Bandpass filter, radio communication module and radio communication device using the bandpass filter

Provided is a bandpass filter which can cope with a wide band and has a large degree of freedom in designing a pass band. Provided also are a radio communication module and a radio communication device using the bandpass filter. The bandpass filter includes a first, a second, and a third resonator (10, 20, 30). The first and the third resonator (10, 30) have a resonance frequency f1 while the second resonator (20) has a resonance frequency f2 which is different from f1. If fe is the even-number mode resonance frequency of a resonance system formed by the first and the third resonator (10, 30) and fo is the odd-number mode resonance frequency, f2 is lower or higher than both of fe and fo. The bandpass filter has a pass band including fe, fo, and f2.

TECHNICAL FIELD

The invention relates to a band-pass filter, and a wireless communication module and a wireless communication apparatus using the band-pass filter, and particularly relates to a band-pass filter that can have a wide band and has high flexibility in designing a passband, and a wireless communication module and a wireless communication apparatus using the band-pass filter.

BACKGROUND ART

In electronic apparatuses such as communication apparatuses, band-pass filters that allow only electric signals of a specific frequency to pass therethrough are used. In particular, band-pass filters are widely used in which a passband including an even mode resonance frequency and an odd mode resonance frequency is formed by using even mode resonance and odd mode resonance in a resonance system in which two resonators having the same resonance frequency are electromagnetically coupled to each other. In such band-pass filters, the difference between the even mode resonance frequency and the odd mode resonance frequency changes in response to the strength of the electromagnetic coupling between the two resonators, thereby determining the width of the passband (e.g., see Patent Literature 1).

CITATION LIST

Patent Literature

SUMMARY OF INVENTION

Technical Problem

However, in the above existing band-pass filters, the passband is formed by using two resonance peaks of the even mode resonance and the odd mode resonance, and thus there is a limit on widening a band. In addition, band-pass filters are also known in which a passband is formed by using three resonance peaks based on three resonance modes in a resonance system in which three resonators having the same resonance frequency are electromagnetically coupled to each other. Such band-pass filters can have wide bands, but it is difficult to optionally and individually set the frequencies of the three resonance peaks, and flexibility in designing a passband is low.

The invention is devised in view of such problems in the existing art, and its object is to provide a band-pass filter that can have a wide band and has high flexibility in designing a passband, and a wireless communication module and a wireless communication apparatus using the band-pass filter.

Solution to Problem

The band-pass filter of the invention comprises a first resonator, a second resonator, and a third resonator that electromagnetically coupled to each other. The first resonator and the third resonator have resonant frequencies f1 equal to each other and the second resonator has a resonance frequency f2 different from f1. Where an even mode resonance frequency of a resonance system constituted of the first resonator and the third resonator is indicated by fe and an odd mode resonance frequency of the resonance system constituted of the first resonator and the third resonator is indicated by fo, the resonance frequency f2 of the second resonator is lower or higher than both fe and fo, and a passband including fe, fo, and f2 is formed by an electric signal being inputted to the first resonator and an electric signal being outputted from the third resonator.

Where: S(k12)=1 when coupling between the first resonator and the second resonator is mainly inductive; S(k12)=−1 when the coupling between the first resonator and the second resonator is mainly capacitive; S(k23)=1 when coupling between the second resonator and the third resonator is mainly inductive; S(k23)=−1 when the coupling between the second resonator and the third resonator is mainly capacitive; S(k13)=1 when coupling between the first resonator and the third resonator is mainly inductive; S(k13)=−1 when the coupling between the first resonator and the third resonator is mainly capacitive; S(δf)=−1 when f2<f1; and S(δf)=1 when f2>f1, it is satisfied that S(k12)×S(k23)×S(k13)×S(δf)=1.

According to the band-pass filter having such a configuration, since the passband is formed by using three resonance peaks located at fe, fo, and f2, a wideband band-pass filter can be obtained. In addition, since the three frequencies fe, fo, and f2 can be optionally set, a band-pass filter having high flexibility in designing a passband can be obtained. Further, according to the band-pass filter having such a configuration, between an electric signal transmitted from the first resonator directly to the third resonator and a signal transmitted from the first resonator through the second resonator to the third resonator, phase inversion does not occur at frequencies between fe, fo and f2, and phase inversion occurs at frequencies outside f2. Therefore, a band-pass filter can be obtained which has good pass characteristics in which there is no attenuation pole in the passband including fe, fo, and f2 and there is an attenuation pole in the outside of f2 that is the outside of the passband.

A wireless communication module of the invention comprises: an RF unit including the above band-pass filter of the invention; and a baseband unit connected to the RF unit.

A wireless communication apparatus of the invention comprises: an RF unit including the above band-pass filter of the invention; a baseband unit connected to the RF unit; and an antenna connected to the RF unit.

According to the wireless communication module and the wireless communication apparatus having such configurations, since the band-pass filter having low loss throughout a wide frequency band used for communication is used as a filter for communication signals, attenuation of communication signals passing through the band-pass filter is reduced. Therefore, a high-performance wireless communication module and wireless communication apparatus having high receiving sensitivity can be obtained.

Advantageous Effects of Invention

According to the band-pass filter of the invention, a band-pass filter that can have a wide band and has high flexibility in designing a passband can be obtained.

DESCRIPTION OF EMBODIMENTS

Hereinafter, a band-pass filter of the invention, and a wireless communication module and a wireless communication apparatus using the band-pass filter will be described in detail with reference to the accompanying drawings.

First Example of Embodiment

FIG. 1is an equivalent circuit diagram showing a band-pass filter of a first example of embodiment of the invention. As shown inFIG. 1, in the band-pass filter of the example, a first resonator10is constituted of a series circuit of inductors L1a, L1b, and L1cand a capacitor C10. A second resonator20is constituted of a series circuit of inductors L2aand L2band a capacitor C20. A third resonator30is constituted of a series circuit of inductors L3a, L3b, and L3cand a capacitor C30. The first resonator10and the second resonator20are electromagnetically coupled to each other with a coupling coefficient k12. The second resonator20and the third resonator30are electromagnetically coupled to each other with a coupling coefficient k23. The first resonator10and the third resonator30are electromagnetically coupled to each other with a coupling coefficient k13. Note that when the sign of each of k12, k23, and k13 is positive, it indicates inductive coupling, and when the sign of each of k12, k23, and k13 is negative, it indicates capacitive coupling. In addition, an input terminal01is grounded through an inductor Lin. An output terminal02is grounded through an inductor Lout. The first resonator10is electromagnetically coupled to the inductor Lin. The third resonator30is electromagnetically coupled to the inductor Lout.

Further, the first resonator10and the third resonator30have resonant frequencies f1 equal to each other, and the second resonator20has a resonance frequency f2 different from f1. Where an even mode resonance frequency of a resonance system constituted of the first resonator10and the third resonator30is indicated by fe and an odd mode resonance frequency of the resonance system constituted of the first resonator10and the third resonator30is indicated by fo, the resonance frequency f2 of the second resonator20is set so as to be lower or higher than both fe and fo. In other words, on a frequency axis, f2 is located not between fe and fo but outside between fe and fo. An electric signal is inputted from the input terminal01through the inductor Lin to the first resonator10, and an electric signal is outputted from the third resonator30through the inductor Lout, whereby a passband including fe, fo, and f2 is formed.

According to the band-pass filter of the example having such a configuration, since the passband is formed by using three resonance peaks located at fe, fo, and f2, a wideband band-pass filter can be obtained. In addition, since the three frequencies fe, fo, and f2 can be optionally set, a band-pass filter having high flexibility in designing a passband can be obtained.

Further, according to the band-pass filter of the example, where: S(k12)=1 when the coupling between the first resonator10and the second resonator20is mainly inductive; S(k12)=−1 when the coupling between the first resonator10and the second resonator20is mainly capacitive; S(k23)=1 when the coupling between the second resonator20and the third resonator30is mainly inductive; S(k23)=−1 when the coupling between the second resonator20and the third resonator30is mainly capacitive; S(k13)=1 when the coupling between the first resonator10and the third resonator30is mainly inductive; S(k13)=−1 when the coupling between the first resonator10and the third resonator30is mainly capacitive; S(δf)=−1 when f2<f1; and S(δf)=1 when f2>f1, it is satisfied that S(k12)×S(k23)×S(k13)×S(δf)=1. By so doing, between an electric signal transmitted from the first resonator10directly to the third resonator30and a signal transmitted from the first resonator10through the second resonator20to the third resonator30, phase inversion does not occur at frequencies in the passband between fe, fo and f2, and phase inversion occurs at frequencies in the outside of f2 that is the outside of the passband. Therefore, a band-pass filter can be obtained which has good pass characteristics in which there is no attenuation pole in the passband including fe, fo and f2 and there is an attenuation pole in the outside of f2 that is the outside of the passband. Note that in the band-pass filter of the example, the magnitudes (absolute values) of the coupling coefficient k12 between the first resonator10and the second resonator20, the coupling coefficient k23 between the second resonator20and the third resonator30, and the coupling coefficient k13 between the first resonator10and the third resonator30are desirably set so as to be substantially equal to each other. By so doing, it is possible to form a desired passband.

As a result of various considerations, the inventor has derived such conditions for obtaining pass characteristics in which there is no attenuation pole in the passband including fe, fo and f2 and there is an attenuation pole in the outside of f2 that is the outside of the passband. The mechanism by which the effect is obtained is inferred as follows by a consideration with a series of circuit simulations conducted by the inventor.

In other words, in the band-pass filter of the example, an electric signal transmitted from the first resonator10directly to the third resonator30and a signal transmitted from the first resonator10through the second resonator20to the third resonator30exist. Here, in a frequency region located in the outside of the even mode resonance frequency fe and the odd mode resonance frequency fo of the resonance system constituted of the first resonator10and the third resonator30, a transmission route of the electric signal transmitted from the first resonator10directly to the third resonator30is equivalent to an inductor when the first resonator10and the third resonator30are mainly inductively coupled to each other, and is equivalent to a capacitor when the first resonator10and the third resonator30are mainly capacitively coupled to each other.

A transmission route of the electric signal transmitted from the first resonator10through the second resonator20to the third resonator30is equivalent to an inductor on the lower frequency side of the resonance frequency f2 of the second resonator20, and is equivalent to a capacitor on the higher frequency side of f2, when both of the main coupling between the first resonator10and the second resonator20and the main coupling between the second resonator20and the third resonator30are inductive or capacitive. In addition, the transmission route of the electric signal transmitted from the first resonator10through the second resonator20to the third resonator30is equivalent to a capacitor on the lower frequency side of the resonance frequency f2 of the second resonator20, and is equivalent to an inductor on the higher frequency side of f2, when either one of the main coupling between the first resonator10and the second resonator20or the main coupling between the second resonator20and the third resonator30is inductive coupling and the other one is capacitive coupling.

Therefore, it is recognized that the two signal transmission routes, that is, the transmission route of the signal transmitted from the first resonator10directly to the third resonator30and the transmission route of the signal transmitted from the first resonator10through the second resonator20to the third resonator30, only need to be set as follows.

First, the case will be considered where the coupling between the first resonator10and the third resonator30is mainly inductive coupling and both of the main coupling between the first resonator10and the second resonator20and the main coupling between the second resonator20and the third resonator30are inductive coupling or capacitive coupling. In the case, the resonance frequency f2 of the second resonator20is set so as to be higher than the resonant frequencies f1 of the first resonator10and the second resonator20. More precisely, f2 is set so as to be higher than both fe and fo. By so doing, phase inversion does not occur between electric signals having passed through the two signal transmission routes in the passband including fe, fo, and f2, and phase inversion occurs between electric signals having passed through the two signal transmission routes on the higher frequency side of f2 that is the outside of the passband. Therefore, a band-pass filter can be obtained which has good pass characteristics in which there is no attenuation pole in the passband including fe, fo, and f2 and there is an attenuation pole on the higher frequency side of f2 that is the outside of the passband.

Next, the case will be considered where the coupling between the first resonator10and the third resonator30is mainly inductive coupling, either one of the main coupling between the first resonator10and the second resonator20or the main coupling between the second resonator20and the third resonator30is inductive coupling, and the other one is capacitive coupling. In the case, the resonance frequency f2 of the second resonator20is set so as to be lower than the resonant frequencies f1 of the first resonator10and the second resonator20. More precisely, f2 is set so as to be lower than both fe and fo. By so doing, phase inversion does not occur between electric signals having passed through the two signal transmission routes in the passband including fe, fo, and f2, and phase inversion occurs between electric signals having passed through the two signal transmission routes on the lower frequency side of f2 that is the outside of the passband. Therefore, a band-pass filter can be obtained which has good pass characteristics in which there is no attenuation pole in the passband including fe, fo, and f2 and there is an attenuation pole on the lower frequency side of f2 that is the outside of the passband.

Next, the case will be considered where the coupling between the first resonator10and the third resonator30is mainly capacitive coupling and both the main coupling between the first resonator10and the second resonator20and the main coupling between the second resonator20and the third resonator30are inductive coupling or capacitive coupling. In the case, the resonance frequency f2 of the second resonator20is set so as to be lower than the resonant frequencies f1 of the first resonator10and the second resonator20. More precisely, f2 is set so as to be lower than both fe and fo. By so doing, phase inversion does not occur between electric signals having passed through the two signal transmission routes in the passband including fe, fo, and f2, and phase inversion occurs between electric signals having passed through the two signal transmission routes on the lower frequency side of f2 that is the outside of the passband. Therefore, a band-pass filter can be obtained which has good pass characteristics in which there is no attenuation pole in the passband including fe, fo, and f2 and there is an attenuation pole on the lower frequency side of f2 that is the outside of the passband.

Next, the case will be considered where the coupling between the first resonator10and the third resonator30is mainly capacitive coupling, either one of the main coupling between the first resonator10and the second resonator20or the main coupling between the second resonator20and the third resonator30is inductive coupling, and the other one is capacitive coupling. In the case, the resonance frequency f2 of the second resonator20is set so as to be higher than the resonant frequencies f1 of the first resonator10and the second resonator20. More precisely, f2 is set so as to be higher than both fe and fo. By so doing, phase inversion does not occur between electric signals having passed through the two signal transmission routes in the passband including fe, fo, and f2, and phase inversion occurs between electric signals having passed through the two signal transmission routes on the higher frequency side of f2 that is the outside of the passband. Therefore, a band-pass filter can be obtained which has good pass characteristics in which there is no attenuation pole in the passband including fe, fo, and f2 and there is an attenuation pole on the higher frequency side of f2 that is the outside of the passband.

To summarize, it is recognized that it is only necessary to satisfy that S(k12)×S(k23)×S(k13)×S(δf)=1, where: S(k12)=1 when the coupling between the first resonator10and the second resonator20is mainly inductive; S(k12)=−1 when the coupling between the first resonator10and the second resonator20is mainly capacitive; S(k23)=1 when the coupling between the second resonator20and the third resonator30is mainly inductive; S(k23)=−1 when the coupling between the second resonator20and the third resonator30is mainly capacitive; S(k13)=1 when the coupling between the first resonator10and the third resonator30is mainly inductive; S(k13)=−1 when the coupling between the first resonator10and the third resonator30is mainly capacitive; S(δf)=−1 when f2<f1; and S(δf)=1 when f2>f1.

Second Example of Embodiment

FIG. 2is an equivalent circuit diagram showing a band-pass filter of a second example of embodiment of the invention. Note that only the difference from the first example described above will be described in the example, the same reference signs are used for like components, and the overlap description is omitted.

The band-pass filter of the example is one specific circuit example of the general equivalent circuit shown inFIG. 1. In the band-pass filter of the example, each of a first resonator10, a second resonator20, and a third resonator30is constituted of a ¼ wavelength line having a grounded end. Electromagnetic coupling k12 between the first resonator10and the second resonator20is constituted of a capacitor C12. Electromagnetic coupling k23 between the second resonator20and the third resonator30is constituted of a capacitor C23. Electromagnetic coupling k13 between the first resonator10and the third resonator30is constituted of a capacitor C13. Coupling Qein between an input terminal01and the first resonator10is constituted of a capacitor Cin. Coupling Qeout between an output terminal02and the third resonator30is constituted of a capacitor Cout. The length of the second resonator20is set so as to be longer than the lengths of the first resonator10and the second resonator20. By so doing, the resonance frequency f2 of the second resonator20is set so as to be lower than the resonant frequencies f1 of the first resonator10and the third resonator30.

In the band-pass filter of the example having such a configuration, since all of k12, k23, and k13 are capacitive and f2<f1, it is satisfied that S(k12)×S(k23)×S(k13)×S(δf)=−1×−1×−1×−1=1. Thus, in the passband including fe, fo, and f2, phase inversion does not occur between an electric signal transmitted from the first resonator10directly to the third resonator30and an electric signal transmitted from the first resonator10through the second resonator20to the third resonator30. On the lower frequency side of f2, phase inversion occurs between an electric signal transmitted from the first resonator10directly to the third resonator30and an electric signal transmitted from the first resonator10through the second resonator20to the third resonator30. Therefore, a band-pass filter can be obtained which has good pass characteristics in which there is no attenuation pole in the passband including fe, fo, and f2 and there is an attenuation pole on the lower frequency side of f2 that is the outside of the passband.

Third Example of Embodiment

FIG. 3is a circuit diagram showing a band-pass filter of a third example of embodiment of the invention. Note that only the difference from the second example described above will be described in the example, the same reference signs are used for like components, and the overlap description is omitted.

In the band-pass filter of the example, as shown inFIG. 3, an input terminal01and a second resonator20are connected to each other through a capacitor C40, the second resonator20and an output terminal02are connected to each other through a capacitor C50, and the input terminal01and the output terminal02are connected to each other through a capacitor C60.

According to the band-pass filter of the example having such a configuration, in the pass characteristics of the band-pass filter, phase inversion occurs between an electric signal transmitted from a first resonator10directly to a third resonator30and an electric signal transmitted from the first resonator10through the second resonator20to the third resonator30, whereby two attenuation poles can be newly formed on the lower frequency side of the attenuation pole formed on the lower frequency side of the resonance frequency f2 of the second resonator20. Therefore, a band-pass filter having better attenuation characteristics on the lower frequency side of the passband can be obtained.

Fourth Example of Embodiment

FIG. 4is a block diagram showing a wireless communication module80and a wireless communication apparatus85of a fourth example of embodiment of the invention.

The wireless communication module80of the example includes a baseband unit81by which a baseband signal is processed, and an RF unit82that is connected to the baseband unit81and by which an RF signal that is after modulation of the baseband signal and before demodulation of the baseband signal is processed. The RF unit82includes a band-pass filter821described above. In the RF unit82, signals in an RF signal obtained by modulating the baseband signal or in a received RF signal and out of a communication band are attenuated by the band-pass filter821.

As a specific configuration, a baseband IC811is provided in the baseband unit81, and an RF IC822is provided in the RF unit82and between the band-pass filter821and the baseband unit81. Note that another circuit may be interposed between these circuits. An antenna84is connected to the band-pass filter821of the wireless communication module80, whereby the wireless communication apparatus85is configured to transmit/receive RF signals.

According to the wireless communication module80and the wireless communication apparatus85of the example having such a configuration, since the band-pass filter821having low loss throughout a wide frequency band used for communication is used as a filter for communication signals, attenuation of communication signals passing through the band-pass filter821is reduced and noise is also decreased. Thus, since the receiving sensitivity improves and the amplification degrees of transmission signals and reception signals can be reduced, the power consumption of an amplifier circuit is reduced. Therefore, the high-performance wireless communication module80and wireless communication apparatus85having high receiving sensitivity and low power consumption can be obtained.

Modified Examples

The invention is not limited to the first to fourth examples of embodiment described above, and various changes and modifications can be made without departing from the gist of the invention.

For example, in the second and third examples of embodiment described above, the ¼ wavelength lines are used as the first resonator10, the second resonator20, and the third resonator30, but the first resonator10, the second resonator20, and the third resonator30are not limited thereto. For example, linear resonators, such as ½ wavelength resonators using micro-strip lines, strip lines, and the like and ring resonators, can be used. In addition, planar resonators, such as dual-mode square resonators and dual-mode circular resonators, can be used. Further, solid resonators and the like, such as dielectric resonators having columnar shapes, rectangular parallelepiped shapes, ring shapes, and the like and coaxial resonators, can be used.

EXAMPLES

Next, specific examples of the band-pass filter of the invention will be described.

Electrical characteristics of the band-pass filter of the second example of embodiment of the invention shown inFIG. 2are calculated by a simulation. The first resonator10, the second resonator20, and the third resonator30are formed by using strip lines. The relative dielectric constant of a dielectric substance is set to 18.7. The interval between upper and lower ground conductors is set to 1 mm. The width of each strip line is set to 0.2 mm. The length of each of the strip lines constituting the first resonator10and the third resonator30is set to 5.4 mm. The length of the strip line constituting the second resonator20is set to 6.85 mm. Each of the capacitors Cin and Cout is set to have a capacitance of 0.6 pF. Each of the capacitors C12, C23, and C13is set to have a capacitance of 0.15 pF.

FIG. 5is a graph showing the simulation result. In the graph, the horizontal axis indicates frequency, and the vertical axis indicates attenuation amount. In addition, the graph inFIG. 5shows the pass characteristics (S21) and the reflection characteristics (S11) of the band-pass filter when the input terminal01is a port1and the output terminal02is a port2. According to the graph shown inFIG. 5, it is recognized that good filter characteristics are obtained in which a flat and wide passband having low loss is formed by using three resonance peaks, an attenuation pole is formed on the lower frequency side of the passband and near the passband, and the attenuation amount greatly changes from an attenuation band to the passband. Thus, effectiveness of the invention can be verified.

Further, electrical characteristics of the band-pass filter of the third example of embodiment of the invention shown inFIG. 3are calculated by a simulation. The first resonator10, the second resonator20, and the third resonator30are formed by using strip lines. The relative dielectric constant of a dielectric substance is set to 18.7. The interval between upper and lower ground conductors is set to 1 mm. The width of each strip line is set to 0.2 mm. The length of each of the strip lines constituting the first resonator10and the third resonator30is set to 5.4 mm. The length of the strip line constituting the second resonator20is set to 6.9 mm. Each of the capacitors Cin and Cout is set to have a capacitance of 0.6 pF. Each of the capacitors C12, C23, and C13is set to have a capacitance of 0.15 pF. Each of the capacitors C40and C50is set to have a capacitance of 0.02 pF. The capacitor C60is set to have a capacitance of 0.001 pF.

FIG. 6is a graph showing the simulation result. In the graph, the horizontal axis indicates frequency, and the vertical axis indicates attenuation amount. In addition, the graph inFIG. 6shows the pass characteristics (S21) and the reflection characteristics (S11) of the band-pass filter when the input terminal01is a port1and the output terminal02is a port2. According to the graph shown inFIG. 6, it is recognized that two attenuation poles are further formed on the lower frequency side of the passband, and the attenuation amount is improved on the lower frequency side of the passband, and better filter characteristics are obtained.

REFERENCE SIGNS LIST