JP5116560B2 - Band pass filter - Google Patents

Description

  The present invention relates to a band pass filter having a relatively wide pass band mainly in a microwave band and a millimeter wave band.

First, a conventional wideband bandpass filter described in, for example, Non-Patent Document 1 below will be described with reference to FIGS. 19 is a circuit diagram, FIG. 20 is an equivalent circuit of FIG. 19 at the center frequency f ₀ of the pass band, and FIG. 21 is a frequency characteristic of the circuit of FIG. In Figure 19, 1A denotes input and output terminals connected to the outside transmission line 2A is having an electrical length of approximately 90 ° at a frequency f _0, 3A is a short-circuited stub having an electrical length of approximately 90 ° at a frequency f _0. The circuit of FIG. 19 is configured by alternately connecting a plurality of transmission lines 2A and a plurality of short-circuited short stubs 3A between input / output terminals 1A.

19, the leading-end short stub 3A is parallel resonance at a frequency _{f 0.} In general, as shown in FIG. 19, a circuit in which a circuit that resonates in parallel at a frequency f ₀ is directly connected by a transmission line can obtain a high coupling amount between the front and rear resonators, and therefore requires a high coupling amount between the resonators. It is known as one of the circuit configurations suitable for realizing a band pass filter having a wide band pass characteristic.

Next, the circuit operation of FIG. 19 will be described with reference to FIG. Figure 20 is a 19, replacing the input and output terminals 1A and terminating resistor 4A, replacing the transmission line 2A ideal J inverter circuit 5A, a circuit is replaced with a parallel resonant circuit 6A a short-circuited stub 3A, at the frequency _{f 0} It is the equivalent circuit of FIG. The ideal J inverter circuit 5A has a characteristic of coupling two parallel resonant circuits 6A connected at the front and rear with a coupling amount corresponding to a required pass bandwidth, and a required pass bandwidth is wide. It is necessary to realize a high binding amount. Parallel resonant circuit 6A to parallel resonance at a frequency _{f 0.} In general, FIG. 20 is known the center frequency of the pass band as a circuit configuration of the band-pass filter of three stages to f _0, the circuit of Figure 19 has a pass band having a center frequency of f _0. In addition to the parallel resonance at the frequency f ₀ , the tip short-circuited stub 3A also performs parallel resonance at the frequency 3f ₀ as a high-order resonance. Therefore, as shown in FIG. 21, the circuit of FIG. 19 has pass characteristics not only near the frequency f ₀ but also near the frequency 3f ₀ . In this way, an unnecessary pass band at a frequency other than the desired frequency is called a spurious response.

  Further, in connection with this type of device, a stripline filter that is connected to a transmission line and a resonator, is easy to process, and is excellent in mass productivity and used in a microwave band and a millimeter wave band is disclosed in, for example, Patent Document 1 below. Has been.

Japanese Patent Laid-Open No. 06-097702 G. Matthaei, L. Young, and E. M. T. Jones, "Microwave Filters, Impedance Matching Networks, and Coupling Structures", McGraw-Hill, 1964, p423

In the conventional bandpass filter as described above, at the frequency 2f ₀ , the transmission line 2A (5A) has an electrical length of about 180 ° and is in series resonance. On the other hand, the tip short-circuit stub 3A (6A) has an electrical length of about 180 °, which is ideally a short-circuit end. However, when a circuit is actually manufactured, it is not an ideal short-circuited end due to the short-circuit structure at the end of the stub, manufacturing errors, and the like, and it is often a minute parallel inductance circuit. A circuit in which series resonance circuits and minute parallel inductance circuits are alternately connected in cascade is known as one of circuit configurations of a band pass filter having a narrow pass band. For this reason, the circuit of FIG. 19 is likely to operate as a bandpass filter having a narrow passband at the frequency 2f ₀ , and as shown in FIG. 22, there is a problem that a local spurious response is likely to occur at the frequency 2f ₀ . It was.

  The present invention reduces the spurious response caused by the series resonance of the transmission line and reduces the broadband low spurious characteristic when a resonant circuit is connected by a transmission line to form a wide band. An object is to obtain a bandpass filter having the same.

  The present invention includes a plurality of parallel resonant circuits that resonate at a use frequency and a plurality of coupling transmission lines, and a band-pass filter configured by cascading the parallel resonant circuits and the coupling transmission lines alternately. In the band-pass filter, the electrical lengths of the plurality of coupling transmission lines have two or more different values.

  According to the present invention, it is possible to provide a bandpass filter having a low-spurious characteristic in a wide band by reducing spurious response due to series resonance of the transmission line.

Embodiment 1 FIG.
1 is a diagram showing a configuration of a band-pass filter according to Embodiment 1 of the present invention. In FIG. 1, 1 is an input / output terminal connected to the outside, 2 is a transmission line, 3 is a tip short-circuited stub, and 4 is a capacitance circuit. The transmission line 2 forms a main line by cascading four transmission lines 2, and both ends of the main line are input / output terminals 1.

The electrical length of each transmission line 2 is such that the two transmission lines 2 connected to the input / output terminal 1 have electrical lengths of about 75 ° and about 100 ° at the center frequency f ₀ of the passband, and the other two transmission line 2 has an electrical length of approximately 90 ° at a frequency f _0. The tip short-circuit stub 3 has an electrical length of about 90 ° at the frequency f ₀ , one end is short-circuited, and the other end is connected to a connection portion between the transmission lines 2. One capacitive circuit 4 is connected in parallel to each end of two transmission lines 2 having electrical lengths of about 75 ° and 100 ° at the frequency f ₀ .

Next, the operation will be described. FIG. 2 shows a π-type circuit conversion of the transmission line 2 having an electrical length of about 90 ° at the frequency f ₀ , a transmission line having a characteristic impedance of Z and an electrical length of 90 ° at the frequency f ₀ , and a characteristic impedance of Z ′. in indicating that electrical length circuit connected parasitic capacitance circuit C _p at both ends of the transmission line is θ at the frequency f ₀ is a possible circuit converts an equivalent in the frequency f _0. Incidentally, in FIG. 2, Z, Z ', θ , C p is the following formula (1), satisfies the equation (2).

Z '= Z / sinθ (1)
2πf ₀ C _p = (cos θ) / Z (2)

  2 is converted into two transmission lines connected to the input / output terminal 1A of the circuit of FIG. 19 with θ being 75 ° and 100 ° in the above formulas (1) and (2). When applied to 2A, the circuit of FIG. 1 is obtained. That is, the circuit of FIG. 1 is a circuit in which π-type circuit conversion is applied to a part of the transmission line 2A of the circuit of FIG.

Next, consider the series resonance of the transmission line 2 in the circuit of FIG. In the circuit of Figure 19, because all of the transmission line 2A to series resonance electrical length at the frequency 2f ₀ is turned approximately 180 °, local spurious response is likely to occur at the frequency 2f ₀ as shown in FIG. 22 There was a problem.

However, in the circuit of FIG. 1, the two transmission lines 2 connected to the input / output terminal 1 have an electrical length of about 180 ° at frequencies of 1.8 f ₀ and 2.4 f ₀ , respectively, and the other two transmission lines 2 Has an electrical length of about 180 ° at a frequency of 2f ₀ . That is, in the circuit of Figure 1, the spurious caused by the series resonance of the transmission line 2 is not concentrated in one frequency three frequency 1.8F _{0, 2f} 0, generated by dispersing the 2.4f _0. Thus, the circuit of Figure 1, as shown in FIG. 3, spurious responses three frequency 1.8F _{0, 2f} 0 due to the series resonance of the transmission line 2, generated by dispersing a 2.4F _0, and Each frequency has a frequency characteristic in which the intensity of the spurious response is suppressed.

  That is, according to the first embodiment, the spurious response due to the series resonance of the transmission line 2 can be distributed and generated in two or more frequencies, and the strength thereof can be suppressed.

  In the first embodiment, θ is set to 75 ° and 100 ° in the above formulas (1) and (2) for two transmission lines 2 connected to the input / output terminal 1 among the four transmission lines 2. Although π-type circuit conversion is applied, it is not necessary to limit θ to 75 ° and 100 ° as long as the value is other than 90 °.

  In the first embodiment, π-type circuit conversion is applied to two transmission lines 2 out of the four transmission lines 2, but one or more transmission lines 2 are represented by the above formulas (1) and (2). The same effect can be obtained by applying π-type circuit conversion in which θ is a value other than 90 °.

  The first embodiment is a three-stage bandpass filter, but the number of filter stages need not be limited to three.

In the first embodiment, the tip short-circuit stub 3 is used as a resonator that resonates at the frequency (operating frequency) f _0. However, if the resonance circuit resonates at the frequency f ₀ , the tip is short-circuited. It is not necessary to limit to. When a resonator having a different form is applied, the frequency corresponding to the spurious response due to the higher-order resonance of the resonator generated at the frequency 3f ₀ in FIG. 3 changes accordingly.

Embodiment 2. FIG.
FIG. 4 is a diagram showing a configuration of a bandpass filter according to the second embodiment of the present invention. In FIG. 4, 1 is an input / output terminal connected to the outside, 2 is a transmission line, 3 is a short-circuited stub, 4 is a capacitance circuit. The transmission line 2 forms a main line by cascading four transmission lines 2, and both ends of the main line are input / output terminals 1.

The electrical length of each transmission line 2 is such that the two transmission lines 2 connected to the input / output terminal 1 have an electrical length of about 45 ° at the frequency f ₀ , and the other two transmission lines 2 have the frequency f ₀ has an electrical length of about 90 °. The tip short-circuit stub 3 is the same as that in the first embodiment shown in FIG. One capacitive circuit 4 is connected in parallel to both ends of each of the two transmission lines 2 having an electrical length of about 45 ° at the frequency f ₀ .

  Next, the operation will be described. First, when the π-type circuit conversion shown in FIG. 2 is applied to two transmission lines 2A connected to the input / output terminal 1A of the circuit of FIG. 19 with θ being 45 °, the circuit of FIG. 4 is obtained. That is, the circuit of FIG. 4 is a circuit in which π-type circuit conversion is applied to a part of the transmission line 2A of the circuit of FIG.

Next, consider the series resonance of the transmission line 2 in the circuit of FIG. In the circuit of FIG. 4, the two transmission lines 2 connected to the input / output terminal 1 have an electrical length of about 180 ° at the frequency 4f ₀ , and the other two transmission lines 2 have an electrical length of about 180 at the frequency 2f ₀ . °. That is, in the circuit of FIG. 4, spurious responses due to series resonance of the transmission line 2 are generated in a distributed manner at two frequencies 2f ₀ and 4f ₀ . Therefore, in the circuit of FIG. 4, as shown in FIG. 5, the spurious response due to the series resonance of the transmission line 2 is generated by being distributed to the two frequencies 2f ₀ and 4f ₀ , and the spurious response of each frequency is It has frequency characteristics with reduced intensity.

That the second embodiment, a spurious response due to the series resonance of the transmission line 2 can be generated at a frequency higher than the frequency 2f _0.

In the second embodiment, π-type circuit conversion in which θ is 45 ° in the above equations (1) and (2) is applied to two transmission lines 2 connected to the input / output terminal 1 among the four transmission lines 2. Although it is applied, if the value is less than 90 °, there is no need to limit θ to 45 °.
In addition, when the spurious response due to the series resonance of the transmission line 2 is generated at a high frequency that does not necessarily require suppression, it is not necessary to suppress the intensity by dispersing the spurious response. The electrical length may be the same value.

  In the second embodiment, π-type circuit conversion is applied to two transmission lines 2 out of the four transmission lines 2, but one or more transmission lines 2 are represented by the above formulas (1) and (2). The same effect can be obtained by applying π-type circuit conversion in which θ is a value greater than or equal to the manufacturing limit value at the time of circuit realization and less than 90 °.

  The second embodiment is a three-stage bandpass filter, but the number of filter stages need not be limited to three.

Further, in the second embodiment, is used a short-circuit stub 3 as a resonator that resonates at a frequency f _0, as long as the resonant circuit which resonates at a frequency f _0, necessary to limit the embodiments to the leading-end short stub There is no. When a resonator having a different form is applied, the frequency corresponding to the spurious response due to the higher-order resonance of the resonator generated at the frequency 3f ₀ also changes in FIG.

Embodiment 3 FIG.
FIG. 6 is a diagram showing a configuration of a band-pass filter according to Embodiment 3 of the present invention. In FIG. 6, 1 is an input / output terminal connected to the outside, 2 is a transmission line, 3 is a short-circuited stub, and 5 is an open stub. The transmission line 2 forms a main line by cascading four transmission lines 2, and both ends of the main line are input / output terminals 1.

The electrical length of each transmission line 2 is that the two transmission lines 2 connected to the input / output terminal 1 have electrical lengths of about 75 ° and 100 ° at the frequency f ₀ , and the other two transmission lines 2 are , Having an electrical length of about 90 ° at frequency f ₀ . The tip short-circuited stub 3 has a characteristic impedance of 60Ω, an electrical length of about 30 ° at the frequency f ₀ , one end is short-circuited, and the other end is connected to a connection point between the transmission lines 2. The tip open stub 5 has an electrical length of less than 90 ° at the frequency f ₀ , has a lower characteristic impedance than the tip short-circuited stub 3, one end is an open end, and the other end is between the transmission lines 2. It is connected to the connection location and is connected in parallel with the tip short-circuit stub 3.

Next, the operation will be described. First, FIG. 7 is parallel with the leading-end short stub having an electrical length of approximately 30 ° at a frequency f ₀ at 60Ω characteristic impedance, the characteristic impedance is the open stub having an electrical length of approximately 45 ° at a frequency f ₀ at 35Ω It is the resonance circuit which connects and comprises. FIG. 8 is an input susceptance B _in as seen from the connection point of the tip short-circuited stub and the tip open stub _in FIG. 7, and the resonant circuit of FIG. 7 resonates in parallel at a frequency at which B _in becomes zero. From FIG. 8, the resonant circuit of Figure 7, parallel resonance at a frequency f _0, it can be seen that the high-order resonance at the frequency 3.7f _0.

As in the resonance circuit of FIG. 7, a resonance circuit in which a tip short-circuit stub having an electrical length of less than 90 ° at a frequency f ₀ and a tip open stub having a lower characteristic impedance than the tip short-circuit stub are connected in parallel. It is known that the frequency of high-order resonance can be shifted to a frequency higher than 3f ₀ while performing parallel resonance at the frequency f ₀ by setting the characteristic impedance and the electrical length of the stub to appropriate values. The configuration is an example.

FIG. 9 is a circuit in which the resonance circuit shown in FIG. 7 is connected as an alternative to the tip short-circuit stub 3 in the circuit of the first embodiment shown in FIG. In FIG. 9, the same or corresponding parts as those in FIG. 1 or FIG. In general, open stub having an electrical length of less than 90 ° at a frequency f ₀ is known to exhibit a capacitive at frequency f _0. In FIG. 9, when the capacitive circuit 4 is replaced with an open-ended stub exhibiting equivalent capacitance at the frequency f ₀ and the portions surrounded by broken lines are combined into one open-ended stub, the circuit shown in FIG. 6 is obtained.

That is, the circuit of FIG. 6 applies the resonance circuit of FIG. 7 as a parallel resonator that resonates at the frequency f _0, and performs series resonance of the transmission line 2 as in the circuit of the first embodiment shown in FIG. due to the three frequency spurious response is generated frequency 1.8f _{0, 2f 0,} a circuit which is distributed to 2.4f _0. Therefore, the circuit of FIG. 6 has the frequency characteristic shown in FIG. 10 in which the spurious response due to the higher-order resonance of the resonator is shifted from the frequency 3f ₀ to the frequency 3.7f _{0 in} the frequency characteristic of the first embodiment shown in FIG. Has characteristics.

That is, according to the third embodiment, the spurious response due to the higher-order resonance of the resonator is shifted to a frequency higher than the frequency 3f ₀ , and the spurious response due to the series resonance of the transmission line 2 is dispersed to increase the strength. Can be suppressed.

  In addition, since the third embodiment is composed only of a transmission line, a tip short-circuit stub, and a tip-open stub, a short-circuit means such as a through hole is used for a microstrip line or a triplate line type circuit made of a strip conductor. With the configuration, the circuit can be easily realized.

  In the third embodiment, as in the first embodiment, θ is set to the two transmission lines 2 connected to the input / output terminal 1 among the four transmission lines 2 in the above formulas (1) and (2). Although π-type circuit conversion with 75 ° and 100 ° is applied, it is not necessary to limit θ to 75 ° and 100 °.

Further, in the third embodiment, the resonance circuit shown in FIG. 7 is applied. However, in the circuit configured by connecting the tip short-circuited stub and the tip-open stub in parallel, the spurious caused by the higher-order resonance of the resonator is used. As long as the response is shifted to a frequency higher than the frequency 3f ₀ , it is not necessary to use the same circuit configuration as the resonance circuit of FIG.

  In the third embodiment, π-type circuit conversion is applied to two transmission lines 2 out of the four transmission lines 2, but in the above formulas (1) and (2), one or more transmission lines 2 are applied. The same effect can be obtained by applying π-type circuit conversion in which θ is a value other than 90 °.

  Further, although the third embodiment is a three-stage bandpass filter, it is not necessary to limit the number of filter stages to three.

Embodiment 4 FIG.
11, FIG. 12, and FIG. 13 are diagrams showing the configuration of a bandpass filter according to Embodiment 4 of the present invention. 11 is a transmission perspective view of the bandpass filter according to the fourth embodiment, FIG. 12 is a transmission top view, and FIG. 13 is a cross-sectional view taken along the line AA ′ of FIG.

  11 to 13, 6 is a multilayer dielectric substrate, 7 is a quasi-lumped constant capacitance circuit, 8 is a short-circuited short stub, 9 is a transmission line, 10 is an input / output line connected to the outside, 11 is a through hole, and 12 is a ground. A conductor layer is shown. In this embodiment, a multi-layer triplate line type band-pass structure in which a ground conductor layer 12 is disposed on a pair of opposing main surfaces (upper and lower surfaces) and a strip conductor is disposed in the formed multilayer dielectric substrate 6 is provided. It is a filter.

The transmission line 9 is formed of a triplate line, and the four transmission lines 9 are connected in cascade to form a main line. Both ends of the main line are input / output lines 10.
The quasi-lumped-constant capacitance circuit 7 has ground electrodes 72 a and 72 b having the same shape grounded to the upper and lower ground conductor layers 12 through the through holes 11 above and below the circular electrode plate 71 connected to the transmission line 9. The capacitor circuit is configured.
The short-circuited short stub 8 grounds one end side of the triplate line arranged in the same layer as the transmission line 9 to the upper and lower ground conductor layers 12 through the through holes 11, and the other end side is a connection point between the transmission lines 9. It is connected to the.

Next, the operation will be described. First, FIG. 14 shows a resonance circuit formed by connecting a tip short-circuited stub having a characteristic impedance of 60Ω and an electrical length of about 30 ° at a frequency f ₀ and a capacitor circuit in parallel. The capacitance circuit has a susceptance value opposite in sign to the susceptance value of the short-circuited short stub at the frequency f _{0. For} example, when the frequency f ₀ is 10 GHz, the capacitance circuit is about 0.5 pF. FIG. 15 shows the input susceptance B _in as seen from the connection point between the short-circuited stub and the capacitance circuit when the frequency f ₀ is 10 GHz. The resonant circuit of FIG. 14 resonates in parallel at a frequency at which B _in becomes zero. . From FIG. 15, it can be seen that the resonant circuit of FIG. 14 resonates in parallel at the frequency f ₀ and the frequency at which the higher-order resonance occurs is at least 4f ₀ or more.

As in the resonance circuit of FIG. 14, a resonance circuit in which a short-circuited short stub having an electrical length of less than 90 ° at a frequency f ₀ and a capacitive circuit are connected in parallel has a characteristic impedance, electrical length, and capacitive circuit of the short-circuited short stub. By setting the capacitance value to an appropriate value, in general, the frequency of higher-order resonance is made higher than that in the case of applying the resonator of FIG. 7 configured by a distributed constant circuit while performing parallel resonance at the frequency f ₀ . It is known that shifting is possible, and the circuit configuration of FIG. 14 is an example.

  FIG. 16 is a circuit in which the resonance circuit shown in FIG. 14 is connected as an alternative to the tip short-circuit stub 3 in the circuit of the first embodiment shown in FIG. In FIG. 16, reference numeral 4 denotes a capacitor circuit, and the other parts are the same as those shown in FIG. In FIG. 16, when the sets of two capacitive circuits 4 surrounded by a broken line are combined into one capacitive circuit, the circuit shown in FIG. 17 is obtained. When the circuit of FIG. The bandpass filter of the fourth embodiment shown in FIG. 13 is obtained.

That is, the band-pass filters of FIGS. 11 to 13 apply the resonance circuit of FIG. 14 as a parallel resonator that resonates at the frequency f _0, and, similarly to the circuit of the first embodiment shown in FIG. three frequency 1.8F ₀ frequency spurious response is generated due to the series resonance of the (2), _2f 0, a circuit which is distributed to 2.4f _0. Accordingly, in the bandpass filters of FIGS. 11 to 13, in the frequency characteristics of the first embodiment shown in FIG. 3, the spurious response due to the higher-order resonance of the resonator is shifted to a higher frequency than in the third embodiment. The frequency characteristics shown in FIG.

  According to the fourth embodiment, the spurious response due to the higher-order resonance of the resonator is shifted to a higher frequency than in the third embodiment, and the spurious response due to the series resonance of the transmission line 9 (2) is changed. It can be dispersed to reduce strength.

  In addition, since the fourth embodiment is composed only of a transmission line, a tip short-circuit stub, and a pseudo lumped constant capacitance circuit, a short-circuit means such as a through hole is added to a microstrip line or a triplate line-type circuit made of a strip conductor. By using the configuration, it is easy to realize a circuit.

  In addition, since the shape of the electrode plate constituting the quasi-lumped constant capacitance circuit 7 is circular in the fourth embodiment, the four circuits of the two transmission lines 9, the tip short-circuit stub 8, and the quasi-lumped constant capacitance circuit 7 are used. Is easily realized.

  In the fourth embodiment, similar to the first embodiment, the two transmission lines 9 connected to the input / output line 10 among the four transmission lines 9 (2) are connected to the above formulas (1) and (2). However, it is not necessary to limit θ to 75 ° and 100 °.

Further, in the fourth embodiment, the resonance circuit shown in FIG. 14 is applied. However, in the circuit configured by connecting the short-circuited tip stub and the quasi-lumped constant capacitance circuit in parallel, the high-order resonance of the resonator is achieved. As long as the resulting spurious response is shifted to a frequency higher than the frequency 3f ₀ , it is not necessary to use the same circuit configuration as the resonance circuit of FIG.

  In the fourth embodiment, π-type circuit conversion is applied to two transmission lines 9 (2) out of four transmission lines 9 (2), but one or more transmission lines 9 (2) are applied. The same effect can be obtained by applying π-type circuit conversion in which θ is a value other than 90 ° in the above formulas (1) and (2).

  Further, although the fourth embodiment is a three-stage bandpass filter, it is not necessary to limit the number of filter stages to three.

  In each embodiment, a transmission line for coupling is constituted by the transmission line 2, and a parallel resonant circuit is constituted by the short-circuited stub 3, the capacitive circuit 4, the open-ended stub 5, and the like.

  Further, for example, about n ° such as electrical length is all about n ° or n °.

It is a circuit diagram of the band pass filter by Embodiment 1 of this invention. It is a figure which shows (pi) type | mold circuit conversion of the transmission line in the band pass filter by Embodiment 1 of this invention. It is a figure which shows the frequency characteristic of the band pass of the band pass filter by Embodiment 1 of this invention. It is a circuit diagram of the band pass filter by Embodiment 2 of this invention. It is a figure which shows the frequency characteristic of the band pass of the band pass filter by Embodiment 2 of this invention. It is a circuit diagram of the band pass filter by Embodiment 3 of this invention. It is a figure which shows the resonance circuit applied to the bandpass filter by Embodiment 3 of this invention. It is a figure which shows the frequency characteristic of the input susceptance which the resonance circuit of FIG. 7 has. It is the circuit diagram which replaced the capacitive circuit of the bandpass filter by Embodiment 1 of this invention with the open end stub. It is a figure which shows the frequency characteristic of the band pass of the band pass filter by Embodiment 3 of this invention. It is a permeation | transmission perspective view of the bandpass filter by Embodiment 4 of this invention. It is a permeation | transmission top view of the bandpass filter by Embodiment 4 of this invention. It is sectional drawing along the AA 'line of FIG. It is a figure of the resonant circuit applied to the bandpass filter by Embodiment 4 of this invention. It is a figure which shows the frequency characteristic of the input susceptance which the resonance circuit of FIG. 13 has. It is a figure which shows the circuit which replaced the front-end | tip short circuit stub of FIG. 1 with the resonance circuit of FIG. It is a figure which shows the circuit obtained from the circuit of FIG. It is a figure which shows the frequency characteristic of the band pass of the band pass filter by Embodiment 4 of this invention. It is a circuit diagram which shows an example of the conventional wide bandpass filter. FIG. 20 is a diagram showing an equivalent circuit of the circuit of FIG. 19 at the frequency f ₀ . It is a figure which shows the frequency characteristic of the band pass of the conventional wide band pass filter. It is a figure which shows the frequency characteristic when a spurious generate | occur | produces locally in FIG.

Explanation of symbols

  1 input / output terminal, 2 transmission line, 3 tip short circuit stub, 4 capacitance circuit, 5 tip open stub, 6 (multilayer) dielectric substrate, 7 quasi-lumped constant capacitance circuit, 8 tip short circuit stub, 9 transmission line, 10 input Output line, 11 through hole, 12 ground conductor layer.

Claims (5)

  A band-pass filter comprising a plurality of parallel resonant circuits that resonate at a use frequency and a plurality of coupling transmission lines, wherein the parallel resonant circuits and the coupling transmission lines are alternately connected in cascade, The band-pass filter characterized in that the electrical lengths of the plurality of coupling transmission lines have two or more different values. A band-pass filter comprising a plurality of parallel resonant circuits that resonate at a use frequency and a plurality of coupling transmission lines, wherein the parallel resonant circuits and the coupling transmission lines are alternately connected in cascade,
The plurality of coupling transmission lines have a characteristic impedance Z and a transmission line whose electrical length is 90 ° at the frequency f ₀ which is the center frequency of the passband, and a characteristic impedance Z ′ and the electrical length is θ at the frequency f ₀ . It is equivalently converted to a π-type circuit in which a parasitic capacitance circuit _Cp is connected to both ends of the transmission line ,
However,
Z ′ = Z / sin θ
2πf0Cp = (cos θ) / Z
A band pass filter, wherein an electrical length of at least one of the plurality of coupling transmission lines is less than 90 ° at a center frequency of a pass band .   The parallel resonant circuit is configured by a circuit in which a tip short-circuit stub and a tip open stub are connected in parallel, and the characteristic impedance of the tip open stub is lower than the characteristic impedance of the tip short-circuit stub. Item 3. A bandpass filter according to Item 1 or 2. The plurality of coupling transmission lines connected in cascade are on a dielectric substrate, a strip conductor passing through the dielectric substrate along the dielectric substrate, and a pair of main surfaces on both sides of the strip conductor of the dielectric substrate. It consists of a formed ground conductor layer,
The plurality of parallel resonant circuits connected in cascade are:
An electrode plate disposed in the same layer as the strip conductor, a first through hole for electrically connecting the strip conductor and the ground conductor layer, and disposed between the electrode plate and each ground conductor layer, respectively. And a second through hole that electrically connects the ground electrode and the ground conductor layer,
The band pass filter according to claim 3, wherein the open-ended stub is configured by a capacitive circuit configured between the electrode plate and the ground electrode. The band pass filter according to claim 4, wherein the electrode plate is a circular electrode plate.

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