PATENT ABSTRACT
A filter includes a circuit board, a low-pass filter circuit, and a high-pass filter circuit. The circuit board includes at least one metal layer, and a dielectric layer attached on the at least one metal layer. The low-pass filter circuit is defined in the metal layer, and includes a main line that has two parallel portions, and a modulating circuit serving as a capacitor connected to the main line of the low-pass filter circuit. The high-pass filter circuit defined in the metal layer includes a main line that has two parallel portions and is connected to the low-pass filter circuit, and a modulating circuit serving as a conductor connected to the main line of the high-pass filter circuit.

PATENT DESCRIPTION
BACKGROUND 
     1. Technical Field 
     The disclosure relates to filters, and more particularly, to a microstrip filter. 
     2. Description of Related Art 
     Vast information interchange occurs via wireless communications systems. During transmission, information may be carried by microwave signals. To achieve successful long distance transmission, high-powered transmitters are used which can result in electromagnetic interference (EMI) in devices such as cellular phones or radio receivers. EMI may interrupt, obstruct, or otherwise degrade or limit the effective performance of the devices. As a result, various microstrip filters have been developed and adopted in the devices as EMI shields. However, a typical microstrip filter consists of discrete elements such as, resistors, capacitors, and inductors, which makes the filter bulky, costly, and environmentally hazardous. 
     What is needed is a microstrip filter addressing the aforedescribed problems. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a microstrip filter according to a first exemplary embodiment. 
         FIG. 2  is top view of a pattern of the microstrip filter of  FIG. 1 . 
         FIG. 3  is a graph of a computer simulated frequency response of the microstrip filter of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIGS. 1 and 2 , a microstrip filter  100  of an exemplary embodiment is shown. The microstrip filter  100  includes a circuit board  110 , a low-pass filter circuit  120 , and a high-pass filter circuit  130 . The low-pass filter circuit  120  and the high-pass filter circuit  130  are formed in the circuit board  110  in series interconnection with each other. 
     The circuit board  110  includes two metal layers  112 , and a dielectric layer  114  below one of the metal layers  112  and above the other one. In one exemplary embodiment, two metal layers  112  are isolated from each other by the dielectric layer  114 . The low-pass filter circuit  120  and the high-pass filter circuit  130  are formed on the metal layers  112  by means of chemical etching, physical etching, or copperplating methods. The low-pass filter circuit  120  and the circuit board  110  cooperatively construct a low-pass filter (not shown). The high-pass filter circuit  130  and the circuit board  110  cooperatively construct a high-pass filter. Either one or both of the filters may be employed in a same device. 
     Referring to  FIG. 2 , the low-pass filter circuit  120  and the high-pass filter circuit  130  both include a main line respectively indicated as  120   a ,  130   a . The linked main lines  120   a  and  130   a , each has two approximately parallel portions to reduce the length of the corresponding lines  120   a  and  130   a . The low-pass filter circuit  120  further includes a modulating circuit  120   b  composed of wide and short low impedance portions connected respectively with the main line  120   a  in parallel or series, which serve as capacitors. The high-pass filter circuit  130  further includes a modulating circuit  130   b  composed of a number of perpendicularly bent branches that extend from the main line  130   a , and that are simultaneously parallel to the longer portion of the main lines  130   a . The branches of the modulating circuit  130   b  serve as conductors. 
     The main line  120   a  of the low-pass filter circuit  120  includes an input portion  122 , a transmission portion  126 , and an output portion  128 . The modulating circuit  120   b  includes a first low impedance portion  123 , a second low impedance portion  124 , a third low impedance portion  125 , and an extending portion  127 . The input portion  122  is connected to the transmission portion  126  via the first low impedance portion  123 . The transmission portion  126  is successively connected to the extending portion  127 . The output portion  128  is successively connected to the transmission portion  126  and the extending portion  127 . The second low impedance portion  124  and the third low impedance portion  125  are correspondingly connected to the extending portion  127  and the output portion  128  through two branching portions that correspondingly perpendicularly extend from the extending portion  127  and the output portion  128 . 
     The input portion  122  includes a first vertical segment  122   a  and a first horizontal segment  122   b . The first horizontal segment  122   b  perpendicularly extends from a terminal end of the vertical segment  122   a  to the first low impedance portion  123 . The first horizontal segment  122   b  is shorter than the first vertical segment  122   a  in length. 
     The transmission portion  126  includes a second horizontal segment  126   a , a second vertical segment  126   b , a third horizontal segment  126   c , and a third vertical segment  126   d . The second horizontal segment  126   a  is connected between the first low impedance portion  123  and the second vertical segment  126   b . The second vertical segment  126   b  perpendicularly extends from an end of the second horizontal segment  126   a . The second vertical segment  126   b  of the transmission portion  126  is parallel to the first vertical segment  122   a  of the input portion  122  but extends along the vertical direction away from the first vertical segment  122   a . In one exemplary embodiment, a total length of the first vertical segment  122   a  and the second vertical segment  126   b  is about 18 mm. The third horizontal segment  126   c  perpendicularly extends from the terminal end of the second vertical segment  126   b . The third horizontal segment  126   c  is parallel to the second horizontal segment  126   a  and extends horizontally towards the first low impedance portions  123 . The third vertical segment  126   d  perpendicularly extends from the terminal end of the third horizontal segment  126   c  towards the first low impedance portion  123 . Exemplarily, the third horizontal segment  126   c  is longer than the second horizontal segment  126   a.    
     The extending portion  127  includes an extending segment  127   a  and a bent segment  127   b . The extending segment  127   a  perpendicularly extends from the terminal end of the third vertical segment  126   d  of the transmission portion  126 , and horizontally extends away from the second vertical segment  126   c  of the transmission portion  126 . The bent segment  127   b  perpendicularly extends from the terminal end of the extending segment  127   a , along a direction away from the third horizontal segment  126   c . The length of the extending segment  127   a  is longer than that of the bent segment  127   b.    
     The output portion  128  includes a fourth horizontal segment  128   a , a fourth vertical segment  128   b  and a fifth horizontal segment  128   c . The fourth horizontal segment  128   a  perpendicularly extends from the terminal end of the third vertical segment  126   d  of the transmission portion  126  towards the second vertical segment  126   b  of the transmission portion  126 . The length of the fourth horizontal segment  128   a  is shorter than that of the third horizontal segment  126   c  of the transmission portion  126 . The fourth vertical segment  128   b  perpendicularly extends from the terminal end of the fourth horizontal segment  128   a  towards the second horizontal segment  126   a  of the transmission portion  126 . The fourth vertical segment  128   b  is longer than the bent segment  127   b  of the extending portion  127 . The fifth horizontal segment  128   c  perpendicularly extends from the terminal end of the fourth vertical segment  128   b  along a direction away from the second vertical segment  126   b  of the transmission portion  126 . The terminal end of the fifth horizontal segment  128   c  extends beyond the terminal end of the bent segment  127   b  of the extending portion  127 . 
     The second low impedance portion  124  is positioned between the extending segment  127   a  of the extending portion  127  and the fifth horizontal segment  128   c  of the output portion  128 , and connected to the fifth horizontal segment  128   c  via a branch perpendicularly extended from the middle of the fifth horizontal segment  128   c . The third low impedance portion  125  is connected to the terminal end of the bent segment  127   b  of the extending portion  127 . 
     In the low-pass filter circuit  120 , sections of the main line  120   a  may be bent to reduce the size of the low-pass filter circuit  120 . The modulating circuit  120   b  serves as capacitors for filtering high frequency signals. 
     The main line  130   a  of the high-pass filter circuit  130  includes a main portion  132  and a terminal portion  136 . The modulating circuit  130   b  of the high-pass filter circuit  130  includes a pair of first coupled lines  133 , a pair of second coupled lines  134 , and a pair of third coupled lines  135 . The main portion  132  is connected to the terminal end of the fifth horizontal segment  128   c  of the output portion  128 , and portion of the main portion  132  is collinear (not shown) to the fifth horizontal segment  128   c . The parallel branches of the main portion  132  comprise a “U” shaped microstrip facing the low-pass filter circuit  130  which significantly saves space. 
     The terminal portion  136  is perpendicularly connected to the terminal end of the main portion  132  and parallel to the first vertical segment  122   a  of the input portion  122 . 
     The first coupled lines  133  symmetrically extend from the two parallel segments of the main portion  132 . Each of the first coupled line  133  includes a first connecting segment  133   a  and a first bent segment  133   b . The first connecting segment  133   a  of each first coupled line  133  perpendicularly extends from the main portion  132  adjacent to the opening of the U-shaped main portion  132 . The first bent segment  133   b  of each first coupled line  133  extends from the terminal end of the first connecting segment  133   a  towards the closed end of the U-shaped main portion  132 . The first bent segments  133   b  of the first coupled line  133  are parallel to the parallel portions of the main portion  132 . 
     The second coupled line  134  symmetrically extend from the two parallel portions of the main portion  132 , adjacent and parallel to the corresponding first coupled line  133 . Each of the second coupled line  134  includes a second connecting segment  134   a  and a second bent segment  134   b . The second connecting segment  134   a  of each second coupled line  134  perpendicularly extends from the main portion  132 , adjacent to the first connecting segment  133   a  of the first coupled line  133 . The length of the second connecting segment  134   a  of the second coupled line  134  is shorter than that of the first connecting segment  133   a  of the first coupled line  133 . The second bent segment  134   b  of each second coupled line  134  perpendicularly extends from the terminal end of the connecting segment  134   a  of the second coupled line  134  towards the closed end of the U-shaped microstrip  132 . The second bent segments  134   b  of the second coupled line  134  are parallel to the parallel portions of the main portion  132 . The length of the second bent segment  134   b  of the second coupled line  134  is shorter than that of the first bent segment  133   b  of the first coupled line  133 . 
     The third coupled line  135  symmetrically extend from the two parallel portions of the main portion  132 , adjacent to the closed end of the U-shaped main portion  132 . Each of the third coupled line  135  includes a third connecting segment  135   a  and third bent segment  135   b . The third connecting segment  135   a  of each third coupled line  135  perpendicularly extends from the main portion  132 . The length of the third connecting segment  135   a  of the third coupled line  135  is shorter than that of the second connecting segment  134   a  of the second coupled line  134 . The third bent segment  135   b  of each third coupled line  135  perpendicularly extends from the terminal end of the third connecting segment  135   a , towards the opening of the main portion  132 . The third bent segments  135   b  of the third coupled line  135  are parallel to the parallel portions of the main portion  132 . The distance between the second connecting segment  134   a  of the second coupled line  134  and the third bent segment  135   b  of the third coupled line  135 , is about two times the length of the third bent segment  135   b  of the third coupled line  135 . 
     The first, second and third coupled lines  133 ,  134 ,  135  serve as conductors to filter low frequency signals. 
     Referring to  FIG. 3 , a computer simulated frequency response of the microstrip filter  100  is shown. Waves whose frequencies fall within 0.8 GHZ˜1.55 GHG pass the microstrip filter  100  low insertion loss. However, waves whose frequencies fall beyond 0.8 GHZ˜1.55 GHG are well filtered. 
     The microstrip filter  100  is composed of interconnecting the low-pass filter circuit  120  and the high-pass filter circuit  130 , in series. The low-pass filter circuit  120  and the high-pass filter circuit  130  are designed small in size by parallely configuring the main lines thereof. The modulating circuit  120   a ,  130   a  are connected respectively to the main line  120   a / 130   a  in series or parallel, thereby forming capacitors and conductors to reduce, or even, eliminate EMI. The microstrip filter  100  is compact, cost-efficient, and environmentally friendly. Accordingly, the low-pass filter circuit  120  and high-pass filter circuit  130  can each be used independently, without prejudice to the present embodiment. 
     It is believed that the present embodiments and their advantages will be understood from the foregoing description, and it will be apparent that various changes may be made thereto without departing from the spirit and scope of the disclosure or sacrificing all of its material advantages, the examples hereinbefore described merely being preferred or exemplary embodiments of the disclosure.