Concentrated constant irreciprocal device

[0002] The present invention relates to a concentrated-constant, non-reciprocal device for use in a microwave band comprising a ferrimagnetic body, particularly to a miniaturized, broad-band concentrated-constant circulator/isolator.

[0003] According to recent miniaturization of semiconductor elements such as ICs, transistors, etc., passive elements such as laminated chip capacitors, laminated chip inductors, chip resisters, etc., microwave devices surface-mounted with such elements are rapidly miniaturized and reduced in thickness. In such circumstances, concentrated-constant, non-reciprocal devices extremely important for microwave devices are also required to be miniaturized and reduced in thickness.

[0004] As a conventional concentrated-constant, non-reciprocal device, there is, for instance, a concentrated-constant isolator having three terminals, one of which is connected to a resister R_o. In the concentrated-constant isolator, a signal does not substantially attenuate in a transmission direction, though it is extremely decreased in the opposite direction. Thus, the concentrated-constant isolator is used for mobile communications equipments such as cellular phones.

[0005] FIGS. 22(a) and (b) schematically show the structure of such a concentrated-constant isolator. This concentrated-constant isolator has a structure in which three sets of center conductors 1a, 1b, 1care intertwined on a ferrimagnetic body 2. One side of the center conductors functions as input/output terminals {circle around (1)}, {circle around (2)}, {circle around (3)}, and the other is connected to a common part 3 (ground conductor in this example), with crossing portions of the center conductors free from short-circuiting by insulating sheets 4. Capacitors C connected between the input/output terminals {circle around (1)}, {circle around (2)}, {circle around (3)} and the common part 3 (ground conductor) determines an operating frequency of the circulator. By applying an external magnetic field 5 to the ferrimagnetic body 2, the circulator is operated at a desired impedance Z₀. Also, to function as an isolator, a resister R₀is connected between the input/output terminal {circle around (3)} and the common part (ground conductor) 3.

[0006] FIG. 23 shows an equivalent circuit of the above concentrated-constant isolator, in which each terminal of an ideal circulator having three input/output terminals {circle around (1)}, {circle around (2)} and {circle around (3)} is connected to an LC parallel resonance circuit. In the figure, the terminal {circle around (1)} is an input terminal; the terminal {circle around (2)} is an output terminal; and the terminal {circle around (3)} is connected to a resister R₀having the same resistance as that of the impedance Z₀. In FIGS. 22 and 23, C is capacitance, and L is inductance in the ferrimagnetic body 2, around which the center conductors are wound. The inductance L changes depending on the external magnetic field 5. In the circulator adjusted to match the external impedance Z₀, the LC resonance circuit is resonant at a center frequency f₀, while input impedance Z₀is zero when a matching load is connected to each terminal.

[0007] As another example of the concentrated-constant, non-reciprocal devices, there is a two-terminal, concentrated-constant isolator schematically shown in FIGS. 24(a) and (b). In the two-terminal, concentrated-constant isolator, two sets of center conductors 1a, 1bare disposed substantially in a perpendicularly crossing manner on the ferrimagnetic body 2. One end of each center conductor is an input/output terminal {circle around (1)}, {circle around (2)}, and the other end is connected to a common part 3 (ground conductor in this example), with crossing portions of the center conductors free from short-circuiting by insulating sheets 4. Capacitors C connected between the input/output terminals {circle around (1)}, {circle around (2)}, and the common part 3 (ground conductor) determines an operating frequency of the circulator. By applying an external magnetic field 5 to the ferrimagnetic body 2, the circulator is operated at a desired impedance Z₀. Also, in the case of transmission in the opposite direction, a resister R₀is connected between the terminals {circle around (1)} and {circle around (2)} for energy absorption.

[0008] FIG. 25 shows an equivalent circuit of the above two-terminal, concentrated-constant isolator, in which each terminal of an ideal non-reciprocal phase shifter having two input/output terminals {circle around (1)} and {circle around (2)} is connected to an LC parallel resonance circuit. In the figure, the terminal {circle around (1)} is an input terminal; the terminal {circle around (2)} is an output terminal; and an ideal non-reciprocal phase shifter is connected in parallel to a resister R₀for absorbing energy at the time of transmission in an opposite direction. This ideal non-reciprocal phase shifter makes the phase advance by 2π when the microwave proceeds in a forward direction, while it makes the phase advance by π when the microwave proceeds in the opposite direction. In FIGS. 24 and 25, C is capacitance, and L is inductance in the ferrimagnetic body, around which the center conductors are wound. The inductance L changes depending on the external magnetic field 5. In the isolator adjusted to match the external impedance Z₀, the LC resonance circuit is resonant at a center frequency f₀, while input impedance Z₀is zero when each terminal is connected to a matching load.

[0009] In general, the sizes of the concentrated-constant, non-reciprocal devices in the above two examples are determined by the sizes of ferrimagnetic bodies (garnet) 2 included therein. The optimum size of the ferrimagnetic body is about ⅛ of a wavelength λg of an electromagnetic wave proceeding in the ferrimagnetic body, at which the ferrimagnetic body is operated in a magnetic field providing the smallest insertion loss. However, because extreme miniaturization makes the ferrimagnetic body 2 much smaller than the optimum size, a magnetic field has to be large, resulting in drastically narrowed bandwidth.

[0010] According to recent increase in the number of users, necessity is generated to cover a wide bandwidth by a single cellular phone. Particularly in a cellular phone usable in a relatively low bandwidth such as 800 MHz, miniaturized, wide-band concentrated-constant circulator/isolators are strongly desired. However, because miniaturization contradicts with the relative bandwidth as described above, the miniaturization of the concentrated-constant, non-reciprocal device simply by reducing the size of the ferrimagnetic body leads to the problem that it fails to cover all the bandwidth necessary for cellular phones.

[0011] Accordingly, an object of the present invention is to overcome the above problems in the prior art technologies, thereby providing a miniaturized, wide-band, concentrated-constant, non-reciprocal device, for instance, a circulator or an isolator.

[0012] The concentrated-constant, non-reciprocal device of the present invention comprises a permanent magnet for applying a DC magnetic field to a ferrimagnetic body; an assembly comprising a plurality of center conductors wound around or at least partly embedded in the ferrimagnetic body, each center conductor having one end as a common terminal and the other end as a first input/output terminal; a plurality of second input/output terminals; a plurality of impedance-converting circuits each connected between the second input/output terminal and the first input/output terminal; and a plurality of load capacitors each connected between the first input/output terminal and the common terminal; wherein the input impedance Z_iof the first input/output terminals and the external impedance Z₀connected to the second input/output terminals meet the relation of 0.05≦Z_i/Z₀≦0.9 at an operating center frequency thereof.

[0013] The impedance-converting circuit preferably comprises inductance connected between each first input/output terminal and each second input/output terminal, and electrostatic capacitance connected between each second input/output terminal and a ground conductor. The impedance-converting circuit preferably has electrostatic capacitance smaller than the load capacitance. The inductance of the impedance-converting circuit is preferably provided by a distributed constant line.

[0014] The number of the second input/output terminals is preferably three, a resister being connected between one of the second input/output terminals and the ground conductor or the common terminal, whereby the concentrated-constant, non-reciprocal device is operated as an isolator. Also, a resister may be connected between one of the three first input/output terminals and the ground conductor or the common terminal, whereby the concentrated-constant, non-reciprocal device is operated as an isolator.

[0015] The inductance and electrostatic capacitance of the impedance-converting circuit and the load capacitance are preferably formed in an integral structure. The integral structure is preferably a ceramic laminate produced by laminating a plurality of green ceramic sheets printed with electrodes, and sintering the resultant laminate at a temperature of 800-1100° C.

[0016] The integral structure may also be produced by forming at least inductance and electrostatic capacitance of the impedance-converting circuits and the load capacitance connected between the first input/output terminals and the common terminal on the same insulating substrate by a thin-film-forming method.

[0017] FIG. 1 is a schematic view showing the equivalent circuit of the concentrated-constant, non-reciprocal device according to one embodiment of the present invention;

[0018] FIG. 2 is a schematic view showing the equivalent circuit of the concentrated-constant, non-reciprocal device according to another embodiment of the present invention;

[0019] FIG. 3 is a schematic view showing the equivalent circuit of the low-impedance circulator according to one embodiment of the present invention;

[0020] FIG. 4 is a graph showing the relations between inductance L₁and electrostatic capacitance C₁and a conversion ratio Z_i/Z₀in the impedance-converting circuit of the concentrated-constant, non-reciprocal device according to one embodiment of the present invention;

[0021] FIG. 5 is a graph showing the relations between a relative bandwidth W and a conversion ratio Z_i/Z₀in the impedance-converting circuit of the concentrated-constant, non-reciprocal device according to one embodiment of the present invention;

[0022] FIG. 6 is a graph showing the relations between a relative bandwidth W and K_i/K (Z_i/Z₀) in the impedance-converting circuit of the concentrated-constant, non-reciprocal device according to one embodiment of the present invention;

[0023] FIG. 7 is a graph showing the variation of electrostatic capacitance C₁and load capacitance C₂with K_i/K in an embodiment of the present invention;

[0024] FIG. 8 is a schematic view showing the equivalent circuit of the concentrated-constant, non-reciprocal device according to a further embodiment of the present invention;

[0025] FIG. 9 is a schematic view showing the equivalent circuit of the concentrated-constant isolator according to a still further embodiment of the present invention;

[0026] FIG. 10 is a schematic view showing the equivalent circuit of the concentrated-constant isolator according to a still further embodiment of the present invention;

[0027] FIG. 11 is an exploded perspective view showing the structure of the concentrated-constant isolator of FIG. 9;

[0028] FIG. 12 is an exploded perspective view showing the structure of the integral laminate in the concentrated-constant isolator of FIG. 11;

[0029] FIG. 13 is a partial cross-sectional view showing the lamination structure of the integral laminate of FIG. 12;

[0030] FIG. 14 is a graph showing the relations between an insertion loss S21 and an opposite-direction loss S12 and a frequency in the isolator of the present invention and a conventional isolator;

[0031] FIG. 15 is a partial cross-sectional view showing the structure of the concentrated-constant isolator according to a still further embodiment of the present invention;

[0032] FIG. 16 is an exploded view showing the structure of the integral laminate in the concentrated-constant isolator of FIG. 15;

[0033] FIG. 17 is a schematic view showing the equivalent circuit of the two-terminal concentrated-constant isolator according to a still further embodiment of the present invention;

[0034] FIG. 18 is a schematic view showing the equivalent circuit of the two-terminal concentrated-constant isolator according to a still further embodiment of the present invention;

[0035] FIG. 19 is an exploded perspective view showing the concentrated-constant isolator in EXAMPLE 1;

[0036] FIG. 20 is an exploded view showing the structure of the integral laminate in the concentrated-constant isolator of FIG. 19;

[0037] FIG. 21 is an exploded perspective view showing another example of an assembly used in the concentrated-constant, non-reciprocal device of the present invention;

[0038] FIG. 22(a) is a plan view showing a conventional concentrated-constant isolator;

[0039] FIG. 22(b) is a cross-sectional view showing the conventional concentrated-constant isolator of FIG. 22(a);

[0040] FIG. 23 is a schematic view showing the equivalent circuit of the conventional concentrated-constant isolator of FIG. 22(a);

[0041] FIG. 24(a) is a plan view showing a conventional two-terminal concentrated-constant isolator;

[0042] FIG. 24(b) is a cross-sectional view showing the conventional two-terminal concentrated-constant isolator of FIG. 24(a); and

[0043] FIG. 25 is a schematic view showing the equivalent circuit of the conventional two-terminal concentrated-constant isolator of FIG. 24(a).

[0044] [1] Ferrimagnetic Body

[0045] The basic principle of miniaturization according to the present invention will be explained, taking a three-terminal concentrated-constant circulator having an objective structure as an example. When an external circuit connected to the circulator has a line impedance Z₀(usually 50 Ω), the relation expressed by the formula (1) below should substantially be satisfied between the saturation magnetization 4πMs and the air-core inductance K of the ferrimagnetic body, to operate the circulator at the desired impedance Z₀by applying an external magnetic field 5 to the ferrimagnetic body 2. The formula (1) is modified from the formula (10) in “Improvement of Temperature Characteristics of VHF-Band, Concentrated-Constant Circulator,” Takeda and Kawashima, The Journal of The Electronic Communications Association, '85/6, Vol. J68-B, No. 6, p.693.

[0046] (H_in+4πMs)K^1/2=A·(Z₀·4πMs)^1/2  (1),

[0047] wherein A is a constant, and H_inis an internal magnetic field expressed by the following formula (2):

[0048] H_in=H_ext−N·4πMs  (2),

[0049] wherein H_extis an external magnetic field, and N is an effective demagnetizing factor.

[0050] With the above formula (1) in mind, problems will be discussed below on the miniaturization (reduction of air-core inductance K) of the circulator. If the line impedance Z₀and the saturation magnetization 4πMs of the ferrimagnetic body are constant, the right side in the formula (1) is constant, resulting in the fact that the internal magnetic field H_inshould be increased to decrease the air-core inductance K. Increase in the internal magnetic field H_inresults in decrease in the inductance L of a portion of the ferrimagnetic body, around which the center conductors are wound, leading to the narrowing of a bandwidth. Therefore, to operate the circulator optimally, it is necessary to keep the internal magnetic field H_inconstant.

[0051] One method for keeping the internal magnetic field H_inconstant even when the air-core inductance K decreases by the miniaturization of the circulator is to reduce the impedance Z₀of the connected external circuit in proportion to the air-core inductance K. That is, when the air-core inductance is reduced from K to K_iby miniaturizing the ferrimagnetic body, the impedance Z₀of the external circuit needs only to be reduced to Z_i. After all, to keep the internal magnetic field H_inconstant, the relation of K_i/K=Z_i/Z₀needs only to be satisfied. In the case of the circulator shown in FIG. 22, for instance, the miniaturized, concentrated-constant circulator can be provided with constant H_in(constant bandwidth), by adjusting the circulator such that the input impedance of the terminal {circle around (1)} is Z_i, when a matching load of impedance Z_iis connected to an air-core inductance K_iand the terminals {circle around (2)}, {circle around (3)} of the miniaturized, concentrated-constant circulator.

[0052] By adjusting the input impedance Z_ito smaller than Z₀according to the principle of the present invention, it is possible to obtain a miniaturized concentrated-constant circulator with reduced air-core inductance K. FIG. 3 shows the equivalent circuit of the concentrated-constant circulator adjusted to have impedance reduced to Z_ismaller than Z₀. In this concentrated-constant circulator, each terminal of an ideal circulator having three input/output terminals {circle around (1)}a, {circle around (2)}a, {circle around (3)}ais connected to a parallel resonance circuit comprising load capacitance C₂and inductance L₂of a portion of the ferrimagnetic body 2 around which center conductors are wound. Hereinafter, Z_iis called internal impedance, and Z₀is called external impedance.

[0053] [2] Impedance-converting Circuit IMT

[0054] (A) Structure of Impedance-converting Circuit IMT

[0055] To use the circulator in a line of impedance Z₀, as shown in FIG. 2, an impedance-converting circuits IMT are inserted between the first input/output terminals {circle around (1)}a, {circle around (2)}a, {circle around (3)}aand the lines of impedance Z₀in the circulator. Here, the input/output terminals {circle around (1)}b, {circle around (2)}b, {circle around (3)}bof the impedance-converting circuits IMT on the side of Z₀are called second input/output terminals, distinguished from the first input/output terminals {circle around (1)}a, {circle around (2)}a, {circle around (3)}a.

[0056] Each impedance-converting circuit IMT is preferably constituted not by a resistance element but by inductance and electrostatic capacitance, taking into consideration the insertion loss of the overall concentrated-constant circulator, to which the impedance-converting circuits IMT are added. As shown in FIG. 1, each impedance-converting circuit IMT is constituted by an L-type circuit comprising electrostatic capacitance C₁between the second input/output terminals {circle around (1)}b, {circle around (2)}b, {circle around (3)}bon the side of larger impedance Z₀and a ground conductor, and inductance L₁between the second input/output terminals {circle around (1)}b, {circle around (2)}b, {circle around (3)}band the first input/output terminals {circle around (1)}a, {circle around (2)}a, {circle around (3)}a.

[0057] To match the external impedance Z₀with the internal impedance Z_iat a center frequency f₀, C₁and L₁should meet the following relations: $\begin{array}{cc}{C}_1=\left(1/\omega ·Z_0\right)\sqrt{\left(Z_0-Z_i\right)/Z_i}\left[F\right]\mathrm{and}& \text{(3),}\\ L_1=\left(Z_i/\omega \right)\sqrt{\left(Z_0-Z_i\right)/Z_i}\left[H\right]& \text{(4),}\end{array}$

[0058] wherein ω is an angular frequency 2πf₀.

[0059] FIG. 4 shows that C₁and L₁[respectively expressed by the formulas (3) and (4)] at f₀=836 MHz and Z₀=50 Ω. C₁linearly increases as Z_i/Z₀decreases, while L₁reaches a maximum value L_1maxat Z_i/Z₀=0.5, indicating a symmetric variation. In general, when the center frequency f₀(MHz) is set, L_1maxis determined as follows:

[0060] L_1max=3979/f₀[nH]  (5)

[0061] It is known from this equation that when increase in insertion loss by the impedance-converting circuit takes place mainly due to the resistance of L₁, the insertion loss has an upper limit, not drastically increasing even at Z_i/Z₀of 0.5 or less.

[0062] (B) Optimum Relation of External Impedance Z₀and Internal Impedance Z_i

[0063] FIG. 5 shows how a relative bandwidth W changes when the conversion ratio Z_i/Z₀of the impedance-converting circuit changes at a constant air-core inductance K (without changing the shapes of the ferrimagnetic body and the center conductor) at f₀=836 MHz and Z₀=50 Ω. When two circulators provided with small ferrimagnetic bodies having air-core inductance K of 1.17 nH and 0.80 nH, respectively are adjusted to match an external impedance Z₀=50Ω (Z_i=Z₀), their relative bandwidths W are 3.9% and 2.4 %, respectively at an opposite-direction insertion loss of 20 dB, failing to achieve a practically required level of 5% or more.

[0064] However, when the impedance-converting circuit of the present invention meeting the formulas (3) and (4) is added to adjust the internal impedance Z_ito 0.85 Z₀or 0.60 Z₀, the relative bandwidth W becomes 5% or more. Thus, even a circulator having a relative bandwidth W lower than the required level can be turned to a circulator satisfying the standard, if the impedance-converting circuit of the present invention is used. Particular effects are obtained at Z_i/Z₀≦0.9.

[0065] FIG. 6 shows how a relative bandwidth W changes, when the conversion ratio Z_i/Z₀of the impedance-converting circuit changes without changing an internal magnetic field H_inin the ferrimagnetic body, even with a small air-core inductance K (even if miniaturized) at f₀=836 MHz. Here, the air-core inductance K is 1.6 nH at a conversion ratio Z_i/Z₀=1, namely when there is no impedance-converting circuit. In FIG. 6, the curve A indicates a case where one ferrimagnetic body is used, and the curve B indicates a case where two ferrimagnetic bodies are used, with a center conductor sandwiched by them. The relative bandwidths W were 5.76% and 8.29%, respectively at Z_i/Z₀32 1.

[0066] As shown in FIG. 6, the relative bandwidth W increases even though the internal magnetic field H_inis constant, when the ferrimagnetic body is miniaturized (K_i/K is reduced), thereby reducing an impedance conversion ratio Z_i/Z₀(=K_i/K). In the case of the curve A, the relative bandwidth W reaches a maximum level of 14% at Z_i/Z₀(=K_i/K)=0.1. Also, in the case of the curve B, the relative bandwidth W reaches a maximum level of 22% at Z_i/Z₀(=K_i/K)=0.2. It is known from FIG. 6 that such increase in the relative bandwidth W can be achieved at Z_i/Z₀of 0.05 or more. Thus, with the impedance-converting circuit of the present invention, the concentrated-constant circulator can be miniaturized and provided with an expanded bandwidth.

[0067] FIG. 7 shows the variation of a load capacitance C₂and an electrostatic capacitance C₁with K_i/K (=Z_i/Z₀) in the case of the curve A in FIG. 6 in which there is one ferrimagnetic body. Though the load capacitance C₂rapidly increases in inverse proportion to decrease in K_i/K, the electrostatic capacitance C₁does not rapidly increase, resulting in the load capacitance C₂always larger than the electrostatic capacitance C₁. Also, the load capacitance C₂is extremely large at Z_i/Z₀<0.05, failing to satisfy practical purposes.

[0068] (C) Structure of Concentrated-constant, Non-reciprocal Device

[0069] Another feature of the present invention is that inductance L₁, electrostatic capacitance C₁and load capacitance C₂are integrated in the impedance-converting circuit IMT.

[0070] In the equivalent circuit of the concentrated-constant, non-reciprocal device shown in FIG. 8, for instance, portions A, B and C (encircled by broken line) in the terminals are integrated separately or together except for inductance L₂in the ferrimagnetic body around which center conductors are wound. Also in the concentrated-constant isolator shown in FIG. 9, a dummy resistance R₀equal to the external impedance Z₀is connected to the second output terminal {circle around (3)}b, and the dummy resistance R₀may be integrated with the above portions A and B as part of the portion C′.

[0071] FIG. 11 is an exploded perspective view showing the structure of the concentrated-constant isolator of FIG. 9. This concentrated-constant isolator comprises a laminate (integral structure) 9 made of a dielectric material, which comprises an impedance-converting circuit having inductance L₁and electrostatic capacitance C₁, and a load capacitance C₂. An assembly 8 constituted by a ferrimagnetic body 2, around which center conductors 1a, 1band 1care wound, is mounted in a through-hole 9aof the laminate 9. The second input/output terminals {circle around (1)}b, {circle around (2)}b, {circle around (3)}band three ground terminals G are thick-film-printed on or transferred to side surfaces of the laminate 9. Further, a chip-shaped dummy resistance 11 is disposed on a connection pattern formed on a top surface of the laminate 9, and the terminal {circle around (3)}bis connected to the ground terminal G. Attached to the ferrimagnetic body 2 are a permanent magnet 7 such as a ferrite magnet for applying the external magnetic field 5, and iron yokes 6, 10 constituting a closed magnetic path.

[0072] The integral laminate 9 is formed by printing a conductor paste such as Ag, Cu, etc. on green ceramic sheets made of a dielectric material having a dielectric constant of 5-15 to form electrodes, laminating and pressure-bonding them, and then burning the resultant laminate at a temperature of 800-1100° C.

[0073] FIG. 12 is an exploded view showing the integral laminate 9 of FIG. 11. This laminate 9 is formed by lamination of green ceramic sheets 1-12. Formed around a center through-hole 9aof the uppermost sheet 1 are lands (first input/output terminals) {circle around (1)}a, {circle around (2)}a, {circle around (3)}afor connecting the center conductors 1a, 1band 1cto the load capacitance C₂. A sheet 2 below the sheet 1 is provided with spiral line electrodes for constituting inductance L₁. Sheets 4, 6 formed with pattern electrodes for constituting load capacitance C₂are alternately laminated with sheets 3, 5 and 7, whose substantially entire surfaces are covered by pattern ground conductors. In this example, the load capacitance C₂is provided by a four-layer laminated capacitor, though the number of lamination may vary depending on the value of a necessary load capacitance C₂. The sheet 8 is formed with a pattern electrode constituting electrostatic capacitance C₁, and sandwiched by sheets 7, 9, on which pattern ground conductors are formed. Disposed on sheets 10-12 are iron yokes 10 produced by punching, etc.

[0074] FIG. 13 schematically shows the cross section of the laminate 9 near the second input/output terminal {circle around (1)}b. Inductance L₁and electrostatic capacitance C₁formed in the laminate 9 are connected to each other on a side surface of the laminate 9, and inductance L₁and load capacitance C₂pass through a through-hole electrode to a top surface of the laminate 9, where they form a first input/output terminal {circle around (1)}aconnected to center conductors of the assembly 8.

[0075] FIG. 14 shows the electric characteristics of a miniaturized, rectangular-shaped isolator of 5 mm in each side having the structure shown in FIG. 11, together with those of the conventional isolator. This miniaturized isolator had f₀=836 MHz, C₂=19 pF, C₁=2.2 pF, and L₁=4.1 nH. With these parameters, the internal input impedance Z_iof the isolator was calculated as 37.4Ω. In FIG. 14, the solid line indicates the electric characteristics of the isolator of the present invention, in which the impedance-converting circuit and the load capacitance C₂are integrated together, and the dotted line indicates the electric characteristics of the conventional isolator matched to an external impedance Z₀without using the impedance-converting circuit. Though both of them have substantially the same insertion loss S21 of about 0.5 dB at a center frequency, the isolator of the present invention is extremely superior to the conventional isolator in that the former has a much wider bandwidth. Also, their comparison in a relative bandwidth at a point where the opposite-direction insertion loss S12 reaches 20 dB or less has revealed that it is 5.8% in the isolator of the present invention, 2% better than 3.8% in the conventional isolator.

[0076] FIG. 15 is an exploded perspective view showing an isolator according to another embodiment of the present invention. Formed by a thin-film-forming method in the integral laminate 12 are an impedance-converting circuits having inductance L₁and electrostatic capacitance C₁, and load capacitance C₂. Also, a ferrimagnetic body 2 and a ground cap 13 made of a conductor plate are disposed in a center through-hole of the integral laminate from a rear side. The integral laminate 12 is mounted onto a resin structure 14 integrally formed with connection terminals by a ball solder. The resin structure 14 is provided with the second input/output terminals {circle around (1)}b, {circle around (2)}b, the exposed isolation terminal {circle around (3)}b, and three ground terminals G.

[0077] FIG. 16 is an exploded view of the integral laminate 12 of FIG. 15. Formed at a center of a conductor pattern 1 formed on an insulating substrate are intertwined center conductors. Formed around the center conductors are spiral conductors having inductance L₁and a resister element 11, constituting an impedance-converting circuit. Other hatched portions are ground conductors. An insulating film pattern 2 is formed on the conductor pattern 1. White circle portions are through-holes free from an insulating film, and arcuate portions a, b, c are through-holes for contacting the end projections of the ground cap 13 to the ground conductor of the conductor pattern 1. 3 indicates a conductor pattern formed on the insulating film pattern 2.

[0078] A center portion of the conductor pattern 3 passes through a through-hole of the insulating film pattern 2 to have connection to the conductor pattern 1, thereby forming intertwined center conductors. The center conductors constitute inductance L₂in the above equivalent circuit. Load capacitance C₂is formed by three large conductor patterns formed on the other ends of the center conductors and a ground conductor of the conductor pattern 1 via the insulating film pattern 2. Also, electrostatic capacitance C₁of the impedance-converting circuit is formed by small conductor patterns in edge portions and the ground conductor of the conductor pattern 1 via the insulating film pattern 2. The inductance L₁is connected between the first input/output terminals {circle around (1)}a, {circle around (2)}a, {circle around (3)}aand the second input/output terminals {circle around (1)}b, {circle around (2)}b, {circle around (3)}bvia through-holes formed in the insulating film pattern 2. Such a structure accelerates further integration, resulting in more miniaturized isolators.

[0079] FIG. 17 shows the equivalent circuit of the two-terminal concentrated-constant isolator according to one embodiment of the present invention. Like the above isolator, the input impedance Z_iis designed to be smaller than the external impedance Z₀, and constituted by inductance L₁and electrostatic capacitance C₁and connected to IMT. As shown in FIG. 18, by integrating portions A and B of each terminal encircled by dotted lines except for center conductor portions L₂of an impedance-converting circuit wound around a ferrimagnetic body, a miniaturized, wide-band, two-terminal, concentrated-constant isolator can be obtained, like the above concentrated-constant isolator.

[0080] The present invention will be explained in further detail by the following EXAMPLES without intention of restricting the scope of the present invention thereto.

[0081] FIG. 19 shows a concentrated-constant isolator in this EXAMPLE, and FIG. 20 shows the internal structure of its integral laminate 9. The integral laminate 9 was produced by forming inductance L₁and electrostatic capacitance C₁of an impedance-converting circuit IMT and load capacitance C₂on sheets made of a dielectric material, and laminating them. The equivalent circuit of this concentrated-constant isolator is the same as shown in FIG. 10. What is different from the concentrated-constant isolator shown in FIG. 9 in terms of an equivalent circuit is that electrostatic capacitance C₁and inductance L₁of an impedance-converting circuit IMT are omitted between the second input/output terminal {circle around (3)}band the first input/output terminal {circle around (3)}a, and that a dummy resistance R_iequal to Z_iis directly connected to the first input/output terminal {circle around (3)}a. Such a constitution makes the structure of the laminate simple, resulting in further miniaturization of the concentrated-constant isolator.

[0082] This laminate 9 was produced by printing a conductor paste based on Ag on green sheets of 50-200 μm in thickness made of low-temperature-sinterable, dielectric ceramic materials to form predetermined electrode patterns, laminating these sheets and burning the resultant laminate for integration. In the concentrated-constant isolator in this EXAMPLE, electrostatic capacitance C₁=2.2 pF, load capacitance C₂=19 pF, and inductance L₁=4.1 nH.

[0083] The internal structure of this laminate 9 will be explained in a lamination order referring to FIG. 20. The lowermost green sheet 10 is formed with electrode patterns 101, 102 for capacitors C₁and electrode patterns 103 for capacitors C₂on a top surface, and a ground conductor (not shown) and a pair of terminal electrodes (not shown) on a rear surface. The green sheet 10 and other green sheets are provided with through-hole electrodes (indicated by black circles in the figure) in their edge portions, such that connection is achieved through the through-hole electrodes between the electrode patterns formed in the laminate 9 and the ground conductors and a pair of terminal electrodes.

[0084] Laminated on the green sheet 10 is a green sheet 9 formed with a ground conductor 104, and further a green sheet 8 formed with an electrode pattern 105 for capacitor C₂. The electrode pattern 105 formed on the green sheet 8 is connected to an electrode pattern 103 formed on a green sheet 10 via through-hole electrodes formed in the green sheet 9. Also, the electrode pattern 105 formed on the green sheet 8 is connected to an electrode pattern 120 for a capacitor C₂formed on a top surface of the green sheet 1 via through-hole electrodes of the green sheets 1-7. Thus, the capacitor C₂inside the dotted line C″ shown in FIG. 10 is formed between the ground conductor on a rear surface and the ground conductors 104, 108 and 117.

[0085] Laminated then is a green sheet 7 formed with line electrodes 106, 107 for inductance L₁and a ground conductor 108. Laminated thereon is a green sheet 6 formed with line electrodes 109, 110 for inductance L₁and a ground conductor 111. Inductance L₁inside the dotted line A shown in FIG. 10 is formed by connecting the line electrodes 106 and 109 via a through-hole electrode, and inductance L₁inside the dotted line B shown in FIG. 10 is formed by connecting the line electrodes 107 and 110 via a through-hole electrode.

[0086] Next, a green sheet 5 formed with electrode patterns 112, 113 for capacitor C₂is laminated. These electrode patterns 112, 113 are connected to ends of line electrodes 109,110 via through-hole electrodes. Further, via through-hole electrodes formed in the green sheets 1-4 electrode patterns 115, 116 for capacitor C₂formed on a green sheet 3 are connected to electrode patterns 118, 119 for capacitor C₂formed on a top surface of a green sheet 1. As a result, capacitor C₂inside the dotted line A in FIG. 10 is formed by the electrode patterns 112, 115, 118, and capacitor C₂inside the dotted line B in FIG. 10 is formed by the electrode patterns 113, 116, 119.

[0087] After laminating the green sheet 4 formed with a ground conductor 114, the green sheet 3, the green sheet 2 formed with a ground conductor 117 and the green sheet 1 in this order and pressure-bonding them, a through-hole 9ais formed in a center portion of the laminate by punching by a die. The resultant green sheet laminate is burned for integration to obtain an integral laminate 9 of 5.0 mm×4.5 mm×0.5 mm in outer size. Mounted onto this laminate 9 is a chip resister 11. As an alternative method for forming a through-hole 9a, green sheets partly punched in advance may be laminated.

[0088] This laminate 9 is mounted in a resin casing 200 equipped with conductor plates for connecting a rear ground conductor to terminal electrodes, and external terminals 202, 204, 205, 206. Disposed in the through-hole 9aof the laminate 9 is an assembly 8 comprising a disc-shaped ferrimagnetic body 2 of 2.2 mm in diameter and 0.5 mm in thickness, and three center conductors 1a, 1b, 1coverlapping on the ferrimagnetic body 2 in an insulated manner. The center conductors are soldered to the conductor patterns 118, 119, 120 of the laminate 9.

[0089] Mounted onto a lower yoke 10 is the resin casing 200 having a bottom shape adapted to the lower yoke 10, and the lower yoke 10 is soldered to the bottom surface of the resin casing 200. Next, the laminate 9 comprising the assembly 8 is soldered to a top surface of the resin casing 200. After a ferrite magnet 7 for applying a DC magnetic field to the ferrimagnetic body 2 in the assembly 8 is fixed to an upper yoke 6, the upper yoke 6 is combined with the lower yoke 10 to complete an isolator.

[0090] In this EXAMPLE, part of the electrode pattern constituting the capacitor C₂is formed on an outer surface of the laminate 9. With such a structure, even when the center frequency f₀deviates from the desired frequency, the electrode patterns can be trimmed to adjust the capacitance of the capacitor C₂, thereby easily adjusting the center frequency f₀.

[0091] When a gap is narrowed between the line electrode constituting inductance L₁and the ground conductor, parasitic capacitance increases due to difference in potential between the line electrode and the ground conductor. Thus, the loss of the impedance-converting circuit IMT increases, which may result in mismatching in impedance with the lines for connecting the concentrated-constant non-reciprocal circuit, and deterioration in electric characteristics. Accordingly, a concentrated-constant isolator comprising a laminate 9 having a ground conductor on a green sheet 4 or a green sheet 5 was produced to evaluate insertion loss characteristics. The results are shown in Table 1.

[0092] As shown in Table 1, the wider the distance in a lamination direction between the electrode and the ground conductor, the smaller the insertion loss. When the distance is 100 μm or more, a desired level of insertion loss can be achieved for practical purposes.

[0093] Though the concentrated-constant isolator in this EXAMPLE comprises one ferrimagnetic body, the same effects can be obtained by sandwiching a plurality of center conductors by two or more ferrimagnetic bodies. The curve B in FIG. 6 shows a case where two ferrimagnetic bodies are used, verifying that further improved effects are obtained by using two ferrimagnetic bodies. The effects of the present invention can also be obtained by using, as a structure in which center conductors are wound around the ferrimagnetic body, a ferrimagnetic body laminate produced by printing metal electrodes for center conductors on thin green sheets made of ferrimagnetic powder, laminating a plurality of printed green sheets, and sintering the resultant laminate at a high temperature of 800° C or higher.

[0094] As described above in detail, in the concentrated-constant, non-reciprocal device of the present invention, the miniaturization of a ferrimagnetic body does not result in deterioration in electric characteristics. Therefore, a miniaturized, wide-band, concentrated-constant circulator/isolator can be provided while maintaining excellent electric characteristics.