JP2022120429A - Side coupling type bandpass filter circuit - Google Patents

Abstract

To facilitate designing and manufacturing of a side coupling type bandpass filter circuit consisting of microstrip lines.SOLUTION: An input side resonance line 13 has a line length of almost a half wavelength, and its first end part is coupled with an end part of an input line 11 and its second end part is opened. An output side resonance line 14 has a line length of almost a half wavelength, and its first end part is opened and its second end part is coupled with an end part of an output line 12. The input side resonance line 13 has: a low-impedance line 31 with a characteristic impedance smaller than that of the input line 11, extending from the first end part over a predetermined line length; and a side coupling line 33 extending from the second end part over a predetermined line length. The output side resonance line 14 has: a side coupling line 42 extending from the first end part over a predetermined line length; and a low-impedance line 41 with a characteristic impedance smaller than that of the output line 12, extending from the second end part over a predetermined line length.SELECTED DRAWING: Figure 1

Description

The present invention relates to a side-coupling band-pass filter circuit composed of microstrip lines.

FIG. 13 of Patent Document 1 discloses a side-coupling bandpass filter. This filter has a structure in which lines with a length of about 1/2 wavelength of the passband are arranged between the input and output portions at both ends on the surface of the substrate so that about 1/4 wavelength of the passband is coupled. A ground plane is formed on the back surface of the substrate. "Side coupling" is a state in which two lines are arranged parallel to each other with a gap in the width direction, and are magnetically coupled.

JP-A-2004-320522

The size of a circuit composed of microstrip lines is proportional to the wavelength of the signal. For example, in a circuit that handles high-frequency signals of 30 GHz (gigahertz) or higher, the length of one wavelength is approximately 5 mm (millimeters) or less.
If the center frequency of the passband of the side-coupling bandpass filter is set to 35 GHz, for example, and its characteristics are to be optimized, the gap between the lines arranged in parallel must be set to 25 μm (micrometers), for example. In some cases, the width of the line must be set to 50 μm, for example. In particular, when the passband width is wide and the external Q of the input is small, the line width becomes narrow and the line spacing becomes narrow. However, it is difficult to form a 25 μm wide gap or a 50 μm wide line when manufacturing such a circuit by etching technology for printed wiring boards. Alternatively, even if it can be formed, the effect of manufacturing errors is large, and it is difficult to realize the characteristics as designed.
An object of the present invention is, for example, to solve such problems.

A side-coupling band-pass filter circuit configured by a microstrip line has a line length of approximately a half wavelength, a first end connected to an end of an input line, and a second end open. and an output-side resonant line having a line length of approximately half the wavelength, a first end being open and a second end being connected to an end of the output line. The input-side resonance line has a characteristic impedance lower than that of the input line, and includes a low-impedance line extending from the first end over a predetermined line length, and a low-impedance line extending from the second end over a predetermined line length. and a side coupling line. The output-side resonance line includes a side coupling line extending from the first end over a predetermined line length, and a characteristic impedance lower than that of the output line and extending from the second end over a predetermined line length. and a low impedance line.
The side coupling lines of the input side resonant line and the output side resonant line may be spaced apart in the width direction and arranged parallel to each other for magnetic coupling.
It may further include one or more intermediate resonant lines having a line length of approximately one-half wavelength and having first and second open ends. Each of the intermediate resonance lines has an input-side coupling line extending over a predetermined line length from the first end and an output-side coupling line extending over a predetermined line length from the second end. You may The side coupling line of the input side resonant lines and one of the input side coupling lines of the intermediate resonant lines may be spaced apart in the width direction and arranged parallel to each other for magnetic coupling. The output side coupling line of one of the intermediate resonance lines and the side coupling line of the output side resonance line may be spaced apart in the width direction and arranged parallel to each other for magnetic coupling.
Each of the side coupling lines of the input side resonant line and the output side resonant line may have a line length of approximately a quarter wavelength.
Each of the low-impedance lines of the input-side resonant line and the output-side resonant line may have a line length of approximately a quarter wavelength.

According to the side-coupling bandpass filter circuit, the input line and the input-side resonance line are connected instead of side-coupling, so there is no need to provide a gap between the input line and the input-side resonance line. . Similarly, since the output-side resonance line and the output line are not side-coupled but connected, there is no need to provide a gap between the output-side resonance line and the output line. Therefore, even if the characteristics are optimized, the gap between lines does not need to be less than 100 μm, and can be easily manufactured. Moreover, since the gap between the lines is large, the influence of manufacturing errors is small, and the characteristics as designed can be easily realized.

The figure which shows an example of the circuit pattern of a side coupling type band-pass filter circuit. The figure which shows an example of the circuit pattern of a side coupling type band-pass filter circuit. The figure which shows an example of the circuit pattern of a side coupling type band-pass filter circuit. The figure which shows an example of the circuit pattern of a side coupling type band-pass filter circuit. The figure which shows an example of the frequency characteristic of a side coupling type bandpass filter circuit. The figure which shows an example of the circuit pattern of a side coupling type band-pass filter circuit. The figure which shows an example of the frequency characteristic of a side coupling type bandpass filter circuit. The figure which shows an example of the circuit pattern of a side coupling type band-pass filter circuit. The figure which shows an example of the circuit pattern of a side coupling type band-pass filter circuit. The figure which shows an example of the circuit pattern of a side coupling type band-pass filter circuit. The figure which shows an example of the frequency characteristic of a side coupling type bandpass filter circuit. The figure which shows an example of the circuit pattern of a side coupling type band-pass filter circuit. The figure which shows an example of the frequency characteristic of a side coupling type bandpass filter circuit. The figure which shows an example of the frequency characteristic of a side coupling type bandpass filter circuit. The figure which shows an example of the circuit pattern of a side coupling type band-pass filter circuit. The figure which shows an example of the circuit pattern of a side coupling type band-pass filter circuit.

The side-coupling bandpass filter circuit 10A shown in FIG. 1 is composed of a conductor pattern (that is, a microstrip line) formed by etching or the like on one surface of a dielectric substrate, for example. On the other surface of the dielectric substrate, for example, a conductor layer is formed over the entire surface.

The side-coupled bandpass filter circuit 10A includes, for example, an input line 11, an output line 12, an input-side resonant line 13, and an output-side resonant line .
The input line 11 is a microstrip line with a characteristic impedance of, for example, 50Ω (ohms), and one end (not shown) is connected to, for example, a signal source circuit (not shown).
The output line 12 is a microstrip line with a characteristic impedance of 50Ω, for example, and one end (not shown) is connected to a load circuit (not shown), for example.

The resonance line 13 on the input side has a length (line length) of about half (for example, about 1/2) of the wavelength λ (for example, about 3.7 mm) of the center frequency f (for example, 38 GHz) of the passband of the band-pass filter circuit. .8 mm) microstrip line. Thereby, it is configured to resonate at a frequency close to the center frequency f.
The resonant line 13 has an input (first) end connected to the other end of the input line 11 and an output (second) end open (that is, connected to nothing). not).

The resonance line 13 has a low impedance line 31 and a side coupling line 33 .
The low-impedance line 31 extends from the input side end of the resonant line 13 to the intermediate portion over a length (line length) of about a quarter of the wavelength λ (for example, about 0.9 mm). The low-impedance line 31 is wider than the input line 11 (eg, three times the width of the input line 11) and has a low characteristic impedance (eg, 10Ω).
The side coupling line 33 extends from the output end of the resonant line 13 to the intermediate portion over a length (line length) of about one quarter of the wavelength λ (for example, about 0.9 mm). The side coupling line 33 is narrower than the low-impedance line 31 (for example, half the width of the input line 11) and has a high characteristic impedance (for example, 100Ω).

Reflection occurs at the boundary between the low-impedance line 31 and the side coupling line 33, and since the line length of the side coupling line 33 is about a quarter of the wavelength λ, the side coupling line 33 has a frequency close to the center frequency f. resonate with Compared to the case where the side-coupled line 33 is directly connected to the input line 11, the difference in the characteristic impedance is large and the reflection is large, resulting in a high Q value. Furthermore, part of the signal transmitted from the side-coupled line 33 to the low-impedance line 31 is reflected at the boundary between the input line 11 and the low-impedance line 31, and the line length of the resonant line 13 is about half the wavelength f. Therefore, the resonance line 13 as a whole resonates at a frequency close to the center frequency f. Thereby, a high Q value can be realized.

The resonant line 14 on the output side is a microstrip line whose length (line length) is about half the wavelength λ (for example, about 1.8 mm). Thereby, it is configured to resonate at a frequency close to the center frequency f.
The resonant line 14 has an output (second) end connected to the other end of the output line 12 and an input (first) end that is open (that is, connected to nowhere). not).

The resonance line 14 has a low impedance line 41 and a side coupling line 42 .
The low-impedance line 41 extends from the output end of the resonant line 14 to the intermediate portion over a length (line length) of about a quarter of the wavelength λ (for example, about 0.9 mm). The low impedance line 41 is wider than the output line 12 (eg, three times the width of the output line 12) and has a low characteristic impedance (eg, 10Ω).
The side coupling line 42 extends from the input side end of the resonant line 14 to the intermediate portion over a length (line length) of about a quarter of the wavelength λ (for example, about 0.9 mm). The side coupling line 42 is narrower than the low-impedance line 41 (for example, half the width of the output line 12) and has a high characteristic impedance (for example, 100Ω).

Reflection occurs at the boundary between the side coupling line 42 and the low impedance line 41, and since the line length of the side coupling line 42 is about a quarter of the wavelength λ, the frequency of the side coupling line 42 is close to the center frequency f. resonate with Compared to the case where the side coupling line 42 is directly connected to the output line 12, the Q value is increased because the difference in characteristic impedance is large and the reflection is large. Furthermore, part of the signal transmitted from the side-coupled line 42 to the low-impedance line 41 is reflected at the boundary between the low-impedance line 41 and the output line 12, and the line length of the resonant line 14 is about half the wavelength f. 1, the entire resonant line 14 resonates at a frequency close to the center frequency f. Thereby, a high Q value can be realized.

The side coupling line 33 of the resonant line 13 and the side coupling line 42 of the resonant line 14 are arranged parallel to each other with a space in the width direction. As a result, the side coupling line 33 and the side coupling line 42 are magnetically coupled to transmit energy.

By configuring as described above, it is possible to obtain circuit characteristics similar to those of the conventional two-stage coupling type bandpass filter circuit. Unlike the conventional side-coupled bandpass filter circuit, current transfers energy from the input line 11 to the input side resonant line 13, so there is no gap between the input line 11 and the input side resonant line 13. Similarly, since energy is transferred from the output-side resonant line 14 to the output line 12 by current, there is no gap between the output-side resonant line 14 and the output line 12 either. Therefore, it is not necessary to provide a gap with a width of less than 100 μm, and it can be manufactured easily. Moreover, since there is no need to provide a portion for coupling to the line through such a gap, there is no need to provide a line with a width of less than 100 μm, and the manufacturing can be facilitated.
In addition, since the designed line width and gap width are 100 μm or more, the influence of manufacturing errors is reduced, and a circuit having characteristics as designed can be easily manufactured.

In the side-coupling bandpass filter circuit 10Z shown in FIG. 2, the input line 11 and the resonant line 13 are connected, like the side-coupling bandpass filter circuit 10A. However, the difference is that the position where the end of the input line 11 is connected to the resonant line 13 is not the end of the resonant line 13 but the intermediate portion. Similarly, the output line 12 and the resonance line 14 are also connected, but the position where the end of the output line 12 is connected to the resonance line 14 is not the end of the resonance line 14 but the middle portion.
Even with such a configuration, circuit characteristics similar to those of the conventional two-stage side coupling type bandpass filter circuit can be obtained without providing a gap with a width of less than 100 μm. However, it is difficult to determine the position at which the input line 11 is connected to the resonant line 13 and the position at which the output line 12 is connected to the resonant line 14, and it is difficult to achieve the characteristics as designed.

On the other hand, in the configuration of the side-coupling bandpass filter circuit 10A, there is no need to adjust the position where the input line 11 is connected to the resonant line 13 and the position where the output line 12 is connected to the resonant line 14. , the characteristics as designed can be easily realized.

A side-coupling bandpass filter circuit 10B shown in FIG. 3 is obtained by adding an intermediate resonance line 15 to the above-described side-coupling bandpass filter circuit 10A. The input-side resonant line 13 and the output-side resonant line 14 are not directly coupled, but are coupled through the resonant line 15 .
The intermediate resonant line 15 is a microstrip line whose length (line length) is about half the wavelength λ (for example, about 1.8 mm). Thereby, it is configured to resonate at a frequency close to the center frequency f.
The resonant line 15 is an isolated line, and both the input (first) and output (second) ends are open.

The resonant line 15 has two side coupling lines 52 and 53 .
The side coupling line 52 extends from the input side end of the resonant line 15 to the intermediate portion over a length (line length) of about a quarter of the wavelength λ (for example, about 0.9 mm).
The side coupling line 53 extends from the output end of the resonant line 15 to the intermediate portion over a length (line length) of about a quarter of the wavelength λ (for example, about 0.9 mm).

The side coupling line 33 of the resonant line 13 and the side coupling line 52 of the resonant line 15 are arranged parallel to each other with a space in the width direction. As a result, the side coupling line 33 and the side coupling line 52 are magnetically coupled to transmit energy.
Further, the side coupling line 53 of the resonant line 15 and the side coupling line 42 of the resonant line 14 are arranged parallel to each other while being spaced apart in the width direction. As a result, the side coupling line 53 and the side coupling line 42 are magnetically coupled to transmit energy.

By configuring as described above, it is possible to obtain circuit characteristics similar to those of a conventional three-stage side coupling type bandpass filter circuit, and it is possible to easily manufacture without providing gaps or lines with a width of less than 100 μm. be able to.

A side-coupling bandpass filter circuit 10C shown in FIG. 4 is obtained by adding an intermediate resonance line 16 to the above-described side-coupling bandpass filter circuit 10B. The intermediate resonant line 15 and the output side resonant line 14 are not directly coupled, but are coupled through the resonant line 16 .
The intermediate resonant line 16 is a microstrip line whose length (line length) is about half the wavelength λ (for example, about 1.8 mm). Thereby, it is configured to resonate at a frequency close to the center frequency f.
The resonant line 16 is an isolated line with both the input (first) and output (second) ends open.

The resonant line 16 has two side-coupled lines 62 and 63 .
The side coupling line 62 extends from the input side end of the resonant line 16 to the intermediate portion over a length (line length) of about a quarter of the wavelength λ (for example, about 0.9 mm).
The side coupling line 63 extends from the output end of the resonant line 16 to the intermediate portion over a length (line length) of about a quarter of the wavelength λ (for example, about 0.9 mm).

The side coupling line 53 of the resonant line 15 and the side coupling line 62 of the resonant line 16 are arranged parallel to each other with a space in the width direction. As a result, the side coupling line 53 and the side coupling line 62 are magnetically coupled to transmit energy.
Further, the side coupling line 63 of the resonant line 16 and the side coupling line 42 of the resonant line 14 are arranged parallel to each other while being spaced apart in the width direction. As a result, the side coupling line 63 and the side coupling line 42 are magnetically coupled to transmit energy.

By configuring as described above, it is possible to obtain circuit characteristics similar to those of the conventional four-stage side coupling type bandpass filter circuit, and it is not necessary to provide a gap or a line with a width of less than 100 μm, making it easy to manufacture. be able to.

FIG. 5 shows a comparison of frequency characteristics of the side-coupling bandpass filter circuits 10A-10C.
Comparing the transmission characteristics 92A to 92C, in the pass band 93, they are all approximately 0 dB, but the attenuation slopes in the transition band are different. That is, the side-coupling bandpass filter circuit 10A has a smaller attenuation slope than the side-coupling bandpass filter circuit 10B. Attenuation gradient is large. That is, the greater the number of stages, the greater the attenuation gradient.
Comparing the reflection characteristics 91A to 91C, the side-coupling bandpass filter circuit 10A has greater attenuation in the transition band than the side-coupling bandpass filter circuit 10B. attenuation in the band is smaller than that of the side-coupling bandpass filter circuit 10B.
As described above, the side-coupling bandpass filter circuits 10A to 10C have different attenuation in the transition region depending on the number of stages, similarly to the conventional side-coupling bandpass filter circuit. can decide.

Similarly, it is possible to construct a side-coupled bandpass filter circuit with a larger number of stages.

Since the side-coupling bandpass filter circuit 10D shown in FIG. 6 is similar to the side-coupling bandpass filter circuit 10B described above, only different parts will be described.
The side coupling line 33 of the resonant line 13 is connected at the input side end to the low impedance line 31, extends rightward from there, immediately bends about 90 degrees and extends downward to the output side. up to the end of
Similarly, the side coupling line 42 of the resonant line 14 is connected at its output side end to the low impedance line 41, extends leftward therefrom, immediately bends about 90 degrees, and extends downward. , to the input end.

The resonance line 15 has a side coupling line 54 and a land 56 . The side coupling line 54 is a microstrip line having a length (line length) of approximately one quarter of the wavelength f (for example, approximately 0.9 mm). The land 56 is provided around a via 55 penetrating the dielectric substrate and is connected through the via 55 to a conductive layer provided on the back side of the dielectric substrate. The diameter of the land 56 is, for example, 1.5 times or more the diameter of the via 55 . The side coupling line 54 has a lower (second) end connected to the land 56 (that is, short-circuited) and an upper (first) end that is open (that is, not connected to anything). . Thereby, it is configured to resonate at a frequency close to the center frequency f.

The side coupling line 33 of the resonant line 13 and the side coupling line 54 of the resonant line 15 are arranged parallel to each other with a space in the width direction. As a result, the side coupling line 33 and the side coupling line 54 are magnetically coupled to transmit energy.
Further, the side coupling line 54 of the resonant line 15 and the side coupling line 42 of the resonant line 14 are arranged in parallel with each other while being spaced apart in the width direction. As a result, the side coupling line 54 and the side coupling line 42 are magnetically coupled to transmit energy.
As a result, the side-coupling bandpass filter circuit 10D operates as a three-stage side-coupling bandpass filter circuit.

FIG. 7 shows a comparison of frequency characteristics between the side coupling bandpass filter circuit 10B and the side coupling bandpass filter circuit 10D.
The resonant line 15 of the side-coupling bandpass filter circuit 10B resonates not only at frequencies close to the center frequency f, but also at frequencies close to integral multiples of the center frequency f. Therefore, as shown by the reflection characteristic 91B and the transmission characteristic 92B, the side-coupling bandpass filter circuit 10B not only has a pass band 93 of frequencies close to the center frequency f, but also has a frequency of, for example, twice the center frequency f. , there is also a band 93B through which the signal passes.
On the other hand, the resonance line 15 of the side coupling bandpass filter circuit 10D does not resonate at frequencies close to even multiples of the center frequency f, and resonates only at frequencies close to odd multiples of the center frequency f. do. Therefore, as shown by the reflection characteristic 91D and the transmission characteristic 92D, there is no signal-passing band at frequencies close to twice the center frequency f. Therefore, it is possible to improve the characteristics in the stopband higher than the designed passband.

Since the side-coupling bandpass filter circuit 10E shown in FIG. 8 is similar to the side-coupling bandpass filter circuit 10C described above, only different parts will be described.
The side-coupled line 33 of the resonant line 13 is similar to the side-coupled bandpass filter circuit 10B in that the input-side end is connected to the output-side end of the low-impedance line 31, but the low-impedance line 31, it is connected particularly to the lower end.
Similarly, the output-side end of the side-coupling line 42 of the resonant line 14 is connected to the upper end of the input-side ends of the low-impedance line 41 .
This makes it possible to secure a space necessary for providing lands 56 and 66, which will be described later.

The resonance line 15 has a side coupling line 54 and a land 56, like the side coupling bandpass filter circuit 10D. The side coupling line 54 is a microstrip line having a length (line length) of approximately one quarter of the wavelength f (for example, approximately 0.9 mm). The land 56 is provided around a via 55 penetrating the dielectric substrate and is connected through the via 55 to a conductive layer provided on the back side of the dielectric substrate. The diameter of the land 56 is, for example, 1.5 times or more the diameter of the via 55 . The side-coupled line 54 has an output side (second) end connected to the land 56 (that is, short-circuited) and an input side (first) end that is open (that is, connected to nothing). do not have). Thereby, it is configured to resonate at a frequency close to the center frequency f.

Similarly, resonant line 16 has side coupling lines 64 and lands 66 . The side coupling line 64 is a microstrip line having a length (line length) of approximately one quarter of the wavelength f (for example, approximately 0.9 mm). The land 66 is provided around a via 65 penetrating the dielectric substrate and is connected through the via 65 to a conductive layer provided on the back side of the dielectric substrate. The diameter of the land 66 is, for example, 1.5 times or more the diameter of the via 65 . The side-coupled line 64 has an input side (second) end connected to the land 66 (that is, shorted) and an output side (first) end that is open (that is, connected to nothing). do not have). Thereby, it is configured to resonate at a frequency close to the center frequency f.

The side coupling line 33 of the resonant line 13 and the side coupling line 54 of the resonant line 15 are arranged parallel to each other with a space in the width direction. As a result, the side coupling line 33 and the side coupling line 54 are magnetically coupled to transmit energy.
Further, the side coupling line 54 of the resonant line 15 and the side coupling line 64 of the resonant line 16 are arranged parallel to each other while being spaced apart in the width direction. As a result, the side coupling line 54 and the side coupling line 64 are magnetically coupled to transmit energy.
The side coupling line 64 of the resonant line 16 and the side coupling line 42 of the resonant line 14 are arranged parallel to each other with a space in the width direction. As a result, the side coupling line 64 and the side coupling line 42 are magnetically coupled to transmit energy.
As a result, the side-coupling bandpass filter circuit 10E operates as a four-stage side-coupling bandpass filter circuit.

With such a configuration, even in the four-stage coupled bandpass filter circuit, it is possible to improve the characteristics in the stopband on the high frequency side.

Similarly, it is also possible to configure a side-coupling bandpass filter circuit with a larger number of stages.
It should be noted that the intermediate resonant lines may not all be 1/4 wavelength shorted resonant lines, but may be mixed with 1/2 wavelength open resonant lines.

Since the side-coupling bandpass filter circuit 10F shown in FIG. 9 is similar to the side-coupling bandpass filter circuit 10E described above, only different parts will be described.
The resonant line 13 on the input side is isolated and not connected to the input line 11 . The resonant line 13 does not have a low impedance line 31 and instead has a side coupling line 32 . The side coupling line 32 extends from the input end of the resonant line 13 to the intermediate portion over a length (line length) of about a quarter of the wavelength λ (for example, about 0.9 mm).
Similarly, the resonant line 14 on the output side is isolated and not connected to the output line 12 . Resonant line 14 does not have low impedance line 41 and instead has side coupling line 43 . The side coupling line 43 extends from the output end of the resonant line 14 to the intermediate portion over a length (line length) of about a quarter of the wavelength λ (for example, about 0.9 mm).

The side-coupling bandpass filter circuit 10F further includes side-coupling lines 18 and 19 .
The side-coupling line 18 is a microstrip line having a length (line length) of about a quarter of the wavelength λ (for example, about 0.9 mm).
The side-coupled line 18 has an input side (first) end connected to the other end of the input line 11 and an output side (second) end open (that is, connected to nowhere). It has not been).
The side-coupling line 19 is a microstrip line having a length (line length) of about a quarter of the wavelength λ (for example, about 0.9 mm).
The side-coupled line 19 has an output side (second) end connected to the other end of the output line 12 and an input side (first) end that is open (that is, connected to nowhere). It has not been).

The side coupling line 18 connected to the input line 11 and the side coupling line 32 of the resonance line 13 are spaced apart in the width direction and arranged parallel to each other. As a result, the side coupling line 18 and the side coupling line 32 are magnetically coupled to transmit energy.
The side coupling line 43 of the resonance line 14 and the side coupling line 19 connected to the output line 12 are arranged parallel to each other with a space in the width direction. As a result, the side coupling line 43 and the side coupling line 19 are magnetically coupled to transmit energy.

With this configuration, the advantage of providing the low-impedance line is lost, but the characteristics on the high frequency side of the passband can be improved.

Since the side coupling bandpass filter circuit 10G shown in FIG. 10 is similar to the side coupling bandpass filter circuit 10B described above, only different parts will be described.
The side-coupling bandpass filter circuit 10B has an input on the left side and an output on the right side in the figure, but the side-coupling bandpass filter circuit 10G is folded in the middle and both the input and the output are on the left side. be.
The intermediate resonant line 15 is substantially U-shaped. It extends rightward from the input side end, bends about 90 degrees in the middle of the side coupling line 52 and extends downward, and further bends about 90 degrees in the middle of the side coupling line 53 to the left. to the end on the output side.
A portion of the side coupling line 52 extending in the left-right direction is arranged parallel to and spaced from the output side portion of the side coupling line 33 in the width direction.
A portion of the side coupling line 53 extending in the left-right direction is arranged parallel to and spaced apart in the width direction from the input side portion of the side coupling line 42 .

Furthermore, the low-impedance line 31 of the resonant line 13 and the low-impedance line 41 of the resonant line 14 are arranged parallel to each other with a space in the width direction. Here, the input side end of the low impedance line 31 and the output side end of the low impedance line 41 are arranged on the same side. As a result, the low-impedance line 31 and the low-impedance line 41 are magnetically coupled in opposite directions to transmit energy.

In the frequency characteristics of the side-coupling bandpass filter circuit 10G shown in FIG. 11, the reflection characteristics 91G are approximately 0 dB except for the passband 93. Conversely, the transmission characteristic 92G is approximately 0 dB in the passband 93 and has a pole 94 in the stopband lower than the passband 93 . This pole 94 is generated by magnetic field coupling of the low-impedance line 31 and the low-impedance line 41 in opposite directions. As a result, the steepness is increased, so that a high steepness can be achieved with a small number of steps.

The length (line length) of the coupling portion between the resonant lines is preferably about one-eighth or less of the wavelength λ. As a result, it is possible to prevent the occurrence of poles other than the pole caused by the coupling between the low-impedance line 31 and the low-impedance line 41 .

The side-coupling bandpass filter circuit 10H shown in FIG. 12 is similar to the side-coupling bandpass filter circuit 10G described above, so only different parts will be described.
The resonant line 15 is substantially L-shaped instead of substantially U-shaped.
The side-coupled bandpass filter circuit 10H further has an intermediate resonant line 16. As shown in FIG. The intermediate resonant line 16 is a microstrip line whose length (line length) is about half the wavelength λ (for example, about 1.8 mm), like the side-coupled bandpass filter circuit 10C. The resonant line 16 is an isolated line with both the input (first) and output (second) ends open. The resonant line 16 has two side-coupled lines 62 and 63 .
The side coupling line 62 extends from the input side end of the resonant line 16 to the intermediate portion over a length (line length) of about a quarter of the wavelength λ (for example, about 0.9 mm).
The side coupling line 63 extends from the output end of the resonant line 16 to the intermediate portion over a length (line length) of about a quarter of the wavelength λ (for example, about 0.9 mm).

The side coupling line 53 of the resonant line 15 and the side coupling line 62 of the resonant line 16 are arranged parallel to each other with a space in the width direction. As a result, the side coupling line 53 and the side coupling line 62 are magnetically coupled to transmit energy.
Further, the side coupling line 63 of the resonant line 16 and the side coupling line 42 of the resonant line 14 are arranged parallel to each other while being spaced apart in the width direction. As a result, the side coupling line 63 and the side coupling line 42 are magnetically coupled to transmit energy.

In the frequency characteristics of the side-coupling bandpass filter circuit 10H shown in FIG. 13, the reflection characteristics 91H are approximately 0 dB except for the passband 93. FIG. Conversely, the transmission characteristic 92H is approximately 0 dB in the passband 93, has a pole 94 in the stopband lower than the passband 93, and is also in the stopband higher than the passband 93. A pole 95 is present. As a result, the steepness is increased, so that a high steepness can be achieved with a small number of steps.

Similarly, it is possible to construct a side-coupled bandpass filter circuit with more stages. As the number of stages is increased, the number of poles is correspondingly increased. As a result, a high steepness can be achieved with a small number of steps.
FIG. 14 shows, as an example, frequency characteristics of a five-stage coupled bandpass filter circuit. It shows an enlarged view near the passband 93 .
As shown in transmission characteristic 92P, the presence of poles 94, 95 just below and just above passband 93 results in a very large attenuation slope. It can be seen that the steepness is higher when compared with the transmission characteristic 92Q in the case of the same five-stage configuration but the low-impedance lines 31 and 32 are not coupled.

Since the side coupling bandpass filter circuit 10J shown in FIG. 15 is similar to the side coupling bandpass filter circuit 10G described above, only different parts will be described.
The intermediate resonant line 15 is not a 1/2 wavelength isolated line, but a 1/4 wavelength short line like the side-coupled bandpass filter circuit 10D. The resonance line 15 has a side coupling line 54 and a land 56 . The side coupling line 54 is a microstrip line having a length (line length) of approximately one quarter of the wavelength f (for example, approximately 0.9 mm). The land 56 is provided around a via 55 penetrating the dielectric substrate and is connected through the via 55 to a conductive layer provided on the back side of the dielectric substrate. The diameter of the land 56 is, for example, 1.5 times or more the diameter of the via 55 . The side-coupled line 54 has a right (second) end connected to the land 56 (that is, shorted) and a left (first) end that is open (that is, not connected to anything).

In this way, even if the intermediate resonant line is a short line with a quarter wavelength instead of an isolated line with a half wavelength, a high steepness can be achieved with a small number of stages. Further, by using a short line of 1/4 wavelength instead of an isolated line of 1/2 wavelength, it is possible to improve the characteristics in the stopband higher than the designed passband.

Since the side coupling bandpass filter circuit 10K shown in FIG. 16 is similar to the side coupling bandpass filter circuit 10G described above, only different parts will be described.
The resonance line 13 on the input side is isolated and not connected to the input line 11, like the side-coupled bandpass filter circuit 10F. The resonant line 13 does not have a low impedance line 31 and instead has a side coupling line 32 . The side coupling line 32 extends from the input end of the resonant line 13 to the intermediate portion over a length (line length) of about a quarter of the wavelength λ (for example, about 0.9 mm). Similarly, the resonant line 14 on the output side is isolated and not connected to the output line 12 . Resonant line 14 does not have low impedance line 41 and instead has side coupling line 43 . The side coupling line 43 extends from the output end of the resonant line 14 to the intermediate portion over a length (line length) of about a quarter of the wavelength λ (for example, about 0.9 mm).

The side-coupling bandpass filter circuit 10K further includes side-coupling lines 18 and 19, like the side-coupling bandpass filter circuit 10F. The side-coupling line 18 is a microstrip line having a length (line length) of about a quarter of the wavelength λ (for example, about 0.9 mm). The side-coupled line 18 has an input side (first) end connected to the other end of the input line 11 and an output side (second) end open (that is, connected to nowhere). It has not been). The side-coupling line 19 is a microstrip line having a length (line length) of about a quarter of the wavelength λ (for example, about 0.9 mm). The side-coupled line 19 has an output side (second) end connected to the other end of the output line 12 and an input side (first) end that is open (that is, connected to nowhere). It has not been).

The side-coupling line 18 connected to the input line 11 and the side-coupling line 19 connected to the output line 12 are spaced apart in the width direction and arranged parallel to each other. Here, the input side end of the side coupling line 18 and the output side end of the side coupling line 19 are arranged on the same side. As a result, the side coupling line 18 and the side coupling line 19 are magnetically coupled in opposite directions to transmit energy. As a result, a high steepness can be achieved with a small number of stages.

The embodiment described above is an example for facilitating understanding of the present invention. The present invention is not limited thereto and includes various modifications, changes, additions or omissions without departing from the scope defined by the appended claims. This can be easily understood by those skilled in the art from the above description.

10A to 10J, 10Z side-coupled bandpass filter circuit, 11 input line, 12 output line, 13 to 16 resonance line, 31, 41 low impedance line, 18, 19, 32, 33, 42, 43, 52 to 54, 62 to 64 side coupling lines, 55, 65 vias, 56, 66 lands, 91A to 91H reflection characteristics, 92A to 92Q transmission characteristics, 93 passband, 93B band, 94, 95 poles.

Claims (5)

In a side-coupled bandpass filter circuit composed of microstrip lines,
an input-side resonant line having a line length of approximately one-half wavelength, a first end connected to an end of the input line, and an open second end;
an output-side resonant line having a line length of approximately one-half wavelength, a first end being open, and a second end being connected to an end of the output line;
The input-side resonant line is
a low-impedance line having a characteristic impedance lower than that of the input line and extending over a predetermined line length from the first end;
a side coupling line extending over a predetermined line length from the second end,
The output-side resonant line is
a side coupling line extending over a predetermined line length from the first end;
a low-impedance line having a characteristic impedance lower than that of the output line and extending from the second end over a predetermined line length;
Side-coupled bandpass filter circuit. The side coupling lines of the input side resonant line and the output side resonant line are spaced apart in the width direction and arranged parallel to each other for magnetic field coupling.
2. The side-coupled bandpass filter circuit of claim 1. further comprising one or more intermediate resonant lines having a line length of approximately half the wavelength and having first and second open ends;
Each of the intermediate resonant lines is
having an input-side coupling line extending over a predetermined line length from the first end,
having an output-side coupling line extending over a predetermined line length from the second end,
the side coupling line of the input side resonant lines and the input side coupling line of one of the intermediate resonant lines are spaced apart in the width direction and arranged parallel to each other for magnetic coupling,
the output-side coupling line of one of the intermediate resonant lines and the side-coupling line of the output-side resonant line are spaced apart in the width direction and arranged parallel to each other and are magnetically coupled,
2. The side-coupled bandpass filter circuit of claim 1. The side coupling lines of the input side resonant line and the output side resonant line each have a line length of approximately a quarter wavelength,
4. A side-coupling bandpass filter circuit according to any one of claims 1 to 3. The low impedance lines of the input side resonant line and the output side resonant line each have a line length of approximately a quarter wavelength,
4. A side-coupling bandpass filter circuit according to any one of claims 1 to 3.

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