the second variable-capacity element. In other words, a switch for switching the inductance of the resonance circuit is constructed without series connection to an inductor. A good phase noise characteristic of the voltage-controlled oscillator is therefore obtained. Furthermore, since the variation range of the resonance frequency is switched without auxiliary inductance, it is possible to downsize the voltage-controlled oscillator.

In addition to the above, on account of the resonance frequency correction section, it is possible to provide a voltage-controlled oscillator in which the ratio of resonance frequencies before and after the switching of the variation range of the resonance frequency does not depend on the control voltage.

The above-described voltage-controlled oscillator is preferably arranged such that the resonance frequency correcting means includes a third variable-capacity element connected in parallel with the second variable-capacity element, and the resonance frequency is corrected by switching the capacity of the third variable-capacity element.

According to this arrangement, the resonance frequency is corrected by switching the capacity of the variable-capacity element. That is, the correction of the resonance frequency is achieved without switching the inductance of the resonance circuit. On this account, it is possible to provide a voltage-controlled oscillator which has a good phase noise characteristic, which is small in size, and whose ratio between resonance frequencies before and after the switching of the variation range of the resonance frequency does not depend on the control voltage.

The above-described voltage-controlled oscillator is preferably arranged such that the resonance frequency correcting means includes a switch that operates together with the resonance frequency range switching means so as to determine whether the third variable-capacity element receives the control voltage or a constant output voltage supplied from a voltage source.

According to this arrangement, the resonance frequency correcting means is constituted by a variable-capacity element, a switch, and a voltage source. It is therefore possible to construct the resonance frequency correcting means with a small number of components and a simple circuit design. On this account, it is possible to provide a voltage-controlled oscillator which is inexpensive and whose ratio between resonance frequencies before and after the switching of the variation range of the resonance frequency does not depend on the control voltage.

The above-described voltage-controlled oscillator is preferably arranged such that the switch includes a transistor.

According to this arrangement, the switching capability to connect/disconnect a current is achieved by the transistor. Since the switching capability is achieved by the transistor, it is possible to realize the downsizing and cost reduction of the resonance frequency correcting means. Also, since the switching capability is achieved by the transistor, it is possible to use a digital signal as the control signal by which the switch is controlled.

The transistor may be a MOS field effect transistor. The switch may be a single-pole double-throw switch made up of a NMOS transistor and a PMOS transistor.

The above-described voltage-controlled oscillator is preferably arranged such that the switch is controlled by a control signal which is a digital signal and an inverse signal which is generated by inverting the control signal by an inverter.

According to this arrangement, a single-pole double-throw switch constituted by a transistor is switched and controlled by a single control signal and an inverse signal generated from the control signal. Moreover, since the inverse signal is generated by an inverter, the circuitry for the switching and control of the switch is simplified, so that the downsizing and cost-reduction of the voltage-controlled oscillator is achieved.

The above-described voltage-controlled oscillator is preferably arranged such that the resonance circuit includes an inductor. As described above, the resonance circuit includes the first variable-capacity element. An LC resonance circuit is therefore constructed because an inductor is included.

The above-described voltage-controlled oscillator may be arranged such that the resonance frequency range switching means includes a switch connected to a terminal of the second variable-capacity element. That is, a specific example of the resonance frequency range switching means is a resonance frequency range switching circuit including the second variable-capacity element and the switch.

Another voltage-controlled oscillator of the present invention includes: a resonance circuit whose resonance frequency changes in accordance with a control voltage applied to a variable-capacity element, the resonance circuit including a resonance frequency range switching circuit which is connected in parallel with the variable capacity element and switches a variation range of the resonance frequency of the resonance circuit; and a resonance frequency correction circuit which corrects the resonance frequency in such a manner as to prevent a ratio between resonance frequencies before and after switching the variation range from depending on the control voltage.

According to this arrangement, a good phase noise characteristic is obtained, downsizing is easily achieved, and the ratio of resonance frequencies before and after the switching of the variation range of the resonance frequency does not depend on the control voltage.

The above-described voltage-controlled oscillator is preferably arranged such that, provided that a variable-capacity element in the resonance circuit, which element contributes to a variation of the resonance frequency, is a first variable-capacity element, the resonance frequency range switching circuit includes (i) a second variable-capacity element connected in parallel with the first variable-capacity element and (ii) a switch that switches capacity of the second variable-capacity element.

The above-described voltage-controlled oscillator is preferably arranged such that, the resonance frequency correction circuit includes a third variable-capacity element connected in parallel with the second variable-capacity element, and the resonance frequency is corrected by switching capacity of the third variable-capacity element.

The above-described voltage-controlled oscillator is preferably arranged such that, the resonance frequency correction circuit includes a switch which operates together with the resonance frequency range switching circuit so as to determine whether the third variable-capacity element receives the control voltage or a constant output voltage supplied from a voltage source.

A transmitter or a receiver of the present invention includes, as a local oscillator, the above-described voltage-controlled oscillator.

The transmitter or receiver including the voltage-controlled oscillator does not require circuitry to prevent the ratio between resonance frequencies before and after the switching of the variation range of the resonance frequency from depending on the control voltage. It is therefore possible to provide a transmitter or a receiver which is inexpensive and has simple circuitry.