Description

BAND PASS FILTER USING 1/4 WAVELENGTH TRANSMISSION LINE

Technical Field

[1] The present invention relates to a miniaturized band pass filter using a miniaturized

1/4 wavelength(λ) transmission line, and more particularly to a miniaturized band pass filter based on the cross-reference applications of Korean Patent Application Nos. 10-2002-0056967 and 10-2004-0063977 filed by the same applicant as the present invention, the contents of which are incorporated by reference. Background Art

[2] Conventional techniques will hereinafter be described with reference to the annexed drawings.

[3] FIGS. 1 and 2 exemplarily show miniaturized equivalent circuits of a conventional

1/4 wavelength transmission line, respectively. FIGS. 3 and 4 are more-miniaturized circuits of a conventional 1/4 wavelength transmission line, respectively. FIG. 5 is a circuit diagram illustrating a filter use for a conventional communication system.

[4] In more detail, FIG. 1 shows a conventional 1/4 wavelength transmission line, and

FIG. 2 shows a circuit diagram in which the 90°transmission line in FIG. 1 is miniaturized to the length of θ.

[5] In this case, the relationship between two transmission lines shown in FIGS. 1 and 2 can be represented by the following Math Figure 1 and 2:

[6] MathFigure 1

Z=Z ₀ /sinθ

[7] MathFigure 2

_ COSθ 1 ωZ ₀

[8] In more detail, as can be seen from Math Figure 1, the shorter the length θ of the miniaturized transmission line, the higher the characteristic impedance value of the transmission line. Provided that a characteristic impedance limit value of a conventional transmission line is 100Ω, it is difficult to reduce the magnitude to 30° or below.

[9] In the meantime, a resonance circuit is artificially inserted into FIG. 3 so as to construct an equivalent circuit of a coupled line having a diagonally short-circuited edge. In this case, a circuit denoted by dotted lines is equivalent to a coupled line

having a short-circuited edge, and the relationship between the coupled line and the above-mentioned circuit can be represented by the following Math Figure 3, 4, 5 and 6: [10] MathFigure 3

7 - 7

[11] MathFigure 4

L ₀ =Z _Oe tanQ/ω

[12] MathFigure 5

ω L ₀

[13] MathFigure 6 c=c _γ +c ₀

[14] In the meantime, the reason why the transmission line of the length θ shown in FIG.

2 is replaced with the above-mentioned coupled line having the short-circuited edge is shown in Math Figure 3. In more detail, as can be seen from Math Figure 3, if the value of Z oe is similar to the value of Z oe regardless of the impedance value Z of the miniaturized coupled line, the resultant value acquired by Math Figure 3 can be reqlaced with the impedance value Z.

[15] FIG. 4 shows a finally-miniaturized 1/4 wavelength transmission line, the concept of which has been disclosed in the above-mentioned Korean Patent Applications filed by the same applicant as the present invention.

[16] In order to describe a commercially-available example of a miniaturized filter, FIG.

5 shows an example of transceiver of a Radio Frequency(RF) communication system equipped with the filter. In the case of a mobile communication field, a switch or duplexer may be connected to the rear of an antenna if required. In this case, microwave filters for use in a variety of systems(e.g., a mobile communication system, a WLAN system, a GPS system, and a satellite DBM system, etc.) have generally used a ceramic filter, a Surface Acoustic Wave(SAW) filter, an LC filter, and Bulk Acoustic Wave(BAW) filter.

[17] In the meantime, a Microwave Monolithic Integrated Circuit(MMIC), wherein a plurality of components other than both a power amplifier and a filter are integrated into one circuit by a semiconductor fabrication, has been generally used as an RF unit of the communication system in recent times. Indeed, it is most difficult to integrate

the filter in the MMIC, resulting in greater inconvenience of use.

[18] Therefore, the conventional techniques are unable to manufacture the filter in the

MMIC, such that the filter must be separated from the MMIC and must be connected to an external part of the MMIC. Disclosure of Invention Technical Problem

[19] Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide a band pass filter capable of being implemented within an MMIC or LTCC(Lo w Temperature Co-fired Ceramic), etc via a miniaturized 1/4 wavelength transmission line.

Technical Solution

[20] In accordance with one aspect of the present invention, the above and other objects can be accomplished by the provision of a band pass filter using a 1/4 wavelegnth transmission line comprising: at least one miniaturized 1/4 wavelength transmission line in which capacitors are connected in parallel to Input/Output connection parts of a coupled line having an edge short-circuited in a diagonal direction, such that the miniaturized 1/4 wavelength transmission line can be implemented within a MMIC(Microwave Monolithic Integrated Circuit) or LTCC(Low Temperature Co- fired Ceramic), etc.

[21] In accordance with another aspect of the present invention, there is provided a band pass filter using a 1/4 wavelength transmission line comprising: at least one miniaturized 1/4 wavelength transmission line in which capacitors are connected in parallel to opposite Input/Output (I/O) ends of a coupled line having an edge short- circuited in the same direction, such that the miniaturized 1/4 wavelength transmission line can be implemented within a (MMIC)Microwave Monolithic Integrated Circuit or LTCC(Low Temperature Co-fired Ceramic), etc. Advantageous Effects

[22] The present invention has the following effects.

[23] Firstly, the MMIC filter according to the present invention can be easily manufactured by the conventional fabrication process widely used in mass production.

[24] Secondly, the present invention is applicable to a variety of fabrication processes, such that a single chip or module can be implemented according to an optimized fabrication.

[25] Thirdly, the present invention can be easily applied to not only the IF filter but also the millimeter band.

[26] Fourthly, the present invention does not use a lumped inductor, so that it can easily manufacture a desired circuit.

Brief Description of the Drawings

[27] The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: [28] FIGS. 1 and 2 exemplarily show miniaturized equivalent circuits of a conventional

1/4 wavelength transmission line; [29] FIGS. 3 and 4 are more-miniaturized circuits of the conventional 1/4 wavelength transmission line;

[30] FIG. 5 is a circuit diagram illustrating a filter usage for a conventional communication system; [31] FIGS. 6 to 10 show a variety of examples of a 1/4 wavelength transmission line according to the present invention; [32] FIG. 11 shows another example of a 1/4 wavelength transmission line according to the present invention; [33] FIG. 12 is a circuit diagram illustrating a band pass filter using the 1/4 wavelength transmission line according to the present invention; [34] FIG. 13 is a graph illustrating an example of the simulation result of the band pass filter using the 1/4 wavelength transmission line according to the present invention; [35] FIGS. 14 and 15 are graphs illustrating other examples of the simulation result of the band pass filter using the 1/4 wavelength transmission line according to the present invention; [36] FIGS. 16 and 17 are cross-sectional views illustrating the substrate of the band pass filter using the 1/4 wavelength transmission line according to the present invention; [37] FIGS. 18 to 21 show a variety of band pass filters using another 1/4 wavelength transmission line according to the present invention; [38] FIGS. 22 to 24 show a variety of band pass filters using still another 1/4 wavelength transmission line according to the present invention; [39] FIGS. 25 and 26 show a variety of band pass filters using still another 1/4 wavelength transmission line according to the present invention; [40] FIG. 27 is a circuit diagram illustrating a typical miniaturized band pass filter having a transmission line, which is inserted between two resonators when the two resonators are connected to each other, according to the present invention; [41] FIGS. 28 to 31 show a variety of band pass filters using still another 1/4 wavelength transmission line according to the present invention; [42] FIG. 32 shows a band pass filter using still another 1/4 wavelength transmission line according to the present invention; and [43] FIGS. 33 and 34 show a variety of examples of a conventional combline filter.

Mode for the Invention

[44] Now, preferred embodiments of the present invention will be described in detail with reference to the annexed drawings. In the drawings, the same or similar elements are denoted by the same reference numerals even though they are depicted in different drawings. In the following description, a detailed description of known functions and configurations incorporated herein will be omitted when it may make the subject matter of the present invention rather unclear.

[45] FIGS. 6 to 10 show a variety of examples of a 1/4 wavelength transmission line according to the present invention.

[46] FIG. 6 is a basic shape of a miniaturized 1/4 wavelength transmission line disclosed in the above-mentioned Korean Patent Applications. FIG. 7 is a more simplified circuit of the 1/4 wavelength transmission line shown in FIG. 6. FIG. 8 is a circuit diagram in which 90°transmission line contained in the transmission line of FIG. 7 is replaced with an inverter.

[47] In this case, it should be noted that artificial resonance circuits are connected to both ends of the miniaturized 1/4 wavelength transmission line as can be seen from FIGS. 6 and 7. It can be recognized that the 90°transmission line is replaced with an admittance inverter as shown in FIG. 8.

[48] FIG. 9 shows a plurality of miniaturized 1/4 wavelength transmission lines of FIG.

8. FIG. 10 is a circuit diagram illustrating a typical band pass filter formed by connecting the miniaturized 1/4 wavelength transmission lines of FIG. 9.

[49] FIG. 11 shows another example of a 1/4 wavelength transmission line according to the present invention. As previously depicted in the above-mentioned Korean Patent Applications, FIG. 11 shows another miniaturized 1/4 wavelength transmission line.

[50] In the meantime, the above-mentioned Korean Patent Applications have disclosed that artificial resonance circuits are connected to both ends of the circuit shown in FIG. 11, and the filter having bandpass characteristics is constructed by repeatedly connecting the right-side circuit of FIG. 11 in several stages.

[51] FIG. 12 is a circuit diagram illustrating a band pass filter using the 1/4 wavelength transmission line according to the present invention. In more detail, FIG. 12 shows a band pass filter configured in four stages at a frequency band of 5.5GHz.

[52] In this case, an electrical length of FIG. 12 is miniaturized to 5°, such that the length of 1/4 wavelength transmission line is reduced to 5.5%. In the meantime, it can be recognized that filter characteristics can be implemented by resonance circuits located at both ends of the circuit of FIG. 12.

[53] FIG. 13 is a graph illustrating an example of the simulation result of the band pass filter using the 1/4 wavelength transmission line according to the present invention. In

other words, FIG. 13 is a graph illustrating the simulation resultant value of variables S

11 and S 21.

[54] The simulation resultant graph of FIG. 13 is acquired when the circuit of FIG. 12 is simulated by ADS software serving as RF-dedicated software manufactured by Agilent Company.

[55] In this case, as shown in FIG. 13, it can be recognized that a coupled line filter having superior skirt characteristics of a fractional bandwidth of 9.5% is acquired. Considering that the miniaturized 1/4 wavelength transmission line has the length of 5°and a four-stage structure, it can be recognized that a total electrical length becomes 20°. Indeed, if the MMIC is designed at a frequency of 5.5GHz, a silicon substrate denoted by ε = 11.7, or GaAs denoted by ε = 12.9, it can be recognized that the length of 20° is set to about 0.8~0.84mm.

[56] If the miniaturized 1/4 wavelength transmission line for a frequency band of 2GHz is set to 2°~ 3°, the minimized implementation can be extended to IMT-2000 and 2.4Ghz ISM band.

[57] FIGS. 14 and 15 are graphs illustrating other examples of the simulation result of the band pass filter using the 1/4 wavelength transmission line according to the present invention. Simulation values of the circuit shown in FIG. 12 are depicted in FIGS. 14 and 15.

[58] In more detail, FIG. 14 is a graph illustrating a variation in S characteristics when the miniaturized 1/4 wavelength transmission line has different lengths. FIG. 15 is a graph illustrating a variation in S characteristics when the miniaturized 1/4 wavelength transmission line has different lengths. Specifically, the graphs of FIGS. 14 and 15 are resultant values acquired when the miniaturized 1/4 wavelength transmission lines of 3°, 5°and 7° are designed in the form of a four-stage filter.

[59] In this case, as shown in FIG. 14, the shorter the length, the shorter the bandwidth, such that a bandwidth can be adjusted by the above-mentioned characteristics.

[60] Furthermore, the bandwidth can also be adjusted by a coupling coefficient of a miniaturized coupled line. In other words, the higher the coupling coefficient, the longer the bandwidth. Likewise, the lower the coupling coefficient, the shorter the bandwidth.

[61] The miniaturized band pass filter is analyzed using electromagnetic waves, instead of using an electronic circuit based on ADS software, so that a plurality of MMIC circuit technologies are introduced, a detailed description thereof will hereinafter be described. In this case, the software for designing an electromagnetic-wave circuit is "HFSS" manufactured by Ansoft Corporation, and the HFSS design tool result closely matches the MMIC circuit manufacturing result.

[62] FIGS. 16 and 17 are cross-sectional views illustrating the substrate of the band pass

filter using the 1/4 wavelength transmission line according to the present invention. In more detail, FIG. 16 is a cross-sectional view illustrating a traveling direction of a coupled line of a resonator, and FIG. 17 is a cross-sectional view illustrating a signal- traveling direction of a transmission line at Input/Output (I/O) ends of the coupled line of the resonator.

[63] It is assumed that the GaAs manufacturing process is used for the structures of

FIGS. 16 and 17. Considering general particulars associated with the height of the MMIC circuit, the height of the MMIC circuit is generally set to 400μm, such that it prevents a current from leaking to ground from the MMIC circuit, and also prevents an unnecessary coupling between the MMIC circuit and the ground under the substrate from being generated. However, in the case of using a high-power signal used for the power amplifier, the height of the MMIC circuit is set to a low value of about lOOμm, resulting in the implementation of a heat sink.

[64] A through-wafer via hole is not currently supported in the manufacturing process based on the Silicon(Si) Substrate. In this case, the circuit can be configured in the form of a coplanar coupled line structure.

[65] In other words, as shown in FIG. 16, a ground pillar is arranged at both sides of a coupled line, so that an aperture is formed. The reason why an insulator called a polyimide is inserted into the structure of FIG. 16 is to prevent a current from flowing down.

[66] However, the present invention is made available without using the above- mentioned polyimide insulator.

[67] Although the present invention uses a GaAs substrate as a substrate, the Si substrate may also be used as the substrate to implement the RF CMOS (Complementary Metal Oxide Semiconductor) process. In this case, oxide is used as the insulator, and a multi- layered circuit may be constructed on the substrate.

[68] In the meantime, if pillars are arranged at both sides of the coupled line as shown in

FIG. 16, and a polyimide layer is deposited on the pillars, the length of a via-hole of a parallel capacitor located at both ends of the miniaturized transmission line is reduced to 20μm, such that a parasitic component of the via-hole is almost disregarded.

[69] As shown in FIG. 17, a 20μm polyimide(=3.5) layer is located under the transmission line connected to the I/O port, a wide ground is located under the 20μm polyimide layer, and the length of the via-hole having a short-circuited edge is 20μm, such that the parasitic component can be disregarded in the same manner as in FIG. 16.

[70] FIGS. 18 to 21 show a variety of band pass filters using another 1/4 wavelength transmission line according to the present invention. FIG. 18 shows an ADS circuit under the condition an intermediate frequency is 1.95GHz and the electric length is 3°. FIG. 19 shows ADS simulation result of the circuit shown in FIG. 18. FIG. 20 shows a

circuit diagram acquired when the ADS result is implemented by the HFSS. FIG. 21 shows the HFSS simulation result of FIG. 20.

[71] In other words, the above-mentioned results of FIGS. 18 to 21 can be acquired when a circuit is designed according to processes shown in FIGS. 16 and 17.

[72] In this case, the above-mentioned circuit is designed under the condition that the intermediate frequency of the circuit is 1.95GHz and the electrical length is 3°(559μm). In addition, the conductor height of the transmission line is 3μm, an even impedance of the coupled line is Z Oe =80, an odd impedance of the coupled line is Z Oo =65, the width of the coupled line is 95μm, a distance between the coupled lines is 316μm, and the length of a total aperture is 800μm.

[73] The capacitor uses a Metal Insulator Metal(MIM) structure, and is designed based on an equation of C=6pF/100xl00μm . In this case, the calculation result of C=6pF/100xl00μm is denoted by C=21.1pF, and the length of one side of a square MIM capacitor is 187.5μm.

[74] If the above-mentioned circuit is constructed as described above, a superior resonator having an insertion loss of -1.93dB can be acquired as shown in FIG. 21.

[75] However, a center frequency is shifted to a lower direction in the above-mentioned circuit. The reason why center frenquency is shifted to the lower direction is that the via-hole length of 20μm cannot be disregarded, a mutual-coupling effect is generated from the transmission line and other components, and the capacitance is slightly increased. Therefore, if the length of the coupled line is reduced or the value of the capacitor is reduced, an intermediate frequency can be controlled. In light of the above-mentioned circuit design result, it can be recognized that the length of the resonator is equal to or less than 600μm. This means that the above-mentioned resonator is the smallest resonator from among currently-developed coupled lines.

[76] FIGS. 22 to 24 show a variety of band pass filters using still another 1/4 wavelength transmission line according to the present invention. In more detail, FIGS 22 to 24 show the circuit implemented when the band pass filters of FIGS. 18 to 21 are connected in two stages.

[77] FIG. 22 shows a circuit configured when the resonator of FIGS. 18-21 is connected in two stages and a short transmission line is connected between the two resonators. FIG. 23 is a graph illustrating band-pass characteristics of the circuit shown in FIG. 18 using the HFSS. FIG. 24 is a graph illustrating broadband characteristics of the circuit shown in FIG. 18 using the HFSS.

[78] The above-mentioned filter of FIGS. 22-24 has been designed for commercial filters based on 1.95GHz IMT2000-, CDMA2000-, UMTS-, and WCDMA TX- technologies. The insertion loss of the two-stage filter circuit is -3.32dB and a reflection coefficient is -18dB, so that there is no difference between the two-stage

filter circuit and its real product. The two-stage filter circuit has the size of 1.6 x 1.1 x 0.43mm = 0.76mm , and the currently-commercial Epcos ceramic filter A360 manufactured by Epcos Company, which has the highest market share in the world in association with a filter-associated technical field. Therefore, it can be recognized that the difference in size between the Epcos ceramic filter A360 and the two- stage filter circuit is about 50 times.

[79] In this case, it should be noted that there is a transmission line between the filters configured in two stages as shown in FIG. 17.

[80] FIGS. 25 and 26 show a variety of band pass filters using still another 1/4 wavelength transmission line according to the present invention. FIG. 25 shows a circuit connected in two stages without using a transmission line between the resonators of FIGS. 18-20. FIG. 26 shows the HFSS simulation result of FIG. 25.

[81] If the circuits are directly connected in two stages without using the transmission line, there arises unexpected distortion as shown in FIG. 26.

[82] The reason why the unexpected distortion occurs is that there arises a mutual- coupling effect between a first-stage circuit and a second-stage circuit when the first- stage circuit is connected to the second-stage circuit, resulting in the creation of signal distortion in individual circuits.

[83] Typically, it can be recognized that electromagnetic waves are concentrated on an insulation layer located at a lower part of the transmission line rather than an upper part of the transmission line. In this case, the use of a ground wall can reduce a parasitic component of the via-hole of the coupled line, and can also reduce interference between resonators, so that it is considered to be an important component.

[84] In the meantime, if the Si-based process is used, the via-hole cannot be formed in the Si substrate as previously stated above. In this case, a coplanar coupled structure or a similar circuit thereof can be configured. In this case, a transmission line may be positioned between two resonators to prevent the coupling effect between the two resonators from being generated.

[85] In conclusion, in order to configure the miniaturized band pass filter, a transmission line having a short length of θ is connected between two resonators, such that it prevents an undesired coupling between two resonators from being generated.

[86] Provided that the circuit having a short-circuited edge in the same direction as the coupled line is designed, it can be recognized that the circuit is the same as those of FIGS. 28-31. The above-mentioned circuit is designed under the condition that a frequency is 1.95GHz and the electrical length is 3°(559μm) in the same manner as in FIGS. 18-21 and FIGS. 22-24.

[87] FIGS. 28 to 31 show a variety of band pass filters using still another 1/4 wavelength transmission line according to the present invention. FIG. 28 shows an ADS circuit of

the miniaturized resonator whose the coupled-line's edge is short-circuited in the same direction. FIG. 29 shows the ADS simulation result of FIG. 28. FIG. 30 shows a circuit diagram configured when the circuit of FIG. 28 is designed in the form of the HFSS. FIG. 31 shows the HFSS simulation result of FIG. 30.

[88] In this case, the process used shown in FIGS. 28-31 is equal to that of FIG. 20, but the aperture length in FIGS. 28-31 is 900μm, differently from FIG. 20.

[89] FIG. 32 shows a band pass filter using still another 1/4 wavelength transmission line according to the present invention. In more detail, FIG. 32 shows the simulation result when the circuit of FIG. 30 is connected in two stages.

[90] Referring to FIGS. 28-31 and 32, it can be recognized that the simulation result of

FIG. 30 is almost equal to those of FIGS. 18-21 and 22-24. In this case, it should be noted that a transmission line is connected between two resonators connected as shown in FIG. 32, so that the transmission line prevents unexpected interference between the two resonators from being generated.

[91] A circuit similar to the coupled line, edges of which are short-circuited in the same direction, has already been developed and introduced to the market as a "Combline Filter" as shown in FIGS. 33-34.

[92] FIGS. 33-34 show a variety of examples of a conventional combline filter. FIG. 33 is a circuit diagram illustrating the combline filter, and FIG. 34 is an equivalent circuit of the combline filter.

[93] As shown in FIGS. 33-34, the above-mentioned combline filter is similar to the circuit of the present invention, so that it can be designed in the form of an MMIC filter. However, indeed, the above-mentioned combline filter has not yet been implemented within an MMIC filter due to the following problems.

[94] Firstly, if the resonators are directly connected in two stages by excessively reducing the size as shown in FIGS. 25 and 26, an unexpected coupling effect occurs, such that an undesired result occurs that is improper for the circuit design purpose. FIGS. 25 and 26 show the filters connected in two stages. If the filters are connected in at least three stages, it can be recognized that an output signal of the resultant circuit is excessively distorted. Therefore, in order to manufacture an excessively-small-sized filter, a transmission line must be connected between two resonators as shown in FIGS. 22, 27, and 32.

[95] Secondly, a transmission line, an edge of which is diagonally short-circuited to achieve the impedance matching of I/O units, is connected to the I/O units as shown in FIG. 33. But, the above-mentioned circuit structure of FIG. 33 is different from the miniaturized circuit structure shown in FIG. ID. In other words, it can be considered that small-sized shunt inductors are located at both ends of the combline filter. In order to remove the influence of the shunt inductors, a shunt capacitor must be connected to

each shunt inductor such that a resonance circuit must be configured. Therefore, the coupled line structure of the I/O unit may be an obstacle to the implementation of the miniaturized structure.

[96] Thirdly, as can be seen from Math Figure 3, the smaller the size, the excessively higher the impedance Z. For example, in the above-mentioned circuit, if the electrical length is 3°, the impedance Z is 953Ω. Most developers have never seen the above- mentioned excessively-high impedance value while developing/studying the MMIC circuit, so that they have a preconceived idea of the combline filter. As a result, indeed, there has been no attempt to implement the combline filter integrated into the MMIC filter. Industrial Applicability

[97] The solution of manufacturing the filter in the form of the MMIC under the millimeter- and X- bands is generally a long-cherished desire in RF technical fields. The present invention has proposed an improved technology capable of designing the MMIC filter under the millimeter- and X- bands, and has implemented a detailed circuit associated with the improved technology, such that it can also be effectively applied to a mobile communication frequency.

[98] Recently, in communication systems, the RF circuit other than the power amplifier and the filter(or duplexer) for use in the communication systems has been configured in the form of a single chip. Specifically, the filter has been implemented with a SAW filter or a ceramic filter, such that it was difficult to implement the filter in the form of a single chip. In order to solve the aforementioned problems, the present invention reduces the size of the filter, and implements a communication system in the form of a single chip using a semiconductor fabrication, so that aforementioned problems cannot be encountered by the circuit of the present invention.

[99] The present invention has the following effects.

[100] Firstly, the MMIC filter according to the present invention can be easily manufactured by the conventional fabrication process widely used in mass production.

[101] Secondly, the present invention is applicable to a variety of fabrication processes, such that a single chip or module can be implemented according to an optimized fabrication.

[102] Thirdly, the present invention can be easily applied to not only the IF filter but also the millimeter band.

[103] Fourthly, the present invention does not use a lumped inductor, so that it can easily manufacture a desired circuit.

[104] Fifthly, a difference between the conventional filter and the inventive MMIC filter is shown in the following Table 1 :

[105] Table 1

[106] Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.