Band pass filter

In a band pass filter, a signal input into a first terminal is transmitted to a first LC resonator and a second LC resonator in this order and is then output from a second terminal. The band pass filter includes a third inductor and a third capacitor. A first end of the third inductor is electrically connected to the ground. The third capacitor is electrically connected between a second end of the third inductor and a node between the first LC resonator and the second LC resonator.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a band pass filter including a plurality of LC resonators.

2. Description of the Related Art

Band pass filters each including a plurality of LC resonators have been known. For example, International Publication No. 2011/114851 discloses a band pass filter including three stages of LC resonators and a ground impedance adjustment circuit. In this band pass filter, an attenuation pole can be provided in an attenuation band near the low-frequency side of a pass band.

Depending on frequency characteristics required for a band pass filter, a case may arise where an attenuation pole is needed near a desired frequency higher than a pass band. However, International Publication No. 2011/114851 does not disclose the concrete configuration with which an attenuation pole is formed near a desired frequency higher than the pass band of a band pass filter.

SUMMARY OF THE INVENTION

Preferred embodiments of the present invention provide attenuation poles near a desired frequency lower than the pass band of a band pass filter and a desired frequency higher than the pass band.

A band pass filter according to a preferred embodiment of the present invention includes a first LC resonator, a second LC resonator, a third inductor, and a third capacitor. The first LC resonator includes a first inductor and a first capacitor electrically connected in parallel with the first inductor. The second LC resonator includes a second inductor and a second capacitor electrically connected in parallel with the second inductor. A first end of the third inductor is electrically connected to a ground. The third capacitor is electrically connected between a second end of the third inductor and a node between the first LC resonator and the second LC resonator. A signal input into a first terminal of the band pass filter is transmitted to the first LC resonator and the second LC resonator in this order and is then output from a second terminal of the band pass filter.

With band pass filters according to preferred embodiments of the present invention, the third capacitor electrically connected between the second end of the third inductor, the first end of which is electrically connected to the ground, and the node between the first LC resonator and the second LC resonator allows attenuation poles to be provided near a desired frequency lower than the pass band of the band pass filter and a desired frequency higher than the pass band.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings. In the drawings, the same reference numeral is used to represent the same or similar portion or a corresponding portion so as to avoid repeated explanation.

First Preferred Embodiment

FIG. 1is an equivalent circuit diagram of a band pass filter100according to a first preferred embodiment of the present invention. As shown inFIG. 1, the band pass filter100includes terminals P1and P2, LC resonators101and102, an inductor13, and a capacitor23.

The LC resonators101and102are electrically connected at a node CP. The LC resonator101includes an inductor11and a capacitor21. Between the terminal P1and the node CP, the inductor11and the capacitor21are electrically connected in parallel with each other. The LC resonator102includes an inductor12and a capacitor22. Between the terminal P2and the node CP, the inductor12and the capacitor22are electrically connected in parallel with each other. The inductor11is magnetically coupled to the inductor12. Magnetic coupling is coupling via a magnetic flux, and a magnetic flux between inductors changes in accordance with the change in a current flowing through one of the inductors and induced electromotive force is generated at the other one of the inductors.

One end of the inductor13is electrically connected to the ground. The capacitor23is electrically connected between the other end of the inductor13and the node CP.

A signal input into the terminal P1is transmitted to the LC resonators101and102in this order and is then output from the terminal P2. A signal input into the terminal P2is transmitted to the LC resonators102and101in this order and is then output from the terminal P1.

FIG. 2is an equivalent circuit diagram of a band pass filter900that is a comparative example of the first preferred embodiment. The equivalent circuit diagram of the band pass filter900inFIG. 2is the same as or similar to the equivalent diagram of the band pass filter100inFIG. 1except that the capacitor23is not included. The other features of the band pass filter900are the same as or similar to that of the band pass filter100, and the description thereof is therefore not repeated.

FIG. 3is a diagram showing an insertion loss IL10of the band pass filter100inFIG. 1and an insertion loss IL90of the band pass filter900inFIG. 2. The pass bands of the band pass filters100and900are preferably the frequency band of f41to f42(>f41), for example.

InFIG. 3, an attenuation (dB) is represented by a negative value at the vertical axis. The greater the absolute value of an attenuation, the larger the insertion loss. An insertion loss is an indicator representing the proportion of signals transmitted to another terminal of an electronic component in signals input into a certain terminal of the electronic component. The larger the insertion loss, the larger the proportion of signals lost in an electronic component in signals input into the electronic component. The same or similar features and advantageous effects are applicable toFIG. 6showing a second preferred embodiment andFIG. 8showing a third preferred embodiment.

Depending on frequency characteristics for a band pass filter, a case may arise where an attenuation pole is needed near a desired frequency outside a pass band. However, in the frequency band shown inFIG. 3, a sufficient attenuation pole is not provided as represented by the insertion loss IL90. On the other hand, attenuation poles are provided at a frequency f51(<f41), a frequency f52(>f42), and a frequency f53(>f52) as represented by the insertion loss IL10.

Regarding the changes in an attenuation in the frequency band of f51to f41and the frequency band of f42to f52, a curve representing insertion loss IL10is steeper than a curve representing the insertion loss IL90. With the band pass filter100, attenuation poles are able to be provided at the divided frequency of a pass frequency (e.g., the center frequency of a pass band) and the multiplied frequency of the pass frequency.

The description will be provided below of the mechanism of providing an attenuation pole near a desired frequency in a band pass filter according to a preferred embodiment of the present invention. The higher a frequency f of a signal passing through an inductor having an inductance L, the higher the impedance (L·2πf) of the inductor. The higher a frequency f of a signal passing through a capacitor having a capacitance C, the lower the impedance (1/(C·2πf)) of the capacitor. When a frequency is lower than the pass band of a band pass filter, the impedance of an inductor becomes relatively lower than that of a capacitor as compared with the case of a frequency in a pass band. On the other hand, when a frequency is higher than the pass band of a band pass filter, the impedance of an inductor becomes relatively higher than that of a capacitor as compared with the case of a frequency in a pass band.

Regarding a signal path to a ground GND at the frequency f51, a signal path passing through the inductor11, the capacitor23, and the inductor13is more predominant than a signal path passing through the capacitor21, the capacitor23, and the inductor13. A signal path passing through the inductor12, the capacitor23, and the inductor13is more predominant than a signal path passing through the capacitor22, the capacitor23, and the inductor13.

By setting the resonant frequency of a series resonator defined by the inductor11, the capacitor23, and the inductor13and the resonant frequency of a series resonator defined by the inductor12, the capacitor23, and the inductor13to frequencies near the frequency f51, an attenuation pole is provided near the frequency f51.

Regarding a signal path to the ground GND at the frequency f53, a signal path passing through the capacitors21and23and the inductor13is more predominant than a signal path passing through the inductor11, the capacitor23, and the inductor13. A signal path passing through the capacitors22and23and the inductor13is more predominant than a signal path passing through the inductor12, the capacitor23, and the inductor13.

By setting the resonant frequency of a series resonator defined by the capacitors21and23and the inductor13and the resonant frequency of a series resonator defined by the capacitors22and23and the inductor13to frequencies near the frequency f53, an attenuation pole is provided near the frequency f53.

Referring toFIGS. 1 and 4, the inductors11and12that are magnetically coupled to each other inFIG. 1are replaced with inductors11A,12A, and112inFIG. 4. Between the terminal P1and the node CP, the inductor112is electrically connected in series with the inductor11A. Between the terminal P2and the node CP, the inductor112is electrically connected in series with the inductor12A. The capacitor21is electrically connected in parallel with the inductors11A and112. The capacitor22is electrically connected in parallel with the inductors12A and112. The LC resonators101and102share the inductor112. By setting the resonant frequency of a parallel resonator defined by the inductor11A and the capacitor21and the resonant frequency of a parallel resonator defined by the inductor12A and the capacitor22to frequencies near the frequency f52, an attenuation pole is provided near the frequency f52.

The frequency f51is a divided frequency (e.g., about half) of the center frequency of the pass band f41to f42of the band pass filter100. Each of the frequencies f52and f53is a multiplied frequency (e.g., about double or about triple) of the center frequency.

With a band pass filter according to the first preferred embodiment, attenuation poles are able to be provided near a desired frequency lower than a pass band and a desired frequency higher than the pass band.

Second Preferred Embodiment

FIG. 5is an equivalent circuit diagram of a band pass filter200according to a second preferred embodiment of the present invention. The equivalent circuit diagram of the band pass filter200inFIG. 5is the same as or similar to the equivalent diagram of the band pass filter100inFIG. 1except that an inductor14is added and the LC resonators101and102are replaced with LC resonators201and202, respectively. The other features of the band pass filter200are the same as or similar to that of the band pass filter100, and the description thereof is therefore not repeated.

As shown inFIG. 5, between the terminal P1and the node CP, the inductor14is electrically connected in series with the inductor11. Between the terminal P2and the node CP, the inductor14is electrically connected in series with the inductor12. The capacitor21is electrically connected in parallel with the inductors11and14. The capacitor22is electrically connected in parallel with the inductors12and14.

Since the LC resonators201and202share the inductor14in the second preferred embodiment, the transmission of a signal through physical connection is more predominant than the transmission of a signal through magnetic coupling. The inductors11and12therefore do not necessarily have to be magnetically coupled to each other.

FIG. 6is a diagram showing insertion losses IL24to IL26of the band pass filter200shown inFIG. 5when the inductance of the inductor14is changed in three stages. The inductance of the inductor14increases in the order of the insertion losses IL24to IL26. A frequency at which an attenuation pole is provided increases in the order of frequencies f60to f66. The pass band of the band pass filter200is the frequency band of f43to f44(>f43).

As shown inFIG. 6, in a frequency band lower than the pass band of f43to f44, attenuation poles are provided near the frequency f60(<f43) in the case of all of the insertion losses IL24to IL26. In a frequency band higher than the pass band of f43to f44, attenuation poles are provided at the frequencies f61and f66in the case of the insertion loss IL24, attenuation poles are provided at the frequencies f62and f65in the case of the insertion loss IL25, and attenuation poles are provided at the frequencies f63and f64in the case of the insertion loss IL26. By changing the inductance of the inductor14, a frequency at which an attenuation pole is provided is able to be changed in a frequency band higher than the pass band of the band pass filter200.

Changing the inductance of the inductor14corresponds to changing the degree of magnetic coupling between the inductors11and12inFIG. 1showing the first preferred embodiment as described with reference toFIG. 4. Changing the inductance of the inductor14providing a physical connection is more stable and easier than changing the degree of magnetic coupling not providing a physical connection. With a band pass filter according to the second preferred embodiment, the frequency of an attenuation pole is able to be stably and easily changed.

With a band pass filter according to the second preferred embodiment, attenuation poles are able to be provided near a desired frequency lower than the pass band of the band pass filter and a desired frequency higher than the pass band.

Third Preferred Embodiment

FIG. 7is an equivalent circuit diagram of a band pass filter300according to a third preferred embodiment of the present invention. The equivalent circuit diagram of the band pass filter300inFIG. 7is the same as or similar to the equivalent diagram of the band pass filter200inFIG. 5except that inductors15and16are added. The other features of the band pass filter300are the same as or similar to that of the band pass filter200, and the description thereof is therefore not repeated.

Between the terminal P1and the node CP, the inductor15is electrically connected in series with the capacitor21. The capacitor21is electrically connected between the inductor15and the node CP. Between the terminal P2and the node CP, the inductor16is electrically connected in series with the capacitor22. The capacitor22is electrically connected between the inductor16and the node CP. The inductance of the inductor15is equal or substantially equal to that of the inductor16, but does not necessarily have to the equal to that of the inductor16.

FIG. 8is a diagram showing insertion losses IL31and IL32of the band pass filter300shown inFIG. 7when the inductances of the inductors15and16are changed in two stages. The inductances of the inductors15and16decrease in the order of the insertion losses IL31and IL32. A frequency at which an attenuation pole is provided increases in the order of frequencies f91to f95.

As shown inFIG. 8, the pass band in the case of the insertion loss IL31is the frequency band of f71to f72(>f71). The pass band in the case of the insertion loss IL32is the frequency band of f81to f82(>f81). The frequency f81is higher than the frequency f71. The frequency f82is higher than the frequency f72. The pass band in the case of the insertion loss IL32is moved to be higher than that in the case of the insertion loss IL31.

In the frequency band lower than the pass band, attenuation poles are provided near the frequency f91in the case of all of the insertion losses IL31and IL32. In the frequency band higher than the pass band, attenuation poles are provided at the frequencies f92(>f72) and f94in the case of the insertion loss IL31and attenuation poles are provided at the frequencies f93(>f83) and f95in the case of the insertion loss IL32. In the frequency band higher than the pass band, the attenuation poles provided at the frequencies f92and f94in the case of the insertion loss IL31are moved to the attenuation poles provided at the higher frequencies f93and f95, respectively in the case of the insertion loss IL32.

In the third preferred embodiment, the case has been described where the inductor15and the capacitor21are electrically connected in series in this order between the terminal P1and the node CP and the inductor16and the capacitor22are electrically connected in series in this order between the terminal P2and the node CP as shown inFIG. 7. The order in which the inductor15and the capacitor21are electrically connected in series and the order in which the inductor16and the capacitor22are electrically connected in series may be opposite to those shown inFIG. 7as shown in the equivalent circuit diagram of a band pass filter310that is a first modification of the third preferred embodiment inFIG. 9.

In the case where the capacitors21to23are electrically connected to one another at the node CP as shown in the equivalent circuit diagram inFIG. 7, one electrodes of the capacitors21to23electrically connected to the node CP are able to be provided by the same capacitor conductor pattern. Accordingly, in the case where a band pass filter according to a preferred embodiment of the present invention is defined by a laminate including a plurality of dielectric layers, design space is able to be effectively used and the band pass filter is able to be significantly reduced in size by employing the equivalent circuit shown inFIG. 7rather than the equivalent circuit shown inFIG. 9.

In the third preferred embodiment, the case has been described where inductors are added in a band pass filter according to the second preferred embodiment to adjust the frequency of an attenuation pole. Inductors that adjust the frequency of an attenuation pole may be added in a band pass filter according to the first preferred embodiment as in a band pass filter320that is a second modification of the third preferred embodiment shown inFIG. 10.

With a band pass filter according to the third preferred embodiment and band pass filters that are the first and second modifications, attenuation poles are able to be provided near a desired frequency lower than the pass band of the band pass filter and a desired frequency higher than the pass band.