Abstract:
A ring millimeter-wave filter is a three-dimensional dual-mode ring filter. The ring millimeter-wave filter makes use of a three-dimensional coupling architecture as the feed of a filter to conquer the limit of the smallest spacing of a planar circuit made by the low-temperature cofired ceramic (LTCC) process so as to achieve the required coupling. Moreover, through the design of an embedded microstrip line, more than 20% of the filter area can be saved to facilitate integration with other components.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a ring millimeter-wave filter and, more particularly, to a ring millimeter-wave filter making use of an embedded microstrip line to realize a three-dimension architecture.  
         [0003]     2. Description of Related art  
         [0004]     Filters play an important role in the wireless communication area. The function of a filter is to pass signals in its pass band and to attenuate signals in its stop band. In other words, filters control the responses near a certain frequency of communication systems.  
         [0005]     After the low-temperature cofired ceramic (LTCC) process has been presented to the public, it has been used to fabricate multi-layer substrates to reduce the whole circuit area. For instance, Taiwan Pat. App. No. 562250 “multi-layer ceramic lowpass filter” and US Pat. App. No. 2005/0012567 A1 “lowpass filter formed in multi-layer ceramic” have disclosed this kind of technology to facilitate integration with more circuits. Moreover, because filters made of inductors and capacitors have a serious high-frequency parasitic effect, they are only suitable to applications in lower microwave bands. Because ring filters use a wavelength transmission line to select the frequency, they can apply to high-frequency or millimeter-wave bands. Moreover, because ring filters have two transmission zero points near their central frequency, they can completely filter out noise of nearby channels and thus are suitable for bandpass filtering applications.  
         [0006]     Conventional planar ring filters use a planar edge coupled method for energy coupling, e.g., US Pat. App. No. 2004/0257173 A1 “apparatus and methods for split feed coupled-ring resonator-pair elliptic-function filters” and U.S. Pat. No. 6,720,848 B2 “dual mode bandpass filter having coupled modes.” For multi-layer package design, however, this planar architecture will occupy most of the surface area and thus is not suitable to small-area designs.  
         [0007]     In the present invention, energy coupling of a multi-layer package three-dimensional structure is used to design a small-area ring filter. The present invention proposes a ring millimeter-wave filter making use of an embedded microstrip line to realize a three-dimensional architecture so as to solve the above problems in the prior art.  
       SUMMARY OF THE INVENTION  
       [0008]     An object of the present invention is to propose a ring millimeter-wave filter, which makes use of a three-dimensional energy coupling method and an embedded microstrip line to reduce the whole filter circuit area so as to facilitate integration with other active and passive circuits.  
         [0009]     Another object of the present invention is to provide a ring millimeter-wave filter, which makes use of the low-temperature cofired ceramic technology to fabricate multi-layer three-dimensional coupling capacitors so as to reduce the ring filter area, hence accomplishing the effect of system packaging.  
         [0010]     To achieve the above objects, a ring millimeter-wave filter of the present invention is made by the low-temperature cofired ceramic (LTCC) multi-layer process. The ring millimeter-wave filter comprises a signal input electrode, a signal output electrode, at least two coupling capacitors, an embedded microstrip line ring, and a perturbation source. The signal input electrode is used for receiving an external signal to be processed. The signal output electrode is used for outputting the processed signal. The magnitudes of coupling capacitance of the coupling capacitors are determined according to the overlap area of an upper metal microstrip line and a lower metal layer electrically connected with the signal input electrode and the signal output electrode, respectively. The embedded microstrip line ring is connected to the lower metal layer. The signal is coupled from the signal input electrode to the embedded microstrip line ring or from the embedded microstrip line ring to the signal output electrode via the coupling capacitors. The perturbation source is located at the symmetric line of the signal input electrode and the signal output electrode and connected to the embedded microstrip line ring. The perturbation source is used to make two orthogonal modes produce coupling so as to excite the required frequency band and bandwidth. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     The various objects and advantages of the present invention will be more readily understood from the following detailed description when read in conjunction with the appended drawing, in which:  
         [0012]      FIG. 1  is a structure diagram of a ring millimeter-wave filter of the present invention;  
         [0013]      FIG. 2  is a side cross-sectional view of a ring millimeter-wave filter of the present invention;  
         [0014]      FIG. 3  is a comparison diagram of the effective dielectric constants of an embedded microstrip line of the present invention and an ordinary microstrip line; and  
         [0015]      FIG. 4  is a diagram showing the simulation and measurement results of a ring millimeter-wave filter of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]     In the present invention, a ring filter conventionally realized in a mono-layer substrate is fabricated by means of the low-temperature cofired ceramic (LTCC) multi-layer substrate process to simplify the whole design flow and also reduce the whole circuit area, hence facilitating integration with other active and passive components. The present invention accomplishes area reduction by means of three dimensional coupling. The present invention uses an embedded microstrip line ring whose effective wavelength, due to its larger dielectric constant, is smaller than that of a microstrip line ring. The whole circuit area can therefore be reduced.  
         [0017]     A ring millimeter-wave filter of the present invention is mainly divided into three parts: an electrode part, a ring part, and a perturbation part. As shown in  FIGS. 1 and 2 , the circumference of an embedded microstrip line ring  10  at layer  6  is a wavelength. A signal input electrode  12  and a signal output electrode  14  at layer  7  are used to receive an external signal to be processed and output the processed signal, respectively. Upper metal microstrip lines  16  and  18  extend from the two electrodes  12  and  14 , respectively. Two coupling capacitors  24  and  26  are formed at the overlap regions between lower metal layers  20  and  22  extending from the embedded microstrip line ring  10  and the upper metal microstrip lines  16  and  18 . The magnitudes of coupling capacitance of the coupling capacitors can be determined according to the overlap area of the upper metal microstrip lines  16  and  18  and the lower metal layers  20  and  22 . A perturbation source  28  is located at the symmetric line of the signal input electrode  12  and the signal output electrode  14  and extends from the embedded microstrip line ring  10 . The perturbation source  28  is used to make two orthogonal modes produce coupling so as to excite the required frequency band and bandwidth. Using the above three dimensional structure, the signal is coupled from the signal input electrode  12  to the embedded microstrip line ring  10  via the coupling capacitor  24  or from the embedded microstrip line ring  10  to the signal output electrode  14  via the coupling capacitor  26 .  
         [0018]     The signal input electrode  12  and the signal output electrode  14  are orthogonal to each other. Exactly below the ring structure, i.e., layer  5 , is a ground layer  30 . In the present invention, the length P of the perturbation source  28  can be changed to change the degree of perturbation so as to vary the central frequency and bandwidth.  
         [0019]     The key to the design of the three dimensional ring filter is the coupling between the input terminal and the output terminal. The magnitude of coupling is determined by the overlap between the upper and lower metal layers (layer  6  and layer  7 ). In order to increase the coupling, the upper metal microstrip lines  16  and  18  at layer  7  and the lower metal layers  20  and  22  at layer  6  have a certain overlap area. As shown in  FIG. 1 , the length a of the upper metal micrpstrip lines  16  and  18  is adjusted to achieve the required coupling of the present invention.  
         [0020]     In practical designs, the required effective wavelength of the embedded microstrip line ring  10  is first designed according the demanded working frequency. Next, the required coupling capacitors  24  and  26  are designed and calculated out by means of three dimensional coupling. The perturbation source  28  is also added to excite the required pass band. The signal input electrode  12  and the signal output electrode  14  are then placed with a spacing of a quarter wavelength, and the perturbation source  28  is placed on the symmetrical line of the signal input electrode  12  and the signal output electrode  14 . The direction of energy transfer is from the signal input electrode  12  via the coupling capacitor  24  to the embedded microstrip line ring  10 , and then via the coupling capacitor  26  to the signal output electrode  14 .  
         [0021]     As shown in  FIG. 3 , because the effective dielectric constant of embedded microstrip line is larger than that of ordinary microstrip line, its wavelength is smaller than that of ordinary microstrip line by about 10%. The filter designed with this wavelength can reduce the whole area by about 20%.  
         [0022]     To exemplify the effect of the present invention, a 3-D LTCC ring filter is made by Formosa Teletek Corporation. The LTCC process has a line-width limit of 3 mils. The thickness of each layer is 3.5 mils. The dielectric constant of the substrate is 7.8. The loss tangent at 10 GHz is 0.015. The software of Agilent ADS 2003 Momentum is used to simulate this architecture. The simulation results are shown in  FIG. 4 . During measurement, TRL calibration is used to eliminate the GSG contact effect. As can be known from  FIG. 4 , the measured insertion loss is 1.5 dB, the bandwidth is 7.5% (2.25 GHz), and the return loss is 12 dB.  
         [0023]     Although the present invention has been described with reference to the preferred embodiment thereof, it will be understood that the invention is not limited to the details thereof. Various substitutions and modifications have been suggested in the foregoing description, and other will occur to those of ordinary skill in the art. Therefore, all such substitutions and modifications are intended to be embraced within the scope of the invention as defined in the appended claims.