Abstract:
A bandpass filter is described herein. The bandpass filter has two parallel signal branches, each connected, on an input side, to an input node and, on an output side, to an output node. The two signal branches form a ring resonator, having a wave mode with a complex amplitude of {right arrow over (U)} CW =|{right arrow over (U)} CW |exp{−jφ CW } propagating in a clockwise direction and a wave mode with a complex amplitude of {right arrow over (U)} CCW =|{right arrow over (U)} CCW |exp{−jφ CCW } propagating in a counterclockwise direction. The vector sum of a resulting wave {right arrow over (U)} out  at the output node of the bandpass filter at two or more stop frequencies is: {right arrow over (U)} out ={right arrow over (U)} CW +{right arrow over (U)} CCW =0, and |{right arrow over (U)} CW |=|{right arrow over (U)} CCW | and |φ CW −φ CCW |=180°. The stop frequencies are arranged such that a passband is formed between two stop frequencies.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Pursuant to 35 USC §120, this application claims the benefit of PCT/DE2006/001695 filed Sep. 26, 2006 which claims the benefit of German Patent Application No. 102005046445.9 filed Sep. 28, 2005. Each of these applications is incorporated by reference in its entirety. 
     TECHNICAL FIELD 
     A bandpass filter that can be inserted, e.g., into a front-end circuit, will be specified. 
     BACKGROUND 
     A bandpass filter is known from publication U.S. Pat. No. 5,191,305. A filter with delay lines is known from publication U.S. Pat. No. 5,301,135. Transversal and recursive filters are known from publication U.S. Pat. No. 5,021,756. 
     SUMMARY 
     A task to be achieved is to specify a bandpass filter with low insertion loss and high insertion loss for interference signals in stop bands. 
     The bandpass filter is suitable for integration in a substrate, especially an LTCC substrate, in which a front-end module is realized. The front-end module can be designed as a multi-band module. The front-end module can be designed, e.g., as a WLAN module with two frequency bands 2.4 . . . 2.5 GHz and 4.9 . . . 5.95 GHz. 
     The transfer function of the specified bandpass filter has steep flanks. 
     A bandpass filter will be specified with signal branches that are connected in parallel and are joined on the input side and on the output side into a common signal path. Two signal branches form a ring resonator, in which a wave mode with a complex amplitude {right arrow over (U)} CW =|{right arrow over (U)} CW |exp{−jφ CW } running in the clockwise direction and a wave mode with a complex amplitude {right arrow over (U)} CCW =|{right arrow over (U)} CCW |exp{−jφ CCW } running in the counterclockwise direction can propagate. For the resulting wave {right arrow over (U)} out  at the output of the bandpass filter, for at least two stop frequencies, the condition {right arrow over (U)} out ={right arrow over (U)} CW +{right arrow over (U)} CCW ≈0 applies, wherein the amplitudes of opposed wave modes are approximately equal |{right arrow over (U)} CW |≈|{right arrow over (U)} CCW |, and wherein the phase difference of these wave modes equals 180°: |φ CW −φ CCW |=180°. 
     The transmission coefficient of each signal branch is equal in both passage directions, so that the two opposed wave modes can circulate. The signal branches feature a frequency-dependent runtime, and each acts as a phase shifter. Two different signal branches preferably feature amplitude characteristic lines and/or phase characteristic lines that are different from each other. 
     In an advantageous variant, the ring resonator includes concentrated LC elements, i.e., capacitors, inductors. In principle, an inductor can be replaced by a section of a delay or transmission line. 
     The ring resonator, however, can also be formed exclusively from line sections. The line sections arranged in the first and/or second signal branch can feature an impedance jump. This can be realized, e.g., by a stub at a suitable position of the line section. 
     The fed electromagnetic wave is divided at the input node of the first and the second signal branch into an incoming wave component running in the clockwise direction and an incoming wave component running in the counterclockwise direction. The incoming wave component led into the first (or second) branch is reflected partially back into this signal branch at the output node, at which the signal branches are joined together again. The wave mode running in the clockwise direction represents the sum of all incoming and reflected wave components rotating to the right. The wave mode running in the counterclockwise direction represents the sum of all incoming and reflected wave components rotating to the left. 
     In one variant, in the first signal branch there is an LCL component made from two series inductors and a capacitor connected to ground. In the second signal branch, there is a CLC component made from two series capacitors and an inductor connected to ground. 
     In one variant, both capacitive and also inductive elements are arranged in the series branch of the first and the second signal branch. Here, the direct-current component of the input signal can be suppressed. 
     In one advantageous variant, in the first signal branch there is a first T connection made from two series inductors and a capacitor connected to ground and a second T-connection connected to the output of the first T-connection and made from two series capacitors and an inductor connected to ground. Here, a first T-connection made from two series capacitors and an inductor connected to ground and a second T connection connected to the output of the first T-connection and made from two series inductors and a capacitor connected to ground are arranged in the second signal branch. 
     The component of the first signal branch connected to the common signal path on the input side is preferably inductive if the component of the second signal branch connected to the common signal path on the input side is capacitive and vice versa. 
     The component of the first signal branch connected to the common signal path on the output side is preferably inductive if the component of the second signal branch connected to the common signal path on the output side is capacitive or vice versa. 
     For the resulting wave {right arrow over (U)} out  at the output node of the bandpass filter, in one variant, the condition {right arrow over (U)} out ={right arrow over (U)} CW +{right arrow over (U)} CCW =0 is fulfilled at three or more stop frequencies, wherein the signals propagating in opposite directions feature approximately the same amplitudes and a phase difference of exactly 180°: |{right arrow over (U)} CW |=|{right arrow over (U)} CCW | and |φ CW −φ CCW |=180°. 
     The amplitude and phase characteristic lines of the signal branches are preferably selected so that the stop frequencies are arranged on both sides of the passband of the bandpass filter and provide steep flanks in the transmission characteristic line of the filter. 
     For the resulting wave {right arrow over (U)} out  at the output node of the bandpass filter, in one variant, the condition {right arrow over (U)} out ={right arrow over (U)} CW +{right arrow over (U)} CCW ≈0 is fulfilled in at least one stop range, wherein the signals propagating in opposite directions propagating in opposite directions have exactly the same amplitudes and a phase difference of approximately 180°: |{right arrow over (U)} CW |≈|{right arrow over (U)} CCW | and |φ CW −φ CCW |≈180°. 
     Preferably, at two or more frequencies from the stop range, the amplitudes of the wave components running in the opposite sense are equal, wherein their phase difference equals 180°: |{right arrow over (U)} CW |=|{right arrow over (U)} CCW | and |φ CW −φ CCW |=180°. 
     A harmonic of the center frequency of the bandpass filter can lie in the stop range or in the vicinity of a stop frequency. This is especially advantageous in the bandpass filters provided for the transmission path. 
     In the common path of the bandpass filter on the input and/or output side, there can be another circuit, e.g., a balanced-unbalanced transformer or an impedance converter, which each preferably include concentrated elements. 
     The bandpass filter is preferably realized in one substrate, e.g., an LTCC substrate by means of the conductor paths and the conductive areas, which are constructed in metallization planes of the substrate. Dielectric layers (for LTCC substrate ceramic layers) are arranged between two metallization layers. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The bandpass filter will be explained below with reference to schematic figures not to scale. Shown are: 
         FIG. 1A , a bandpass filter with two T-connections, which has two signal branches connected in parallel, 
         FIG. 1B , the bandpass filter according to  FIG. 1A  with measurement arrangements arranged in its signal branches; 
         FIG. 2 , a bandpass filter with four T-connections, which has two T-connections for each signal branch; 
         FIG. 3 , scattering parameters of the filter according to  FIG. 1A  at the input and output (top), amplitude response (middle) measured at the ports of the measurement arrangement, phase response (bottom) measured at the ports of the measurement arrangement; 
         FIG. 4 , perspective view of the metallization planes of a substrate with a filter according to  FIG. 2  integrated in this substrate; 
         FIG. 5 , an embodiment in which the layer configuration according to  FIG. 4  is divided into two parts arranged one next to the other. 
     
    
    
     DETAILED DESCRIPTION 
     The filter shown in  FIG. 1A  includes a signal path, which is arranged between a terminal of the input gate IN and a terminal of the output gate OUT of the filter. The signal path includes signal branches  11 ,  12  (series branches) connected in parallel. Each branch includes a circuit, which here contains only electrically passive components, i.e., capacitors, inductors, and optionally line sections, that is, no semiconductor elements, e.g., amplifier elements. Therefore, each signal branch is reciprocal, i.e., its transmission characteristic is equal in both directions, so that an electromagnetic wave can propagate in the closed loop formed by two branches connected in parallel. This loop is preferably an oscillating circuit or a ring resonator. 
     The first branch  11  includes a T-connection with two series capacitors C 1 , C 3  and one parallel inductor L 2 . The second branch  12  includes a T-connection with two series inductors L 1 , L 3  and one parallel capacitor C 2 . The series capacitors and inductors C 1 , C 3 , L 1 , L 3  form a ring resonator, in which an electromagnetic wave can circulate. 
     For N series branches connected in parallel, the signal applied to the input is divided into N parts, which are joined together again on the output side. Each signal branch represents a delay line for the corresponding sub-signal, wherein τ n  is the runtime of the signal in the n-th branch and a n  is the transmission coefficient of the n-th branch. The resulting time signal y(t) is calculated as 
               y   ⁡     (   t   )       =       ∑     n   =   1     N     ⁢       a   n     ⁢       x   ⁡     (     t   -     τ   n       )       .               
Here t is the time. The corresponding transmission characteristic S 21 (f) in the frequency range is calculated as
 
                 S   21     ⁡     (   f   )       =       ∑     n   =   1     N     ⁢       A   n     ⁢       exp   ⁡     (       -   j2π     ⁢           ⁢   f   ⁢           ⁢     τ   n       )       .               
Here f is the frequency and A n  is the transfer function of the n-th branch.
 
     It is advantageous if the amplitude responses (and/or phase responses) of different series branches of the filter are different from each other. They can be adapted to each other in such a way that a passband in the given frequency range and notches at the given stop frequencies are formed in the transfer function of the total filter. 
     The incoming electromagnetic wave applied to the input IN is divided at the electrical node  1  into an incoming component U CW,inc  running in the clockwise direction and an incoming component U CCW,inc  running in the counterclockwise direction. 
     At the electrical node  2 , a part of the component U CW,inc  passes into the series branch  12 , a part of this component is decoupled into the output path and a part is reflected back into the branch  11 , wherein a reflected component U CCW,ref  running in the counterclockwise direction is formed. At the electrical node  2 , a part of the component U CCW,inc  passes into the series branch  11 , a part of this component is decoupled into the output path and a part is reflected back into the branch  12 , wherein a reflected component U CW,ref  running in the clockwise direction is formed. 
     The sum of all wave components running in the clockwise direction forms a wave mode U CW  running in the clockwise direction: 
     
       
         
           
             
               
                 U 
                 → 
               
               CW 
             
             = 
             
               
                 ∑ 
                 k 
               
               ⁢ 
               
                 
                   { 
                   
                     
                       
                         ( 
                         
                           
                             U 
                             → 
                           
                           
                             CW 
                             , 
                             inc 
                           
                         
                         ) 
                       
                       k 
                     
                     + 
                     
                       
                         ( 
                         
                           
                             U 
                             → 
                           
                           
                             CW 
                             , 
                             ref 
                           
                         
                         ) 
                       
                       k 
                     
                   
                   } 
                 
                 . 
               
             
           
         
       
     
     k is the count of the components of the wave mode. 
     The sum of all wave components running in the counterclockwise direction forms a wave mode U CCW  running in the counterclockwise direction: 
     
       
         
           
             
               
                 U 
                 → 
               
               CCW 
             
             = 
             
               
                 ∑ 
                 k 
               
               ⁢ 
               
                 
                   { 
                   
                     
                       
                         ( 
                         
                           
                             U 
                             → 
                           
                           
                             CCW 
                             , 
                             inc 
                           
                         
                         ) 
                       
                       k 
                     
                     + 
                     
                       
                         ( 
                         
                           
                             U 
                             → 
                           
                           
                             CCW 
                             , 
                             ref 
                           
                         
                         ) 
                       
                       k 
                     
                   
                   } 
                 
                 . 
               
             
           
         
       
     
     The resulting signal {right arrow over (U)} out =|{right arrow over (U)} out |exp{−jφ out } at the output node  2  represents the vector sum of the opposed wave modes at this node: {right arrow over (U)} out ={right arrow over (U)} CW +{right arrow over (U)} CCW . 
     The magnitude of the resulting signal at a given frequency depends on the amplitude and phase relationships of the opposed wave modes {right arrow over (U)} CW  and {right arrow over (U)} CCW  at this frequency. A notch, i.e., a break in the transmission characteristic of the filter, is formed, for example, at a frequency at which the vector sum of the opposed wave modes at the output node  2  is equal to zero: {right arrow over (U)} CW +{right arrow over (U)} CCW =0 (destructive interference). This is the case when the two opposed wave modes {right arrow over (U)} CW  and {right arrow over (U)} CCW  have the same amplitude |{right arrow over (U)} CW |=|{right arrow over (U)} CCW | but a phase difference of 180°. The equality of the amplitudes means that the transmission coefficients in the two signal branches are equal at the given stop frequency. 
     Transmission takes place when the opposed wave modes at the node  2  have different amplitudes from each other, so that their sum is not equal to zero: {right arrow over (U)} CW +{right arrow over (U)} CCW ≠0. This is the case when the signal branches have different transmission coefficients from each other at the pass frequency. 
     The complex amplitude of the incoming and reflected waves is measured by means of an ideal measurement arrangement, which has a part M 1  arranged in the first signal branch and a part M 2  arranged in the second signal branch. The signal passes through this measurement arrangement without loss. 
     Each part of the measurement arrangement counts the sum of all wave components running in a certain direction—in the clockwise or counterclockwise direction. In  FIG. 3 , the measurement results are presented in the middle and at the bottom. 
     Between the nodes  1  and  3 , the amplitude response S 31  and the phase response φ 31  of the wave component U CW,inc  are detected. Between the nodes  1  and  4 , the amplitude response S 41  and the phase response φ 41  of the wave component U CCW,inc  are detected. Between the nodes  1  and  5 , the amplitude response S 51  and the phase response φ 51  of wave component U CCW,ref  reflected at node  2  are detected and between the nodes  1  and  6 , the amplitude response S 61  and the phase response φ 61  of the wave component U CW,ref  reflected at this node are detected. 
     The S-matrix of the measurement arrangement is given by 
     
       
         
           
             
               S 
               ⁡ 
               
                 [ 
                 0 
                 ] 
               
             
             = 
             
               
                 [ 
                 
                   
                     
                       0 
                     
                     
                       1 
                     
                     
                       1 
                     
                     
                       0 
                     
                   
                   
                     
                       1 
                     
                     
                       0 
                     
                     
                       0 
                     
                     
                       1 
                     
                   
                   
                     
                       1 
                     
                     
                       0 
                     
                     
                       0 
                     
                     
                       0 
                     
                   
                   
                     
                       0 
                     
                     
                       1 
                     
                     
                       0 
                     
                     
                       0 
                     
                   
                 
                 ] 
               
               . 
             
           
         
       
     
     In  FIG. 2 , a filter with four T-connections is shown. Here, on the input side a CLC element is arranged in the first branch  11 , and an LCL element is arranged in the second branch  12 . The CLC element arranged in the first branch is formed by the series capacitors C 1 , C 3  and a parallel inductor L 2 . The LCL element arranged in the second branch is formed by the series inductors L 1 , L 3  and a parallel capacitor C 2 . On the output side, an LCL element L 4 , C 5 , L 6  is arranged in the first branch, and a CLC element C 4 , L 5 , C 6  is arranged in the second branch. 
     Also like the bandpass filter shown in  FIG. 1A , the transmission characteristic of this bandpass filter features three notches. The variant according to  FIG. 2  distinguishes itself also in that the direct-current component is suppressed, because series capacitors are arranged in the two series branches connected in parallel. 
     In one variant, the signal branches can be constructed in such a way that more than three notches are generated in the transmission characteristic of the filter. 
     At least one of the inductors, e.g., L 1 -L 3  in  FIGS. 1A  and L 1 -L 6  in  FIG. 2 , arranged in the signal branches  11 ,  12  of the filter can have an inductance value of approx. zero. 
     In  FIG. 3  at the top, the transmission characteristic S 21  and the reflection coefficient S 11  of the filter are shown. The transmission characteristic S 21  shows a passband at approx. 5.5 GHz and three notches at approx. 2.6 GHz, 7.45 GHz, and 9 GHz. 
       FIG. 4  shows a component, in which the filter is realized according to  FIG. 2 . The LC elements arranged in the branches  11 ,  12  are constructed as conductor paths and conductive areas in the metallization planes of a substrate. The LC elements, especially the inductors, can also be realized, in principle, by means of via contacts, which connect two metallization planes of the substrate. 
     The capacitor C 1  is formed between the conductive areas  441  and  451 . The capacitor C 3  is formed between the conductive areas  451  and  461 . The inductor L 2  is formed by the conductor path  421 . The inductor L 4  is realized by the via contact DK 4 . The capacitor C 5  is formed between the conductive areas  402  and  491 . The inductor L 6  is realized by the conductor path  481 . 
     The inductor L 1  is realized by the conductor path  431 , and the inductor L 3  is realized by the conductor path  432 . The capacitor C 2  is formed between the conductive surfaces  401  and  411 . The inductor L 4  is formed by the via contact DK 1 , which connects the conductor path  432  and the area  452  conductively. The capacitor C 4  is formed between the conductive areas  452  and  462  and the capacitor C 6  is formed between the surfaces  462  and  471 . The inductor L 5  is formed by the conductor path  482  and the via contacts DK 2  and DK 3 . 
     The conductive areas  401  and  402  arranged in the outer metallization planes are set to ground and are used for shielding the LC elements formed in the inner metallization planes. 
     In  FIG. 5 , a variant of the realization of the filter presented in  FIG. 4  is shown. The layer configuration according to  FIG. 4  is divided into two parts, which are arranged preferably one next to the other.  501 ,  502 , and  503  are electrical connections. 
     The area  401  is here divided into the areas  401   a  and  401   b , which are connected conductively to each other by means of an electrical connection  504  and which are arranged in one metallization plane. The area  402  is divided into the areas  402   a  and  402   b , which are connected conductively to each other by means of an electrical connection  502  and which are arranged in one metallization plane. 
     
       
         
               
             
               
               
             
           
               
                   
               
               
                 List of reference symbols 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 Terminal node of the input-side signal path 
               
               
                 2 
                 Terminal node of the output-side signal path 
               
               
                 3-6 
                 Ports of the measurement device 
               
               
                 11 
                 First signal branch 
               
               
                 12 
                 Second signal branch 
               
               
                 C1-C6 
                 Capacitor 
               
               
                 DK1-DK4 
                 Via contact 
               
               
                 in 
                 Input 
               
               
                 L1-L6 
                 Inductor 
               
               
                 M1, M2 
                 Measurement arrangement 
               
               
                 out 
                 Output 
               
               
                 S11, S22 
                 Reflection coefficient at node 1 
               
               
                 S21 
                 Transfer function of the circuit between the nodes 1 and 2 
               
               
                 S31, S41, 
                 Transfer function measured between the node 1 and the nodes 
               
               
                 S51, S61 
                 3, 4, 5, and 6, respectively 
               
               
                 φ31, φ41, 
                 Phase response measured between the node 1 and the nodes 3, 
               
               
                 φ51, φ61 
                 4, 5, and 6, respectively 
               
               
                 U CW, inc   
                 Incoming wave mode running in the clockwise direction 
               
               
                 U CW, ref   
                 Reflected wave mode running in the clockwise direction 
               
               
                 U CCW, inc   
                 Incoming wave mode running in the counterclockwise 
               
               
                   
                 direction 
               
               
                 U CCW, ref   
                 Reflected wave mode running in the counterclockwise 
               
               
                   
                 direction