Small size cross-coupled trisection filter

[0002] 1. Field of the Invention

[0003] The present invention relates to a cross-coupled trisection filter, with inductance and capacitance devices, thereby reducing its physical size and increasing the production yield.

[0004] 2. Description of Related Art

[0005] According to the filter design specification, if the degree of the resonator is increased, the selectivity of the frequency band is increased. However, this is accompanied with bandpass attenuation C and an increase in physical size. Refer to FIG. 1 for a prototype of a cascade trisection bandpass filter. As shown in FIG. 1, any cascade trisection bandpass filter generally provides asymmetric frequency response. Conventional bandpass filters with asymmetric response are further described in “Microstrip Cross-coupled trisection bandpass filters with asymmetric frequency characters” by J. -S. Hong and M. J. Lancaster, as shown in FIG. 2a, and in “Microstrip Cascade Trisection Filter” by Chu-Chen Yang and Chin-Yang Chang, as shown in FIG. 2b. The resonators R1a, R2a, and R3ain FIG. 2aare construed on a substrate sun, wherein the resonator R1ahas an input port IN and the resonator R3ahas an output port OUT. The resonators R1b, R2b, R3b, R4b, and R5bin FIG. 2bare construed on a substrate (not shown), wherein the resonator R5ahas an input port P1 and the resonator R3ahas an output port P2. As shown in FIG. 2a, the 3-pole filter structure is composed of three λ/2-line open-loop resonators R1a, R2a, R3aon one side of the dielectric substrate SUB with a ground plane on the other side. The cross coupling between resonators R1aand R3aexists because of their proximity. An attenuation pole of finite frequency exists on the high side of the pass band due to the cross-coupling. As shown in FIG. 2b, the 5-pole filter with two λ/2-line open-loop resonators and three hairpin resonators has mixed (electric and magnetic) couplings between resonators R1band R2band between resonators R2band R3b, the mixed couplings between resonators R3band R4band between resonators R4band R5b. The lower attenuation pole is due to the nonadjacent magnetic coupling between resonators R1band R3b, and the upper attenuation pole is due to the nonadjacent electric coupling between resonators R3band R5b. Thus, both FIGS. 2aand 2bcan achieve a higher selectivity without increasing the degree of poles, i.e. the number of resonators. However, such a structure exhibits increased size and easily suffers spurious effect on odd frequencies of the band pass (see the appendix A), so the required level of filtration is not achieved.

[0006] Accordingly, an object of the invention is to provide a filtering structure, which adds a serial capacitance device into each resonator of the filter in FIG. 1 to reduce the filter size.

[0007] Another object of the invention is to provide a small size cross-coupled trisection filtering structure, which uses the semi-lumped LC resonator to avoid the spurious effect and also keep the attenuation pole on the high frequency during the band pass.

[0008] Another object of the invention is to provide a small size cross-coupled trisection filtering structure, which only couples to the high impedance transmission portion of the resonators, thereby fitting a multilayer and easily adjusting the frequency of an attenuation pole by changing the high impedance transmission distance of the first and third poles without changing the bandpass characteristics.

[0009] The invention provides a small size cross-coupled trisection filter structure, including a first resonance unit; a second resonance unit; and a third resonance unit. Each of the units includes an inductance device, e.g. a transmission line, and a capacitance device, e.g. a capacitor, wherein the high impedance transmission portions of two of the units are coplanar and one has an input while the other has an output.

[0010] The invention will become apparent by referring to the following detailed description of a preferred embodiment with reference to the accompanying drawings, wherein:

[0011] FIG. 1 is a prototype illustrating a cascade trisection bandpass filter;

[0012] FIG. 2ais a typical equivalent circuit of FIG. 1;

[0013] FIG. 2bis another typical equivalent circuit of FIG. 1;

[0014] FIG. 3 is an equivalent circuit of the invention;

[0015] FIG. 4 is an embodiment of FIG. 3 according to the invention;

[0016] FIG. 5 is another embodiment of the high impedance transmission portion of FIG. 3 according to the invention;

[0017] Appendix A is a curve illustrating typical spurious effect appearing on the structure of FIG. 1;

[0018] Appendix B is curves illustrating the frequency change of attenuation poles generated by the structure of FIG. 4 according to the invention; and

[0019] Appendix C is a curve illustrating a filtering response without spurious effect appearing on the structure of FIG. 3 according to the invention.

[0020] Refer to FIG. 3, an equivalent circuit of the invention, which is designed by using a semi-lumped LC resonator and according to a prototype of the cascade trisection (CT) bandpass filter structure. In FIG. 3, the circuit includes three resonance units, each having a high impedance transmission line and a serial capacitance device, wherein every high impedance transmission line can consist of two inductance devices.

[0021] As shown in FIG. 3, the equivalent circuit of a 3-pole bandpass filter is shown. In such an equivalent circuit, the high impedance transmission portion of the resonator is cross-coupled. A first trisection bandpass resonance unit includes high impedance transmission lines L11, L12 and a capacitance device C1. A second trisection bandpass resonance unit includes high impedance transmission lines L21, L22 and a capacitance device C2. A third trisection bandpass resonance unit includes high impedance transmission lines L31, L32 and a capacitance device C3. The coupling of lines L11 L22 and the coupling of lines L21, L32 are mainly coupled while the coupling of lines L11 L32 and the coupling of the first and third resonance units are cross-coupled. Also, capacitance devices C1, C2, C3 are the ground capacitance. The port Port1 is located between lines L11 and L12, in order to input the signal to the circuit, the port Port2 is located between lines L31 and L32, in order to output the signal of the circuit.

[0022] [First Embodiment]

[0023] Refer to FIG. 4, an embodiment of FIG. 3. In FIG. 4, a low temperature cofire ceramic technique is carried to a filtering structure with a size 3.2 mm×2.5 mm×1.3 mm and having operating frequency 2.1 GHz, also its explored members included.

[0024] As shown in FIG. 4, the embodiment uses nine dielectric layers, with ten metal circuit layers 1-10 with thicknesses of the dielectric layers from top to bottom of 3.6, 3.6, 3.6, 3.6, 7.2, 11.8, 7.2, 3.6 and 3.6 (mil). The ten metal circuit layers 1-4 form a first capacitance layer, the layers 5-8 form an inductance layer, and the layers 9-10 form a second capacitance layer. However, the first capacitance layer can only include the layers 1 and 2, depending on the desired capacitance. The 1, 3, 5, 8 and 10 metal circuit layers are grounded for isolation. The metal line can be silver, copper or any conductive material. The grounding capacitance mentioned above is carried by a metal-insulator-metal (MIM) structure in the embodiment. For example, the capacitance device C2 is an MIM structure forming of the metal layers 8, 9, 10 and insulating dielectric layers to be placed in the middle. Moreover, the capacitance devices C1 and C3 of FIG. 3 are constructed the same as the capacitance C2 of FIG. 4. A need to enlarge the capacitance values is created by increasing the number of layers. The coupling portion (inductance) of the high impedance transmission line mentioned above is achieved by conjoining the layers 5, 6. The main couplings of lines L11, L22 and lines L21, L32 are achieved by the non-coplanar coupling lines. The coupling value is decided by the requirement of bandwidth of the bandpass filter such that the coupling value is changed by the coupled overlap width or the dielectric thickness between the coupling lines. The couplings of lines L11, L12 and lines L31, L32 are carried by edge coupling of the coplanar coupling lines. Such a coupling value can adjust the frequencies of attenuation poles without changing the bandwidth and central frequency of the bandpass filter (see the appendix B, from the point A with 1.7 GHz shift to the point B with 1.45 GHz). Every line can be any conductive material, such as gold, copper, tin or others. The combination of every layer is achieved by vias, e.g. using lines XR through the corresponding vias between the layers, as shown in FIG. 4.

[0025] In a multilayer structure, the coupling line used in the embodiment has the advantages of small size and high yield.

[0026] [Second Embodiment]

[0027] Refer to FIG. 5, further illustrating another embodiment of the high impedance transmission portion (i.e., the layers 4-8 of FIG. 4 can be replaced by this embodiment and the remaining layers are the same) of FIG. 3. The implementation of the capacitance devices C1, C2, C3 of the embodiment is omitted because they are the same as the implementation of the capacitance of FIG. 4. The implementation of the high impedance transmission line follows.

[0028] As shown in FIG. 5, the high impedance transmission lines (inductive portion of the resonator) are between top metal plate and bottom metal plate forming strip-lines. The two metal plates use the edge grounding for isolation. The metal line can be copper or any conductive material. The layer 2 is a line layer with the layout of the transmission degrees L1, L2, L3 inside. The transmission lines L1, L2, L3 use edge couple differ from the first embodiment case. The advantage of this embodiment is that the structure can be implemented by a single-layer double sided PCB. Such a coupling value can adjust the frequencies of attenuation poles without changing the bandwidth and central frequency of the bandpass filter. Every line can be any conductive material, e.g. gold, copper, tin or others.

[0029] The advantage of the embodiment is its simple structure, which can be implemented by a two-face single board due to the coplanar layout of the capacitors.

[0030] Briefly, the resonator with the input port and the resonator with the output port have to implement in coplanar, and the metal layer and the insulator layer are interlaced in implementation. Therefore, various alterations and modifications in the circuit layout of the invention can be made.

[0031] Accordingly, the invention provides a small size cross-coupled trisection filtering structure, which minimizes bandpass filtering structure using a multilayer configuration, and adjusts the attenuation pole on both sides of the band pass to avoid the spurious effect appearing on odd frequencies of the bandpass (see appendix C, only one bandpass). Thus the filtering design will satisfy the specific demands.

[0032] Although the present invention has been described in its preferred embodiment, it is not intended to limit the invention to the precise embodiment disclosed herein. Those who are skilled in this technology can still make various alterations and modifications without departing from the scope and spirit of this invention. Therefore, the scope of the present invention shall be defined and protected by the following claims and their equivalents.