JP4284245B2 - Distributed constant filter - Google Patents

Description

  The present invention relates to a distributed constant filter having excellent frequency selectivity that operates in a microwave or millimeter wave band, which requires the use of a distributed constant circuit.

  In a conventional low-pass filter, a circuit in which a low-impedance line and a high-impedance line are cascade-connected on the same straight line has been used (for example, see Non-Patent Document 1). A high-impedance microstrip line and a low-impedance microstrip line are cascade-connected to each other on a dielectric substrate.

  Here, the low-impedance line can also be regarded as an open-ended stub. The high impedance line functions as a series inductance element and the low impedance line functions as a capacitance element between the signal line and the ground plane, and both exhibit low-pass characteristics. However, the amount of passage becomes a relatively gradual monotonous decrease at a frequency equal to or higher than the cutoff frequency, and it is generally difficult to obtain a steep attenuation characteristic.

  As means for solving the above problem, there is known a method of forming a so-called attenuation pole in which the filter passage amount is theoretically 0 in the attenuation region (see, for example, Non-Patent Document 2). The polarized low-pass filter forming such an attenuation pole has a folded symmetrical structure composed of a capacitance element and a J inverter circuit, and the J inverter circuit is used for coupling the capacitance elements. It has been.

  Since the passing phase of the J inverter changes to ± 90 ° in accordance with the sign of the J value, two signal propagation paths having opposite passing phases are formed. Therefore, the frequency characteristic of such a polarized low-pass filter can form an attenuation pole with a passing amount of zero. Here, the negative coupling by the J inverter is generally referred to as non-adjacent coupling. For this non-adjacent coupling, when the coupling amount of the negative coupling is sufficiently small, a capacitive coupling line can be used, which makes it possible to configure a polar low-pass filter with a distributed constant circuit. .

G. Matthaei et al., “Microwave Filters, Impedance-Matching Networks, and Coupling Structures”, Arttech House, 1980. I. By Hunter, “Theory and Design of Microwave Filters”, IEEE, 2001.

  However, the prior art has the following problems. In order to make the attenuation characteristic of the low-pass filter steep, it is necessary to provide an attenuation pole and make the attenuation pole as close as possible to the cutoff frequency of the filter. However, the amount of coupling required for non-adjacent coupling having a negative polarity is increased accordingly, which increases the coupling capacitance value required for capacitive coupling between lines, which makes it difficult to realize this. It was.

  The present invention has been made to solve the above-described problems. A distributed constant filter that easily realizes a non-adjacent coupling having a large coupling amount required when the attenuation pole is close to the cutoff frequency of the filter. The purpose is to obtain.

The distributed constant filter according to the present invention is configured so that each of both ends of the first transmission line and the second transmission line having a length of about ¼ wavelength at the cutoff frequency is longer than the ¼ wavelength. Closed loop circuit configured by connecting with a small high impedance transmission line and inside the closed loop circuit to suppress unnecessary electromagnetic coupling between the first transmission line, the second transmission line, and the high impedance transmission line Provided with via holes provided and open-ended stubs connected to both ends of the second transmission line, with one end of the second transmission line serving as an input end and the other end of the second transmission line serving as an output end It is.

  According to the present invention, a non-adjacent coupling having a large coupling amount required when the attenuation pole is close to the cutoff frequency of the filter by using a configuration including a positive J inverter without using a capacitive coupling line. Can be obtained.

  Hereinafter, preferred embodiments of the distributed constant filter of the present invention will be described with reference to the drawings. As a transmission line in the following description, a microstrip line formed on a dielectric substrate is used, but a coplanar line or a slot line can also be used.

Embodiment 1 FIG.
FIG. 1 shows a circuit configuration diagram of a distributed constant filter according to Embodiment 1 of the present invention. A first microstrip line 2 and a second microstrip line 3 having a length of about ¼ wavelength at the cutoff frequency of the filter are formed on the dielectric substrate 1, and the first microstrip line is further formed. Both ends of the second and second microstrip lines 3 are respectively connected by a high impedance microstrip line 4.

  Further, open ends stubs 5 are connected to both ends of the second microstrip line 3, and input / output terminals corresponding to the input end and the output end are connected. A via hole 6 is disposed to suppress unnecessary electromagnetic coupling between the first microstrip line 2, the second microstrip line 3, and the high impedance microstrip line 4.

  Next, the operation of the distributed constant filter according to Embodiment 1 of the present invention will be described below in comparison with a conventional polar low-pass filter. FIG. 2 is a circuit configuration diagram of a conventional polarized low-pass filter, which has a folded symmetrical structure including a capacitance element and a J inverter circuit.

Here, the values of J ₁ and J ₂ of the J inverter circuit are (−J ₁ ) <0, + J ₂ > 0. Since the passing phase of the J inverter changes to ± 90 ° in accordance with the sign of the J value, two signal propagation paths having opposite passing phases are formed. Therefore, the frequency characteristic of such a polarized low-pass filter can form an attenuation pole with a passing amount of zero.

  The distributed constant circuit according to the first embodiment has a circuit equivalent to the circuit of the conventional polar low-pass filter shown in FIG. 2, and is obtained by circuit modification of the circuit of FIG. It shows below using FIG. FIG. 3 is a diagram showing a plurality of equivalent circuits for obtaining the circuit configuration of the distributed constant filter according to the first embodiment of the present invention.

First, as shown in FIG. 3 (a), the circuit of the conventional polarized low-pass filter shown in FIG. 2 has the filter characteristics even if the signs of the J inverter (−J ₁ ) and + J ₂ are simultaneously inverted. Since they do not change, they can be + J ₁ and (−J ₂ ).

Next, since the negative J inverter (−J ₂ ) <0 in FIG. 3A is equivalent to the cascade connection of the positive J inverters J = 1 and + 1 / J ₂ , FIG. The circuit shown in FIG. Further, the capacitance C ₂ is connected to J inverters with J = 1 and J ₁₂ = 1 at both ends, and is equivalent to the series inductor L ₂ = C ₂ , so that the circuit shown in FIG. Will be obtained.

That is, by using a part of the capacitance element in the filter as a series inductance element, the negative J inverter included in the conventional polarized low-pass filter circuit of FIG. 2 can be eliminated. Distributed constant filter shown in FIG. 1 is different from the circuit of FIG. 3 (c), in open stub 5 a capacitance element C _1, microstrip high impedance less than the length 1/4 wavelength inductance element L ₂ The line 4 is a circuit in which the J inverter is approximately realized by the first microstrip line 2 and the second microstrip line 3 that are quarter wavelength lines.

  The positive J inverter can be easily realized by a quarter wavelength line, and can realize a larger coupling amount than the capacitive coupling of the coupling line. As a result, according to the first embodiment, a low-pass filter having an attenuation pole in the very vicinity of the cutoff frequency of the filter can be configured.

  Therefore, as shown in FIG. 1, both ends of the first microstrip line 2 and the second microstrip line 3 having a length of about ¼ wavelength at the cutoff frequency of the filter are connected to the high impedance microstrip line. 4 is a distributed constant low-pass filter including a closed loop circuit configured by connecting with an inductance element composed of 4 and a circuit composed of open-ended stubs 5 connected to both ends of the second microstrip line 3 as its constituent elements. By configuring the non-adjacent coupling necessary for forming the attenuation pole can be realized without using the negative coupling.

  According to the first embodiment, by using a configuration including a positive J inverter without using a capacitive coupling line, non-adjacent coupling with a large coupling amount can be easily realized, and the cutoff frequency of the filter is extremely low. A distributed constant filter having a low-pass characteristic with a steep attenuation characteristic having an attenuation pole in the vicinity can be obtained.

  In the above description, an inductance element made of a high-impedance microstrip line is used. However, the present embodiment is not limited to this. The same effect can be obtained even if a lumped constant inductor such as a coil or a chip inductor is used as the inductance element.

Embodiment 2. FIG.
In the first embodiment, a distributed constant filter having a low-pass characteristic with a steep attenuation characteristic has been described. In the second embodiment, a distributed constant filter having a high-pass characteristic with a steep decay characteristic will be described.

  FIG. 4 shows a circuit configuration diagram of the distributed constant filter according to the second embodiment of the present invention. A first microstrip line 12 and a second microstrip line 13 having a length of about ¼ wavelength at the cutoff frequency of the filter are formed on the dielectric substrate 11, and the first microstrip line is further formed. Both ends of 12 and the second microstrip line 13 are connected by MIM (Metal-Insulator-Metal) capacitors 14, respectively.

  Further, both ends of the second microstrip line 13 are connected to a short-circuited short stub composed of a high impedance microstrip line 15 and a ground via hole 16a, and input / output terminals corresponding to an input terminal and an output terminal. Connected. A via hole 16b is disposed in order to suppress unnecessary electromagnetic coupling between the first microstrip line 12 and the second microstrip line 13.

  Next, the operation of the distributed constant filter according to the second embodiment of the present invention will be described. The distributed constant circuit according to the second embodiment is obtained by replacing the capacitance element and the inductance element of the distributed constant circuit according to the first embodiment with an inductance element and a capacitance element, respectively. That is, both ends of the first microstrip line 12 and the second microstrip line 13 are connected by an MIM capacitor 14 corresponding to a capacitance element instead of the high impedance microstrip line 4 corresponding to an inductance element. . In addition, a tip short-circuit stub corresponding to an inductance element is connected to both ends of the second microstrip line 13 instead of a tip-open stub 5 corresponding to a capacitance element.

  By applying such a substitution to the low-pass filter, the frequency characteristics of the distributed constant filter become a high-pass filter, and the configuration shown in FIG. 4 has a high-pass having an attenuation pole in the immediate vicinity of the cutoff frequency of the filter. A filter can be obtained.

  Therefore, as shown in FIG. 4, both ends of the first microstrip line 12 and the second microstrip line 13 having a length of about ¼ wavelength at the cutoff frequency of the filter are connected to the capacitance formed by the MIM capacitor 14. By configuring a distributed constant high-pass filter that includes a closed loop circuit configured by connecting with elements and a circuit composed of a short-circuited stub connected to both ends of the second microstrip line 13 as its constituent elements Thus, the non-adjacent coupling necessary for the formation of the attenuation pole can be realized without using the negative coupling.

  According to the second embodiment, by using a configuration including a positive J inverter without using a capacitive coupling line, non-adjacent coupling with a large coupling amount can be easily realized, and the cut-off frequency of the filter is extremely low. A distributed constant filter having a high-pass characteristic with a steep attenuation characteristic having an attenuation pole in the vicinity can be obtained.

  In the above description, the MIM capacitor 14 is used as a capacitance element that connects both ends of the first microstrip line 12 and the second microstrip line 13, but the present embodiment is not limited to this. . The same effect can be obtained even if other capacitance elements such as a chip capacitor are used.

Embodiment 3 FIG.
In the first embodiment, a distributed constant filter having a low-pass characteristic with a steep attenuation characteristic has been described. In the third embodiment, a distributed constant filter in which attenuation characteristics are further steepened with respect to such a distributed constant filter will be described.

  FIG. 5 shows a circuit configuration diagram of the distributed constant filter according to the third embodiment of the present invention. A first microstrip line 22 and a second microstrip line 23 having a length of about ¼ wavelength at the cutoff frequency of the filter are formed on the dielectric substrate 21, and the first microstrip line is further formed. Both ends of 22 and the second microstrip line 23 are connected by a high impedance microstrip line 24, respectively.

  Further, open end stubs 25 are connected to both ends of the second microstrip line 23. Further, high-impedance microstrip lines 27a, 27b, and 27c and low-impedance microstrip lines 28a and 28b are alternately connected to both ends of the second microstrip line 23 via a low-pass filter. The input / output terminals corresponding to the input terminal and the output terminal are connected.

  A via hole 26 is arranged to suppress unnecessary electromagnetic coupling between the first microstrip line 22, the second microstrip line 23, and the high impedance microstrip line 24.

  Next, the operation of the distributed constant filter according to the third embodiment of the present invention will be described. The distributed constant circuit according to the third embodiment further includes a low-pass filter composed of a circuit in which a low-impedance line and a high-impedance line are cascade-connected to the low-pass filter shown in the first embodiment. It is added. Since the filter generally has a steeper attenuation characteristic by increasing the number of circuit stages, a low-pass filter having a steeper attenuation characteristic can be obtained by adopting the configuration shown in FIG. .

  Further, as shown in FIG. 5, both ends of the first microstrip line 22 and the second microstrip line 23 having a length of about ¼ wavelength at the cutoff frequency of the filter are connected to the high impedance microstrip line. A distributed constant low-pass filter including a closed loop circuit configured by connecting using inductance elements composed of 24 and a circuit composed of open-ended stubs 25 connected to both ends of the second microstrip line 23 as its constituent elements By configuring the non-adjacent coupling necessary for forming the attenuation pole can be realized without using the negative coupling.

  According to the third embodiment, by using a configuration including a positive J inverter without using a capacitive coupling line, non-adjacent coupling with a large coupling amount can be easily realized, and the cut-off frequency of the filter is extremely low. A distributed constant filter having a low-pass characteristic with a steep attenuation characteristic having an attenuation pole in the vicinity can be obtained. Furthermore, by adding another low-pass filter composed of high and low impedance lines to such a distributed constant filter, it is possible to further steepen the attenuation characteristic.

  In the above description, the distributed constant filter has been described in which the attenuation characteristic is sharpened by cascading distributed constant filters having low-pass characteristics. Similarly, a distributed constant filter having a steep attenuation characteristic can be obtained by cascading distributed constant filters having high-pass characteristics. For example, in a high band, a general high-pass filter composed of a ladder circuit of a series capacitor and a parallel inductor and a distributed constant filter having the high-pass characteristic described in the second embodiment are connected in cascade. In addition, the attenuation characteristic can be further sharpened.

  Further, the distributed constant filter having the low-pass characteristic having the steep attenuation characteristic described in the first embodiment and the distributed constant filter having the high-pass characteristic having the steep attenuation characteristic described in the second embodiment are connected in cascade. Thus, it is possible to obtain a distributed constant filter having steep attenuation characteristics in both the high frequency region and the low frequency region.

1 is a circuit configuration diagram of a distributed constant filter according to a first embodiment of the present invention. FIG. It is a circuit block diagram of the conventional polarized low-pass filter. It is the figure which showed the some equivalent circuit for obtaining the circuit structure of the distributed constant filter in Embodiment 1 of this invention. FIG. 3 is a circuit configuration diagram of a distributed constant filter according to a second embodiment of the present invention. FIG. 10 is a circuit configuration diagram of a distributed constant filter according to a third embodiment of the present invention.

Explanation of symbols

  1, 11, 21 Dielectric substrate, 2, 12, 22 First microstrip line 3, 13, 23 Second microstrip line 4, 15, 24 High impedance microstrip line 5, 25 Open end Stub, 6, 16a, 16b, 26 Via hole, 14 MIM capacitor, 27a, 27b, 27c High impedance microstrip line, 28a, 28b Low impedance microstrip line.

Claims (8)

Each end of the first transmission line and the second transmission line having a length of about ¼ wavelength at the cut-off frequency is connected by a high impedance transmission line having a length smaller than that of the ¼ wavelength. A closed loop circuit comprising:
A via hole provided inside the closed loop circuit to suppress unwanted electromagnetic coupling between the first transmission line, the second transmission line, and the high impedance transmission line;
Open-ended stubs connected to both ends of the second transmission line;
With
A distributed constant filter, wherein one end of the second transmission line is an input end and the other end of the second transmission line is an output end. The distributed constant filter according to claim 1,
The distributed impedance filter, wherein the high impedance transmission line is composed of a lumped constant inductor element. The distributed constant filter according to claim 1 or 2,
A low-pass filter formed by alternately cascading high-impedance lines and low-impedance lines is added between the input end and the output end and both ends of the second transmission line. Distributed constant filter. A closed-loop circuit configured by connecting both ends of the first transmission line and the second transmission line having a length of about ¼ wavelength at a cutoff frequency by a capacitance element;
A via hole provided inside the closed-loop circuit to suppress unwanted electromagnetic coupling between the first transmission line and the second transmission line;
A tip short-circuit stub connected to both ends of the second transmission line;
With
A distributed constant filter, wherein one end of the second transmission line is an input end and the other end of the second transmission line is an output end. The distributed constant filter according to claim 4,
2. The distributed constant filter according to claim 1, wherein the capacitance element includes an MIM capacitor or a chip capacitor in which a dielectric is loaded between two metals. The distributed constant filter according to claim 4 or 5,
A distributed constant filter, wherein a high-pass filter comprising a ladder circuit of a series capacitor and a parallel inductor is added between the input end and the output end and both ends of the second transmission line.   A distributed constant filter comprising the distributed constant filter according to claim 1 and the distributed constant filter according to claim 4 connected in cascade.   8. The distributed constant filter according to claim 1, wherein the transmission line used in the distributed constant filter is a microstrip line, a coplanar line, or a slot line formed on a dielectric substrate. .

Images