Patent Publication Number: US-6215371-B1

Title: Non-reciprocal circuit element with a capacitor between the shield conductor and ground to lower the operating frequency

Description:
TECHNICAL FIELD 
     The present invention relates to a non-reciprocal circuit element used in a microwave band radio device, for example in a mobile communication device such as a portable telephone. 
     BACKGROUND ART 
     In accordance with recent downsizing of mobile communication devices, demand for downsizing of non-reciprocal circuit elements such as isolators or circulators used in the communication devices has increased. 
     A conventional lumped element type circulator has an assembled circulator element with a circular plane shape and a basic structure as shown in an exploded oblique view of FIG.  1 . 
     In the figure, a reference numeral  100  denotes a circular substrate made of a non-magnetic material such as a glass-reinforced epoxy. Center conductors (inner conductors)  101  and  102  are formed on the top face and next to the bottom face of the non-magnetic material substrate  100 , respectively. These inner conductors  101  and  102  are electrically connected with each other by via holes  103  passing through the substrate  100 . Circularly shaped members  104  and  105  made of a ferromagnetic material are attached to the both faces of the non-magnetic material substrate  100  having the inner conductors  101  and  102  so that rotating RF (Radio Frequency) magnetic fluxes are induced In these ferromagnetic members  104  and  105  due to an RF power applied to the inner conductors  101  and  102 . The conventional circulator element of the circulator has a circular plane shape and is constructed by assembling, namely piling and bonding, the ferromagnetic members  104  and  105  on the both sides of the non-magnetic material substrate  100 . 
     The circulator as a whole is constructed, as shown in its exploded oblique view of FIG. 2, by stacking and fixing in sequence the ferromagnetic members  104  and  105 , grounding conductor electrodes  106  and  107 , exciting permanent magnets  108  and  109  and a metal housing separated to upper and lower parts  110  and  111  on the both side of the non-magnetic material substrate  100  having the inner conductors  101  ( 102 ), respectively. The housing parts  110  and  111  form a magnetic path of the magnetic flux from and to the exciting permanent magnets  108  and  109 . 
     If a RF power Is applied to the inner conductors  101  and  102  through terminal circuits not shown, RF magnetic flux rotating around the inner conductors  101  and  102  will be produced In the ferromagnetic members  104  and  105 . Under this state, If a dc magnetic field perpendicular to the RF magnetic flux is applied from the permanent magnets  108  and  109 , the ferromagnetic members  104  and  105  present different permeability μ +  and μ −  depending upon rotating sense of the RF magnetic flux, as shown in FIG. 3. A circulator utilizes this difference of the permeability depending upon the rotating sense. Namely, a propagation velocity of the RF signal in the circulator element will differ in accordance with the rotating sense and thus the signals transmitting to the opposite directions will cancel each other, thereby preventing the propagation of the signal to a particular port. 
     A non-propagating port is determined in accordance with its angle against a driving port due to the permeability μ +  and μ −  of the ferromagnetic member. For example, if ports A, B and C are arranged in this order along a certain rotating sense, the port B will be determined as the non-propagating port against the driving port A and the port C will be determined as the non-propagating port against the driving port B. Terminating one port of thus arranged circulator might constitute an isolator. Termination of the port can be realized by connecting to the port a matched resistor such as a chip resistor, or a thick or thin film resistor formed on a substrate for providing a resonance capacitor. 
     In such non-reciprocal circuit element, the ratio of volume occupied by the permanent magnet(s) is typically larger than that of another components. This has made difficult to downsize the non-reciprocal circuit element. 
     Most of conventional lumped element circulators may have a structure represented by an equivalent circuit shown in FIG.  4 . In this case, one end (outer conductor)  400  of each inductor of the circulator is directly connected to the ground. 
     Known in this field is, in order to widen frequency band of a circulator, to insert a serial resonance circuit  501  for adjusting eigen values of in-phase (equal phase) excitation between a common connection point (outer conductor)  500  to which one end of each inductor of the circulator is commonly connected and the ground, as shown in an equivalent circuit of FIG.  5 . 
     In general, to obtain three-port circulator operation, it is necessary to keep those admittances at in-phase excitation, positive phase excitation and negative phase excitation thereof have relationship of angular difference of 120 degrees with each other. The admittances at the positive phase excitation and the negative phase excitation will generally vary depending upon frequency change but admittance at the in-phase excitation will never change. Thus, if the frequency changes greatly, it is impossible to fees the relationship of angular difference of 120 degrees in the admittances causing that circulator operation cannot be expected. As a result, the operation frequency band of the circulator is limited to a narrower band. 
     Contrary to this, as aforementioned, by additionally inserting the serial resonance circuit for adjusting eigen values of in-phase excitation, the relationship of angular difference of 120 degrees in the admittances can be kept for a long time resulting the operation frequency band of the circulator to widen. However, the addition of the LC serial resonance circuit results of increase in the number of components of the circulator and therefore invites difficulty of downsizing of the circulator. In addition, since it is very difficult to make a small and high-performance inductor, the LC serial resonance circuit to be added will become large in size. 
     Japanese Patent Publication No.49(1984)-28219 discloses a circulator with capacitors each of which is inserted between one end of each inner conductor and the grounded conductor. An equivalent circuit of this circulator is shown in FIG.  6 . As will be understood from the figure, in the circulator, capacitors  601 ,  602  and  603  are connected to respective ends of three inner conductors. However, according to this structure, these capacitors will exert an influence upon not only eigen values of In-phase excitation but also eigen values of both positive and negative phase excitations. Therefore, as well as the conventional art shown in FIG. 4, when the frequency changes greatly, it is impossible to keep the relationship of angular difference of 120 degrees in the admittances causing that circulator operation cannot be expected. As a result, the operation frequency band of the circulator is limited to a narrower band. 
     Temperature characteristics of the non-reciprocal circuit element will be discussed hereinafter. 
     There are various factors that will effect on the temperature characteristics of a non-reciprocal circuit element such as a circulator. It is considered that the main factor is temperature characteristics of saturation magnetization in the ferromagnetic material such as YIG (yttrium iron garnet) used for the circulator element, or the temperature characteristics of the permanent magnet(s) for providing bias magnetic field. In general, change in the temperature characteristics of the ferromagnetic material such as YIG used is larger than that of the bias magnetic field. Thus, the higher the temperature of the circulator, the higher its operation frequency becomes. This causes effective frequency band to be used to become narrower. Thus, in general, gadolinium is substituted in YIG to improve the temperature characteristics of saturation magnetization in YIG. However, the substitution of gadolinium causes loss of YIG to increase and therefore invites increased insertion loss of the circulator. Also, such substitution cannot perfectly adjust the temperature characteristics. 
     As aforementioned, with the spread of and downsizing of recent mobile communication devices, the non-reciprocal circuit elements themselves are requested to be manufactured in smaller size, in lighter weight and in lower height. In order to satisfy these requirements, it is important to make components of the non-reciprocal circuit element, particularly permanent magnet(s), in smaller size. 
     The conventional art has another problem that if the non-reciprocal circuit element is made in smaller size, its operation frequency will increase and thus it is difficult to obtain a desired operation frequency. 
     DISCLOSURE OF INVENTION 
     It is therefore an object of the present invention to provide a non-reciprocal circuit element with smaller size, lighter weight and lower height by lowering operation magnetic field of the non-reciprocal circuit element to downsize its permanent magnet(s), and by lowering operation frequency. 
     Another object of the present invention is to provide a non-reciprocal circuit element that can be fabricated without changing material used and can optionally adjust temperature characteristics without inviting increased insertion loss. 
     According to the present invention, a non-reciprocal circuit element includes a capacitor connected between a shield conductor and a ground of the non-reciprocal circuit element, for adjusting only eigen values of in-phase excitation. 
     Also, according to the present invention, a non-reciprocal circuit element includes a plurality of inner conductors intersecting such that they remain insulated from each other, a shield conductor connected in common to one end of each of the inner conductors, and a capacitor connected between the shield conductor and a ground of the non-reciprocal circuit element, for adjusting only eigen values of in-phase excitation. 
     Since a capacitor is connected between a shield conductor that is commonly connected to one ends of inner conductors and a ground, for adjusting only eigen values of in-phase excitation, both center frequency of isolation and applied bias magnetic field can be simultaneously decreased. By lowering the operation frequency, a smaller sized circulator element can be used. As a result, a non-reciprocal circuit element with smaller size, lighter weight and lower height can be realized. In addition, by lowering operation magnetic field, a smaller sized permanent magnet can be used, resulting further downsizing of the non-reciprocal circuit element to realize. Furthermore, since such effects can be obtained by merely adding a capacitor, downsizing of the non-reciprocal circuit element will be expedited. 
     Selecting the capacitance value of this additional capacitor can optionally change the amount of frequency change per unit of magnetic field dF/dH. If dF/dH increases, the temperature characteristics of the non-reciprocal circuit element is affected more strongly by the temperature characteristics of the bias magnetic field and thus there occurs an effect as if the temperature characteristics of the bias magnetic field increases. As a result, the temperature characteristics of the circulator can be improved. The dF/dH can be optionally changed depending upon the capacitance value of the additional capacitor. Thus, the temperature characteristics of the circulator can be optionally adjusted by selecting the capacitance value. If the capacitance value is determined to an optimum value, a circulator with substantially constant temperature characteristics may be realized. 
     It is preferred that the additional capacitor is a capacitor with a capacitance value of Cs [pF] which satisfies Cs×C≦1500, where C [pF] is a parallel resonance capacitance value of the non-reciprocal circuit element. More preferably, the additional capacitor is a capacitor with a capacitance value of Cs [pF] which satisfies Cs×C≦900. 
     In an embodiment of the present invention, the inner conductors are strip lines folded on the ferromagnetic material body. In this case, the additional capacitor preferably includes the shield conductor, the ground and a resin material that is inserted between the shield conductor and the ground as a dielectric material. 
     In another embodiment of the present invention, the inner conductors are conductors formed integrally in the ferromagnetic material body. In this case, the additional capacitor preferably includes the shield conductor, the ground and a ceramic material that is inserted between the shield conductor and the ground as a dielectric material. 
     In a further embodiment of the present invention, the additional capacitor is a capacitor formed integrally with the ferromagnetic material body. 
     It is preferred that input/output capacitors are formed between input/output ports and the ground, or between input/output ports and the shield conductor. 
     Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is an exploded oblique view showing the already described circulator element of the conventional lumped element type circulator; 
     FIG. 2 is an exploded oblique view illustrating the assemble of the already described conventional circulator; 
     FIG. 3 shows characteristics of gyromagnetic permeability of the ferromagnetic material; 
     FIG. 4 is an equivalent circuit diagram of the already described conventional circulator; 
     FIG. 5 is an equivalent circuit diagram of the already described conventional circulator with the added serial resonance circuit for adjusting eigen values of in-phase excitation; 
     FIG. 6 is an equivalent circuit diagram of the already described conventional circulator described in Japanese Patent Publication No.49(1984)-28219; 
     FIG. 7 is an exploded oblique view schematically illustrating whole configuration and assembling order of a lumped element type isolator as a preferred embodiment of a non-reciprocal circuit element according to the present invention; 
     FIG. 8 is a plan view illustrating expanded state before folding with respect to inner conductors and a shield conductor of the embodiment shown in FIG. 7; 
     FIG. 9 is a plan view illustrating an assembly constituted by folding the inner conductors of the embodiment shown in FIG. 7 on a ferrite core; 
     FIG. 10 is an oblique view illustrating an assembled lumped element type isolator of the embodiment shown in FIG. 7; 
     FIG. 11 is an equivalent circuit diagram of the non-reciprocal circuit element of the embodiment shown in FIG. 7; 
     FIG. 12 illustrates isolation characteristics when one of capacitors with various capacitance values Cs is added; 
     FIG. 13 illustrates isolation characteristics when a capacitor with a capacitance value Cs is added and applied magnetic field is optimized; 
     FIG. 14 illustrates change in operation frequency characteristics when the capacitance value Cs is varied; 
     FIG. 15 illustrates change in applied magnetic field characteristics when the capacitance value Cs is varied; 
     FIG. 16 illustrates change in dF/dH when the capacitance value Cs is varied; 
     FIG. 17 illustrates change in isolation when a capacitor with a capacitance value Cs=1 pF is added and applied magnetic field is varied; 
     FIG. 18 illustrates change in isolation when no capacitor with a capacitance value Cs is added and applied magnetic field is varied; 
     FIG. 19 is an oblique view schematically illustrating configuration of a circulator element part of a lumped element type isolator as another embodiment of a non-reciprocal circuit element according to the present invention; 
     FIG. 20 is an A—A sectional view of FIG. 19; 
     FIG. 21 is an exploded oblique view schematically illustrating whole configuration of the embodiment shown in FIG. 19; 
     FIG. 22 is an exploded oblique view schematically illustrating whole configuration of a lumped element type isolator as a further embodiment of a non-reciprocal circuit element according to the present invention; and 
     FIG. 23 is an equivalent circuit diagram of the non-reciprocal circuit element of the embodiment shown in FIG.  22 . 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Hereinafter, an example of a lumped element type isolator as a preferred embodiment of a non-reciprocal circuit element according to the present invention will be described. Although this embodiment is in a case of the lumped element type isolator, the present invention can be applied to a distributed element type isolator, a lumped element type circulator and a distributed element type circulator. 
     FIG. 7 is an exploded oblique view schematically illustrating whole configuration and assembling order of the lumped element type isolator as a preferred embodiment of a non-reciprocal circuit element according to the present invention, FIG. 8 is a plan view illustrating expanded state before folding with respect to inner conductors and a shield conductor of the embodiment shown in FIG. 7, FIG. 9 is a plan view illustrating an assembly constituted by folding the inner conductors of the embodiment shown in FIG. 7 on a ferrite core, and FIG. 10 is an oblique view illustrating the assembled lumped element type isolator of the embodiment shown in FIG.  7 . 
     In these figures, reference numeral  700  denotes a shield conductor (shield plate),  701   a ,  701   b  and  701   c  denote strip lines which constitute the three inner conductors, and  702  denotes the circular plate shaped ferrite core made of YIG, respectively. 
     The shield conductor  700  and the strip lines  701   a ,  701   b  and  701   c  are formed by stamping of a copper foil, as shown in FIG. 8, so that the three strip lines  701   a ,  701   b  and  701   c  are elongated and protruded from the shield conductor  700  in radial directions. The end portions of the strip lines  701   a  and  701   b  are used as input/output terminals and the end portion of the strip line  701   c  is terminated. As shown in FIGS. 7 and 9, the shield conductor  700  (FIG.  8 )is formed in a circular shape with substantially the same size as that of the ferrite core  702  disposed thereon. 
     The assembly  703  consisting of the strip lines as for the three inner conductors and the circular ferrite core is formed as follows. First, the circular ferrite core  702  is disposed on the shield conductor  700 . Thereafter, one of strip lines  701   a  and  701   b  with the input/output terminals is folded along the peripheral edge of the ferrite core  702 , and then the other one is also folded. Finally, the strip line  701   c  with the terminal to be connected to a terminating resistance along the peripheral edge of the ferrite core  702 . Thus, as shown in FIGS. 7 and 9, the assembly  703  with three strip lines  701   a ,  701   b  and  701   c  folded on the upper face of the circular ferrite core  702  to cross with each other is formed. 
     Although it is not shown in the figures, when the strip lines  701   a ,  701   b  and  701   c  are folded on the circular ferrite core  702 , insulating sheets made of polyimide material are inserted between the strip lines  701   a ,  701   b  and  701   c  to make electrical insulation among them. 
     As will be understood from FIGS. 7 and 10, the lumped element type isolator has, other than the assembly  703 , an inner substrate  704  with the terminating resistor and necessary capacitors, a resin housing  705  shaped in a rectangular frame, a permanent magnet  706  for applying DC magnetic field to the assembly  703  in the thickness direction of the ferrite core  702 , upper and lower covers  707  and  708  attached in integral to the resin housing  705  to cover upper and lower sides of the housing  705 , which operate as soft magnetic yokes, a terminal substrate  709  used for plane-mounting, and an insulating sheet  710  for forming an additional capacitor (capacitance value of Cs) according to the present invention, which will adjust only eigen values of in-phase excitation. 
     The dielectric insulating sheet  710  is inserted between the assembly  703  and the lower cover  708  so as to form the additional capacitor with the capacitance value Cs, in which the shield conductor  700  of the assembly  703  and the under cover  708  operate, as capacitor electrodes. The insulating sheet  710  can be made of any dielectric material other than resin material such as polyimide. 
     The inner substrate  704  made of dielectric material has a through hole  711  at its center portion for holding the assembly  703  inserted therein. On the top face of the substrate  704 , capacitor electrodes  704   a ,  704   b  and  704   c  with predetermined shapes, to which the end portions of the strip lines  701   a ,  701   b  and  701   c  are electrically connected, and a shield electrode  704   d  are formed. On the top face, furthermore, a terminating resistor  712  made of for example ruthenium oxide is formed by a thick-film printing. The terminating resistor  712  is connected between the capacitor electrode  704   c  connected with the end portion of the strip line  701   c  and the shield electrode  704   d . Although it is not shown in the figures, next to the bottom face of the substrate  704 , a ground electrode that forms input/output capacitors between it and the capacitor electrodes  704   a ,  704   b  and  704   c  is formed. This ground electrode is directly grounded. 
     The assembly  703  is fitted in the hole  711  of the substrate  704  and then the end portions of the strip lines  701   a ,  701   b  and  701   c  are electrically connected to the capacitor electrodes  704   a ,  704   b  and  704   c  on the substrate  704 , respectively. 
     The inner substrate  704  with the fitted assembly  703  is disposed on the lower cover  708  made of soft magnetic metal material such as iron via the insulating sheet  710 . 
     The rectangular frame shaped housing  705  has two connection electrodes  705   a  and  705   b  at positions corresponding to the end portions or input/output terminals of the two strip lines  701   a  and  701   b , respectively. The housing  705  also has a ground connection electrode  705   d  for grounding one end of the terminating resistor  712 , at a position of the ground electrode  704   d . To the bottom side of the resin housing  705 , the under cover  708  with the assembly  703  attached thereto is assembled. Soldering to the inner end portions of the connection electrodes  705   a  and  705   b  respectively connects the end portions of the strip lines  701   a  and  701   b  and also the capacitor electrodes  704   a  and  704   b . Soldering to the inner end portion of the ground connection electrode  705   d  connects the ground electrode  704   d.    
     The permanent magnet  706  is fixed in the upper cover  707  made of soft magnetic metal material such as iron. The upper cover  707  containing the permanent magnet  706  is assembled on the resin housing  705 , and the upper cover  707  and the lower cover  708  are caulked with each other to make them in one piece. Thus, the permanent magnet  706  and the ferrite core  702  with the strip lines  701   a ,  701   b  and  701   c  formed thereon are arranged inside and surrounded by a magnetic yoke constituted by these upper and lower covers  707  and  708 . 
     The terminal substrate  709  has next to its bottom face two plane-mounting terminal electrodes  709   a  and  709   b  used for connection with external circuits at positions corresponding to the input/output terminal end portions of the two strip lines  701   a  and  701   b , and a ground electrode  709   d . The terminal substrate  709  also has on its top face electrodes  709   a ′ and  709   b ′ which are respectively connected to the plane-mounting terminal electrodes  709   a  and  709   b  through via holes (not shown), and an electrode  709   d ′ which is connected to the ground electrode  709   d  through a via hole (not shown). This terminal substrate  709  is mounted next to the bottom face of the under cover  708 . The electrodes  709   a ′ and  709   b ′ are connected by soldering to the outer end portions of the connection electrodes  705   a  and  705   b  of the resin housing  705 , respectively. The electrode  709   d ′ is connected by soldering to the bottom face of the under cover  708 . 
     Thus, the lumped element type isolator in which the input/output terminal end portions of the two strip lines  701   a  and  701   b  are electrically connected to the plane-mounting terminal electrodes  709   a  and  709   b  of the terminal substrate  709 , and the end portion of the strip line  701   c  is terminated by being connected to the ground electrode  709   d  through the terminating resistor  712  is provided. 
     A plurality of samples with the same structure as the above-mentioned lumped element type isolator but with different values of Cs×C were fabricated where C is input/output capacitance. The size of the circular ferrite core  702  is 3.5 mm in diameter and 0.4 mm in thickness. 
     For these samples, center frequency of isolation, relative intensity of applied bias magnetic field, and changed amount of center frequency of isolation when the temperature varies from −25° C. to 85° C. were measured, respectively. The measured results are indicated in Table 1. For comparison, a sample of the isolator with no additional capacitor was fabricated and the above-mentioned characteristics were also measured (Cs×C=0). 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 Center 
                   
                 Changed 
               
               
                   
                   
                 Frequency 
                   
                 Amount 
               
               
                   
                   
                 of 
                 Applied 
                 of Center 
               
               
                   
                   
                 Isolation 
                 Magnetic 
                 Frequency 
               
               
                   
                 Cs × C 
                 (MHz) 
                 Field 
                 (MHz) 
               
               
                   
                   
               
             
            
               
                   
                  0 
                 936 
                 1.00 
                 35 
               
               
                   
                 580  
                 892 
                 0.99 
                 33 
               
               
                   
                 390  
                 875 
                 0.99 
                 33 
               
               
                   
                 50 
                 848 
                 0.96 
                 33 
               
               
                   
                 20 
                 830 
                 0.95 
                 33 
               
               
                   
                 10 
                 815 
                 0.95 
                 33 
               
               
                   
                   
               
            
           
         
       
     
     Other samples with the size of the circular ferrite core  702  of 2.5 in diameter and 0.4 mm in thickness were fabricated and similar measurements were executed. The measured results are indicated in Table 2. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
                 Center 
                   
                 Changed 
               
               
                   
                   
                 Frequency 
                   
                 Amount 
               
               
                   
                   
                 of 
                 Applied 
                 of Center 
               
               
                   
                   
                 Isolation 
                 Magnetic 
                 Frequency 
               
               
                   
                 Cs × C 
                 (MHz) 
                 Field 
                 (MHz) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                  0 
                 1007 
                 1.00 
                 6.75 
               
               
                   
                 40 
                  920 
                 0.91 
                 −5.5 
               
               
                   
                   
               
            
           
         
       
     
     As will be apparent from Tables 1 and 2, addition of the capacitor with the capacitance value Cs will present not only lowering of center frequency of isolation and lowering of applied bias magnetic field but also improvement of temperature characteristics of the lumped element type isolator. 
     The isolation characteristics and temperature characteristics of the non-reciprocal circuit element according to the present invention will be described hereinafter with reference to calculation result in its simulation. 
     In general, an admittance of in-phase excitation y 1 , an admittance of positive phase excitation y 2  and an admittance of negative phase excitation y 3  with respect to a three-port non-reciprocal circuit element can be indicated as:          y   1     =     jωC   +     1     jωL   1                   y   2     =     jωC   +     1     jωL   2                   y   3     =     jωC   +     1     jωL   3                         
     where C is a parallel resonance capacitance, L 1  is an inductance of in-phase excitation, L 2  is an inductance of positive phase excitation, and L 3  is an inductance of negative phase excitation. 
     By measuring C L 1 , L 2  and L 3 , the admittances y 1 , y 2  and y 3  can be calculated from these equations, and then isolation characteristics can be calculated from the following equations:          s   i     =         y   0     -     y   1           y   0     -     y   1                         
     
       
           S   31 =⅓( s   1   +s   2   e   j2π/3   +s   3   e   −j2π/3)   
       
     
     where y 0  is an eigen admittance of the circuit, s is eigen values of a scattering matrix and S 31  is isolation. 
     An equivalent circuit of the non-reciprocal circuit element or the circulator in this embodiment is shown in FIG. 11 in comparison with that of the conventional circulator shown in FIG.  4 . As will be apparent by comparing these figures, according to this embodiment, ends of the three inner conductors which consist of three inductors connected together and a capacitor  1100  with a capacitance value Cs for adjusting the eigen values of in-phase excitation is additionally connected between the connected ends of the three inner conductors and the ground. The non-grounded electrode of the capacitance  1100  shown in FIG. 11 corresponds to the shield conductor  700 . In this case, the capacitance value Cs acts only the admittance of in-phase excitation and represented as follows.          y   1     =     jωC   -     j        1       ω                   L   1       -     3     ω                 Cs                               
     FIG. 12 shows calculation results of isolation characteristics when a capacitance value Cs of the additional capacitor  1100  is varied. The isolation characteristics shown in this figure are calculated from the measured C L 1 , L 2  and L 3  in case Cs×C=30, 300 and 3000 [(pF) 2 ] and in case the additional capacitor  1100  is omitted. 
     As shown in FIG. 12, by forming the additional capacitor  1100  at this position, the center frequency of isolation lowers. 
     However, in the case of FIG. 12, since the isolation is calculated under assumption that the applied magnetic field is kept constant, the maximum value of each isolation characteristics becomes smaller when the capacitance decreases. 
     FIG. 13 shows calculation results of adjusted isolation characteristics when the applied magnetic field is reduced so that the maximum isolation value of each case becomes its largest value. As will be noted from this figure, by reducing the applied magnetic field, the center frequency of the isolation more lowers. 
     FIG. 14 shows relationship between Cs×C and the center frequency of isolation and FIG. 15 shows relationship between Cs×C and applied magnetic field. These figures illustrates characteristics of not only this embodiment but also another embodiment shown in FIG.  22 . As will be apparent from these figures, by adding the capacitor  1100  with the capacitance value Cs, both the operation frequency of the circulator and the magnetic field to be applied thereto can be lowered. It can be noted from FIG. 14 that the operation frequency will greatly lower when Cs×C≦1500 [(pF) 2 ]. Thus, a desired range of Cs×C will be equal to or less than 1500 [(pF) 2 ]. It can also be noted from FIG. 15 that the applied magnetic field will greatly lower when Cs×C≦900 [(pF) 2 ]. Thus, a more desired range of Cs×C will be equal to or less than 900 [(pF) 2 ]. 
     In general, size of the circulator element is inversely proportional to its operation frequency. Namely, if the operation frequency increases, a smaller sized circulator element can be used and therefore downsizing of overall circulator can be expected. In addition, since a smaller sized permanent magnet can be used when the applied magnetic field decreases, the circulator can be further downsized. 
     FIG. 16 shows a relationship between Cs×C and amount of frequency change per unit magnetic field dF/dH as a result of calculation of the frequency change when the applied magnetic field and also Cs×C are varied. As will be apparent from the figure, by adding the capacitor  1100  with the capacitance value Cs, dF/dH becomes larger than that when no capacitor is added. The smaller capacitance value Cs will result the larger dF/dH (the amount of change in frequency with respect to the amount of change in applied magnetic field). The dF/dH can be optionally changed by appropriately selecting the value of Cs. 
     There may be various factors that exert influence upon temperature characteristics of a non-reciprocal circuit element such as a circulator. Two main factors are temperature characteristics of magnetization saturation of the ferromagnetic material such as YIG, utilized in a circuit element and temperature characteristics of the permanent magnet for providing bias magnetic field. Typically, since the temperature characteristics of the ferromagnetic material such as YIG is larger than that of the bias magnetic field, the operation frequency of the conventional circulator will increase when the temperature rises causing the available frequency band to limit in fact. 
     However, according to the present invention, dF/dH increases by adding the capacitor  1100  with the capacitance value Cs as aforementioned. This means that the temperature characteristics of the circulator is affected more strongly by the temperature characteristics of the bias magnetic field. In other words, according to the present invention, since there occurs an effect as if the temperature characteristics of the bias magnetic field increases, the temperature characteristics of the circulator can be improved. The dF/dH can be optionally changed depending upon the capacitance value Cs. Thus, the temperature characteristics of the circulator can be optionally adjusted by selecting the capacitance value Cs. If the value Cs is determined to an optimum value, a circulator with substantially constant temperature characteristics may be realized. 
     FIG. 17 shows isolation characteristics in case a capacitor  1100  with a capacitance value Cs=1 pF is added and applied magnetic field is varied. For comparison, isolation characteristics in case the capacitor  1100  with a capacitance value Cs is not added is shown in FIG.  18 . It is understood from these figures that deterioration of the maximum value of the isolation when the capacitor  1100  is added is smaller than that when the capacitor  1100  is not added. Thus, by adding the capacitor  1100  with the capacitance value Cs, deterioration of frequency bandwidth of the isolation can be prevented and also the temperature characteristics of the circulator can be improved. 
     FIG. 19 is an oblique view schematically illustrating configuration of a circulator element part of a lumped element type isolator as another embodiment of a non-reciprocal circuit element according to the present invention, FIG. 20 is an A—A sectional view of FIG. 19, and FIG. 21 is an exploded oblique view schematically illustrating whole configuration of the embodiment shown in FIG.  19 . Although this embodiment is in a case of the lumped element type isolator, the present invention can be applied to a distributed element type isolator, a lumped element type circulator and a distributed element type circulator. 
     In these figures, reference numeral  1900  denotes a circulator element formed by integrating and sintering ferromagnetic material body and inner conductors (center conductors)  1901  with a trigonally symmetric pattern,  1902  denotes a shield conductor formed next to whole bottom face and on a part of the side faces of the circulator element  1900 ,  1903   a ,  1903   b  and  1903   c  denote terminal electrodes formed on the side faces of the circulator element  1900  and connected to each one of the ends of the respective inner conductors  1901 ,  1904  denotes an inner substrate,  1905  denotes an exciting permanent magnet,  1906  denotes a yoke made of soft magnetic metal such as iron, and  1907  denotes a dielectric material layer formed next to the bottom face of the shield conductor  1902  for forming an additional capacitor (capacitance value of Cs) according to the present invention, which will adjust only eigen values of in-phase excitation, respectively. 
     The dielectric material layer  1907  is inserted between the shield conductor  1902  and one face of the yoke  1906  located under the conductor  1902  so as to form the additional capacitor with the capacitance value Cs, in which the shield conductor  1902  of the circulator element  1900  and the one face of the yoke  1906  operate as capacitor electrodes. The dielectric material layer  1907  can be made of any dielectric material other than ceramic. 
     The inner substrate  1904  made of dielectric material has a through hole  1908  at its center portion for holding the circulator element  1900  inserted therein. On the top face of the substrate  1904 , capacitor electrodes  1904   a ,  1904   b  and  1904   c  with predetermined shapes, to which the terminal electrodes  1903   a ,  1903   b  and  1903   c  of the circulator element  1900  are electrically connected, respectively are formed. On the top face, furthermore, a terminating resistor  1909  made of for example ruthenium oxide is formed by a thick-film printing. The terminating resistor  1909  is connected between the capacitor electrode  1904   c  connected with the terminal electrode  1903   c  and a ground electrode  1904   d . Although it is not shown in the figures, next to the whole bottom face of the substrate  1904 , a ground electrode that forms input/output capacitors between it and the capacitor electrodes  1904   a ,  1904   b  and  1904   c  is formed. The capacitor electrodes  1904   a  and  1904   b  also constitute an input terminal and an output terminal, and the ground electrode  1904   d  also constitutes a ground terminal. 
     Hereinafter, fabrication of the circulator element  1900  will be described in detail. First, yttrium oxide (Y 2 O 3 ) material powder and iron oxide material (Fe 2 O 3 ) powder are mixed together in a molar ratio of 3:5, and then the mixed powder is calcinated at 1200° C. Thus a ball mill crushes obtained calcination powder, and then ferromagnetic material slurry is fabricated by adding an organic binder and a solvent thereto. Thus obtained ferromagnetic material slurry is formed into green sheets by using a doctor blade. Then, via holes are formed in the green sheet by means of a punching machine. Thereafter, a pattern of the inner conductors  1901  is formed by a conductive material by using a thick-film printing, and simultaneously the via holes are filled by the conductive material. The conductive material used may be silver paste for example. 
     The green sheets with thus formed inner conductors and via holes are stacked with each other and then the stacked sheets are hot-pressed. And then, the hot-pressed sheets are diced and separated into discrete circulator elements. The separated elements are then sintered at 1480° C.. Baking silver paste next to the whole bottom face of the sintered element forms the shield conductor  1902 . The terminal electrodes  1903   a ,  1903   b  and  1903   c , and connection electrodes for connecting the other ends of the inner conductors with the shield conductor  1902  are also formed by baking silver paste on the side faces of the sintered element. As a result, the circulator element  1900  is completed. 
     Thereafter, the dielectric material layer  1907  is formed by printing ceramic paste on the face of the shield conductor  1902  of the circulator element  1900  and by firing them. 
     A lumped element type isolator can be fabricated by assembling the inner substrate  1904 , the permanent magnet  1905  and the upper and lower yoke  1906  with thus obtained circulator element  1900  as shown in FIG.  21 . 
     An additional capacitor with a capacitance value Cs is formed by the shield conductor  1902  and one face of the yoke  1906  between which the dielectric material layer  1907  made of ceramic material is sandwiched. The value of Cs×C of this isolator was 50 [(pF) 2 ]. 
     For this sample, center frequency of isolation, relative intensity of applied bias magnetic field, and changed amount of center frequency of isolation when the temperature varies from −25° C. to +85° C. were measured, respectively. The measured results are indicated in Table 3. For comparison, a sample of the isolator with no additional capacitor was fabricated and the above-mentioned characteristics were also measured (Cs×C=0). 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                   
                 Center 
                   
                 Changed 
               
               
                   
                   
                 Frequency 
                   
                 Amount 
               
               
                   
                   
                 of 
                 Applied 
                 of Center 
               
               
                   
                   
                 Isolation 
                 Magnetic 
                 Frequency 
               
               
                   
                 Cs × C 
                 (MHz) 
                 Field 
                 (MHz) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                  0 
                 883.5 
                 1.00 
                 14.5 
               
               
                   
                 50 
                 802.3 
                 0.93 
                 6.83 
               
               
                   
                   
               
            
           
         
       
     
     As will be apparent from this Table 3, addition of the capacitor with the capacitance value Cs will present not only lowering of center frequency of isolation and lowering of applied bias magnetic field but also improvement of temperature characteristics of the lumped element type isolator as well as in the previous embodiment. 
     FIG. 22 is an oblique view schematically illustrating configuration of a circulator element part of a lumped element type isolator as a further embodiment of a non-reciprocal circuit element according to the present invention. Although this embodiment is in a case of the lumped element type isolator, the present invention can be applied to a distributed element type isolator, a lumped element type circulator and a distributed element type circulator. 
     In the figure, reference numeral  2200  denotes a circulator element formed by integrating and sintering ferromagnetic material body and inner conductors (center conductors) with a trigonally symmetric pattern,  2202  denotes a shield conductor formed next to whole bottom face and on a part of the side faces of the circulator element  2200 ,  2203   a ,  2203   b  and  2203   c  denote terminal electrodes formed on the side faces of the circulator element  2200  and connected to one ends of the respective inner conductors,  2204  denotes an inner substrate,  2205  denotes an exciting permanent magnet,  2206  denotes a yoke made of soft magnetic metal such as iron,  2207  denotes a dielectric material layer formed next to the bottom face of the shield conductor  2202  for forming an additional capacitor (capacitance value of Cs) according to the present invention, which will adjust only eigen values of in-phase excitation,  2210  denotes another shield conductor, respectively. The another shield conductor  2210  is inserted between the shield conductor  2202  formed next to the bottom face of the circulator element  2200  and a shield electrode (not shown) formed next to the bottom face of the inner substrate  2204  so as to connect with the shield conductor  2202  and the shield electrode. 
     The dielectric material layer  2207  is inserted between the another shield conductor  2210  and one face of the yoke  2206  located under the conductor  2210  so as to form the additional capacitor with the capacitance value Cs, in which the another shield conductor  2210  and the one face of the yoke  2206  operate as capacitor electrodes. The dielectric material layer  2207  can be made of any dielectric material other than ceramics. 
     The inner substrate  2204  made of dielectric material has a through hole  2208  at its center portion for holding the circulator element  2200  inserted therein. On the top face of the substrate  2204 , capacitor electrodes  2204   a ,  2204   b  and  2204   c  with predetermined shapes, to which the terminal electrodes  2203   a ,  2203   b  and  2203   c  of the circulator element  2200  are electrically connected, respectively are formed. On the top face, furthermore, a terminating resistor  2209  made of for example ruthenium oxide is formed by a thick-film printing. The terminating resistor  2209  is connected between the capacitor electrode  2204   c  connected with the terminal electrode  2203   c  and a ground electrode  2204   d . Although it is not shown in the figure, next to the whole bottom face of the substrate  2204 , a shield electrode that forms input/output capacitors between it and the capacitor electrodes  2204   a ,  2204   b  and  2204   c  is formed. The capacitor electrodes  2204   a  and  2204   b  also constitute an input terminal and an output terminal, and the ground electrode  2204   d  also constitutes a ground terminal. 
     Hereinafter, fabrication of the circulator element  2200  will be described in detail. First, yttrium oxide (Y 2 O 3 ) material powder and iron oxide material (Fe 2 O 3 ) powder are mixed together in a molar ratio of 3:5, and then the mixed powder is calcinated at 1200° C. Thus a ball mill crushes obtained calcination powder, and then ferromagnetic material slurry is fabricated by adding an organic binder and a solvent thereto. Thus obtained ferromagnetic material slurry is formed into green sheets by using a doctor blade. Then, via holes are formed in the green sheet by means of a punching machine. Thereafter, a pattern of the inner conductors is formed by a conductive material by using a thick-film printing, and simultaneously the via holes are filled by the conductive material. The conductive material used may be silver paste for example. 
     The green sheets with thus formed inner conductors and via holes are stacked with each other and then the stacked sheets are hot-pressed. And then, the hot-pressed sheets are diced and separated into discrete circulator elements. The separated elements are then sintered at 1480° C.. Baking silver paste next to the whole bottom face of the sintered element forms the shield conductor  2202 . The terminal electrodes  2203   a ,  2203   b  and  2203   c , and connection electrodes for connecting the other ends of the inner conductors with the shield conductor  2202  are also formed by baking silver paste on the side faces of the sintered element. As a result, the circulator element  2200  is completed. 
     Thus fabricated circulator element  2200  is attached to the inner substrate  2204 , and then the another shield conductor  2210  which is connected to the whole shield electrode and to the shield electrode formed next to the bottom face of the inner substrate  2204  and the dielectric material layer  2207  is stacked in this order. Thereafter, by assembling the permanent magnet  2205  and the upper and lower yoke  2206  with them as shown in FIG. 22, a lumped element type isolator can be fabricated. 
     An additional capacitor with a capacitance value Cs is formed by the shield conductor  2210  and one face of the yoke  2206  between which the dielectric material layer  2207  made of ceramic material is sandwiched. 
     FIG. 23 shows an equivalent circuit diagram of the non-reciprocal circuit element (isolator) of this embodiment shown in FIG.  22 . 
     One end of the three inner conductors which consist of three inductors connected together and a capacitor  2300  with a capacitance value Cs for adjusting the eigen values of in-phase excitation is additionally connected between the connected ends of the three inner conductors and the ground. In this case, the capacitance value Cs acts only the admittance of in-phase excitation and represented as follows.          y   1     =     j       3     ω                   C   s         +     1       ω                 C     -     1     ω                   L   1                                 
     In this embodiment, one electrode of the input/output capacitors are not directly grounded but connected to the another shield conductor  2210 , and therefore one electrodes of the input/output capacitors are grounded via the additional capacitor  2300 . Ungrounded electrode of the additional capacitor  2300  shown in FIG. 23 corresponds to the another shield conductor  2210  and the above-mentioned one electrode connected thereto. 
     As will be apparent from FIGS. 14 and 15, by adding the capacitor  2300  with the capacitance value Cs, both the operation frequency of the circulator and the magnetic field to be applied thereto can be lowered. It can be noted from FIG. 14 that the operation frequency will greatly lower when Cs×C≦1500 [(pF) 2 ]. Thus, a desired range of Cs×C will be equal to or less than 1500 [(pF) 2 ]. It can also be noted from FIG. 15 that the applied magnetic field will greatly lower when Cs×C≦900 [(pF) 2 ]. Thus, a more desired range of Cs×C will be equal to or less than 900 [(pF) 2 ]. 
     Addition of the capacitor with the capacitance value Cs will present not only lowering of center frequency of isolation and lowering of applied bias magnetic field but also improvement of temperature characteristics of the lumped element type isolator as well as in the previous embodiment. 
     Many widely different embodiments of the present invention may be constructed without departing from the spirit and scope of the present invention. It should be understood that the present invention is not limited to the specific embodiments described in the specification, except as defined in the appended claims. 
     As described in detail, according to the present invention, since a capacitor is connected between a shield conductor which is commonly connected to one ends of inner conductors and an ground, for adjusting only eigen values of in-phase excitation, both center frequency of isolation and applied bias magnetic field can be simultaneously decreased. By lowering the operation frequency, a smaller sized circulator element can be used. As a result, a non-reciprocal circuit element with smaller size, lighter weight and lower height can be realized. In addition, by lowering operation magnetic field, a smaller sized permanent magnet can be used, resulting further downsizing of the non-reciprocal circuit element to realize. Furthermore, since such effects can be obtained by merely adding a capacitor, downsizing of the non-reciprocal circuit element will be expedited. 
     Selecting the capacitance value of this additional capacitor can optionally change the amount of frequency change per unit of magnetic field dF/dH. If dF/dH increases, the temperature characteristics of the non-reciprocal circuit element are affected more strongly by the temperature characteristics of the bias magnetic field and thus there occurs an effect as if the temperature characteristics of the bias magnetic field increase. As a result, the temperature characteristics of the circulator can be improved. The dF/dH can be optionally changed depending upon the capacitance value of the additional capacitor. Thus, the temperature characteristics of the circulator can be optionally adjusted by selecting the capacitance value. If the capacitance value is determined to an optimum value, a circulator with substantially constant temperature characteristics may be realized. In other words, temperature characteristics can be optionally adjusted without changing material used and without inviting increased insertion loss.