JP2013005121A - High frequency filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a small high frequency filter circuit having attenuation poles on both sides of the passband of a bandpass filter.SOLUTION: A filter circuit 1 comprises a bandpass filter 40 having a predetermined passband, a first lowpass filter 20 connected between the bandpass filter 40 and an input terminal 10 to form a first attenuation pole on the high frequency side of the passband, and a second lowpass filter 30 connected between the bandpass filter 40 and an output terminal 12 to form a second attenuation pole on the high frequency side of the passband. A distributed constant line 23 included in the first lowpass filter 20 and a distributed constant line 33 included in the second lowpass filter 20 are coupled electromagnetically. As a result, a third attenuation pole is formed on the low frequency side of the passband of the bandpass filter 40.

Description

  The present invention relates to a high frequency filter.

  In a wireless communication device such as a mobile phone, a band pass filter that passes only frequency components of a pass band of an input signal is used. In recent years, since the multiband of wireless communication devices has progressed, it is desirable to attenuate components in the stopband other than the passband over a wide band, and to remove signal contamination in a wireless system that uses the signal in the stopband. ing.

  A low pass filter (LPF) is effective for attenuating the high frequency side of the pass band over a wide area, and a high pass filter (HPF) is effective for attenuating the low frequency side of the pass band over a wide area. For example, JP 2003-163503 A (Patent Document 1) discloses an example of an LPF.

  A method of adding attenuation poles on the high frequency side and low frequency side of the pass band using interlaced coupling has also been proposed. For example, H. Shaman and J.-S. Hong, “Input and output cross-coupled wideband bandpass filter,” IEEE Trans. Microw. Theory Tech., Vol. 55, no. 12, pp. 2562-2568, Dec. In 2007 (Non-Patent Document 1), a distributed constant line having a quarter wavelength of the pass frequency is arranged between the band-pass filter and the input terminal and the output terminal, and this set of distributed constant lines is electromagnetically coupled to each other. By doing so, it is disclosed that attenuation poles are added to the low frequency side and the high frequency side of the passband.

JP 2003-163503 A

H. Shaman and J.-S. Hong, “Input and output cross-coupled wideband bandpass filter,” IEEE Trans. Microw. Theory Tech., Vol. 55, no. 12, pp. 2562-2568, Dec. 2007

  In the bandpass filter described in Non-Patent Document 1, harmonic resonance occurs at a frequency that is three times the center frequency of the pass band, and thus the high frequency side of the pass band cannot be attenuated over a wide area. If an LPF is provided in the bandpass filter described in Non-Patent Document 1, there is a possibility that both attenuation on the low frequency side of the passband of the bandpass filter and attenuation over a wide area on the high frequency side of the passband can be achieved. However, if both the LPF and the distributed distributed line for coupling are provided, the circuit becomes larger and the insertion loss is deteriorated due to the larger size. Various embodiments of the present invention provide a small high frequency filter circuit that realizes attenuation on the low frequency side of the passband of the bandpass filter and attenuation over a wide area on the high frequency side of the passband. Other problems of the present invention will be understood from the description of this specification and the accompanying drawings.

  A filter circuit according to an embodiment of the present invention includes a bandpass filter having a predetermined passband, a bandpass filter connected between the bandpass filter and an input terminal, and a first attenuation pole on the high frequency side of the passband. A first low-pass filter to be formed; and a second low-pass filter that is connected between the band-pass filter and an output terminal and forms a second attenuation pole on the high-frequency side of the passband. In one aspect of the present invention, the first filter element included in the first low-pass filter and the second filter element included in the second low-pass filter form a third attenuation pole on the low frequency side of the pass band. To be electromagnetically coupled.

  Various embodiments of the present invention provide a small high-frequency filter circuit that realizes attenuation on the low-frequency side of the passband of the bandpass filter and attenuation over a wide area on the high-frequency side of the passband.

1 is a circuit diagram showing a high-frequency filter circuit according to an embodiment of the present invention. The circuit diagram which shows the high frequency filter circuit which concerns on one Embodiment of this invention in detail Graph showing the frequency characteristics of the high-frequency filter circuit shown in FIG. The graph which shows the frequency characteristic of the high frequency filter which is a comparative example 6 is a graph showing frequency characteristics of a high-frequency filter circuit according to various embodiments of the present invention. The top view which shows the structure of the high frequency filter circuit which concerns on one Embodiment of this invention The graph which shows the result of the electromagnetic field simulation of the frequency characteristic of the high frequency filter circuit shown in FIG. The graph which shows the experimental result of the frequency characteristic of the high frequency filter circuit shown in FIG. The circuit diagram which shows the high frequency filter circuit which concerns on other embodiment of this invention. The graph which shows the frequency characteristic of the high frequency filter circuit shown in FIG. The graph which shows the frequency characteristic of the high frequency filter which is a comparative example The circuit diagram which shows the high frequency filter circuit which concerns on other embodiment of this invention. The graph which shows the frequency characteristic of the high frequency filter circuit shown in FIG. The graph which shows the frequency characteristic of the high frequency filter which is a comparative example

  Various embodiments of the present invention will be described with reference to the accompanying drawings. FIG. 1 is a circuit diagram showing a filter circuit 1 according to an embodiment of the present invention. As illustrated, the filter circuit 1 according to the embodiment of the present invention includes a first low-pass filter 20, a second low-pass filter 30, and a band-pass filter 40. Hereinafter, for the sake of omission, the low pass filter may be referred to as “LPF” and the band pass filter may be referred to as “BPF”. The first LPF 20 is disposed between the input terminal 10 and the BPF 40, and the second LPF 30 is disposed between the BPF 40 and the output terminal 12. The first LPF 20 and the second LPF 30 are arranged so that a part of the first LPF 20 and a part of the second LPF 30 are electromagnetically coupled. The filter circuit 1 extracts a passband signal from the multiband signal input from the input terminal 10 and outputs the signal from the output terminal 12 to a subsequent circuit. The filter circuit 1 is used by being incorporated into a high frequency module for a UWB wireless system, for example.

  In one aspect, the filter circuit 1 is configured symmetrically with respect to a vertical bisector (filter circuit 1 center line) connecting the input terminal 10 and the output terminal 10. For example, the first LPF 20 is configured to be axisymmetric with respect to the second LPF 30 and the center line of the filter circuit 1, and each element constituting the BPF 40 is arranged to be axisymmetric with respect to the center line. The

  FIG. 2 is a circuit diagram showing the filter circuit 1 according to an embodiment of the present invention in more detail. The first LPF 20 is configured by connecting a capacitor 21 and distributed constant lines 22 to 24 in parallel between the input terminal 10 and the BPF 40. The LPF 30 is configured by connecting a capacitor 31 and distributed constant lines 32 to 34 in parallel between the BPF 40 and the output terminal 12. The distributed constant line 23 and the distributed constant line 33 are electromagnetically coupled, and an attenuation pole is formed on the low frequency side of the pass band of the bandpass filter 40 by this electromagnetic coupling. In one aspect of the present invention, the distributed constant lines 23 and 33 are arranged so that the even mode admittance and the odd mode admittance of the filter circuit 1 are equal.

  The BPF 40 includes a first resonator 50, a second resonator 60, and a third resonator 80. One end of the first resonator 50 is connected to the first LPF 20, and the other end is connected to the second resonator 60 via the capacitor 42. One end of the third resonator 80 is connected to the second LPF 30, and the other end is connected to the resonator 60 via the capacitor 46. The second resonator 60 has a wide pass band, and forms attenuation poles at both ends of the pass band. The first resonator 50 and the third resonator 80 each form an attenuation pole on the high frequency side of the passband of the second resonator 60.

  The first resonator 50 includes a short stub 52 and an open stub 54, and the third resonator 80 includes a short stub 82 and an open stub 84. An impedance matching capacitor 44 is connected between the connection point of the resonator 50 and the capacitor 42 and the ground. An impedance matching capacitor 48 is connected between the connection point of the resonator 80 and the capacitor 46 and the ground. The second resonator 60 includes distributed constant lines 62 and 64, a capacitor 66 connected in parallel to the distributed constant lines 62 and 64, a connection point between the distributed constant line 62 and the distributed constant line 64, and ground. And a distributed constant line 68 connected therebetween. A wavelength shortening capacitor 70 is disposed between the distributed constant line 68 and the ground.

  Thus, in the high frequency filter circuit 1 according to the embodiment of the present invention, the first LPF 20 and the second LPF 30 form an attenuation pole on the high frequency side of the pass band of the BPF 40, and the first LPF 20 Attenuation poles can also be added to the low-frequency side of the passband of the BPF 40 by electromagnetic coupling between a part of (the distributed constant line 23) and a part of the second LPF 30 (the distributed constant line 33). In one aspect of the present invention, the filter circuit 1 including the distributed constant line 23 and the distributed constant line 33 has an attenuation pole on the low frequency side of the passband of the BPF 40 when the even mode admittance and the odd mode admittance are equal. . As described above, in one embodiment of the present invention, a part of the first LPF 20 and the second LPF 30 for attenuating the high frequency side are coupled to each other without adding an element for generating interlaced coupling. Thus, the attenuation pole can be added to the low frequency side, and therefore the attenuation pole can be added to the low frequency side by a small filter circuit. In other words, since the distributed constant line 23 and the distributed constant line 33 are used in combination as an LPF element and an electromagnetic coupling element, it is not necessary to newly provide an electromagnetic coupling element. The filter circuit of Non-Patent Document 1 forms an attenuation pole on the low frequency side that electromagnetically couples 1/4 wavelength distributed constant lines connected to the input terminal and the output terminal, respectively. The high-frequency filter circuit 1 according to the embodiment can be reduced in size by a size corresponding to a set of 1/4 wavelength distributed constant lines as compared with the filter circuit of Non-Patent Document 1.

  The high frequency filter circuit 1 shown in FIG. 2 is configured by a strip line, and a simulation of frequency characteristics was performed. In this simulation, the impedances of the input terminal 10 and the output terminal 12 are both 50Ω, and the characteristic impedances of the distributed constant lines 22, 24, 32, 34, the short stubs 52, 82, the distributed constant line 68, and the open stubs 54, 84. 46.3Ω, the characteristic impedance of the distributed constant lines 62 and 64 is 38.6Ω, the characteristic impedance of the odd mode of the distributed constant lines 23 and 33 is 45.9Ω, and the characteristic impedance of the even mode of the distributed constant lines 23 and 33 Was 45.7Ω. Also, the reference frequency is 4.0 GHz, the electrical length of the distributed constant lines 22 and 32 is 5 degrees, the electrical length of the distributed constant lines 24 and 34 is 18 degrees, and the electrical length of the distributed constant lines 23 and 33 is All are 10 degrees, the electrical length of the distributed constant line 68 is 50.9 degrees, the electrical lengths of the short stubs 52 and 82 are both 26.9 degrees, the electrical length of the open stub 54 is 43.9 degrees, The electrical length was 38.4 degrees, and the electrical lengths of the distributed constant lines 62 and 64 were both 15.7 degrees. Further, the capacitance value of the capacitor 21 is 0.25 pF, the capacitance value of the capacitor 31 is 0.27 pF, the capacitance values of the capacitors 42 and 46 are both 0.75 pF, and the capacitance values of the capacitors 44 and 48 are both 0.5 pF, The capacitance value of the capacitor 66 was 1.0 pF, and the capacitance value of the capacitor 70 was 2.7 pF. The dielectric constant was 7.1 and the dielectric loss tangent was 0.005.

The result of this simulation is shown in FIG. FIG. 3 is a graph showing simulation results of the pass characteristic (S21) and the reflection characteristic (S11) of the high frequency filter circuit 1 according to the embodiment of the present invention. FIG. 4 is a graph showing simulation results of the pass characteristic (S21) and the reflection characteristic (S11) when the BPF 40, the first LPF 20, and the second LPF 30 are arranged so as not to be electromagnetically coupled to each other. As shown in FIG. 3, attenuation poles f ₁ and f ₂ are formed at both ends of the pass band, attenuation poles f ₃ and f ₄ are formed on the high frequency side of the pass band, and attenuation is on the low frequency side of the pass band. it was confirmed that the electrode f ₅ is formed. The attenuation poles f ₁ and f ₂ are formed by the second resonator 60, and the attenuation poles f ₃ and f ₄ are formed by the first resonator 50 and the third resonator 80. f ₅ is formed by electromagnetic coupling between the distributed constant line 23 and the distributed constant line 33. The attenuation poles corresponding to the attenuation poles f ₁ , f ₂ , f ₃ , and f ₄ can also be confirmed in the graph of FIG. On the other hand, in the simulation result of FIG. 4, no attenuation pole is formed on the low frequency side of the pass band. Thus, by a part of the first LPF20 and the part of the second LPF30 are electromagnetically coupled, it was confirmed that the addition of attenuation pole f ₅ on the low frequency side of the pass band of the BPF 40.

  It has been confirmed that the position of the attenuation pole added to the low frequency side changes depending on the degree of coupling between the distributed constant line 23 and the distributed constant line 33. The degree of coupling between the distributed constant line 23 and the distributed constant line 33 is that a via electrode connected to the ground electrode is disposed between the distributed constant line 23 and the distributed constant line 33, and the odd mode of the distributed constant lines 23 and 33 is changed. Adjustment is possible by changing the characteristic impedance. The arrangement of the via electrode will be described later. FIG. 5 is a graph showing a simulation result of the pass characteristic of the high-frequency filter circuit 1. The graph of FIG. 5 shows the filter when the odd-mode characteristic impedance Zso is 45.7Ω, 44.7Ω, and 43.7Ω in the distributed constant lines 23 and 33 having the even-mode characteristic impedance Zse of 45.9Ω. The pass characteristics of the circuit 1 are shown respectively. As shown in the figure, it has been confirmed that the position of the attenuation pole appearing on the low frequency side of the passband changes depending on the value of the odd-mode characteristic impedance Zso (that is, depending on the arrangement of the via electrode).

  Next, the structure of the filter element 100 that realizes the high-frequency filter circuit 1 according to an embodiment of the present invention will be described with reference to FIG. The filter element 100 according to an embodiment of the present invention includes, for example, a plurality of insulator layers on an inner layer of a low temperature co-fired ceramic (hereinafter sometimes referred to as “LTCC”) substrate. It is configured by laminating and forming a conductor pattern between the insulating layers. FIG. 6 is a plan view showing the structure of the filter element 100 according to an embodiment of the present invention. As illustrated, the filter element 100 is configured by laminating insulator layers L1, L2, and L3. Conductor patterns constituting the high-frequency filter circuit 1 are formed on the insulator layers L1, L2, and L3. Although not shown, a ground electrode GND1 is formed on the lower surface of the insulator layer L3. In addition, an insulator layer L0 (not shown) that functions as an upper cover is provided on the upper surface of the insulator layer L1, and a ground electrode GND2 is also formed on the upper surface of the insulator layer. The insulator layer L3 functions as a lower cover. The conductor patterns and ground electrodes of the insulator layers L1 to L3 are connected to each other by via electrodes that extend through the insulator layers in the stacking direction.

  A conductor pattern including a lead conductor 102 connected to the input terminal 10 and a capacitor electrode 106 disposed in the vicinity of the lead conductor 102 is formed on the input end side of the insulating layer L3. The capacitor electrode 106 is connected to the ground electrode GND1 formed on the lower surface of the insulator layer L3 through the via electrode VH10. On the other hand, a conductor pattern including an extraction conductor 104 connected to the output terminal 12 and a capacitor electrode 108 disposed in proximity to the extraction conductor 104 is formed on the output end side of the insulator layer L3. The capacitor electrode 108 is connected to the ground electrode GND1 through the via electrode VH11. A capacitor electrode 110 is formed between the input side and output side conductor patterns. A capacitor electrode 112 is formed on the right end of the insulator layer L3. The capacitor electrode 112 is connected to the ground electrode GND1 through the via electrode VH12.

  On the input end side of the insulating layer L2, a strip line 112 connected to the extraction electrode 102 via the via electrode VH1, a capacitor electrode 116 connected to the other end of the strip line 112, and a right end side from the capacitor electrode 116 Strip lines 120 and 124 are formed to extend respectively. The right end of the strip line 120 is connected to the ground electrode GND1 through the via electrode VH4. On the output end side of the insulating layer L2, on the strip line 114 connected to the extraction electrode 104 via the via electrode VH2, the capacitor electrode 118 connected to the other end of the strip line 114, and the right end side from the capacitor electrode 118 Strip lines 122 and 126 are formed respectively extending in the direction. The right end of the strip line 122 is connected to the ground electrode GND1 through the via electrode VH5. The upper side of the strip line 112 and the lower side of the strip line 114 are arranged at positions facing each other, and a via hole VH3 is formed between the upper side of the strip line 112 and the lower side of the strip line 114. The interval between the upper side of the strip line 112 and the lower side of the strip line 114 is appropriately adjusted so that the degree of coupling between the upper side of the strip line 112 and the lower side of the strip line 114 becomes a desired value. The degree of coupling between the upper side of the strip line 112 and the lower side of the strip line 114 can also be adjusted by the position of the via hole VH3. A capacitor electrode 128 is disposed between the capacitor electrode 116 and the capacitor electrode 118. A U-shaped strip line composed of a pair of strip lines 130 and 132 is formed on the right side of the capacitor electrode 128. The left end of the strip line 130 is connected to the lower right end of the capacitor electrode 128, and the left end of the strip line 132 is connected to the capacitor electrode 110 of the insulator layer L3 via the via electrode VH9. The via electrode VH9 extends in the direction of the insulator layer L1, and is connected to a connection point between a capacitor electrode 140 and a capacitor electrode 144 described later in the insulator layer L1. A strip line 134 extends rightward from a connection point between the strip line 130 and the strip line 132. A capacitor electrode 136 is connected to the right end of the strip line 134.

  A capacitor electrode 138 connected to the left end of the strip line 130 via the via electrode VH8 is formed on the input end side of the insulating layer L1. A capacitor electrode 140 is formed on the output end side of the insulator layer L1. A capacitor electrode 144 is disposed between the capacitor electrode 138 and the capacitor electrode 140. The capacitor electrode 144 is electrically connected to the capacitor electrode 140. A connection point between the capacitor electrode 140 and the capacitor electrode 144 is connected to the left end of the strip line 132 of the insulating layer L2 through the via electrode VH9. A capacitor electrode 142 is formed on the right end of the insulator layer L1.

  The ground electrode GND2 on the upper surface of the insulator layer L0 and the ground electrode GND1 formed on the lower surface of the insulator layer L3 are electrically connected via via electrodes VH3, VH6, and VH7. The ground electrode GND2 and the capacitor electrode 142 of the insulator layer L1 are connected to each other via a via electrode (not shown).

  A correspondence relationship between each element of the filter element 100 configured as described above and each component of the filter circuit 1 of FIG. 2 will be described. First, the distributed constant lines 22 to 24 of the first LPF 20 are configured by the extraction electrode 102 and the strip line 112, and the capacitor 21 includes a portion formed in a wide width of the extraction electrode 102 and a portion near the left end of the capacitor electrode 116. It consists of. Further, the distributed constant lines 32 to 34 of the second LPF 30 are configured by the extraction electrode 104 and the strip line 114, and the capacitor 31 includes a portion formed in a wide width of the extraction electrode 104 and a portion near the left end of the capacitor electrode 118. It consists of.

  The short stub 52 of the first resonator 50 corresponds to the strip line 120, and the open stub 54 corresponds to the strip line 124. The short stub 82 of the third resonator 80 corresponds to the strip line 122, and the open stub 84 corresponds to the strip line 126. The capacitor 42 is constituted by a part of the capacitor electrode 116 and the capacitor electrode 138, and the capacitor 46 is constituted by a part of the capacitor electrode 118 and the capacitor electrode 140. The capacitor 44 is configured by the capacitor electrode 106 and a part of the capacitor electrode 116, and the capacitor 48 is configured by the capacitor electrode 108 and a part of the capacitor electrode 118. The distributed constant line 62 of the second resonator 60 corresponds to the strip line 130, the distributed constant line 64 corresponds to the strip line 132, and the distributed constant line 68 corresponds to the strip line 134. The capacitor 66 includes capacitor electrodes 110, 128, and 144. The capacitor 70 includes capacitor electrodes 112, 136 and 142.

  An electromagnetic field simulation of the frequency characteristics of the filter element 100 configured as described above was performed. In this simulation, the dimensions of the filter element 100 were set to 6.2 × 2.7 × 0.366 mm, and the pass characteristic and reflection characteristic of the filter element 100 were simulated using an electromagnetic field simulator HFSS of Ansys Inc. FIG. 7 shows the simulation results of the transmission characteristics and reflection characteristics of the filter element 100. As shown in the figure, it was confirmed that the filter element 100 has a wide pass band and can suppress the high frequency side of the pass band over a wide band. Moreover, it was confirmed that an attenuation pole appears on the low frequency side (about 0.3 GHz position) of the pass band.

  Further, an experiment for examining the frequency characteristics of the filter element 100 was performed. The filter element 100 of 6.2x2.7x0.366mm is built in the LTCC board of the shape of 8.0x5.0x0.63mm, and the filter element 100 is passed using the network analyzer N5230A PNA-L made by Agilent Technologies Inc. Characteristics and reflection characteristics were measured. FIG. 8 shows the measurement results of the transmission characteristics and reflection characteristics of the filter element 100. As shown in the figure, it was confirmed that the harmonic component of the passing signal can be suppressed to less than 20 dB up to at least 16 GHz. It was also confirmed that an attenuation pole appeared on the low frequency side of the passband.

  FIG. 9 is a circuit diagram showing a high-frequency filter circuit 1A according to another embodiment of the present invention. In the high-frequency filter circuit 1A, a band-pass filter 150 including a one-stage Chebyshev filter is disposed between the first LPF 20 and the second LPF 30 instead of the band-pass filter 40 of the high-frequency filter circuit 1. The LPF 20 is configured by removing the distributed constant lines 22 and 24 from the LPF 20 shown in FIG. 2, and the LPF 30 is configured by removing the distributed constant lines 32 and 34 from the LPF 30 shown in FIG. That is, the LPF 20 includes a distributed constant line 23 and a capacitor 21, and the LPF 30 includes a distributed constant line 33 and a capacitor 31. The high frequency filter circuit 1A has the same configuration as that of the high frequency filter circuit 1 except that a band pass filter 150 is provided instead of the band pass filter 40. The band pass filter 150 includes a capacitor 152, a capacitor 154, and a capacitor 156 and an inductor 158 arranged in parallel between a connection point between the capacitor 152 and the capacitor 154 and the ground.

  The frequency characteristics of the high frequency filter circuit 1A shown in FIG. 9 were simulated. In this simulation, the center frequency of the passing signal is 2.45 GHz, the capacitance values of the capacitors 152 and 154 are about 0.91 pF, the capacitance value of the capacitor 156 is about 1.96 pF, and the inductance value of the inductor 158 is about 1. 33 nH. Further, the capacitance values of the capacitor 21 of the first LPF 20 and the capacitor 31 of the second LPF 30 are each 0.9 pF, the electrical lengths of the distributed constant lines 23 and 33 are both 50 degrees, and the distributed constant lines 23 and 33 The even mode characteristic impedance was 40Ω, and the odd mode characteristic impedance was 36Ω. The simulation result is shown in FIG. FIG. 10 is a graph showing simulation results of the pass characteristic (S21) and the reflection characteristic (S11) of the high frequency filter circuit 1A according to the embodiment of the present invention. FIG. 11 shows the frequency characteristics of the single-stage band-pass filter 150 alone. As shown in the figure, in the high frequency filter circuit 1A according to one embodiment of the present invention, attenuation poles are formed at both ends of the pass band, at the high frequency side of the pass band, and at the low frequency side of the pass band, respectively. It was confirmed that the filter characteristics were improved in each stage as compared with.

  FIG. 12 is a circuit diagram showing a high-frequency filter circuit 1B according to another embodiment of the present invention. In the high frequency filter circuit 1B, a band pass filter 160 is disposed between the first LPF 20 and the second LPF 30 instead of the band pass filter 40 of the high frequency filter circuit 1. The LPF 20 is configured by removing the distributed constant lines 22 and 24 from the LPF 20 shown in FIG. 2, and the LPF 30 is configured by removing the distributed constant lines 32 and 34 from the LPF 30 shown in FIG. That is, the LPF 20 includes a distributed constant line 23 and a capacitor 21, and the LPF 30 includes a distributed constant line 33 and a capacitor 31. The high frequency filter circuit 1B has the same configuration as the high frequency filter circuit 1 except that a band pass filter 160 is provided instead of the band pass filter 40. The band-pass filter 160 connects the first SAW resonator 162 and the second SAW resonator 164 in series, and a connection point between the first SAW resonator 162 and the second SAW resonator 164 and the ground. Inductor 166 and capacitor 168 are arranged between each of them.

  The frequency characteristics of the high frequency filter circuit 1B shown in FIG. 12 were simulated. In this simulation, the pass band of the pass signal was set to around 1.5 GHz, the inductance value of the inductor 166 was about 2.3 nH, and the capacitance value of the capacitor 168 was about 3.0 pF. The capacitance values of the capacitor 21 of the first LPF 20 and the capacitor 31 of the second LPF 30 are 1.0 pF, the electrical lengths of the distributed constant lines 23 and 33 are both 25 degrees, and the distributed constant lines 23 and 33 The even mode characteristic impedance was 50Ω, and the odd mode characteristic impedance was 40Ω. The simulation result is shown in FIG. FIG. 13 is a graph showing simulation results of the pass characteristic (S21) and the reflection characteristic (S11) of the high frequency filter circuit 1B according to the embodiment of the present invention. FIG. 14 shows a simulation result of frequency characteristics of the band-pass filter 160 alone. As shown in the figure, in the high frequency filter circuit 1B according to an embodiment of the present invention, attenuation poles are formed at both ends of the pass band, the high frequency side of the pass band, and the low frequency side of the pass band, respectively. Compared to the case, it was confirmed that the filter characteristics were improved in each stage.

  The embodiments of the present invention are not limited to the aspects explicitly described in the present specification, and various modifications can be made to the embodiments in the specification. For example, the first LPF 20 and the second LPF 30 are arbitrary as long as at least a part of the first LPF 20 and the second LPF 30 can be electromagnetically coupled to form an attenuation pole on the low frequency side of the pass band. The configuration can be taken. Moreover, various band pass filters can be used in the high frequency filter circuit of the present invention in addition to those specified in the present specification. The characteristic impedance, the electrical length, the capacitance value, the inductance value, the center frequency of the passing signal, and the like described in this specification are merely examples, and are not intended to limit the present invention. The high frequency filter circuit of the present invention can also be mounted on a multilayer substrate other than LTCC. In addition, various modifications can be made to the above-described embodiment without departing from the spirit of the present invention.

1, 1A, 1B High-frequency filter circuit 20 First low-pass filter 30 Second low-pass filter 40, 150, 160 Band-pass filter 23, 33 Distributed constant line 100 Filter element

Claims (6)

A bandpass filter having a predetermined passband;
A first low-pass filter connected between the band-pass filter and the input terminal and forming a first attenuation pole on the high-frequency side of the passband;
A second low-pass filter connected between the band-pass filter and an output terminal and forming a second attenuation pole on the high-frequency side of the passband;
With
The first filter element included in the first low-pass filter and the second filter element included in the second low-pass filter are electromagnetically coupled so as to form a third attenuation pole on the low frequency side of the pass band. Filter circuit.   The filter circuit according to claim 1, wherein the even mode admittance and the odd mode admittance are equal.   The filter circuit according to claim 1, wherein the band-pass filter includes a SAW resonator.   The filter circuit according to claim 1, wherein a position of the third attenuation pole changes in accordance with a degree of coupling between the first filter element and the second filter element.   The band-pass filter and the first and second low-pass filters are formed inside a multilayer substrate composed of a plurality of insulator layers, and are between the first filter element and the second filter element in the multilayer substrate. The filter circuit according to claim 1, further comprising a via hole extending in a stacking direction of the insulator layers.   The filter circuit according to claim 5, wherein ground electrodes are provided on both end faces in the stacking direction of the multilayer substrate, and the via hole is electrically connected to at least one of the ground electrodes.

Images