JP4501729B2 - High frequency filter - Google Patents

Description

  The present invention relates to a high frequency filter mainly used in a microwave band and a millimeter wave band.

First, the configuration of a conventional high frequency filter will be described. 12 is a transmission diagram of a conventional high-frequency filter, FIG. 13 is a view seen below the AA ′ plane in FIG. 12, and FIG. 14 is a pass characteristic of the high-frequency filter of FIGS. This filter is composed of a triplate line. 12 and 13, 1 is a ground conductor, 2 is a dielectric substrate, and 3 is a strip conductor formed on the dielectric substrate 2. 4 is an input / output line composed of a ground conductor 1, a dielectric substrate 2, and a strip conductor 3, and is connected to an external circuit, and 5 is a resonator composed of the ground conductor 1, the dielectric substrate 2, and the strip conductor 3. Is a coupling portion for coupling the input / output line 4 and the resonator 5 or the resonators 5 to each other. That is, FIG. 12 and FIG. 13 show a high frequency filter in which three resonators are cascade-connected in three stages.
Next, the operation of this high frequency filter will be described. The resonance frequency of the resonator 5 and the degree of coupling of the coupling unit 6 are adjusted so that a high frequency wave such as a microwave passes with the frequency f _M as the center frequency. These adjustments are mainly performed by adjusting the electrical length in the line direction of the resonator or the interval between the coupling portions.
RF frequency away from the center frequency f _M is the case where incident from the input-output line 4a to filter, since the resonator 5 does not resonate, the high-frequency incident is reflected. On the other hand, when the frequency of the incident high frequency is f _M or a frequency in the vicinity thereof, the resonator 5 resonates. Therefore, the input / output line 4a and the resonator 5a are connected via the coupling portion 6a, the resonator 5a and the resonator 5b are connected via the coupling portion 6b, the resonator 5b and the resonator 5c are connected via the coupling portion 6c, and the resonator 5c The input / output lines 4b are strongly coupled via the coupling part 6d. As a result, the frequency f _M or high frequency of the vicinity will be propagated to the output line 4b from the output line 4a. That is, as shown in FIG. 14, the pass characteristics of the high frequency filter shown in FIGS. 12 and 13 are a pass band having a frequency band centered on f _M and a pass characteristic on both sides of the high frequency side and the low frequency side. Is a pass characteristic of a so-called band-pass filter. In FIG. 14, f _L represents the low-frequency cutoff frequency, and f _H represents the high-frequency cutoff frequency. (For example, see Non-Patent Document 1)

Seymour B. Cohn et al. "Parallel-Coupled Transmission-Line-Resonator Filters" (1958 Transactions on IEEE Microwave Theory and Techniques 6.2 (Apr. 1958 [T-MTT]): 223-231)

In general, in a band pass filter in which a plurality of resonators are electromagnetically coupled as shown in a conventional high frequency filter, the insertion loss in the pass band basically depends on the Q value of each resonator, and the Q value is The larger the value, the smaller the insertion loss.
The Q value of each resonance circuit basically increases as the volume occupied by the resonator portion increases, increases as the conductivity of the constituent material of the strip conductor increases, and the dielectric loss tangent of the dielectric substrate decreases. It has the property of becoming larger.
Therefore, when a conventional high-frequency filter is configured in a structure using a thin dielectric substrate or a resin substrate that is inexpensive but has a relatively large dielectric loss tangent, the Q value of the resonator becomes small for the above reason. Therefore, there is a problem that the insertion loss of the pass band becomes large and it is difficult to obtain a low-loss band pass filter. In many cases, the dielectric loss tangent of the dielectric substrate generally increases as the input signal becomes higher in frequency, and the above problem becomes more noticeable at high frequencies such as millimeter waves.
In the case of a conventional band pass filter, the suppression of the pass characteristics on the high frequency side and the low frequency side of the pass band is continuous, and the amount of suppression increases as the distance from the pass band increases. On the other hand, if a continuous stopband is not always required, such as when a narrow stopband requires multiple discrete stopbands, an excessive stopband width and attenuation amount relative to the required stopband width and attenuation amount. As a result, the insertion loss of the pass band is often increased.
In addition, in a band pass filter in which a plurality of resonators are electromagnetically coupled as in a conventional band pass filter, the input wave is a coupling part of the resonator in the process from reaching the output end to the output end. Since it is necessary to pass through the conductor gap, when the input wave has high energy, a high voltage is applied between the electrodes sandwiching the conductor gap, which may cause dielectric breakdown and damage the substrate.

  The present invention has been made to solve the above-described problems, and has stopbands having necessary and sufficient bandwidths and attenuation amounts on the high-frequency side and the low-frequency side of the frequency band as the passband. An object of the present invention is to construct a high-frequency filter having a small loss in the passband with a resin-based substrate or the like that is difficult to construct a resonator having a high Q value.

  It is another object of the present invention to obtain a high-frequency filter that eliminates a conductor gap in the process from the input end to the output end and prevents the substrate from being damaged even when the input wave has high energy.

A high-frequency filter according to the present invention includes a plurality of parallel capacitance circuits having an open stub and exhibiting a predetermined parallel capacitance at a predetermined cutoff frequency, and a plurality of series inductance circuits exhibiting a predetermined series inductance at the predetermined cutoff frequency; And at least one of the open stubs included in the parallel capacitance circuit, wherein the parallel capacitance circuit, the series inductance circuit, and the parallel capacitance circuit are alternately connected in an odd number in the order from the input end to the output end. An open stub having an electrical length of ¼ wavelength at a first frequency lower than the predetermined cutoff frequency, having a vicinity of the predetermined cutoff frequency as a pass band, and the plurality of parallel regions in a frequency region other than the vicinity of the cutoff frequency It has band-pass characteristics in which attenuation poles are formed at multiple frequencies determined based on the capacitance circuit

  Since the high frequency filter according to the present invention is configured as described above, the vicinity of a predetermined cutoff frequency is a pass band, and the frequency is determined based on the plurality of parallel capacitance circuits in a frequency region other than the vicinity of the cutoff frequency. A high frequency filter having an attenuation pole and having a band pass characteristic can be obtained, and since there is no conductor gap between the input end and the output end, a high frequency filter excellent in power durability can be easily obtained. Further, since a resonator that resonates in the pass band is not used, the loss in the pass band can be reduced as compared with the conventional band pass filter.

Embodiment 1 FIG.
FIG. 1 is a configuration explanatory view for explaining the configuration of a high-frequency filter according to Embodiment 1 for carrying out the present invention. In FIG. 1, 8 is an input / output terminal, 9 is a parallel capacitance circuit, 10 is a series inductance circuit, 11 is an open stub having a characteristic impedance Z ₀ Ω and electrical lengths θf ₁ , θf ₂ , and θf ₃ , respectively. As shown in FIG. 1, the high-frequency filter of the present embodiment has three parallel capacitance circuits 9 and two series inductance circuits 10 alternately connected in cascade, and the open stub is connected in parallel to the connection portion of the parallel capacitance circuit 9 in the cascade connection. 11 is connected, and the three parallel capacitance circuits 9 and the two series inductance circuits 10 are alternately connected in cascade, and the input / output line 4 for connecting to an external circuit is connected to both ends.

  Next, the operation of the high frequency filter according to this embodiment will be described. First, in the high frequency filter according to the present embodiment, the entire circuit includes the input / output terminal 8a, the open stub 11a, the parallel capacitance circuit 9a, the open stub 11b, the series inductance circuit 10a, the open stub 11c, the parallel capacitance circuit 9b, and the open stub 11d. A series inductance circuit 10b, an open stub 11e, a parallel capacitance circuit 9c, an open stub 11f, and an input / output terminal 8b are cascaded in this order.

Here, the series inductance circuits 10a and 10b are adjusted to have a specific series inductance value at the cutoff frequency fc, and the electrical length of the open stubs 11a and 11f is 1 / at a specific frequency f1 lower than the cutoff frequency fc. The electrical length is adjusted to four wavelengths θf1. The electrical lengths of the open stubs 11b and 11e are adjusted to an electrical length θf2 of a quarter wavelength at a specific frequency f2 higher than the cutoff frequency fc, and the electrical lengths of the open stubs 11c and 11d are longer than the cutoff frequency fc. It is adjusted to an electrical length θf3 of a quarter wavelength at a high specific frequency f3. Then, the parallel capacitance circuit 9a is adjusted to act as a parallel capacitance circuit in which the circuit in conjunction with 3 parts of an open stub 11a parallel capacitance circuit 9 a and the open stub 11b has a specific parallel capacitance value at the cut-off frequency fc Similarly, the parallel capacitance circuit 9b is adjusted so that the circuit including the three parts of the open stub 11c, the parallel capacitance circuit 9b, and the open stub 11d functions as a parallel capacitance circuit having a specific parallel capacitance value at the cutoff frequency fc. In addition, the parallel capacitance circuit 9c is adjusted so that the circuit combining the three parts of the open stub 11e, the parallel capacitance circuit 9c, and the open stub 11f functions as a parallel capacitance circuit having a specific parallel capacitance value at the cutoff frequency fc. ing.

  By the above adjustment, the high frequency filter according to the present embodiment becomes substantially equivalent to the circuit shown in FIG. 2 at the cutoff frequency fc. In FIG. 2, 8 is an input / output terminal, 9 is a parallel capacitance circuit, and 10 is a series inductance circuit. The input / output terminals 8a and 8b in FIG. 2 correspond to the input / output terminals 8a and 8b in FIG. 1, respectively, and the series inductance circuits 10a and 10b in FIG. 2 correspond to the inductance circuits 10a and 10b in FIG. 2 corresponds to the circuit including the open capacitance stubs 11a and 11b and the parallel capacitance circuit 9a enclosed by the broken line in FIG. 1 and the parallel capacitance circuit 9e in FIG. 2 and the broken line in FIG. The enclosed circuit including the open stubs 11c and 11d and the parallel capacitance circuit 9b corresponds to the parallel capacitance circuit 9f in FIG. 2 and the open stubs 11e and 11f and the parallel capacitance circuit 9c enclosed in broken lines in FIG. The corresponding circuit corresponds. The circuit of FIG. 2 is a circuit in which parallel capacitance circuits and series inductance circuits are alternately connected in cascade, and constitutes a so-called low-pass filter having a frequency fc as a cutoff frequency.

Next, the pass characteristics of the high frequency filter according to this embodiment will be described. From the above description, the high-frequency filter according to the present embodiment is substantially equivalent to a low-pass filter having a cutoff frequency of the frequency fc in the vicinity of the frequency fc. Have In the frequency region other than the vicinity of the cut-off frequency fc, the open stub 11 in FIG. 1 has attenuation poles at frequencies that are odd multiples of the frequencies f ₁ , f ₂ , and f _{3 at which} the electrical lengths of the open stubs 11 are ¼ wavelengths.
That is, according to the present embodiment, a so-called band-pass filter is substituted with a pass band in the vicinity of the frequency fc and attenuation poles in the vicinity of the frequencies f ₁ , f ₂ , f ₃ . A high-frequency filter having pass characteristics that can be obtained can be obtained. This transmission characteristic is shown by the solid line in FIG. Note that the broken line in FIG. 3 is a pass characteristic of a general low-pass filter having the frequency fc as a cutoff frequency.

  Further, in the high frequency filter according to the present embodiment, since there is no conductor gap between the input end and the output end, there is an advantage that the power resistance is superior to the conventional band pass filter configured by coupling between resonators. is there.

  In addition, since the high-frequency filter according to the present embodiment does not use a resonator that resonates in the passband, which is applied in the conventional bandpass filter, the loss in the passband is smaller than that of the conventional bandpass filter. can do.

  In the high frequency filter according to the present embodiment, two open stubs adjusted to have an attenuation pole on the low frequency side of the pass band and two open stubs adjusted to have an attenuation pole on the high frequency side of the pass band. Although four stubs are applied and each characteristic impedance is made equal, if the same effect as this embodiment can be obtained, it is not necessary to limit the number of each to two and four. The characteristic impedances need not be equal.

In addition, the high-frequency filter according to the present embodiment is configured by a combination of three parallel capacitance circuits, two series inductance circuits, and six open stubs, but the number of each is limited to three, two, and six. is necessary to not, be configured by a combination satisfying the same condition as this embodiment, Ru same effect as this embodiment can be obtained.

Embodiment 2
FIG. 4 is a perspective view for explaining the structure of the high-frequency filter according to Embodiment 2 for carrying out the present invention. FIG. 5 shows a top view of the high frequency filter. In the present embodiment, a specific structure of the high-frequency filter based on the configuration explanatory diagram of the high-frequency filter exemplified in the first embodiment is illustrated. A dielectric substrate is disposed on a ground conductor, and a strip is formed thereon. This is a so-called microstrip line type filter configured by arranging conductors.

4 and 5, 1 is a ground conductor, 2 is a dielectric substrate, and 3 is a strip conductor formed on the dielectric substrate 2. 4, 5, 6, 7 are each composed of a ground conductor 1, a dielectric substrate 2, and a strip conductor 3, where 4 is an input / output line connected to an external circuit, and 5 is a line width wider than that of the input / output line 4. A wide low-impedance line, 6 is a high-impedance line with a narrower line width than the input / output line 4, 7 is a line with a characteristic impedance of Z ₀ Ω, and an open stub with electrical lengths θf ₁ , θf ₂ , and θf ₃ , respectively. Forming. In the high frequency filter of the present embodiment, three low impedance lines 5 and two high impedance lines 6 are alternately connected in cascade, and two open stubs 7 are connected to the cascade connection portions of the low impedance lines 5, respectively. An input / output line 4 for connecting to an external circuit is connected to both ends of the connection.

  In general, the high-frequency filter having such a configuration can be represented by an equivalent circuit using distributed constant lines. FIG. 6 shows the high-frequency filter according to the present embodiment expressed using distributed constant lines. In FIG. 6, 8 is an input / output terminal, 11 is an open stub represented by a distributed constant line, 19 is a low impedance line represented by a distributed constant line, and 20 is a high impedance line represented by a distributed constant line.

Next, the operation of the high frequency filter according to the above embodiment will be described with reference to FIG. In the high frequency filter according to the present embodiment, the entire circuit includes the input / output terminal 8a, the open stub 11a, the low impedance line 19a, the open stub 11b, the high impedance line 20a, the open stub 11c, the low impedance line 19b, the open stub 11d, and the high An impedance line 20b, an open stub 11e, a low impedance line 19c, an open stub 11f, and an input / output terminal 8b are cascaded in this order. Here, the high impedance lines 20a and 20b are adjusted to work as a series inductance circuit having a specific series inductance value at the cutoff frequency fc, and the electrical lengths of the open stubs 11a and 11f are lower than the cutoff frequency fc. is adjusted to an electrical length .theta.f ₁ quarter wavelength at the frequency f _1. Also, the open stub 11b, the electrical length of 11e is adjusted to an electrical length .theta.f ₂ 1/4 wavelength at certain high frequency f ₂ than the cut-off frequency fc, the open stub 11c, the electrical length of 11d are cut-off frequency fc It is adjusted to an electrical length θf ₃ of a quarter wavelength at a specific frequency f ₃ higher than that. The low impedance line 19a is adjusted so that the circuit combining the three parts of the open stub 11a, the low impedance line 19a, and the open stub 11b acts as a parallel capacitance circuit having a specific parallel capacitance value at the cutoff frequency fc. Similarly, the low impedance line 19b is adjusted so that the circuit combining the three parts of the open stub 11c, the low impedance line 19b, and the open stub 11d functions as a parallel capacitance circuit having a specific parallel capacitance value at the cutoff frequency fc. In addition, the low impedance line 19c is adjusted so that the circuit including the open stub 11e, the low impedance line 19c, and the open stub 11f is a parallel capacitance circuit having a specific parallel capacitance value at the cutoff frequency fc. Yes. These adjustments are mainly performed by adjusting the characteristic impedance determined by the physical length of each line in the line direction and the line width.

By adjusting as described above, the high frequency filter according to the present embodiment is substantially equivalent to the circuit shown in FIG. 2 at the cutoff frequency fc.
In FIG. 2, 8 is an input / output terminal, 9 is a parallel capacitance circuit, and 10 is a series inductance circuit. The input / output terminals 8a and 8b in FIG. 2 correspond to the input / output terminals 8a and 8b in FIG. 6, respectively, and the series inductance circuits 10a and 10b in FIG. 2 correspond to the high impedance lines 20a and 20b in FIG. 2 corresponds to the circuit including the open capacitance stubs 11a and 11b and the low impedance line 19a surrounded by the broken line in FIG. 6 and the parallel capacitance circuit 9e in FIG. 2 and the broken line in FIG. The open stubs 11c and 11d and the parallel capacitance circuit 9b, which are enclosed in the figure, correspond to the parallel capacitance circuit 9f in FIG. 2 and the open stubs 11e and 11f and the parallel capacitance circuit 9c in the broken line in FIG. The combined circuit corresponds. Therefore, like the circuit shown in FIG. 2, the circuit shown in FIG. 6 is a circuit in which parallel capacitance circuits and series inductance circuits are alternately connected in cascade, and a so-called low-pass filter having a frequency fc as a cutoff frequency is provided. Constitute.

  That is, the high-frequency filter according to the present embodiment is the high-frequency filter according to the first embodiment, the series inductance circuit 10 in FIG. 2 is configured by the high impedance line 20 in FIG. 6, and the parallel capacitance circuit 9 in FIG. The low impedance line 19 in FIG. 6 is used. Therefore, the pass characteristic of the high frequency filter according to the present embodiment is the same as that of the high frequency filter according to the first embodiment.

  The high frequency filter according to the present embodiment is composed only of distributed constant lines without applying a lumped constant element such as a chip capacitor, so that it has low loss and small characteristic variation, and is easy to manufacture and inexpensive. There are advantages such as being.

  Further, in the high frequency filter according to the present embodiment, since there is no conductor gap between the input end and the output end, the conventional high frequency filter configured by coupling between resonators is the same as the high frequency filter according to the first embodiment. There is an advantage that the power durability is superior to that of the band-pass filter.

  In addition, the high-frequency filter according to the present embodiment does not use a resonator that resonates in the pass band, which is applied in the conventional band-pass filter, and therefore, similarly to the high-frequency filter according to the first embodiment, The loss in the pass band can be made smaller than that of the band pass filter.

Embodiment 3 FIG.
FIG. 7 is a perspective view for explaining the structure of a high-frequency filter according to Embodiment 3 for carrying out the present invention. FIG. 8 shows a top view of the high frequency filter.
In the present embodiment, a specific structure of a high-frequency filter different from the high-frequency filter illustrated in the second embodiment is illustrated. A dielectric substrate is disposed on a ground conductor, and a strip conductor is disposed thereon. This is a so-called microstrip line type filter.

7 and 8, 1 is a ground conductor, 2 is a dielectric substrate, and 3 is a strip conductor formed on the dielectric substrate 2. 4, 5, 6, 7, 12, and 13 are each composed of a ground conductor 1, a dielectric substrate 2, and a strip conductor 3. 4 is an input / output line connected to an external circuit, and 5 is more than the input / output line 4. A low-impedance line with a wide line width, 6 is a high-impedance line with a narrower line width than the input / output line 4, 7 is a line with a characteristic impedance of Z ₀ Ω, and an open stub with electrical lengths θf ₂ and θf ₃ respectively. Forming. Also, 12 is a transmission line whose both ends are connected to separate circuits, 13 is a transmission line whose tip is an open end, and 14 is composed of a combination of transmission line 12 and transmission line 13 A branch stub having a branch point in the middle of the track. Here, the branch stub 14 having such a branched shape is hereinafter referred to as a branch stub 17. Further, in the branch stub 17, the part constituting the middle of the branch stub 17 like the transmission line 12 is called a trunk stub 15, and the part constituting the end of the branch stub 17 like the transmission line 13 is called the branch stub 16. Call. As can be seen from FIGS. 7 and 8, the high-frequency filter of the present embodiment has three low-impedance lines 5 and two high-impedance lines 6 alternately connected in cascade. The two open stubs 7 or the branch stub 17 and the open stub 7 are connected to each other, and the input / output line 4 for connecting to an external circuit is connected to both ends of the cascade connection.

In general, the high-frequency filter having such a configuration can be expressed using a distributed constant line, and FIG. 9 shows the high-frequency filter according to the present embodiment expressed using a distributed constant line. In FIG. 9, 8 is an input / output terminal, 11 is an open stub represented by a distributed constant line, 19 is a low impedance line represented by a distributed constant line, 20 is a high impedance line represented by a distributed constant line, and 15 is a distributed constant line. Is a branch stub represented by a distributed constant line, and 17 is a branch stub represented by a distributed constant line.
As can be seen from FIG. 9, the high frequency filter according to the present embodiment is obtained by replacing the open stubs 11a and 11f with the branch stubs 17a and 17b in the high frequency filter according to the second embodiment shown in FIG. It is.

Here, first, the operation of the branch stub 17 will be described. First, for comparison, an input susceptance Bin of a normal open stub having no branching structure is shown _in Formula 1. Note that (l, Z) represents the physical length in the line direction and the characteristic impedance of the line. Next, FIG. 10 shows a distributed constant line diagram of a branch stub having one branch point in the middle of the line, like the branch stub 17 of the high-frequency filter according to the present embodiment. In FIG. 10, 15 is a trunk stub, 16 is a branch stub, and 18 is a parallel connection. At this time, as shown in FIG. 10, the physical length in the line direction of the trunk stub 15 and the branch stubs 16a and 16b and the characteristic impedance of the line are (l ₀ , Z ₀ ), (l ₁ , Z ₀ ), ( l ₂ , Z ₀ ), the input susceptance B _in of the branch stub at the frequency f when viewed from the parallel connection unit 18 is given by Equation 2. Each stub has an attenuation pole at a frequency at which the input susceptance B _in of Equations 1 and 2 diverges.

From Equation 1, in a normal open stub, the input susceptance diverges at a periodic frequency of f _ant , 3 f _ant , 5 f _ant (2n + 1) f _ant (n is an integer). On the other hand, in Equation 2, the input susceptance does not necessarily diverge at a periodic frequency, and the physical length of the trunk stub and the branch stub, which are constituent elements of the branch stub, and the characteristic impedance of the line are adjusted. The input susceptance can be diverged at a plurality of relatively arbitrary frequencies that are not periodic.
In other words, by applying a branch stub and adjusting the component stem stub and branch stub, it is possible to set attenuation poles at a plurality of relatively arbitrary frequencies compared to applying a normal open stub. . These adjustments are made by adjusting the characteristic lengths determined by the physical lengths of the trunk stub and the branch stub in the line direction and the line widths.

Next, the operation of the high frequency filter will be described.
The circuit of the high frequency filter exemplified in the third embodiment includes an input / output terminal 8a, a branch stub 17a, a low impedance line 19a, an open stub 11a, a high impedance line 20a, an open stub 11b, a low impedance line 19b, an open stub 11c, The high impedance line 20b, the open stub 11d, the low impedance line 19c, the branch stub 17b, and the input / output terminal 8b are connected in cascade. Here, the high impedance lines 20a and 20b are adjusted to work as a series inductance circuit having a specific inductance value at the cutoff frequency fc, and the electrical length of the open stubs 11a and 11d is higher than the cutoff frequency fc. is adjusted to an electrical length .theta.f ₂ quarter wavelength at the frequency f _2, the open stub 11b, the electrical length of 11c, the electrical length of a quarter wavelength at a particular frequency f ₃ greater than the cutoff frequency fc .theta.f ₃ Has been adjusted. The electrical lengths and characteristic impedances of the trunk stub 15a, branch stubs 16a and 16b, and the trunk stub 15b and branch stubs 16c and 16d, which are the components of the branch stubs 17a and 17b, are as follows. The attenuation pole is adjusted to be set at a specific frequency f _1a lower than the cutoff frequency fc and at specific frequencies f _1b and f _1c higher than the frequency fc. Further, the low impedance line 19a is adjusted so that the circuit including the three parts of the branch stub 17a, the low impedance line 19a, and the open stub 11a works as a parallel capacitance circuit having a specific parallel capacitance value at the cutoff frequency fc. Similarly, the low-impedance line 19b is a circuit in which the three parts of the open stub 11b, the low-impedance line 19b, and the open stub 11c function as a parallel capacitance circuit having a specific parallel capacitance value at the cutoff frequency fc. The low-impedance line 19c is a circuit in which the open stub 11d, the low-impedance line 19c, and the branch stub 17b are combined to serve as a parallel capacitance circuit having a specific parallel capacitance value at the cutoff frequency fc. Have been adjusted so that. These adjustments are mainly performed by adjusting characteristic impedances determined by the physical lengths and line widths of the various lines shown in FIGS.

  By adjusting as described above, the high frequency filter according to the present embodiment is substantially equivalent to the circuit shown in FIG. 2 at the cutoff frequency fc. In FIG. 2, 8 is an input / output terminal, 9 is a parallel capacitance circuit, and 10 is a series inductance circuit. The input / output terminals 8a and 8b in FIG. 2 correspond to the input / output terminals 8a and 8b in FIG. 9, respectively, and the series inductance circuits 10a and 10b in FIG. 2 correspond to the high impedance lines 20a and 20b in FIG. 2 corresponds to the circuit including the branch stub 17a, the low impedance line 19a, and the open stub 11a surrounded by a broken line in FIG. 9, and the parallel capacitance circuit 9e in FIG. The open stubs 11b, 11c and the parallel capacitance circuit 9b, which are surrounded by a broken line in FIG. 2, correspond to the parallel capacitance circuit 9f in FIG. 2, and the open stub 11d, the parallel capacitance circuit 9c, A circuit combined with the branch stub 17b corresponds. The circuit of FIG. 2 is a circuit in which parallel capacitance circuits and series inductance circuits are alternately connected in cascade, and constitutes a so-called low-pass filter having a frequency fc as a cutoff frequency. That is, the high-frequency filter according to the third embodiment is the same as the high-frequency filter described in the second embodiment, except that the open stubs 11a and 11f shown in FIG. 6 are constituted by the branch stubs 17a and 17b in FIG. The characteristics in the vicinity of the cut-off frequency fc are the same as those of the high-frequency filter according to the first and second embodiments.

Next, the pass characteristics of the high frequency filter according to this embodiment will be described. From the above description, the high-frequency filter according to the present embodiment has the pass characteristic of the low-pass filter having the cutoff frequency as fc in the vicinity of the cutoff frequency fc, similar to the high-frequency filters according to the first and second embodiments. . In the frequency region other than the vicinity of the cut-off frequency fc, the frequencies f ₂ and f _{3 at which} the electrical lengths of the open stubs 11 in FIG. 9 are 1/4 wavelengths, and the branch stubs 17 in FIG. The attenuation poles are set at the frequencies f _1a , f _1b , and f _1c set with the attenuation poles.
That is, the high frequency filter according to the third embodiment uses the vicinity of the cutoff frequency fc as a pass band, and attenuation poles at frequencies f _1a , f _1b , f _1c , f ₂ , f ₃ . It is possible to obtain a high-frequency filter having a pass characteristic that can be used as a substitute for a so-called band-pass filter. FIG. 11 shows this pass characteristic with a solid line. For comparison, FIG. 11 shows a pass characteristic of a general low-pass filter having a frequency fc as a cutoff frequency by a broken line.
As described above, in the high frequency filter according to the third embodiment, among the six open stubs in the high frequency filter according to the second embodiment, the branch stub is applied instead of the two open stubs, thereby the second embodiment. This makes it possible to set attenuation poles at a plurality of relatively arbitrary frequencies as compared with the high-frequency filter according to (1).

Since the high frequency filter according to the present embodiment is composed of only distributed constant lines without applying a lumped constant element such as a chip capacitor, it has low loss and characteristics similar to the high frequency filter according to the second embodiment. There are advantages such as small variations, easy manufacture and low cost.
Further, in the high-frequency filter according to the present embodiment, since there is no conductor gap between the input end and the output end, the coupling between the resonators is the same as the high-frequency filter according to the first and second embodiments. There is an advantage that the power durability is superior to that of the conventional band-pass filter constituted by.
In addition, since the high-frequency filter according to the present embodiment does not use a resonator that resonates in the pass band, which is applied in the conventional band-pass filter, the high-frequency filter according to the first and second embodiments. Similarly, the loss in the pass band can be reduced as compared with the conventional band pass filter.

In the high frequency filter exemplified in the present embodiment, four open stubs adjusted to have an attenuation pole on the high frequency side of the passband are applied, and each characteristic impedance is made equal. If the same effect as that of the embodiment can be obtained, it is not necessary to limit the number of applied open stubs to four, and similarly, it is not necessary to equalize the characteristic impedance.
In addition, in the high frequency filter according to the present embodiment, two branch stubs adjusted to have attenuation poles on both sides of the high and low frequencies of the passband are applied, and the characteristic impedance of the trunk stub and the branch stub that are constituent elements of the branch stub However, if the same effect as this embodiment can be obtained, it is not necessary to limit the number of branch stubs to be applied to two. Similarly, the characteristic impedances of the trunk stub and the branch stub are equal. do not have to.
The high-frequency filter according to the present embodiment is configured by a combination of three parallel capacitance circuits, two series inductance circuits, two branch stubs, and four open stubs. However, the same effect as the present embodiment can be obtained. If possible, there is no need to limit each number to three, two, two, or four.

FIG. 3 is a configuration explanatory diagram illustrating a configuration of a high-frequency filter according to Embodiment 1. 3 is an equivalent circuit at a cutoff frequency fc of the high frequency filter according to the first embodiment. It is a characteristic view explaining the pass characteristic of the high frequency filter concerning this invention. FIG. 6 is a perspective view illustrating the structure of a high frequency filter in a second embodiment. 6 is a top view of a high-frequency filter according to Embodiment 2. FIG. FIG. 6 is an equivalent circuit diagram illustrating the high frequency filter according to the second embodiment using distributed constant lines. FIG. 10 is a perspective view illustrating the structure of a high frequency filter in a third embodiment. 6 is a top view of a high-frequency filter according to Embodiment 3. FIG. FIG. 6 is an equivalent circuit diagram illustrating a high frequency filter according to a third embodiment using distributed constant lines. It is a distributed constant track diagram of the branch stub which has one branch point in the middle of a track. FIG. 10 is a characteristic diagram for explaining pass characteristics of the high-frequency filter according to the third embodiment. It is a permeation | transmission figure explaining the structure of the conventional high frequency filter. It is a top view of the conventional high frequency filter. It is a characteristic view explaining the pass characteristic of the conventional high frequency filter.

Explanation of symbols

  1 ground conductor, 2 dielectric substrate, 3 strip conductor, 4 input / output line, 5 low impedance line, 6 high impedance line, 7 open stub, 8 input / output terminal, 9 parallel capacitance circuit, 10 series inductance circuit, 11 distributed constant Open stub by track, 12 trunk stub, 13 branch stub, 14 branch stub, 15 trunk stub by distributed parameter track, 16 branch stub by distributed parameter track, 17 branch stub by distributed parameter track, 18 parallel connection, 19 distributed parameter track Low impedance line by 20, high impedance line by 20 distributed constant lines.

Claims (5)

A plurality of parallel capacitance circuits each having an open stub and exhibiting a predetermined parallel capacitance at a predetermined cutoff frequency, and a plurality of series inductance circuits exhibiting a predetermined series inductance at the predetermined cutoff frequency, from the input end to the output end The odd number of the parallel capacitive circuit, the series inductance circuit, and the parallel capacitive circuit are alternately connected in cascade , and at least one of the open stubs included in the parallel capacitive circuit is lower than the predetermined cutoff frequency. An open stub having an electrical length of ¼ wavelength at a frequency of 1 and having a vicinity of the predetermined cutoff frequency as a pass band, and a plurality of determined based on the plurality of parallel capacitance circuits in a frequency region other than the vicinity of the cutoff frequency A high-frequency filter having a band-pass characteristic in which an attenuation pole is formed at a frequency. A high frequency filter formed in a microstrip line type by providing a ground conductor on one surface of a dielectric substrate and a strip conductor on the other surface for connecting to an external circuit provided at an input end and an output end An input / output line, a low-impedance line having a line width wider than the input / output line, and a stub circuit having an open stub connected in parallel to each end of the low-impedance line, and having a predetermined parallel capacitance at a predetermined cutoff frequency And a plurality of high-impedance lines exhibiting a predetermined inductance at the predetermined cutoff frequency and having a line width narrower than the input / output line, and the input / output line at the input end and the output The low impedance line, the high impedance from the input end to the output end between the input and output lines at the end Line, the odd number alternately in the order of low impedance line are connected in cascade, the electrical length at least one predetermined lower cut-off frequency a first frequency of the said open stub connected in parallel to the low impedance line is An open stub having a quarter wavelength, in which the vicinity of the predetermined cutoff frequency is a pass band, and attenuation poles are formed at a plurality of frequencies determined based on the plurality of parallel capacitance circuits in a frequency region other than the vicinity of the cutoff frequency. High-frequency filter with bandpass characteristics. First, second, and third parallel capacitance circuits that each exhibit a predetermined parallel capacitance at a predetermined cutoff frequency; and first and second series inductance circuits that each exhibit a predetermined series inductance at the predetermined cutoff frequency; The first parallel capacitance circuit, the first series inductance circuit, the second parallel capacitance circuit, the second series inductance circuit, and the third parallel capacitance circuit from the input end to the output end The first parallel capacitor circuit is connected to the first parallel capacitor circuit in the order of the first frequency lower than the predetermined cutoff frequency, the second frequency higher than the cutoff frequency, and the third frequency. An open stub having an electrical length of ¼ wavelength at a frequency of 1, an open stub having an electrical length of ¼ wavelength at the second frequency, and a first capacitor having one end grounded in parallel. And forming the second parallel capacitance circuit by parallel connection of two open stubs having an electrical length of ¼ wavelength at the third frequency and a second capacitance having one end grounded, An open stub having an electrical length of ¼ wavelength at the first frequency, an open stub having an electrical length of ¼ wavelength at the second frequency, and a third grounded at one end. Formed in parallel connection with a capacity, and a pass band in the vicinity of the predetermined cutoff frequency, and an odd multiple of the first frequency, the second frequency, and the third frequency in a frequency region other than the vicinity of the cutoff frequency A high-frequency filter having a band-pass characteristic in which an attenuation pole is formed at a frequency. A high frequency filter formed in a microstrip line type by providing a ground conductor on one surface of a dielectric substrate and a strip conductor on the other surface for connecting to an external circuit provided at an input end and an output end The input / output lines are connected in parallel to the first, second, and third low-impedance lines that are wider than the input / output lines, and to both ends of the first, second, and third low-impedance lines, respectively. A first, second and third parallel capacitance circuit comprising an open stub and exhibiting a predetermined parallel capacitance at a predetermined cutoff frequency; a predetermined series inductance at the predetermined cutoff frequency; and a line width from the input / output line First and second high-impedance lines having a narrow width between the input and output lines of the input end and the input and output lines of the output end. The first low-impedance line, the first high-impedance line, the second low-impedance line, the second high-impedance line, and the third low-impedance line are cascaded in this order, and from the predetermined cutoff frequency For the first frequency that is lower, the second frequency that is higher than the cut-off frequency, and the third frequency, the first parallel capacitance circuit is electrically long at the first low-impedance line and the first frequency. Is formed of an open stub having a quarter wavelength, and an open stub having an electrical length of ¼ wavelength at the second frequency, and the second parallel capacitance circuit is formed of the second low impedance line, the third The electrical length is formed by two open stubs having a quarter wavelength, and the third parallel capacitor circuit is electrically connected to the third low impedance line and the first frequency. / 4 wavelength open stub, and the second frequency is formed of an open stub having an electrical length of ¼ wavelength. A high-frequency filter having a band-pass characteristic in which an attenuation pole is formed at a frequency that is an odd multiple of the first frequency, the second frequency, and the third frequency. A high frequency filter formed in a microstrip line type by providing a ground conductor on one surface of a dielectric substrate and a strip conductor on the other surface for connecting to an external circuit provided at an input end and an output end An open stub connected in parallel to each of the input / output lines, the first, second and third low impedance lines each having a wider line width than the input / output lines and both ends of the first and third low impedance lines. A first stub, a second stub, a third stub, and an open stub connected in parallel to both ends of the second low-impedance line, and exhibiting a predetermined parallel capacitance at a predetermined cutoff frequency; First and second high impedance lines that exhibit a predetermined series inductance at the predetermined cutoff frequency and have a line width narrower than the input / output line; The first low-impedance line, the first high-impedance line, and the second high-impedance line from the input end toward the output end between the input / output line at the input end and the input / output line at the output end. A trunk stub in which a low-impedance line, the second high-impedance line, and the third low-impedance line are cascaded in this order, and the branch stub is connected in parallel to the end portions of the first and third low-impedance lines. And an input susceptance of the branch stub when viewed from a parallel connection portion of the stem stub to the first and third low impedance lines. The first cutoff frequency is lower than the cutoff frequency of the first cutoff frequency, and the second cutoff frequency is higher than the cutoff frequency. With respect to the fourth frequency and the fifth frequency higher than the number, the first parallel capacitance circuit has an electrical length of ¼ wavelength at the first low impedance line, the branch stub, and the fourth frequency. And the second parallel capacitance circuit is formed by the second low impedance line and two open stubs having an electrical length of ¼ wavelength at the fifth frequency, 3 parallel capacitance circuits are formed by the third low impedance line, the branch stub, and an open stub having an electrical length of ¼ wavelength at the fourth frequency, and the vicinity of the predetermined cutoff frequency is defined as a pass band. A high-frequency filter having bandpass characteristics in which attenuation poles are formed at frequencies that are odd multiples of the first, second, and third frequencies and the fourth and fifth frequencies in a frequency region other than the vicinity of the cutoff frequency.

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