Patent Publication Number: US-9406989-B2

Title: Two-port non-reciprocal circuit element

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to two-port non-reciprocal circuit elements, and more specifically to a two-port non-reciprocal circuit element such as an isolator preferably for use in a microwave band. 
     2. Description of the Related Art 
     Non-reciprocal circuit elements such as isolators or circulators generally have a characteristic of transmitting a signal only in a predetermined specific direction and not transmitting a signal in the opposite direction. With the use of this characteristic, for example, an isolator is used in a transmission circuit unit of a wireless communication system such as a cellular phone. 
     A known two-port non-reciprocal circuit element of this type is described in Japanese Patent No. 4197032. The described two-port isolator includes a ferrite to which a direct-current magnetic field is applied by a permanent magnet, a first central electrode and a second central electrode which are disposed on the ferrite so as to be insulated from each other, a first capacitor electrically connected between an input port and an output port, a resistor electrically connected between the input port and the output port, a second capacitor electrically connected between the output port and a ground port, an input terminal, and an output terminal. An impedance matching capacitor is electrically connected at least between the input port and the input terminal or between the output port and the output terminal, and a coupling capacitor is electrically connected between the input terminal and the output terminal. 
     The coupling capacitor is configured to adjust an insertion loss characteristic and an isolation characteristic using the trade-off between them. However, the coupling capacitor has an impedance that decreases as the operating frequency increases, and thus, at a high operating frequency, the input port and the output port are substantially directly coupled to each other in a harmonic frequency band, resulting in it being difficult to obtain a desired harmonic attenuation. In the future, it is expected to implement a wireless communication system for high-frequency applications, and the problem described above is considered to become serious. Adding a trap circuit enables an improvement in harmonic attenuation, whereas the complexity of a structure or a circuit increases. There is also a problem of degradation in insertion loss. 
     SUMMARY OF THE INVENTION 
     Preferred embodiments of the present invention provide a two-port non-reciprocal circuit element capable of achieving a good insertion loss characteristic and a good harmonic attenuation characteristic without significantly increasing the complexity of a structure or a circuit. 
     A two-port non-reciprocal circuit element according to a first aspect of various preferred embodiments of the present invention includes a permanent magnet, a ferrite to which a direct-current magnetic field is applied by the permanent magnet, a first central electrode disposed on the ferrite and including an end electrically connected to an input port and another end electrically connected to an output port, a second central electrode disposed on the ferrite so as to intersect the first central electrode while being electrically insulated from the first central electrode, the second central electrode including an end electrically connected to the output port and another end electrically connected to a ground port, a first capacitor electrically connected between the input port and the output port, a resistor electrically connected between the input port and the output port, a second capacitor electrically connected between the output port and the ground port, an input terminal, and an output terminal, wherein an impedance matching capacitor is electrically connected at least between the input port and the input terminal or between the output port and the output terminal, and a coupling capacitor and a coupling inductor are connected in series between the input terminal and the output terminal. 
     In a second aspect of various preferred embodiments of the present invention, the coupling capacitor and the coupling inductor may be connected in series between the input terminal and the output port. 
     In a third aspect of various preferred embodiments of the present invention, the coupling capacitor and the coupling inductor may be connected in series between the input port and the output terminal. 
     In the two-port non-reciprocal circuit element described above, a series circuit including the coupling capacitor and the coupling inductor and the first capacitor define a parallel resonant circuit, and the parallel resonant circuit has a high impedance around a resonant frequency. For this reason, matching the resonant frequency of the parallel resonant circuit to a harmonic frequency which requires attenuation achieves a good harmonic attenuation characteristic. In addition, since the coupling capacitor is connected in parallel to the first capacitor, a good insertion loss characteristic is achieved. The impedance of the coupling inductor is small enough to be negligible around the operating center frequency, with substantially no degradation in insertion loss. 
     In the two-port non-reciprocal circuit element described above, furthermore, only the addition of the coupling inductor will not result in an increase in the complexity of a structure or a circuit. Additionally, since the coupling inductor is connected in series to the coupling capacitor, the coupling capacitor is sufficient to have a small capacitance value, leading to a reduction in the size of the coupling capacitor. 
     According to various preferred embodiments of the present invention, it is possible to achieve a good insertion loss characteristic and a good harmonic attenuation characteristic without significantly increasing the complexity of a structure or a circuit. 
     The above and other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an electrical equivalent circuit diagram illustrating a two-port non-reciprocal circuit element according to a first exemplary embodiment of the present invention. 
         FIG. 2  is an electrical equivalent circuit diagram illustrating a two-port non-reciprocal circuit element according to a second exemplary embodiment of the present invention. 
         FIG. 3  is an electrical equivalent circuit diagram illustrating a two-port non-reciprocal circuit element according to a third exemplary embodiment of the present invention. 
         FIG. 4  is a circuit diagram illustrating a parallel resonant circuit including a matching capacitor and a coupling capacitor. 
         FIG. 5  is an exploded perspective view of a two-port non-reciprocal circuit element. 
         FIGS. 6A and 6B  are graphs illustrating the characteristics of the two-port non-reciprocal circuit element according to the first exemplary embodiment, in which  FIG. 6A  illustrates a harmonic attenuation characteristic and  FIG. 6B  illustrates an insertion loss characteristic. 
         FIG. 7  is a graph illustrating relationships between Q factors of a coupling inductor and insertion loss in the band of 3200 MHz to 3800 MHz. 
         FIG. 8  is a graph illustrating a relationship between the Q factor of the coupling inductor and insertion loss at 3500 MHz. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Two-port non-reciprocal circuit elements according to exemplary embodiments of the present invention will be described hereinafter with reference to the accompanying drawings. 
       FIG. 1  to  FIG. 3  illustrate equivalent circuits of two-port non-reciprocal circuit elements according to first to third exemplary embodiments. The illustrated two-port non-reciprocal circuit elements are lumped-constant isolators. 
     A two-port isolator  1 A according to the first exemplary embodiment illustrated in  FIG. 1  includes a first central electrode L 1  including an end electrically connected to an input port P 1  and another end electrically connected to an output port P 2 . A second central electrode L 2  includes an end electrically connected to the output port P 2  and another end electrically connected to a ground port P 3 . A resonance capacitor C 1  and a terminating resistor R are connected electrically in parallel between the input port P 1  and the output port P 2 . A resonance capacitor C 2  is electrically connected between the output port P 2  and the ground port P 3 . Matching capacitors Cs 1  and Cs 2  are electrically connected between the input port P 1  and an input terminal  14  and between the output port P 2  and an output terminal  15 , respectively, to match impedances. A coupling capacitor Cj and a coupling inductor Lj are further electrically connected in series between the input terminal  14  and the output port P 2 . 
     Further, the first central electrode L 1  and the resonance capacitor C 1  define a parallel resonant circuit between the input port P 1  and the output port P 2 . The second central electrode L 2  and the resonance capacitor C 2  define a parallel resonant circuit between the output port P 2  and the ground port P 3 . 
     A two-port isolator  1 B according to the second exemplary embodiment illustrated in  FIG. 2  is configured such that the coupling capacitor Cj and the coupling inductor Lj are electrically connected in series between the input port P 1  and the output terminal  15 , and the other configuration is similar to that in the first exemplary embodiment. 
     A two-port isolator  1 C according to the third exemplary embodiment illustrated in  FIG. 3  is configured such that the coupling capacitor Cj and the coupling inductor Lj are electrically connected in series between the input terminal  14  and the output terminal  15 , and the other configuration is similar to that in the first exemplary embodiment. 
       FIG. 5  illustrates a schematic configuration of the isolator  1 A, and the isolator  1 A preferably includes at least a yoke  10 , a multilayer substrate  20 , a central electrode assembly  30  including a ferrite  31 , and permanent magnets  41  to apply a direct-current magnetic field to the ferrite  31 . The central electrode assembly  30  is configured such that the first central electrode L 1  and the second central electrode L 2 , which are electrically insulated from each other, are disposed on front and rear surfaces of the microwave ferrite  31  having a rectangular or substantially rectangular parallelepiped shape. A specific configuration of the central electrode assembly  30  is described in detail in, for example, Japanese Patent No. 4197032, and is well known so that it is not described here. 
     The coupling inductor Lj and the terminating resistor R each include a chip-type element. The other capacitors are incorporated into the multilayer substrate  20 . The multilayer substrate  20  is constructed by sintering a stack of a plurality of dielectric sheets on which electrodes having a predetermined shape, which define various capacitors, and interlayer connection conductors (via-hole conductors) are provided. The multilayer substrate includes, on a front surface thereof, electrodes  21  to  25 , and, on a rear surface thereof, electrodes defining and functioning as the input terminal  14  and the output terminal  15  and a ground electrode (not illustrated in  FIG. 5 ). The inductor Lj and the terminating resistor R, which are illustrated as chip-type elements in  FIG. 5 , may be incorporated into the multilayer substrate  20 , and the other capacitors may be configured as chip-type elements. 
     Prior to connecting the coupling capacitor Cj and the coupling inductor Lj to the isolator, the phase of a transmission signal at the output terminal  15  is ahead of the phase of a transmission signal at the input terminal  14  during forward transmission, whereas the phase of a transmission signal at the input terminal  14  is ahead of the phase of a transmission signal at the output terminal  15  during reverse transmission. The coupling capacitor Cj also advances the phase of a transmission signal regardless of forward transmission or reverse transmission. After the coupling capacitor Cj has been added to the isolator, accordingly, during forward transmission, a signal to be transmitted by magnetic coupling between the central electrodes L 1  and L 2  and a signal to be transmitted via the coupling capacitor Cj are strengthened by each other, resulting in an increase in the transmission signal as a whole. That is, a forward transmission characteristic with a wide bandwidth and low insertion loss is achieved. This effect becomes pronounced in accordance with an increase in the capacitance of the coupling capacitor Cj. 
     Consequently, it is not necessary to increase the length of the second central electrode L 2  to increase the inductance of the second central electrode L 2 , resulting in a reduction in the size of the isolator. In addition, since it is not necessary to increase the inductance of the second central electrode L 2 , it is not necessary to reduce the capacitance value of the resonance capacitor C 2  to such an extent that measurement or adjustment of the capacitance value of the resonance capacitor C 2  is disabled. Such an isolator thus easily supports a communication system in a high-frequency band exceeding 3000 MHz. 
     In a relatively high high-frequency frequency band, the central electrode L 1  has a high impedance and is therefore substantially electrically open. In this case, a series connection circuit in which the capacitor Cs 1  and the capacitor C 1  are connected in series is connected in parallel to the series connection circuit of the capacitor Cj and the inductor Lj (see  FIG. 4 ), resulting in a parallel resonant circuit being provided. The parallel resonant circuit has a high impedance around a resonant frequency, and thus a signal to be transmitted around the resonant frequency is significantly reduced. Matching the resonant frequency to a harmonic frequency which requires attenuation achieves a good harmonic attenuation characteristic. 
     The harmonic attenuation characteristic and the insertion loss characteristic of the isolator  1 A according to the first exemplary embodiment described above are indicated by a curved line A in  FIG. 6A  and a curved line A in  FIG. 6B , respectively. Curved lines B in the respective drawings indicate the characteristics in a comparative example in which the coupling inductor Lj is not included. 
     The characteristics described above are obtained from data of simulation with the following specifications. 
     Capacitor C 1 : 1.95 pF 
     Capacitor C 2 : 0.45 pF 
     Capacitor Cs 1 : 0.80 pF 
     Capacitor Cs 2 : 1.55 pF 
     Resistor R: 320Ω 
     Inductor Lj: 1 nH 
     Capacitor Cj: 0.40 pF 
     The isolators  1 A,  1 B, and  1 C are each configured such that only the coupling inductor Lj is added to the isolator described in Japanese Patent No. 4197032, which will not result in a significant increase in the complexity of a circuit or a structure. In addition, around a center frequency at which each of the isolators  1 A,  1 B, and  1 C operates as a non-reciprocal circuit element, the impedance of the coupling inductor Lj is small enough to be negligible, with minor degradation in insertion loss due to the addition of the inductor Lj. 
     A forward transmission characteristic with a wide bandwidth and low insertion loss is achieved, whereas an isolation characteristic with a narrow bandwidth is achieved. The reason for this is that, during reverse transmission, a reverse signal to be transmitted by magnetic coupling between the central electrodes L 1  and L 2  and a reverse signal to be transmitted via the coupling capacitor Cj are also strengthened by each other as in forward transmission, resulting in an increase in the reverse transmission signal as a whole. However, the recent specification requirements for isolators have a tendency to emphasize insertion loss over isolation, and an isolation characteristic with a narrow bandwidth often becomes less problematic. 
     A series connection of the inductor Lj and the capacitor Cj causes the impedance of a circuit to be lower than that in the case where only the capacitor Cj is connected. In order to obtain the same impedance, it is necessary to reduce the capacitance value of the capacitor Cj. Accordingly, in a case where the inductor Lj is connected, the capacitance value of the capacitor Cj is made smaller than that in the case where only the capacitor Cj is connected. In particular, in a case where the capacitor Cj is incorporated into the multilayer substrate  20 , the area of a capacitance electrode of the capacitor Cj is reduced, making it possible to reduce the size of the isolator. 
     Next, the Q factor of the coupling inductor Lj in each of the isolators  1 A,  1 B, and  1 C will be described. The Q factor of the inductor Lj is preferably greater than or equal to 10 at an operating center frequency.  FIG. 7  illustrates relationships between Q factors of the inductor Lj and insertion loss in the band of 3200 MHz to 3800 MHz, which are indicated by a curved line C for a Q factor of 10, a curved line D for a Q factor of 20, and a curved line E for a Q factor of 30.  FIG. 8  illustrates a relationship between the Q factor of the inductor Lj and insertion loss at 3500 MHz. Table 1 given below shows degradations (dB) with respect to the respective Q factors on the basis of the characteristics illustrated in  FIG. 8 . 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Lj-Q (3.5 GHz) 
                 Insertion Loss (dB) 
                 Degradation (dB) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 5 
                 0.49 
                 0.05 
               
               
                 10 
                 0.47 
                 0.03 
               
               
                 15 
                 0.46 
                 0.02 
               
               
                 20 
                 0.45 
                 0.02 
               
               
                 25 
                 0.45 
                 0.02 
               
               
                 30 
                 0.45 
                 0.02 
               
               
                 35 
                 0.45 
                 0.01 
               
               
                 No Lj 
                 0.43 
                 0.00 
               
               
                   
               
            
           
         
       
     
     As revealed in Table 1, if the Q factor of the coupling inductor Lj is greater than or equal to 10, the degradation in insertion loss due to the connection of the inductor Lj is less than or equal to 0.03 dB, achieving a low insertion loss characteristic as well as a good harmonic attenuation characteristic. 
     The coupling capacitor Cj may be configured as a chip-type element. In this case, the capacitor Cj preferably has a self-resonant frequency that is twice or more as high as the operating center frequency. That is, the chip capacitor Cj serves as an inductor at a frequency greater than or equal to the self-resonant frequency, and defines a parallel resonant circuit together with the capacitors Cs 1 , Cs 2 , and C 1 . The parallel resonant circuit has a resonant frequency that is twice or more as high as the center frequency of the isolator. The harmonic attenuation characteristics are generally required in a frequency band of the second harmonic or more. 
     The configuration described above improves the attenuation in a frequency band of the second harmonic or more. In addition, there is no need for a chip inductor or an electrode pattern to implement the inductor Lj, achieving a reduction in the size and cost of an isolator. Furthermore, the chip capacitor Cj defines and functions as a capacitor at the center frequency of the isolator, making it possible to make an insertion loss characteristic and an isolation characteristic be in a trade-off relationship with each other. 
     A two-port non-reciprocal circuit element according to the present invention is not limited to those in the exemplary embodiments described above, and a variety of changes can be made within the scope of the present invention. 
     As described above, exemplary embodiments of the present invention is suitable for use in a two-port non-reciprocal circuit element such as an isolator used in a microwave band, and is advantageous particularly to achieving a good insertion loss characteristic and a good harmonic attenuation characteristic without significantly increasing the complexity of a structure or a circuit. 
     While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.