Frequency-variable LC filter and high-frequency front end circuit

A first series arm LC filter circuit includes a capacitor and an inductor connected in series to provide a series circuit between a first connection terminal and a second connection terminal, a capacitor connected in parallel to the series circuit, and an inductor and a variable capacitor connected in parallel between a connection point of the capacitor and the inductor and a ground potential. A first parallel arm LC filter circuit is connected between the first connection terminal and the ground potential. A second parallel arm LC filter circuit is connected between the second connection terminal and the ground potential. The inductor is directly connected to the second connection terminal or is connected to the second connection terminal with another inductor interposed therebetween.

BACKGROUND

Technical Field

The present disclosure relates to a frequency-variable LC filter including a resonance circuit of an inductor and a variable capacitor.

Patent Document 1 discloses a frequency-variable LC filter using an inductor and a variable capacitor. The frequency-variable LC filter in Patent Document 1 includes a first LC parallel circuit, a second LC parallel circuit, a third series circuit, and a fourth series circuit. Both of the first LC parallel circuit and the second LC parallel circuit include parallel circuits of inductors and variable capacitors. One ends of the first LC parallel circuit and the second LC parallel circuit are connected to each other with a coupling inductor and the other ends thereof are connected to a ground potential.

The third series circuit includes a variable capacitor, and one end thereof is connected to the first LC parallel circuit and the other end thereof is connected to a first connection terminal. The fourth series circuit includes a variable capacitor, and one end thereof is connected to the second LC parallel circuit and the other end thereof is connected to a second connection terminal.

The one end (end portion at the side at which the coupling inductor is connected) of the first LC parallel circuit is connected to the first connection terminal with a first variable capacitor interposed therebetween. The one end (end portion at the side at which the coupling inductor is connected) of the second LC parallel circuit is connected to the second connection terminal with a second variable capacitor interposed therebetween. The first connection terminal and the second connection terminal are connected with a fixed capacitor interposed therebetween.

With this configuration, capacitances of the variable capacitors of the first and second parallel circuits and capacitances of the variable capacitors of the third and fourth series circuits are changed to adjust bandpass characteristics.

BRIEF SUMMARY

However, the frequency-variable LC filter disclosed in Patent Document 1 uses four variable capacitors to adjust the characteristics, resulting in increase in a circuit size.

Furthermore, the frequency-variable LC filter disclosed in Patent Document 1 has the configuration in which two variable capacitors (“54 and 55” in Patent Document 1) are connected in series between the first connection terminal as an input terminal and the second connection terminal as an output terminal, resulting in increase in loss of bandpass characteristics.

Moreover, the frequency-variable LC filter disclosed in Patent Document 1 has a large difference between steepness of attenuation characteristics at the low frequency side of a pass band and steepness of the attenuation characteristics at the high frequency side thereof.

Accordingly, the present disclosure provides a frequency-variable LC filter having a simple configuration capable of providing steep attenuation characteristics at both sides of a pass band with low loss of bandpass characteristics.

A frequency-variable LC filter according to an aspect of the disclosure includes an input terminal, an output terminal, a first series arm LC filter circuit, and first and second parallel arm LC filter circuits. The first series arm LC filter circuit is connected between the input terminal and the output terminal. The first parallel arm LC filter circuit is a circuit both ends of which are one end of the first series arm LC filter circuit and a ground potential. The second parallel arm LC filter circuit is a circuit both ends of which are the other end of the first series arm LC filter circuit and the ground potential. Each of the first parallel arm LC filter circuit and the second parallel arm LC filter circuit includes a variable capacitor and an inductor connected in series. The first series arm LC filter circuit includes a fixed capacitor, an LC series circuit, and an LC parallel circuit. The fixed capacitor is connected in parallel to the LC series circuit. Both ends of the LC series circuit are the input terminal and the output terminal and the LC series circuit includes a fixed capacitor and an inductor connected in series. The LC parallel circuit includes a variable capacitor and an inductor connected in parallel. The inductor included in the LC series circuit is directly connected to the output terminal or is connected to the output terminal with another inductor interposed therebetween.

With this configuration, no variable capacitor is connected in series between the input terminal and the output terminal. Furthermore, three variable capacitors are provided. Therefore, the circuit configuration is simplified while suppressing loss of bandpass characteristics.

In the frequency-variable LC filter in the aspect of the disclosure, a resonant frequency of the first parallel arm LC filter circuit can be lower than a center frequency of a pass band of the frequency-variable filter and a resonant frequency of the second parallel arm LC filter circuit can be higher than the center frequency.

With this configuration, steepness of attenuation characteristics is improved at both sides of the pass band and a frequency range providing desired attenuation is enlarged.

The frequency-variable LC filter in the aspect of the disclosure can have the following configuration. A resonant frequency of the LC parallel circuit in the first series arm LC filter circuit is higher than the resonant frequency of the first parallel arm LC filter circuit and is lower than the center frequency. A resonant frequency of the LC series circuit in the first series arm LC filter circuit and a resonant frequency by the inductor and the fixed capacitor of the LC series circuit are higher than the center frequency and are lower than the resonant frequency of the second parallel arm LC filter circuit.

With this configuration, the steepness of the attenuation characteristics is further improved at both sides of the pass band and the frequency range providing the desired attenuation is further enlarged.

Furthermore, the frequency-variable LC filter in the aspect of the disclosure can have the following configuration. Inductances of the inductor of the LC series circuit, the inductor of the LC parallel circuit, and the inductor of the first parallel arm LC filter circuit are larger than 20 nH. Capacitances of the variable capacitor of the first parallel arm LC filter circuit and the variable capacitor of the second parallel arm LC filter circuit are smaller than 20 pF.

With this configuration, the steepness of the attenuation characteristics is further improved at both sides of the pass band and the frequency range providing the desired attenuation is further enlarged.

In the frequency-variable LC filter in the aspect of the disclosure, a capacitance of the variable capacitor of the first series arm LC filter circuit can be smaller than 20 pF.

With this configuration, the steepness of the attenuation characteristics is improved at both sides of the pass band and the frequency range providing the desired attenuation is enlarged.

Moreover, in the frequency-variable LC filter in the aspect of the disclosure, at least one inductor of the inductor of the first parallel arm LC filter circuit, the inductor of the LC series circuit, the inductor of the LC parallel circuit, and the inductor of the second parallel arm LC filter circuit can be magnetically coupled with another inductor differing from the at least one inductor among the above-described inductors.

With this configuration, the steepness of the attenuation characteristics is improved.

Furthermore, in the frequency-variable LC filter in the aspect of the disclosure, the inductor of the first parallel arm LC filter circuit can be capacitively coupled with at least one of the inductor of the LC series circuit, the inductor of the LC parallel circuit, and the inductor of the second parallel arm LC filter circuit.

With this configuration, the capacitance of the fixed capacitor can be decreased or the fixed capacitor can be omitted, thereby further reducing the frequency-variable LC filter in size.

The frequency-variable LC filter in the aspect of the disclosure further can include a fixed capacitor connected between the first series arm LC filter circuit and the input terminal, an LC series circuit connected between a connection point of the fixed capacitor and the input terminal and the ground potential, and a fixed capacitor one end of which is connected to the output terminal and the other end of which is connected to the ground potential. The second parallel arm LC filter circuit includes an inductor connected in parallel to the variable capacitor and the inductor connected in series.

With this configuration, not only the steepness of the attenuation characteristics is improved at both sides of the pass band but also the attenuation of a harmonic and moreover the attenuation at the lower frequency side are increased.

The frequency-variable LC filter in the aspect of the disclosure can further include two fixed capacitors connected in series between the first series arm LC filter circuit and the input terminal, and an inductor and a fixed capacitor connected in parallel between the first series arm LC filter circuit and the output terminal. The second parallel arm LC filter circuit includes an inductor connected in parallel to the variable capacitor and the inductor connected in series, and the first parallel arm LC filter circuit is connected between a path between the two fixed capacitors and the ground potential.

With this configuration, the bandpass characteristics are further improved while maintaining the attenuation characteristics.

Furthermore, the frequency-variable LC filter in the aspect of the disclosure can be set as follows. A center frequency f0of a pass band, which is defined by the first series arm LC filter circuit, a frequency fp2of an attenuation pole, which is defined by the first series arm LC filter circuit and is higher than the pass band, a frequency fp1of an attenuation pole, which is defined by the first parallel arm LC filter circuit and is lower than the pass band, and a frequency fp3of an attenuation pole, which is defined by the second parallel arm LC filter circuit and is higher than the frequency fp2, satisfy the following three equations.
f0/2<fp1<f0   Equation 1
f0<fp2<3×f0/2   Equation 2
2×f0<fp3<3×f0   Equation 3

For example, capacitances of the respective variable capacitors are set so as to satisfy the above-described equations 1 to 3. Satisfaction of these three equations enables the frequency-variable LC filter to maintain the desired bandpass characteristics and maintain the desired attenuations at around the attenuation poles at both sides of the pass band and at around a frequency of the harmonic even when the pass band is changed.

Furthermore, another aspect of the disclosure relates to a high-frequency front end circuit and has the following features. The high-frequency front end circuit performs wireless communication by selecting a usage channel from vacant communication channels of a plurality of communication channels in a communication band that is configured by the plurality of communication channels in a specific frequency band that is used in the system. The high-frequency front end circuit includes a fixed filter and first and second variable filters. The fixed filter attenuates high-frequency signals outside the specific frequency band that is used in the system. The first variable filter attenuates high-frequency signals of unnecessary waves in the specific frequency band, which vary depending on the usage channel. The second variable filter is configured by a frequency-variable LC filter and attenuates IMD (intermodulation distortion) in the specific frequency band. The second variable filter is the frequency-variable LC filter according to any one of the above-described filters.

This configuration improves insertion loss of the second variable filter and makes attenuation characteristics steep. Transmission characteristics as the high-frequency front end circuit are therefore improved.

Furthermore, the high-frequency front end circuit in the aspect of the disclosure can have the following configuration. The first variable filter includes an input terminal, an output terminal, a series arm resonance circuit, and first and second parallel arm resonance circuits. The series arm resonance circuit is connected in series between the input terminal and the output terminal. The first parallel arm resonance circuit is a circuit both ends of which are one end of the series arm resonance circuit and a ground potential. The second parallel arm resonance circuit is a circuit both ends of which are the other end of the series arm resonance circuit and the ground potential. The series arm resonance circuit includes a fixed capacitor having a fixed capacitance. Each of the series arm resonance circuit, the first parallel arm resonance circuit, and the second parallel arm resonance circuit includes a variable capacitor, an inductor, and an elastic wave resonator. The variable capacitor, the inductor, and the elastic wave resonator in the series arm resonance circuit are connected in parallel. The variable capacitor, the inductor, and the elastic wave resonator in each of the first parallel arm resonance circuit and the second parallel arm resonance circuit are connected in series. The fixed capacitor in the series arm resonance circuit is connected to the parallel arm resonance including the elastic wave resonator having a lower impedance of an impedance of the elastic wave resonator of the first parallel arm resonance circuit and an impedance of the elastic wave resonator of the second parallel arm resonance circuit.

This configuration makes the attenuation characteristics at both sides of the pass band of the first variable filter steep. The transmission characteristics as the high-frequency front end circuit are therefore improved.

Furthermore, the high-frequency front end circuit in the aspect of the disclosure can include a detector detecting respective reception levels of a plurality of vacant communication channels when there is the plurality of vacant communication channels, and a determination unit selecting, as the usage channel, a vacant communication channel having the highest reception level among the plurality of detected reception levels.

With this configuration, high-frequency signals can be transmitted and received using an optimum communication channel. The transmission characteristics of the high-frequency front end circuit can therefore be further improved.

Furthermore, the high-frequency front end circuit in the aspect of the disclosure can further include an amplification-side amplification circuit.

Furthermore, the high-frequency front end circuit in the aspect of the disclosure can further include a signal processor.

The present disclosure can provide a frequency-variable LC filter capable of decreasing loss of bandpass characteristics and decreasing a difference between steepness of attenuation characteristics at the low frequency side of a pass band and steepness of the attenuation characteristics at the high frequency side thereof with a simple configuration reduced in size.

DETAILED DESCRIPTION

A frequency-variable LC filter according to a first embodiment of the present disclosure will be described with reference to the drawings.FIG. 1is a circuit diagram of the frequency-variable LC filter in the first embodiment of the present disclosure.

A frequency-variable LC filter40includes a first series arm LC filter circuit41, a first parallel arm LC filter circuit42, a second parallel arm LC filter circuit43, a first connection terminal P401, and a second connection terminal P402.

The first series arm LC filter circuit41is connected between the first connection terminal P401as an input terminal and the second connection terminal P402as an output terminal. The first parallel arm LC filter circuit42is connected between the first series arm LC filter circuit41at the first connection terminal P401side and a ground potential. The second parallel arm LC filter circuit43is connected between the first series arm LC filter circuit41at the second connection terminal P402side and the ground potential.

The first series arm LC filter circuit41includes capacitors411and413, inductors412and414, and a variable capacitor415.

The capacitor411and the inductor412are connected in series between the first connection terminal P401and the second connection terminal P402. In this case, the inductor412is connected directly in series to the second connection terminal P402. The capacitor413is connected in parallel to a series circuit of the capacitor411and the inductor412. The inductor414and the variable capacitor415are connected in parallel. This parallel circuit is connected between a connection point between the capacitor411and the inductor412and the ground potential.

A resonant frequency f41of the first series arm LC filter circuit41formed by the above-described circuit configuration mainly contributes to a frequency of a pass band of the frequency-variable LC filter40and a frequency of an attenuation pole at the high frequency side of the pass band. In this case, when a center frequency of the pass band is f0, a resonant frequency f412of the parallel circuit of the inductor414and the variable capacitor415is set to be lower than the center frequency f0(f412<f0). A resonant frequency f411of the parallel circuit of the inductor412and the capacitor413is set to be higher than the center frequency f0(f411>f0). Furthermore, a resonant frequency f413of the series circuit of the capacitor411and the inductor412is set to be higher than the center frequency f0(f413>f0).

The first parallel arm LC filter circuit42includes an inductor421and a variable capacitor422.

A series circuit of the inductor421and the variable capacitor422is connected between the first series arm LC filter circuit41at the first connection terminal P401side and the ground potential.

A resonant frequency f42of the first parallel arm LC filter circuit42formed by the above-described circuit configuration mainly contributes to a frequency of an attenuation pole at the low frequency side of the pass band of the frequency-variable LC filter40. In this case, the resonant frequency f42is set to be lower than the center frequency f0(f42<f0). To be more specific, the resonant frequency f42is set to be lower than the resonant frequency f412(f42<f412).

The second parallel arm LC filter circuit43includes an inductor431and a variable capacitor432.

A series circuit of the inductor431and the variable capacitor432is connected between the first series arm LC filter circuit41at the second connection terminal P402side and the ground potential.

A resonant frequency f43of the second parallel arm LC filter circuit43formed by the above-described circuit configuration mainly contributes to a frequency of an attenuation pole at the high frequency side of the pass band of the frequency-variable LC filter40. In this case, the resonant frequency f43is set to be higher than the center frequency f0(f43>f0). To be more specific, the resonant frequency f43is set to be higher than the resonant frequencies f411and f413(f42>f411, f413).

The above-described configuration can realize a band pass filter having the pass band and the attenuation poles frequencies which are changeable by changing the capacitances of the variable capacitors415,422, and432.

FIG. 2is a graph illustrating bandpass characteristics of the frequency-variable LC filter in the first embodiment of the present disclosure. As illustrated inFIG. 2, usage of the frequency-variable LC filter40can form the attenuation poles at both sides of the pass band with a pass band width of approximately 100 [MHz]. Furthermore, the attenuation can be made difficult to be decreased in frequency bands at the opposite sides to the pass band with respect to the respective attenuation poles and desired attenuation can be realized in a wide frequency band. In addition, the attenuation can be increased at both of the high frequency side and the low frequency side of the pass band and the frequency band in which the desired attenuation is provided can be enlarged.

In particular, as illustrated inFIG. 1, the inductor412of the first series arm LC filter circuit41is directly connected to the second connection terminal P402without necessarily the capacitor interposed therebetween, thereby making attenuation characteristics steep.

In other words, the attenuation characteristics can be made steeper as illustrated inFIG. 3by connecting the inductor412of the first series arm LC filter circuit41directly to the second connection terminal P402or connecting it to the second connection terminal P402with another inductor interposed therebetween.

This effect is considered to be provided for the following reason.

The capacitor that is directly connected to the inductor has frequency characteristics of attenuating low frequency and allowing high frequency to pass like a high pass filter, and therefore causes deterioration in attenuation of the high frequency.

On the other hand, the inductor that is directly connected to the connection terminal has frequency characteristics of attenuating the high frequency and allowing the low frequency to pass like a low pass filter.

FIG. 3is a graph illustrating bandpass characteristics of the frequency-variable LC filter in the first embodiment of the present disclosure and an existing configuration. The existing configuration illustrated inFIG. 3is the circuit configuration described in the background art, that is, the configuration in which the variable capacitors are connected to the respective connection terminals and the inductors are connected between the variable capacitors. AlthoughFIG. 3illustrates setting of only one type of capacitances of the variable capacitors, the same result can be provided for other capacitances.

As illustrated inFIG. 3, usage of the frequency-variable LC filter40in the present application can decrease loss in the pass band and can make the attenuation characteristics at both sides (high frequency side and low frequency side) of the pass band steep.

As described above, usage of the configuration in the embodiment can decrease the loss of the bandpass characteristics and decrease the difference between the steepness of the attenuation characteristics at the low frequency side of the pass band and the steepness of the attenuation characteristics at the high frequency side thereof. Moreover, this configuration can reduce the number of variable capacitors configuring the circuit and realize the frequency-variable LC filter with a simple configuration.

The inductances of the inductors412,414, and421can be larger than 20 [nH]. The attenuation characteristics can be further improved by using the inductors having the above-described inductances.

Furthermore, the capacitances of the variable capacitors422and432can be smaller than 20 [pF]. The capacitance of the variable capacitor415can be smaller than 20 [pF]. The attenuation characteristics can be further improved by adjusting the capacitances of the variable capacitors in the above-described capacitance range.

Next, frequency-variable LC filters according to a second embodiment of the present disclosure will be described with reference to the drawings.FIGS. 4A, 4B, and 4Care circuit diagrams of the frequency-variable LC filters in the second embodiment of the present disclosure.FIGS. 4A, 4B, and 4Cillustrate modes in which places of magnetic field coupling are different from one another.

Frequency-variable LC filters40A,40B, and40C in the embodiment are different from the frequency-variable LC filter40in the first embodiment in a point that inductors are magnetically coupled. Other configurations thereof are the same as those of the frequency-variable LC filter40in the first embodiment.

As illustrated inFIG. 4A, the inductor412and the inductor414are magnetically coupled with each other in the frequency-variable LC filter40A. As illustrated inFIG. 4B, the inductor412and the inductor421are magnetically coupled with each other in the frequency-variable LC filter40B. As illustrated inFIG. 4C, the inductor412and the inductor431are magnetically coupled with each other in the frequency-variable LC filter40C.

The bandpass characteristics as illustrated inFIG. 5are provided by causing the inductor412that is directly connected to the second connection terminal P402to be magnetically coupled with another inductor as described above.FIG. 5is a graph illustrating the bandpass characteristics of the frequency-variable LC filter in the second embodiment of the present disclosure. As illustrated inFIG. 5, usage of the configuration of the frequency-variable LC filter40A,40B, or40C can make the attenuation characteristics steeper while maintaining low loss in the pass band in comparison with a mode that is not magnetically coupled.

The inductor412is caused to be magnetically coupled with another inductor in the embodiment. However, inductors that are caused to be magnetically coupled with each other among the inductor in the first parallel arm LC filter circuit, the inductor in the LC series circuit, the inductor in the LC parallel circuit, and the inductor in the second parallel arm LC filter circuit may be combined in a desirable pattern.

Next, frequency-variable LC filters according to a third embodiment of the present disclosure will be described with reference to the drawings.FIGS. 6A and 6Bare circuit diagrams of the frequency-variable LC filters in the third embodiment of the present disclosure.FIGS. 6A and 6Billustrate modes in which places of capacitive coupling are different from each other.

Frequency-variable LC filters40D and40E in the embodiment are different from the frequency-variable LC filter40in the first embodiment in a point that inductors are caused to be capacitively coupled with each other. Other configurations thereof are the same as those in the frequency-variable LC filter40in the first embodiment.

As illustrated inFIG. 6A, the inductor421and the inductor431are capacitively coupled with each other in the frequency-variable LC filter40D. With this configuration, at least a part of the capacitance of the capacitor413can be provided by the capacitive coupling between the inductor421and the inductor431. Accordingly, the capacitance of the capacitor413can be decreased. Alternatively, the capacitor413can be omitted.

As illustrated inFIG. 6B, the inductor421and the inductor412are capacitively coupled with each other in the frequency-variable LC filter40E. With this configuration, at least a part of the capacitance of the capacitor411can be provided by the capacitive coupling between the inductor421and the inductor412. Accordingly, the capacitance of the capacitor411can be decreased. Alternatively, the capacitor411can be omitted.

With this configuration in the embodiment, the capacitance of the fixed capacitor configuring the frequency-variable LC filter can be decreased or the fixed capacitor can be omitted, thereby reducing the frequency-variable LC filter in size.

The frequency-variable LC filter in each of the above-described embodiments can be used in a high-frequency front end circuit, which will be described as follows.FIG. 7is a functional block diagram of a high-frequency front end circuit according to an embodiment of the present disclosure.

A high-frequency front end circuit10includes an antenna ANT, an antenna matching circuit20, a frequency-fixed filter30, the frequency-variable LC filter40, a diplexer50, frequency-variable filters61and62, a transmission-side amplification circuit71, a reception-side amplification circuit72, a signal processor80, a transmission circuit91, and a reception circuit92. The signal processor80includes a transmission signal generator801, a demodulator802, and a channel determination unit810. The frequency-fixed filter30corresponds to a “fixed filter” in the present disclosure. The frequency-variable LC filter40corresponds to a “second filter” in the present disclosure. The frequency-variable filters61and62correspond to a “first filter” in the present disclosure. It is sufficient that the high-frequency front end circuit10includes at least the frequency-fixed filter30, the frequency-variable LC filter40, and the frequency-variable filter61. In this case, the frequency-fixed filter30, the frequency-variable LC filter40, and the frequency-variable filter61are connected in series in this order. Some or all components of the diplexer50, the frequency-variable filter62, the transmission-side amplification circuit71, the reception-side amplification circuit72, and the signal processor80can be omitted.

The antenna ANT is connected to the antenna matching circuit20. The antenna matching circuit20is connected to the frequency-fixed filter30. The frequency-fixed filter30is connected to the frequency-variable LC filter40. The frequency-variable LC filter40is connected to an antenna-side terminal of the diplexer50. A transmission-side terminal of the diplexer50is connected to the frequency-variable filter61. The frequency-variable filter61is connected to the transmission-side amplification circuit71. The transmission-side amplification circuit71is connected to the transmission circuit91. The transmission circuit91is connected to the transmission signal generator801of the signal processor80. The reception-side terminal of the diplexer50is connected to the frequency-variable filter62. The frequency-variable filter62is connected to the reception-side amplification circuit72. The reception-side amplification circuit72is connected to the reception circuit92. The reception circuit92is connected to the demodulator802of the signal processor80.

The high-frequency front end circuit10transmits and receives high-frequency signals using a vacant communication channel in a communication band configured by a plurality of communication channels. For example, the high-frequency front end circuit10transmits and receives the high-frequency signals in accordance with specifications of a TV white space. With the specifications of the TV white space, channels through which no signal of television broadcasting is transmitted among the plurality of communication channels set to a UHF (ultra high frequency) band of the television broadcasting, that is, a communication band of 470 [MHz] to 790 [MHz] and having frequency band widths of 6 [MHz] are used as vacant communication channels.

FIG. 8is a graph illustrating bandpass characteristics of the high-frequency front end circuit in the embodiment of the present disclosure.FIG. 8illustrates relations between the communication band and the respective communication channels. It should be noted thatFIG. 8illustrates the case in which a communication channel CH64is a selected channel (vacant communication channel that is used for communication in the high-frequency front end circuit10).

The antenna matching circuit20performs impedance matching between the antenna ANT and a circuit at the signal processor80side from the frequency-fixed filter30. The antenna matching circuit20is configured by an inductor and a capacitor. For example, in the antenna matching circuit20, element values of the inductor and the capacitor are set such that return loss of the antenna ANT is equal to or lower than a predetermined value in the communication band overall.

The frequency-fixed filter30is configured by an inductor and a capacitor. That is to say, the frequency-fixed filter30is a frequency-fixed-type LC filter. In the frequency-fixed filter30, element values of the inductor and the capacitor are set such that a frequency band of the communication band is within a pass band thereof and frequency bands outside the communication band are within attenuation bands thereof. For example, the frequency-fixed filter30is configured by a low pass filter. As indicated by filter characteristics SF30in FIG.8, the frequency-fixed filter30is configured such that the frequency band of the communication band is within the pass band thereof and the frequency band which is higher than the frequency band of the communication band is within the attenuation band thereof. The frequency-fixed filter30therefore transmits high-frequency signals in the communication band with low loss and attenuates high-frequency signals outside the communication band.

As the frequency-variable LC filter40, any one of the frequency-variable LC filters inFIG. 1,FIGS. 4A, 4B, and 4C, andFIGS. 6A and 6Bin the above-described embodiments is employed.

The frequency-variable LC filter40changes the pass band and the attenuation band thereof in accordance with the selected channel. In this case, the pass band contains the frequency band of the selected channel. As indicated by filter characteristics SF40inFIG. 8, the frequency band width of the pass band of the frequency-variable LC filter40is larger than the frequency band width of the selected channel. For example, the frequency band width of the pass band of the frequency-variable LC filter40is approximately 10 times as large as the frequency band width of the selected channel.

The frequency-variable LC filter40has the attenuation poles at both sides of the pass band on a frequency axis. As indicated by the filter characteristics SF40inFIG. 8, the attenuation band of the frequency-variable LC filter40contains no frequency band in which the attenuation is largely decreased and a predetermined attenuation can be provided at all of the frequencies in the communication band outside the pass band.

The frequency-variable LC filter40therefore transmits high-frequency signals in the frequency band for a plurality of channels including the selected channel with low loss and attenuates high-frequency signals in the other frequency bands. Accordingly, the frequency-variable LC filter40can attenuate unnecessary waves present at frequencies separated from the frequencies for the selected channel in the communication band. In particular, the frequency-variable LC filter40can enlarge a frequency range of the attenuation band in comparison with the frequency-variable filters61and62using resonators, which will be described later. Therefore, the frequency-variable LC filter40is effective for attenuation of IMD that can be generated in a large frequency band in the communication band, and that varies in accordance with the usage communication channel (selected channel).

The diplexer50is configured by a circulator, a duplexer, or the like. The diplexer50outputs transmission signals (high-frequency signals) input from the transmission-side terminal to the antenna-side terminal and outputs reception signals (high-frequency signals) input from the antenna-side terminal to the reception-side terminal.

Each of the frequency-variable filters61and62includes at least an elastic wave resonator and a variable capacitor. The elastic wave resonator is a resonator using elastic waves, which is used in SAW (surface acoustic waves), BAW (bulk acoustic waves), and the like. Furthermore, each of the frequency-variable filters61and62includes at least one of an inductor and a capacitor in accordance with filter characteristics. That is to say, each of the frequency-variable filters61and62is a frequency-variable-type resonator filter. Each of the frequency-variable filters61and62is a band pass filter using a resonance point and an anti-resonance point of the resonator. The specific circuit configurations of the frequency-variable filters61and62will be described later. The frequency-variable filters61and62have the same basic configuration and the frequency-variable filter61is therefore described below.

The frequency-variable filter61changes a pass band and an attenuation band thereof in accordance with the selected channel. In this case, the pass band contains the frequency band of the selected channel. As indicated by filter characteristics SF61inFIG. 8, the frequency band width of the pass band of the frequency-variable filter61is substantially the same as the frequency band width of the selected channel.

The frequency-variable filter61has attenuation poles at both sides of the pass band on the frequency axis. The frequency-variable filter61is the resonator filter and the attenuation characteristics of the pass band are steeper than those of the LC filter, as indicated by the filter characteristics SF61inFIG. 8. The frequency-variable filter61therefore transmits high-frequency signals of the selected channel with low loss and attenuates high-frequency signals of adjacent communication channels.

As indicated by the filter characteristics SF61inFIG. 8, the attenuation band of the frequency-variable filter61has frequency bands in which the attenuation is decreased at the opposite sides to the pass band with respect to the attenuation poles. However, the sufficient attenuation can be provided with the frequency-variable LC filter40and the frequency-fixed filter30even in the frequency bands in which the attenuation cannot be provided with the frequency-variable filter61because the frequency-variable filter61, the frequency-variable LC filter40, and the frequency-fixed filter30are connected in series in a transmission path of the high-frequency signals.

Therefore, the high-frequency signals of the selected channel can be transmitted with low loss and the high-frequency signals in frequency bands other than the selected channel, which include the adjacent channels, can be attenuated as indicated by total filter characteristics SFtx inFIG. 8. The same action effects can be provided even when the selected channel is switched.

The transmission-side amplification circuit71includes an amplification element. The specific circuit configuration of the transmission-side amplification circuit71will be described later. The transmission-side amplification circuit71amplifies a transmission signal generated by the transmission signal generator801and outputs it to the frequency-variable filter61. The reception-side amplification circuit72includes a so-called LNA (low noise amplifier). The reception-side amplification circuit72amplifies a reception signal output from the frequency-variable filter62and outputs it to the demodulator802.

The channel determination unit810of the signal processor80detects the vacant communication channels in the communication band. For example, the channel determination unit810acquires a map of the vacant channels from the outside and detects the vacant channels based on the map. The channel determination unit810selects at least one of the vacant communication channels and sets it as the selected channel. The channel determination unit810outputs the selected channel to the transmission signal generator801. The transmission signal generator801generates the transmission signal with a high-frequency signal having a frequency of the selected channel and outputs it to the transmission-side amplification circuit71. Although not illustrated in the drawing, the channel determination unit810outputs the selected channel to the demodulator802. The demodulator802demodulates the reception signal with a local signal based on the selected channel.

The channel determination unit810also outputs the selected channel to the frequency-variable LC filter40, the transmission-side amplification circuit71, the frequency-variable filter61, and the frequency-variable filter62. The frequency-variable LC filter40, the frequency-variable filter61, and the frequency-variable filter62achieve the above-described filter characteristics using the selected channel. The transmission-side amplification circuit71performs amplification processing on the transmission signal using the selected channel.

As described above, when wireless communication is performed using the selected communication channel (selected channel) in the communication band configured by the plurality of communication channels, usage of the configuration of the high-frequency front end circuit10in the embodiment can achieve the wireless communication with low loss using the selected channel.

It should be noted that the communication channel may be determined by the following method. The high-frequency front end circuit includes a detector. The detector may be connected to the diplexer50at the antenna ANT side and may be connected to another antenna for reception level detection. When there is the plurality of vacant communication channels, the detector detects respective reception levels of the plurality of vacant communication channels. The detector outputs the reception levels to the channel determination unit810. The channel determination unit810selects, as the communication channel, the vacant communication channel having the highest reception level among the plurality of detected reception levels.

It should be noted that the frequency-variable filters61and62can have the following circuit configuration.FIG. 9is a circuit diagram of the frequency-variable filter of the resonator filter type in the embodiment of the present disclosure. As described above, the frequency-variable filters61and62have the same basic configuration excluding setting of the frequency and the frequency-variable filter61is therefore described.

The frequency-variable filter61includes a series arm resonance circuit601, a first parallel arm resonance circuit602, a second parallel arm resonance circuit603, a connection terminal P601as an input terminal, and a connection terminal P602as an output terminal.

The series arm resonance circuit601is connected between the connection terminal P601and the connection terminal P602. The first parallel arm resonance circuit602is connected between the series arm resonance circuit601at the connection terminal P601side and a ground potential. The second parallel arm resonance circuit603is connected between the series arm resonance circuit601at the connection terminal P602side and the ground potential.

The series arm resonance circuit601includes a capacitor610, an elastic wave resonator611, an inductor612, and a variable capacitor613. The elastic wave resonator611, the inductor612, and the variable capacitor613are connected in parallel. The capacitor610is connected in series to the parallel circuit. The resonance circuit is connected between the connection terminal P601and the connection terminal P602. In this case, the capacitor610is connected to the connection terminal P601, that is, connected to the first parallel arm resonance circuit602.

The first parallel arm resonance circuit602includes an elastic wave resonator621, an inductor622, and a variable capacitor623. The elastic wave resonator621, the inductor622, and the variable capacitor623are connected in series. The series resonance circuit is connected between the connection terminal P601and the ground potential.

The second parallel arm resonance circuit603includes an elastic wave resonator631, an inductor632, and a variable capacitor633. The elastic wave resonator631, the inductor632, and the variable capacitor633are connected in series. The series resonance circuit is connected between the connection terminal P602and the ground potential.

The series arm resonance circuit601and the first and second parallel arm resonance circuits602and603are band pass filters using resonance points and anti-resonance points of the elastic wave resonators611,621, and631, respectively. The frequency-variable filter61functions as a band pass filter having the pass band that is changed by changing the capacitances of the variable capacitors613,623, and633.

An impedance of the elastic wave resonator621is lower than an impedance of the elastic wave resonator631.

FIG. 10is a graph illustrating bandpass characteristics of the frequency-variable filter illustrated inFIG. 9. As illustrated inFIG. 10, usage of the frequency-variable filter61can achieve filter characteristics having the pass band width of approximately 10 [MHz] and having the attenuation poles at both sides of the pass band. In particular, the attenuation poles having steep attenuation characteristics and large attenuations can be formed at both sides of the pass band on the frequency axis by connecting the capacitor to the series arm resonance circuit601at the first parallel arm resonance circuit602side, in other words, by connecting the capacitor to the side of the resonance filter including the elastic wave resonator having a smaller impedance, as illustrated inFIG. 9. Therefore, the high-frequency signals in the frequency bands of the channels adjacent to the selected channel can be largely attenuated.

Next,FIG. 11Ais a circuit diagram of a frequency-variable LC filter40F according to a fourth embodiment of the present disclosure.

The frequency-variable LC filter40F is different from the frequency-variable LC filter40in the first embodiment in a point that it includes a first parallel arm LC filter circuit42F in order to further improve the steepness of the attenuation characteristics. The first parallel arm LC filter circuit42F is configured by adding a fixed capacitor423to the first parallel arm LC filter circuit42.

As illustrated inFIG. 11A, the fixed capacitor423is connected in parallel to a series circuit formed by the inductor421and the variable capacitor422. A frequency at an anti-resonant point of a circuit formed by the inductor421and the fixed capacitor423is set to the low frequency side of a pass band of the frequency-variable LC filter40F. This makes the attenuation characteristics of the frequency-variable LC filter40F at the low frequency side of the pass band steeper as illustrated in the following characteristic graph.

FIG. 11Bis a graph illustrating bandpass characteristics of the frequency-variable LC filter40F and bandpass characteristics of a frequency-variable LC filter according to a comparative example. In the characteristic graph inFIG. 11B, a solid line and a dotted line indicate the bandpass characteristics of the frequency-variable LC filter40F and a dashed line and a long dashed double-dotted line indicate the bandpass characteristics of the frequency-variable LC filter in the comparative example. The dotted line and the long dashed double-dotted line indicate characteristic graphs when the pass bands indicated by the solid line and the dashed line are respectively changed to the higher frequency side. It should be noted that the frequency-variable LC filter40in the first embodiment is used as the frequency-variable LC filter in the comparative example.

As indicated by the solid line and the dashed line in FIG.11B, the frequency-variable LC filter40F can transmit signals in the pass band with low loss at substantially the same degree as the frequency-variable LC filter40in the comparative example. The frequency (approximately 400 MHz) of the attenuation pole of the frequency-variable LC filter40F at the low frequency side is closer to the pass band than the frequency (approximately 380 MHz) of the attenuation pole of the frequency-variable LC filter in the comparative example at the low frequency side. The attenuation (dB) of the frequency-variable LC filter40F is larger than the attenuation of the frequency-variable LC filter in the comparative example at frequencies around the frequency of the attenuation pole at the low frequency side. That is to say, the attenuation characteristics of the frequency-variable LC filter40F in the embodiment are steeper than the attenuation characteristics of the frequency-variable LC filter in the comparative example. As indicated by the dotted line and the long dashed double-dotted line inFIG. 11B, this improvement in the steepness can also be provided even when the pass band is changed to the higher frequency side.

FIG. 12Ais a circuit diagram of a frequency-variable LC filter40G according to a fifth embodiment of the present disclosure andFIG. 12Bis a graph illustrating bandpass characteristics of the frequency-variable LC filter40G and bandpass characteristics of a frequency-variable LC filter according to a comparative example. It should be noted that the frequency-variable LC filter40is used as the frequency-variable LC filter in the comparative example.

The frequency-variable LC filter40G is different from the frequency-variable LC filter40in a point that it includes a first series arm LC filter circuit41G in order to further improve the attenuation characteristics for harmonics of signals in the pass band. The first series arm LC filter circuit41G is configured by adding an inductor416to the first series arm LC filter circuit41.

As illustrated inFIG. 12A, the inductor416is connected in series to the variable capacitor415. The series circuit is connected in parallel to the inductor414. A frequency at a resonant point of the series circuit is set to the high frequency side of the pass band of the frequency-variable LC filter40G. Therefore, the attenuation characteristics of the frequency-variable LC filter40G for the harmonics are improved as illustrated in the following characteristic graph.

As indicated by a solid line and a dashed line inFIG. 12B, the attenuation of the frequency-variable LC filter40G is larger than the attenuation of the frequency-variable LC filter in the comparative example at a frequency (approximately 900 MHz) that is 2-fold of the center frequency of the pass band of the frequency-variable LC filter40G. As indicated by a dotted line and a long dashed double-dotted line inFIG. 12B, increase in the attenuation for the second harmonic can also be provided even when the pass band is changed to the high frequency side.

Next,FIG. 13Ais a circuit diagram of a frequency-variable LC filter40H according to a sixth embodiment of the present disclosure andFIG. 13Bis a graph illustrating bandpass characteristics of the frequency-variable LC filter and bandpass characteristics of a frequency-variable LC filter according to a comparative example. It should be noted that the frequency-variable LC filter40is used as the frequency-variable LC filter in the comparative example.

The frequency-variable LC filter40H is different from the frequency-variable LC filter40in a point that it includes a fixed capacitor424and a second parallel arm LC filter circuit43H in order to further improve the attenuation characteristics at the low frequency side and the high frequency side of the pass band. The second parallel arm LC filter circuit43H is configured by adding an inductor433to the second parallel arm LC filter circuit43.

As illustrated inFIG. 13A, the fixed capacitor424is connected in series between the first connection terminal P401and the first series arm LC filter circuit41. The first parallel arm LC filter circuit42, the fixed capacitor411, and the fixed capacitor424form a T-shaped high pass filter circuit. The T-shaped high pass filter circuit improves the steepness of the attenuation characteristics at the low frequency side of the pass band as illustrated in the following characteristic graph.

The inductor433of the second parallel arm LC filter circuit43H is connected in parallel to the series circuit formed by the inductor431and the variable capacitor432. A frequency at an anti-resonant point of a parallel circuit formed by the inductor433and the variable capacitor432is set to the high frequency side of the pass band of the frequency-variable LC filter40H. This improves the steepness of the attenuation characteristics at the high frequency side of the pass band as illustrated in the following characteristic graph.

As indicated by a solid line and a dashed line inFIG. 13B, the frequency-variable LC filter40H can transmit signals in the pass band with low loss at substantially the same degree as the frequency-variable LC filter40in the comparative example. The frequency (approximately 390 MHz) of the attenuation pole of the frequency-variable LC filter40H at the low frequency side is closer to the pass band than the frequency (approximately 380 MHz) of the attenuation pole of the frequency-variable LC filter in the comparative example at the low frequency side. The attenuation (dB) of the frequency-variable LC filter40H is larger than the attenuation of the frequency-variable LC filter in the comparative example at frequencies around the frequency of the attenuation pole at the low frequency side. The frequency (approximately 620 MHz) of the attenuation pole of the frequency-variable LC filter40H at the high frequency side is closer to the pass band than the frequency (approximately 650 MHz) of the attenuation pole of the frequency-variable LC filter in the comparative example at the high frequency side. The attenuation (dB) of the frequency-variable LC filter40H is larger than the attenuation of the frequency-variable LC filter in the comparative example at frequencies around the frequency of the attenuation pole at the high frequency side. That is to say, the attenuation characteristics of the frequency-variable LC filter40H in the embodiment are steeper than the attenuation characteristics of the frequency-variable LC filter in the comparative example. As indicated by the dotted line and the long dashed double-dotted line inFIG. 13B, this improvement in the steepness can also be provided even when the pass band is changed to the high frequency side.

Next,FIG. 14is a circuit diagram of a frequency-variable LC filter401according to a seventh embodiment of the present disclosure.FIGS. 15A and 15Bare graphs illustrating bandpass characteristics of the frequency-variable LC filter40I in the seventh embodiment of the present disclosure and bandpass characteristics of a frequency-variable LC filter according to a comparative example. It should be noted that the frequency-variable LC filter40is used as the frequency-variable LC filter in the comparative example.

The frequency-variable LC filter40I is configured by adding an inductor441, a fixed capacitor442, and a fixed capacitor443to the frequency-variable LC filter40H. With this configuration, the frequency-variable LC filter40I not only improves the steepness of the attenuation characteristics but also increases the attenuation for harmonics of signals in the pass band and increases the attenuation at the lower frequency side than the attenuation pole at the low frequency side.

One end of a series circuit formed by the inductor441and the fixed capacitor442is connected to a path between the first connection terminal P401and the fixed capacitor424and the other end thereof is connected to the ground potential. A frequency at a resonant point of this series circuit is, for example, 150 MHz, and is set to be lower than the frequency of the attenuation pole at the low frequency side. Therefore, the attenuation of the frequency-variable LC filter40I is increased at the lower frequency side than the attenuation pole at the low frequency side as illustrated in the following characteristic graph. One end of the fixed capacitor443is connected to a path between the second connection terminal P402and the second parallel arm LC filter circuit43H and the other end thereof is connected to the ground potential. The fixed capacitor443increases the attenuation of the frequency-variable LC filter401for the harmonics as illustrated in the following characteristic graph.

As indicated by a solid line and a dashed line inFIG. 15A, the frequency-variable LC filter40I can transmit signals in the pass band with low loss at substantially the same degree as the frequency-variable LC filter40in the comparative example. The frequency (approximately 410 MHz) of the attenuation pole of the frequency-variable LC filter40I at the low frequency side is closer to the pass band than the frequency (approximately 380 MHz) of the attenuation pole of the frequency-variable LC filter in the comparative example at the low frequency side. The attenuation (dB) of the frequency-variable LC filter40I is larger than the attenuation of the frequency-variable LC filter in the comparative example at frequencies around the frequency of the attenuation pole at the low frequency side. The frequency (approximately 620 MHz) of the attenuation pole of the frequency-variable LC filter40I at the high frequency side is closer to the pass band than the frequency (approximately 650 MHz) of the attenuation pole of the frequency-variable LC filter in the comparative example at the high frequency side. The attenuation (dB) of the frequency-variable LC filter40I is larger than the attenuation of the frequency-variable LC filter in the comparative example at frequencies around the frequency of the attenuation pole at the high frequency side. That is to say, the attenuation characteristics of the frequency-variable LC filter40I in the embodiment are steeper than the attenuation characteristics of the frequency-variable LC filter in the comparative example. As indicated by a dotted line and a long dashed double-dotted line inFIG. 15A, this improvement in the steepness can also be provided even when the pass band is changed to the high frequency side.

As indicated by the solid line and the dashed line inFIG. 15A, the attenuation of the frequency-variable LC filter40I is larger than the attenuation of the frequency-variable LC filter in the comparative example in a band of equal to or higher than 100 MHz and equal to or lower than 300 MHz. As indicated by the dotted line and the long dashed double-dotted line inFIG. 15A, increase in the attenuation at the lower frequency side relative to the attenuation pole at the low frequency side can also be provided even when the pass band is changed to the high frequency side.

As indicated by a solid line and a dashed line inFIG. 15B, the attenuation of the frequency-variable LC filter40I is larger than the attenuation of the frequency-variable LC filter in the comparative example at a 2-fold frequency (approximately 900 MHz) of the center frequency of the pass band of the frequency-variable LC filter40I. As indicated by a dotted line and a long dashed double-dotted line inFIG. 15B, the increase in the attenuation for the second harmonic can also be provided even when the pass band is changed to the high frequency side.

Next,FIG. 16Ais a circuit diagram of a frequency-variable LC filter40J according to an eighth embodiment of the present disclosure andFIG. 16Bis a graph illustrating bandpass characteristics of the frequency-variable LC filter40J and bandpass characteristics of a frequency-variable LC filter according to a comparative example. It should be noted that the frequency-variable LC filter40I is used as the frequency-variable LC filter in the comparative example.

The frequency-variable LC filter40J is different from the frequency-variable LC filter40I in a point that it includes a first series arm LC filter circuit41J. The first series arm LC filter circuit41J is configured by adding an inductor417and a switch418to the first series arm LC filter circuit40I. The frequency-variable LC filter40J provides desired attenuation characteristics by short-circuiting or opening of the switch418with change of the pass band.

As illustrated inFIG. 16A, a series circuit formed by the inductor417and the switch418is connected in parallel to the fixed capacitor413. In other words, the series circuit is connected in parallel to the series circuit formed by the inductor412and the fixed capacitor411.

The switch418is short-circuited when the pass band of the frequency-variable LC filter40J is changed to the high frequency side. The switch418is opened when the pass band of the frequency-variable LC filter40J is changed to the low frequency side. It should be noted that the switch418is short-circuited or opened based on information of the selected channel, which is output from the channel determination unit810. The switch418is therefore short-circuited or opened together with change of the capacitances of the variable capacitors422,415, and432.

In the characteristic graph inFIG. 16B, a solid line indicates the bandpass characteristics of the frequency-variable LC filter40J when the pass band is changed to the low frequency side and the switch418is opened. A dotted line indicates the bandpass characteristics of the frequency-variable LC filter40J when the pass band is changed to the high frequency side and the switch418is short-circuited. As indicated by a long dashed double-dotted line inFIG. 16B, when the pass band of the frequency-variable LC filter in the comparative example is changed to the high frequency side, the attenuation is a minimum value at a frequency of around 450 MHz which is lower than the attenuation pole at the low frequency side. As indicated by the dotted line inFIG. 16B, when the pass band of the frequency-variable LC filter40J in the embodiment is changed to the high frequency side, the attenuation is a minimum value at a frequency of around 510 MHz which is lower than the attenuation pole at the low frequency side, in the same manner. At the low frequency side of the pass band, the minimum value (approximately 40 dB) of the attenuation of the frequency-variable LC filter40J is larger than the minimum value (approximately 38 dB) of the attenuation of the frequency-variable LC filter in the comparative example. In other words, the frequency-variable LC filter40J can ensure the attenuation, which is larger than the minimum attenuation of the frequency-variable LC filter in the comparative example, at the low frequency side of the pass band.

When the pass band is changed to the low frequency side and the switch418is opened, the circuit configuration of the frequency-variable LC filter40J becomes equal to the circuit configuration of the frequency-variable LC filter40I. Therefore, the bandpass characteristics of the frequency-variable LC filter40J also become equal to the bandpass characteristics of the frequency-variable LC filter40I. That is to say, the switch418is opened when the pass band is changed to the low frequency side, so that the frequency-variable LC filter40J can keep the bandpass characteristics of the frequency-variable LC filter40I.

In the embodiment, the switch418is short-circuited only when the pass band is changed to the high frequency side. Alternatively, the frequency-variable LC filter40J may employ a mode in which the switch418is short-circuited only when the pass band is changed to the low frequency side depending on respective element values.

Next,FIG. 17is a circuit diagram of a frequency-variable LC filter40K according to a ninth embodiment of the present disclosure.FIG. 18is a graph illustrating bandpass characteristics of the frequency-variable LC filter40K and bandpass characteristics of a frequency-variable LC filter according to a comparative example. It should be noted that the frequency-variable LC filter40is used as the frequency-variable LC filter in the comparative example.

The frequency-variable LC filter40K is configured by adding an inductor451and a fixed capacitor452to the frequency-variable LC filter40I in order to increase the attenuation for a third harmonic of the signal in the pass band.

As illustrated inFIG. 17, one end of the inductor451is connected to the second connection terminal P402and the other end thereof is connected to the first series arm LC filter circuit41(or the second series arm LC filter circuit43H). The fixed capacitor452is connected in parallel to the inductor451. A frequency at a resonant point of a parallel circuit formed by the inductor451and the fixed capacitor452is set to be higher than the pass band of the frequency-variable LC filter40K. To be more specific, the frequency at the resonant point is, for example, 2300 MHz and is higher than a 3-fold frequency (1350 MHz) of the center frequency of the pass band.

As indicated by a solid line and a dashed line in the characteristic graph inFIG. 18, the attenuation of the frequency-variable LC filter40K is larger than the attenuation of the frequency-variable LC filter in the comparative example at a frequency of around 1350 MHz.

As indicated by a dotted line and a long dashed double-dotted line inFIG. 18, the attenuation of the frequency-variable LC filter40K is also larger than the attenuation of the frequency-variable LC filter in the comparative example at a frequency of around 1950 MHz even when the pass band is changed to the high frequency side. It should be noted that the center frequency of the pass band changed to the high frequency side is approximately 650 MHz(=1950/3 MHz).

Next,FIG. 19is a circuit diagram of a frequency-variable LC filter40L according to a tenth embodiment of the present disclosure.FIG. 20is a graph illustrating bandpass characteristics of the frequency-variable LC filter40L in the tenth embodiment of the present disclosure and bandpass characteristics of a frequency-variable LC filter according to a comparative example. It should be noted that the frequency-variable LC filter40is used as the frequency-variable LC filter in the comparative example.

The frequency-variable LC filter40L is configured by adding an inductor461, an inductor462, and a fixed capacitor463to the frequency-variable LC filter40I in order to increase the attenuation for the third harmonic of the signal in the pass band.

One end of the inductor461is connected to the second connection terminal P402and the other end thereof is connected to the first series arm LC filter circuit41(or the second series arm LC filter circuit43H). A series circuit formed by the inductor462and the fixed capacitor463is connected between the second connection terminal P402and the ground potential. The inductors461and462and the fixed capacitor463form a low pass filter having a cutoff frequency of approximately 2000 MHz.

As indicated by a solid line and a dashed line in the characteristic graph inFIG. 20, the attenuation (42 dB) of the frequency-variable LC filter40L is larger than the attenuation (30 dB) of the frequency-variable LC filter in the comparative example at a frequency of around 1350 MHz.

As indicated by a dotted line and a long dashed double-dotted line inFIG. 20, the attenuation (43 dB) of the frequency-variable LC filter40L is also larger than the attenuation (34 dB) of the frequency-variable LC filter in the comparative example at a frequency of around 1950 MHz even when the pass band is changed to the high frequency side. It should be noted that the center frequency of the pass band changed to the high frequency side is approximately 650 MHz(=1950/3 MHz).

Next,FIG. 21is a circuit diagram of a frequency-variable LC filter40M according to an eleventh embodiment of the present disclosure.FIG. 22Ais a graph illustrating bandpass characteristics of the frequency-variable LC filter40M in the eleventh embodiment of the present disclosure and bandpass characteristics of a frequency-variable LC filter according to a comparative example, andFIG. 22Bis a partially enlarged view of the characteristic graph illustrated inFIG. 22A. It should be noted that the frequency-variable LC filter40K is used as the frequency-variable LC filter in the comparative example.

The frequency-variable LC filter40M is configured by changing the circuit configuration in a path from the first connection terminal P401to the first series arm LC filter circuit41in the frequency-variable LC filter40K. To be specific, as illustrated inFIG. 21, a series circuit formed by fixed capacitors426and425is connected between the first connection terminal P401and the first series arm LC filter circuit41. The first parallel arm LC filter circuit42M is different from the first parallel arm LC filter circuit42only in a connection point. The first parallel arm LC filter circuit42M is connected between a path between the fixed capacitor426and the fixed capacitor425and the ground potential. With this configuration, the fixed capacitors425and426and the first parallel arm LC filter circuit42M form a T-shaped high pass filter.

The frequency-variable LC filter40M does not include the parallel arm LC filter circuit formed by the inductor441and the fixed capacitor442, which are included in the frequency-variable LC filter40K, between the first connection terminal P401and the first series arm LC filter circuit41. The frequency-variable LC filter40M thus forms the T-shaped high pass filter without necessarily providing elements of the parallel arm LC filter circuit. With this, the frequency-variable LC filter40M allows signals to pass in the pass band with lower loss than the frequency-variable LC filter40K.

As indicated by a solid line and a dotted line inFIG. 22A, the frequency-variable LC filter40M ensures the attenuation of equal to or higher than 35 dB in bands other than the pass band. As illustrated in the enlarged view inFIG. 22B, the frequency-variable LC filter40M allows the signals to pass in the pass band with lower loss than the frequency-variable LC filter40K. These effects of the frequency-variable LC filter40M can also be provided even when the pass band is changed to the high frequency side as indicated by a dashed line and a long dashed double-dotted line inFIG. 22AandFIG. 22B.

Next,FIG. 23is a circuit diagram of a frequency-variable LC filter40N according to a twelfth embodiment of the present disclosure.FIG. 24is a graph illustrating bandpass characteristics of the frequency-variable LC filter40N in the twelfth embodiment of the present disclosure and bandpass characteristics of a frequency-variable LC filter according to a comparative example. It should be noted that the frequency-variable LC filter40K is used as the comparative example.

The frequency-variable LC filter40N is configured by adding an inductor462and a fixed capacitor463to the frequency-variable LC filter40M in order to increase the attenuation for the third harmonic of the signal in the pass band. A parallel arm LC filter circuit formed by the inductor462and the fixed capacitor463is connected between the second connection terminal P402and the ground potential.

As indicated by a solid line and a dashed line in the characteristic graph inFIG. 24, the attenuation of the frequency-variable LC filter40N is larger than the attenuation of the frequency-variable LC filter in the comparative example at a frequency of around 1350 MHz when the pass band is changed to the low frequency side. In the same manner, as indicated by a dotted line and a long dashed double-dotted line inFIG. 24, the attenuation of the frequency-variable LC filter40N is larger than the attenuation of the frequency-variable LC filter in the comparative example at a frequency of around 1950 MHz when the pass band is changed to the high frequency side.

Next,FIG. 25is a circuit diagram of a frequency-variable LC filter400according to a thirteenth embodiment of the present disclosure. The frequency-variable LC filter400in the embodiment is different from the frequency-variable LC filter40in a point that inductors in first and second parallel arm LC filter circuits420and430are switched by switches472and476, respectively. The frequency-variable LC filter400is configured by increasing the change width of the pass band with the change of the capacitances of variable capacitors422,415, and432by switching the inductors in the first and second parallel arm LC filter circuits420and430.

To be specific, as illustrated inFIG. 25, the first parallel arm LC filter circuit420includes the variable capacitor422, the switch472, an inductor421, and an inductor471. One end of the variable capacitor422is connected to the first connection terminal P401. The switch472includes one common terminal and two individual terminals. The common terminal of the switch472is connected to the other end of the variable capacitor422. One individual terminal of the switch472is connected to one end of the inductor421. The other individual terminal of the switch472is connected to one end of the inductor471. The other ends of the inductors421and471are connected to the ground potential.

As illustrated inFIG. 25, the second parallel arm LC filter circuit430includes the variable capacitor432, the switch476, an inductor431, and an inductor475. One end of the variable capacitor432is connected to the second connection terminal P402. The switch476includes one common terminal and two individual terminals. The common terminal of the switch476is connected to the other end of the variable capacitor432. One individual terminal of the switch476is connected to one end of the inductor431. The other individual terminal of the switch476is connected to one end of the inductor475. The other ends of the inductors431and475are connected to the ground potential.

When the common terminal and the individual terminal at the inductor421side are connected in the switch472and the common terminal and the individual terminal at the inductor431side are connected in the switch476, the circuit configuration of the frequency-variable LC filter400is equivalent to the circuit configuration of the frequency-variable LC filter40.

An inductance (L471) of the inductor471is set to be larger than an inductance (L421) of the inductor421(L471>L421). An inductance (L475) of the inductor475is set to be smaller than an inductance (L431) of the inductor431(L475<L431).

FIG. 26is a graph illustrating bandpass characteristics of the frequency-variable LC filter400. A dashed line and a long dashed double-dotted line inFIG. 26indicate the bandpass characteristics of the frequency-variable LC filter40. That is to say, the dashed line and the long dashed double-dotted line inFIG. 26indicate the bandpass characteristics when the inductors421and431are used. As illustrated inFIG. 26, a change width wc1of the pass band of the frequency-variable LC filter40is approximately 160 MHz.

When connection of the switch472is switched and the inductor471having a large inductance is used, as indicated by a solid line inFIG. 26, the pass band is changed to the low frequency side in comparison with the case in which the inductor421having a small inductance is used (indicated by the dashed line in the drawing). When connection of the switch476is switched and the inductor475having a small inductance is used, as indicated by a dotted line inFIG. 26, the pass band is changed to the high frequency side in comparison with the case in which the inductor421having a large inductance is used (indicated by the long dashed double-dotted line in the drawing). As illustrated inFIG. 26, a change width wc2of the pass band of the frequency-variable LC filter400is approximately 230 MHz and is larger than the change width wc1.

Furthermore, the frequency-variable LC filter400ensures the attenuation of equal to or larger than 35 dB in a frequency band of 100 MHz to 900 MHz. Furthermore, the frequency-variable LC filter400allows signals to pass in the pass band with low loss at substantially the same degree as the frequency-variable LC filter40.

As described above, the frequency-variable LC filter400can increase the change width of the pass band while maintaining the bandpass characteristics and the attenuation characteristics.

A frequency-variable LC filter in the existing technique cannot provide desired bandpass characteristics and attenuation characteristics when the pass band is changed in some cases.

In order to provide the desired bandpass characteristics and attenuation characteristics even when the pass band is changed, the capacitances of the respective variable capacitors422,415, and432can be set as follows.

FIG. 27is a graph illustrating bandpass characteristics of the frequency-variable LC filter40in the first embodiment of the present disclosure for explaining a setting example of the capacitances of the respective variable capacitors422,415, and432. Description is made below while the frequency of the attenuation pole at the low frequency side of the pass band having a center frequency f0is a frequency fp1, the frequency of the attenuation pole at the high frequency side is a frequency fp2, and the frequency of the attenuation pole at the higher frequency side than the frequency fp2is a frequency fp3inFIG. 27. The attenuation pole at the frequency fp1is formed by the first parallel arm LC filter circuit42. The attenuation pole at the frequency fp2is formed by the first series arm LC filter circuit41. The attenuation pole at the frequency fp3is formed by the second parallel arm LC filter circuit43. Furthermore, the center frequency f0is set by the first series arm LC filter circuit41.

The inventor of the present application have found that desired bandpass characteristics and attenuation characteristics are provided by setting such that the frequencies fp1, fp2, and fp3satisfy the following three inequalities with reference to the center frequency f0.
f0/2<fp1<f0   Equation 1:
f0<fp2<3×f0/2   Equation 2:
2×f0<fp3<3×f0   Equation 3:

The capacitances of the variable capacitors422,415, and432are set so as to satisfy these three inequalities. Then, as illustrated inFIG. 27, signals in the pass band can be made to pass therethrough with low loss even when the pass band is changed. In addition, the attenuations of equal to or larger than approximately 30 dB can be ensured at around the frequencies fp1and fp2of the attenuation poles at both sides of the pass band even when the pass band is changed. Furthermore, the attenuations of equal to or larger than approximately 25 dB can be ensured for the second and third harmonics of the signal in the pass band.

FIG. 28Ais a graph illustrating bandpass characteristics of the frequency-variable LC filter40when the capacitances of the respective variable capacitors422,415, and432are changed andFIG. 28Bis an enlarged view of a range indicated by a dotted line in the characteristic graph inFIG. 28A.

Solid lines inFIGS. 28A and 28Bindicate the bandpass characteristics of the frequency-variable LC filter40satisfying all of the above-described equations 1 to 3. Dashed lines indicate the bandpass characteristics of a frequency-variable LC filter when the above-described equations 2 and 3 are satisfied and the frequency fp1in the above-described equation 1 is set to be lower than 1/2-fold of the center frequency f0.

As illustrated inFIGS. 28A and 28B, the frequency-variable LC filter in the comparative example, which does not satisfy the above-described equation 1, increases a signal loss amount in the pass band. Furthermore, the frequency-variable LC filter in the comparative example, which does not satisfy the above-described equation 1, decreases the attenuation in a frequency band of 470 MHz to 580 MHz at the low frequency side of the pass band. When a threshold value of the attenuation is assumed to be 25 dB, the frequency-variable LC filter in the comparative example allows signals to pass in an unnecessary band of 520 MHz to 570 MHz. Accordingly, when the frequency-variable LC filter in the comparative example, which does not satisfy the above-described equation 1, is used for wireless communication of the TV white space, an S/N (signal to noise) ratio is deteriorated and it allows unnecessary signals to pass for 8 channels. On the other hand, the frequency-variable LC filter40which satisfies all of the above-described equations 1 to 3 does not allow the signals to pass in the unnecessary band of 520 MHz to 570 MHz.

Next,FIG. 29Ais a graph illustrating bandpass characteristics of the frequency-variable LC filter40when the capacitances of the respective variable capacitors422,415, and432are changed andFIG. 29Bis an enlarged view of a range indicated by a dotted line in the characteristic graph inFIG. 29A.

Solid lines inFIGS. 29A and 29Bindicate the bandpass characteristics of the frequency-variable LC filter40satisfying all of the above-described equations 1 to 3. Dashed lines indicate the bandpass characteristics of a frequency-variable LC filter when the above-described equations 1 and 3 are satisfied and the frequency fp2in the above-described equation 2 is set to be higher than 3/2-fold of the center frequency f0.

As illustrated inFIGS. 29A and 29B, the frequency-variable LC filter in the comparative example, which does not satisfy the above-described equation 2, increases the signal loss amount in the pass band. Furthermore, the frequency-variable LC filter in the comparative example, which does not satisfy the above-described equation 2, decreases the attenuation in a frequency band of 700 MHz to 800 MHz at the high frequency side of the pass band. When the threshold value of the attenuation is assumed to be 25 dB, the frequency-variable LC filter in the comparative example allows signals to pass in an unnecessary band of 760 MHz to 800 MHz. Accordingly, when the frequency-variable LC filter in the comparative example, which does not satisfy the above-described equation 2, is used for the wireless communication of the TV white space, the S/N ratio is deteriorated and it allows unnecessary signals to pass for 6 channels. On the other hand, the frequency-variable LC filter40which satisfies all of the above-described equations 1 to 3 does not allow the signals to pass in the unnecessary band of 760 MHz to 800 MHz.

FIG. 30is a graph illustrating bandpass characteristics of the frequency-variable LC filter40when the capacitances of the respective variable capacitors422,415, and432are changed.

A solid line inFIG. 30indicates the bandpass characteristics of the frequency-variable LC filter40satisfying all of the above-described equations 1 to 3. A dashed line indicates the bandpass characteristics of a frequency-variable LC filter when the above-described equations 1 and 2 are satisfied and the frequency fp3in the above-described equation 3 is set to be higher than 3/2-fold of the center frequency f0and be lower than 2-fold of the center frequency f0.

As illustrated inFIG. 30, the frequency-variable LC filter in the comparative example, which does not satisfy the above-described equation 3, increases the signal loss amount in the pass band. Accordingly, when the frequency-variable LC filter in the comparative example, which does not satisfy the above-described equation 3, is used for the wireless communication of the TV white space, the S/N ratio is deteriorated.

Next,FIG. 31is a graph illustrating bandpass characteristics of the frequency-variable LC filter40when the capacitances of the respective variable capacitors422,415, and432are changed.

A solid line inFIG. 31indicates the bandpass characteristics of the frequency-variable LC filter40satisfying all of the above-described equations 1 to 3. A dashed line indicates the bandpass characteristics of a frequency-variable LC filter when the above-described equations 1 and 2 are satisfied and the frequency fp3in the above-described equation 3 is set to be higher than 3-fold of the center frequency f0.

As illustrated inFIG. 31, the frequency-variable LC filter in the comparative example, which does not satisfy the above-described equation 3, increases the signal loss amount in the pass band. Furthermore, the frequency-variable LC filter in the comparative example, which does not satisfy the above-described equation 3, extremely decreases the attenuation to be 10 dB at a frequency of around 1100 MHz. Accordingly, when the frequency-variable LC filter in the comparative example, which does not satisfy the above-described equation 3, is used for the wireless communication of the TV white space, the S/N ratio is deteriorated.

As described above, by setting the capacitances of the respective variable capacitors422,415, and432so as to satisfy the above-described equations 1 to 3, the bandpass characteristics and the attenuation characteristics of the frequency-variable LC filter40are maintained even when the pass band is changed.

REFERENCE SIGNS LIST

10HIGH-FREQUENCY FRONT END CIRCUIT

20ANTENNA MATCHING CIRCUIT

40,40A,40B,40C,40D,40E,40F,40G,40H,401,40J,40K,40L,40M,40N,400FREQUENCY-VARIABLE LC FILTER

41,41J FIRST SERIES ARM LC FILTER CIRCUIT

42,42M,420FIRST PARALLEL ARM LC FILTER CIRCUIT

43,43H,430SECOND PARALLEL ARM LC FILTER CIRCUIT

601SERIES ARM RESONANCE CIRCUIT

602FIRST PARALLEL ARM RESONANCE CIRCUIT

603SECOND PARALLEL ARM RESONANCE CIRCUIT

801TRANSMISSION SIGNAL GENERATOR

810CHANNEL DETERMINATION UNIT

ANT ANTENNA

P401FIRST CONNECTION TERMINAL

P402SECOND CONNECTION TERMINAL