Patent Abstract:
A dual-band bandpass filter with stepped-impedance resonators uses only one circuit to generate dual-band effect. It adopts the principle of stepped-impedance resonator, which contains a connecting section and two coupling sections. The impedance and electrical length of the connecting section and coupling sections conforms to a selected condition to generate two passbands at desired frequencies. A multi-layer broadside-coupled parallel lines structure may be applied to increase coupling-amount between the parallel lines so that the dual-band bandpass filters have broader bandwidth and less loss.

Full Description:
FIELD OF THE INVENTION 
     The present invention relates to a dual-band bandpass filter adopted for use in wireless communication and particularly to a dual-band bandpass filter with stepped-impedance resonators. 
     BACKGROUND OF THE INVENTION 
     Wireless communication has had a tremendous growth in recent years. Developments of wireless transceivers have been gradually directed to multiple bandwidths to provide more flexibility. By means of this technology, users can access different services through one multi-mode, multi-band terminal. In the previous technology, GSM and WCDMA communication systems achieve the dual-band operation by switching two separated transceivers. Such architecture requires two transceivers operating in different frequency. Hence, it requires higher cost, greater circuit area, and more power consumption. To overcome these drawbacks, a so-called concurrent dual-band architecture has been introduced. In this architecture, one transceiver can simultaneously operate in two passbands, where the key building blocks, such as low noise amplifier and bandpass filter, have two concurrent passbands and adequate the stop-band suppression. The concurrent dual-band low noise amplifier has been designed to achieve the required effect, but the dual-band bandpass filter is still not yet reported H. Miyake, S. Kitazawa, T. Ishizaki, T. Yamada, and Y. Nagatomi, “A miniaturized monolithic dual band filter using ceramic lamination technique for dual mode portable telephones,” 1997 IEEE MTT-S Int. Microwave Symp. Dig., vol. 2, pp. 789–792, June 1997, a dual-band bandpass filter was fabricated in low temperature co-fired ceramic processes. However, its structure actually included two separated filters. The filter layout at the upper four layers was designed for the pass-band of 900 MHz and layout at the lower four layers was for the pass-band of 1800 MHz. Although these two circuits were fabricated at the same low temperature co-fired ceramic chip, they had individual output and input ports, hence required additional input and output combination circuits to transmit the signal through a single pair of input and output ports. In practice, it still does not effectively reduce the circuit area and cost. 
     SUMMARY OF THE INVENTION 
     To resolve the foregoing problems, a dual-band bandpass filter with stepped-impedance resonators was provided and it requires only one circuit to generate a concurrent dual-passband effect. 
     The dual-band bandpass filter with stepped-impedance resonators according to the invention includes a circuit board, input end, output end and at least two stepped-impedance resonators. The input end, output end and resonators are mounted onto the circuit board. The input end receives signals and the output end output signals respectively. Each resonator includes a connecting section which had two ends connected respectively to a coupling section. 
     Moreover, the coupling sections of the resonators are coupled with each other. One coupling section is coupled respectively with the input end and the output end to filter input signals. Also, the multi-layer broadside-coupled parallel lines structure can be applied to implement dual-band filters with broader bandwidth and less loss. 
     The foregoing, as well as additional objects, features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are schematic diagrams of the invention. 
         FIG. 2A  is a chart showing the relationship between impendence ratio and first two resonant frequencies of the resonator according to the invention. 
         FIG. 2B  is a chart showing a full-wave simulation result of the filter of the invention. 
         FIG. 3  is a schematic diagrams of the invention adopted on a two-layer circuit board. 
         FIGS. 4A ,  4 B and  4 C are schematic diagrams of a second embodiment of the resonator of the invention. 
         FIGS. 5A and 5B  are schematic views of a third embodiment of the resonator of the invention. 
         FIG. 6  is a schematic view of the invention adopted on a multi-layer circuit board. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 1A , the dual-band bandpass filter equipped with stepped-impedance resonators according to the invention includes a circuit board  10 , an input end  21 , an output end  22 , a first resonator  30  and a second resonator  40 . The input end  21 , the output end  22 , the first resonator  30  and the second resonator  40  are mounted onto the circuit board  10 . The input end  21  receives signals to be filtered. After the signals have been filtered, they are transmitted outwards through the output end  22 . 
     The first resonator  30  has a first coupling section  31  coupling with the input end  21  and a second coupling section  32  coupling with a third coupling section  41  of the second resonator  40 . The second resonator  40  has a fourth coupling section  42  coupling with the output end  22 . Hence signals received from the input end  21  are transmitted outwards through the output end  22  through the coupling relationships set forth above. Meanwhile, each of the coupling sections can be in a broadside-coupled structure to increase the coupling. The first resonator  30  and the second resonator  40  have the same structure. The first resonator  30  is used as an example below for more details. 
     The first resonator  30  includes two symmetrical coupling sections  31  and  32  at two ends, and a connecting section  33  to bridge the two coupling sections. They are all transverse electromagnetic wave (TEM) or quasi-TEM transmission lines. Referring to  FIG. 1B , define the impedance ratio of the transmission line is: Z 2 /Z 1 =R and total electric length is: θ T =2(θ 1 +θ 2 ) 
     By means of the even-mode and odd-mode analysis method, the odd resonance condition at first resonance frequency f 1  is as follows: 
     
       
         
           
             
               
                 
                   
                     θ 
                     T 
                   
                   = 
                   
                     2 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       
                         tan 
                         
                           - 
                           1 
                         
                       
                       ⁡ 
                       
                         [ 
                         
                           
                             1 
                             
                               1 
                               - 
                               R 
                             
                           
                           ⁢ 
                           
                             ( 
                             
                               
                                 R 
                                 
                                   tan 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   
                                     θ 
                                     1 
                                   
                                 
                               
                               + 
                               
                                 tan 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   θ 
                                   1 
                                 
                               
                             
                             ) 
                           
                         
                         ] 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
             
               
                 
                   
                     θ 
                     1 
                   
                   = 
                   
                     
                       tan 
                       
                         - 
                         1 
                       
                     
                     ⁡ 
                     
                       ( 
                       
                         R 
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     The even resonance condition at second resonance frequency f 2  is as follows: 
     
       
         
           
             
               
                 
                   
                     tan 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       θ 
                       1 
                     
                   
                   = 
                   
                     
                       
                         ∞ 
                       
                       
                         
                           
                             
                               θ 
                               1 
                             
                             = 
                             
                               
                                 n 
                                 2 
                               
                               ⁢ 
                               π 
                             
                           
                           , 
                           
                             n 
                             = 
                             1 
                           
                           , 
                           2 
                           , 
                           
                             3 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             … 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     When θ 1 =θ 2 , the relationship of the ratio of first resonance frequency and the second resonance frequency and the impendence ratio R can be further derived as below: 
                       f   2       f   1       =         θ     1   ⁢   S         θ   1       =     π     2   ⁢           ⁢     tan     -   1       ⁢     R                   (   4   )                 ⇒   R     =       (     tan   ⁢       π   ⁢           ⁢     f   1         2   ⁢           ⁢     f   2           )     2             (   5   )               
where f 2  is the second resonance frequency of the resonator, and f 1  is the first resonance frequency. Hence altering the value of R may control the frequencies of two passbands, and the required dual passbands may be achieved (referring to  FIG. 2A ). Take the dual-band bandpass filter used in the wireless local area network (WLAN) of 2.4/5.2 GHz for example:
 
                 f   2       f   1       =       5.2   2.4     =     π     2   ⁢           ⁢     tan     -   1       ⁢     R                         hence   ⁢           ⁢   R     =     0.785   .           
When θ 1 =½θ 2 , the relationship of the ratio of first resonance frequency and the second resonance frequency and R may be indicated as follow:
 
                 f   2       f   1       =         tan     -   1       ⁢         R   +   2     R             tan     -   1       ⁢       R     R   +   2                   
When the circuit is complemented with a two-layer circuit board  10 , there is a first layer  11  and a second layer  12  (referring to  FIG. 3 ). The input end  21  and output end  22  are located on the first layer  11 , while the first resonator  30  and the second resonator  40  are located on the second layer  12 . The coupling relationship is still maintained. The difference between structures in  FIG. 3  and  FIG. 1  is that the input end  21  is coupled with the first coupling section  31  of the first resonator  30  through the circuit board  10 , and the fourth coupling section  42  of the second resonator  40  is coupled with the output end  22  through the circuit board  10 . As seen from the top view, the input end  21  and the first coupling section  31 , the output end  22  and the fourth coupling section  42  alike, can be fully overlapped to reduce insertion loss.
 
     Besides the example set forth above where the connecting section  33  of the first resonator  30  is collinear with the coupling sections  31  and  32 , a design of U-shaped resonator may also be formed as shown in a second embodiment in  FIGS. 4A ,  4 B and  4 C. Namely, the connecting section  33  is bent and located on one side of the coupling sections  31  and  32 . The first resonator  30  and the second resonator  40  are coupled together in the same orientation (referring to  FIG. 4A ), or in the opposite orientation (as shown in  FIG. 4B ). Furthermore, the coupling sections  31  and  32  can be also located respectively on opposite sides of the connecting section (as shown in  FIG. 4C ). Refer to  FIGS. 5A and 5B  for a third embodiment of the invention. The first coupling section  31  of the first resonator  30  is located on a first layer  11  and connecting section  33  located on both the layer  11  and the layer  12 , the second coupling section  32  is located on the second layer  12  of the circuit board  10 , and the coupling sections  31  and  32  are unoverlapped (referring to  FIG. 5A ) or overlapped—(referring to  FIG. 5B ). 
     The invention can be adopted on a multi-layer circuit board  10  as shown in third embodiment in  FIG. 6  (also referring to  FIGS. 5A and 5B ). The input end  21  is coupled with the first coupling section  31  of the first resonator  30  on a third layer  13 , the second coupling section  32  of the first resonator  30  is coupled with the third coupling section  41  of the second resonator  40  on a second layer  12 , and the fourth coupling section  42  of the second resonator  40  is coupled with the output end  22  on a first layer  11 . 
     While the preferred embodiments of the invention have been set forth for the purpose of disclosure, modifications of the disclosed embodiments of the invention as well as other embodiments thereof may occur to those skilled in the art. Accordingly, the appended claims are intended to cover all embodiments, which do not depart from the spirit and scope of the invention.

Technology Classification (CPC): 7