Patent Publication Number: US-6222425-B1

Title: Nonreciprocal circuit device with a dielectric film between the magnet and substrate

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a nonreciprocal circuit device for use in a microwave band such as, for instance, an isolator or a circulator. 
     2. Description of the Related Art 
     Generally, a nonreciprocal circuit device, such as a lumped constant isolator or a circulator, has low attenuation of signals in the forward direction and high attenuation of signals in the reverse direction, and is used in a transmission circuit of a communications unit such as, for instance, a mobile telephone. 
     However, linear distortion in an amplifier integrated into a communications unit causes radiation (spurious emissions, especially at two and three times the fundamental frequency). Since this radiation can cause interference and irregular operation of a power amplifier, it must be kept below a fixed level. Radiation is sometimes prevented by using an amplifier with excellent linearity, or by using an extra filter to attenuate radiated waves. 
     However, an amplifier with excellent linearity is expensive, and using an extra filter increases the number and cost of components, and in addition, increases the overall size of the communications equipment. For these reasons, these measures cannot easily be used in mobile telephones and the like, where there is a strong demand for smaller and less expensive devices. 
     On the other hand, a lumped constant isolator functions as a bandpass filter in the forward direction, and consequently it has large attenuation in the forward direction in frequency bands distant from the pass band. It may be envisaged that radiation can be attenuated by utilizing these characteristics to block spurious emissions outside the pass band. However, since conventional isolators were not originally designed to obtain attenuation outside the pass band, their capability for this purpose is limited. 
     Accordingly, the present applicants devised an experimental isolator (not yet publicly known) which contains a circuit element comprising a low-pass filter. As shown in FIG. 12, this isolator includes an inductor L 1  which is a constituent element of a low-pass filter. This inductor L 1  is patterned on a dielectric substrate  18  which is provided between a magnetic assembly  4  and a magnet  6 , and connected between an input port and a matching capacitor Co′. 
     Consequently, as shown in the equivalent circuit diagrams of FIG.  13  and FIG. 14, a π-type low-pass filter, comprising the connection of C 1 -L 1 -C 2 , is connected to the input port. Here, since C 1  is provided by a part of the capacitance of the matching capacitor Co′ of the isolator, it does not need to be provided separately. C 2  is formed by externally appending a capacitance to the isolator. 
     According to the above mentioned isolator containing a low-pass filter, attenuation outside the pass band can be increased, and interference and irregular operation caused by radiation can be prevented. The low-pass filter has a simple constitution and is inexpensive, making an expensive amplifier and an extra filter unnecessary, and enables the device to be made small-scale at low cost. 
     However, when the above low-pass filter is provided on a dielectric substrate, the magnet is in contact with the dielectric substrate, and consequently there is a concern that the high-frequency material characteristics of the magnet, particularly the tangent δ or Dissipation Factor (Dissipation Factor=tangent δ×100[%]), will have an adverse effect on the insertion loss of the isolator. 
     In general, commercially available mass-produced magnets were not developed for high-frequency components, and they are consequently liable to have a considerable dissipation factor (loss tangent). Therefore, it can be expected that the insertion loss of the isolator will increase when a circuit element on the dielectric substrate is in contact with the magnet. A further problem is that the magnet has a high dielectric constant, making it difficult to form inductance. 
     SUMMARY OF THE INVENTION 
     The present invention has been realized in consideration of these problems, and is able to provide a nonreciprocal circuit device which is capable of reducing the insertion loss of an isolator when a circuit element is provided on a dielectric substrate. 
     The nonreciprocal circuit device of the present invention comprises a magnetic assembly comprising a plurality of central conductors arranged so as to intersect adjacent to a ferrite body, a dielectric substrate disposed between a magnet and said magnetic assembly, said magnet applying a dc magnetic field to said magnetic assembly; wherein a circuit element is provided by patterning on said dielectric substrate, and a dielectric film or layer is disposed at least between said circuit element on said dielectric substrate and said magnet. 
     Alternatively, the dielectric film may be affixed to the magnet, or to the dielectric substrate. 
     In other embodiments of the present invention, the circuit element is provided by patterning on a laminated dielectric substrate, and at least one dielectric layer of said laminated substrate is disposed between at least said circuit element and said magnet. 
     In an alternative arrangement, a circuit element may be provided by patterning on said dielectric substrate, and a dielectric film may cover at least one part of the surface of said circuit element. 
     Preferably, the circuit element may comprise all or part of an inductor; a π-type low-pass filter; an LC series bandpass filter comprising an inductor and a capacitor; a phase-shift circuit comprising a micro-stripline; a phase-shift circuit comprising a stripline; a directional coupler; a capacitance coupler comprising a capacitor; or a band-elimination filter. Each of these circuit elements is known to the art. Each is formed by patterning as described herein. 
     Other features and advantages of the present invention will become apparent from the following description of embodiments of the invention which refers to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exploded perspective view to explain a lumped constant type isolator according to a first embodiment of the present invention; 
     FIGS. 2A and 2B are diagrams showing an inductor on the dielectric substrate of the isolator shown in FIG. 1; 
     FIG. 3 is a characteristics diagram showing effects of the first embodiment; 
     FIGS. 4A and 4B are diagrams showing a dielectric substrate according to another embodiment of the present invention; 
     FIG. 5 is an equivalent circuit diagram of the isolator of the embodiment shown in FIGS. 4A and 4B; 
     FIG. 6 is an equivalent circuit diagram of part of the isolator of the embodiment shown in FIGS. 4A and 4B; 
     FIG. 7 is an exploded perspective view of a lumped constant type isolator according to a third embodiment of the present invention; 
     FIG. 8 is an exploded perspective view of a lumped constant type isolator according to a fourth embodiment of the present invention; 
     FIG. 9 is an exploded perspective view of a dielectric substrate according to another embodiment of the present invention; 
     FIG. 10 is an exploded perspective view of a dielectric substrate according to another embodiment of the present invention; 
     FIGS. 11A and 11B are diagrams showing a dielectric substrate according to another embodiment of the present invention; 
     FIG. 12 is an exploded perspective view of an experimental isolator to explain the background of the present invention; 
     FIG. 13 is an equivalent circuit diagram of the isolator shown in FIG. 12; and 
     FIG. 14 is an equivalent circuit diagram of part of the isolator shown in FIG.  12 . 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION 
     Preferred embodiments of the present invention will be described with reference to the accompanying drawings. 
     FIGS. 1,  2 A and  2 B are diagrams to explain a lumped constant type isolator according to a first embodiment of the present invention, FIG. 1 being an exploded perspective view of the isolator, FIG. 2A, a plan view of an inductor provided on a dielectric substrate, and FIG. 2B, a perspective plan view of an electrode provided on the back face of the dielectric substrate. 
     In FIG. 1, a lumped constant isolator  1  comprises a terminal block  3  provided on the bottom surface  2   a  of a case  2  made of magnetic metal, a magnetic assembly  4  provided on the terminal block  3 , a box-like cap  5  made of the same magnetic metal as the case  2 , a rectangular permanent magnet  6  affixed to the inner surface of the cap  5 , forming a magnetic circuit, wherein the permanent magnet  6  applies a dc magnetic field to the magnetic assembly  4 . 
     The magnetic assembly  4  comprises three central conductors  8 ,  9  and  10 , which intersect at angles of  120  degrees and are provided on the upper surface of a circular disk-like ferrite  7 , with an interposed insulating sheet (not shown in the diagram), and a ground  11  connected to the central conductors  8 - 10  abutting on the lower surface of the ferrite  7 . 
     The terminal block  3  is made of electrically insulating resin, and comprises rectangular frame-like side walls  3   a  integrally provided with a bottom wall  3   b , a through hole  3   c  being provided in the bottom wall  3   b . Recessed portions  3   d  are formed in the bottom wall  3   b  surrounding the through hole  3   c . The recessed portions  3   d  accommodate single plate matching capacitors  12   a - 12   c  and a single plate terminal resistor R. 
     The magnetic assembly  4  is inserted through the through hole  3   c , so that the ground  11  of the magnetic assembly  4  connects to the bottom surface  2   a  of the case  2 . 
     Input/output terminals  15  for surface mounting and a ground terminal  16  are provided on the outer surfaces of the left and right side walls  3   a  of the terminal block  3 , and the input/output terminals  15  lead out at corners of the upper surface of the bottom wall  3   b . Furthermore, the ground terminal  16  leads out at each of the recessed portions  3   d , and is connected to one end of the lower surface electrode of each of the capacitors  12   a - 12   c  and the terminal resistor R. The terminals  15  and  16  are each partially insert-molded in the terminal block  3 . 
     Input/output ports P 1 -P 3  of the central conductors  8 - 10  are connected to the electrodes on the upper surfaces of the capacitors  12   a - 12   c . The tip of the port P 2  is connected to the output terminal  15 , and the tip of the port P 3  is connected to the terminal resistor R. 
     A rectangular plate-like dielectric substrate  18  is provided on the upper surface of the magnetic assembly  4 . When the cap  5  and the permanent magnet  6  are attached to the case  2 , the dielectric substrate  18  electrically and mechanically holds the magnetic assembly  4  and the terminal block  3  to the case  2 , and holds the ports P 1 -P 3  of the central conductors  8 - 10  to the capacitors  12   a - 12   c . Furthermore, a hole  18   a  is provided in the center of the dielectric substrate  18  to correspond to the magnetic assembly  4 , and a notch  18   b  is provided in a corner of the dielectric substrate  18  to correspond to the terminal resistor R. 
     An inductor L 1  is provided by patterning on the upper surface of the dielectric substrate  18 , to form a circuit element  20  comprised in a π-type low-pass filter. A first end of the inductor L 1  connects via a through hole electrode  21  to a connection electrode  22  on the lower surface of the dielectric substrate  18 , and a second end of the inductor L 1  similarly connects via a through hole electrode  23  to an input electrode  24  on the lower surface. The first end of the inductor L 1  is connected by the connection electrode  22  to the port P 1  of the central conductor  8 , and the second end is connected by the input electrode  24  to the input terminal  15 . 
     Further, a dielectric film  25  is provided between the dielectric substrate  18  and the permanent magnet  6 , the dielectric film  25  being sandwiched between the permanent magnet  6  and the dielectric substrate  18 . The dielectric film  25  is rectangular, so as to completely cover the lower surface of the permanent magnet  6 , and has low dielectric constant and low dissipation factor. 
     Next, the effects and advantages of the present invention will be described. 
     According to the lumped constant isolator  1  of the present invention, an inductor L 1  is provided by patterning on a dielectric substrate  18 , and the inductor L 1 , a capacitor  12   a  and an external capacitor comprise a π-type low-pass filter, whereby attenuation outside the pass band can be increased and interference and irregular operation caused by unnecessary radiation can be prevented. Consequently, it is possible to realize a low-pass filter of simple structure which is inexpensive, making the expensive amplifier and extra filter described above unnecessary, and contributing to down-sizing and cost reduction. 
     In the above-described experimental device, there was a concern that insertion loss of the isolator would increase when the permanent magnet  6  contacted the inductor L 1  on the dielectric substrate  18 . By contrast, in the present embodiment, a dielectric film  25  having low dielectric constant and low loss tangent (dissipation factor) is sandwiched between the dielectric substrate  18  and the permanent magnet  6 , enabling the inductor L 1  to be separated from the permanent magnet  6 , which has a high dielectric constant and a high loss tangent. The inductance thereby increases and the insertion loss decreases. Thus, the Q of the inductor can be improved and, as a result, the insertion loss of the isolator can be reduced. 
     The present embodiment has described a rectangular dielectric film  25  which completely covers the lower surface of the permanent magnet  6 . However, the advantages of the present invention are achieved merely by the separation of the inductor from the permanent magnet, having high dielectric constant and high loss tangent, by inserting a dielectric layer of low dielectric constant and low loss tangent therebetween. Therefore, there are no particular limitations on the shape and size of the inserted dielectric. 
     For instance, since air is also a dielectric of low dielectric constant and low tangent, a layer of air can be provided between the magnet and the inductor by providing a hole in the portion of the dielectric film which contacts the inductor L 1 , achieving the same effects as the embodiment already described. Furthermore, when using a dielectric film with a hole provided therein, it is possible to use a dielectric of high dielectric constant and tangent. 
     Polyimide, Teflon, epoxy, glass epoxy or the like is used as the material for the dielectric film  25 . Furthermore, other non-conductive insulating materials other than those mentioned above can be used as the dielectric film  25 . 
     FIG. 3 is a characteristics diagram showing measurements of insertion loss taken to confirm the effects of the above lumped constant isolator. The permanent magnet used in this experimentation has relative dielectric constant of  25 , and tangent of 1×10 −2 , and the dielectric film has relative dielectric constant of 3.5, tangent of 2×10 −3 , and thickness of 50 μm. For comparison, similar measurements were taken for an isolator with no dielectric film (in FIG. 3, the alternate long and short dash line represents the comparative example, and the solid line represents the present embodiment). As is clearly shown in FIG. 3, insertion loss can be improved by roughly 0.05 dB when the dielectric film is used. 
     The above embodiment describes a case where the inductor L 1  constituting a low-pass filter is provided on a dielectric substrate  18 , but the circuit element of the present invention is not restricted to this, and it is acceptable to use, for instance, an LC series bandpass filter, a micro-stripline phaseshift circuit, a stripline phase-shift circuit, a directional coupler, a capacitance coupler, or a band-elimination filter, known as a BEF, trap filter or notch filter, or the like, and these achieve substantially the same effects as in the above embodiment. 
     FIGS. 4A to  6  are diagrams explaining other embodiments of the present invention described above, FIG. 4A being a plan view of a capacitor and an inductor provided on a dielectric substrate, FIG. 4B being a perspective plan view of an electrode provided on the rear surface of the dielectric substrate, and FIG.  5  and FIG. 6 being their respective equivalent circuits. In these diagrams, identical and corresponding parts to those in FIG. 2, FIG.  13  and FIG. 14 are designated by identical reference characters. 
     The isolator of the present embodiment comprises an inductor L 1  and a capacitor  30 , provided by patterning on the upper surface of a dielectric substrate  18  to form a circuit element, comprising a low-pass filter. The port P 1  of a central conductor  8  is connected via a through hole electrode  21  and a connection electrode  22  to a first end of the inductor L 1 . 
     A first capacitor electrode  30   a  is connected to a second end of the inductor L 1  and connected to an input electrode  24  via the through hole electrode  21 . On the rear surface of the dielectric substrate  18 , a second capacitor electrode  30   b  is provided at the portion facing the first capacitor electrode  30   a , and this second capacitor electrode  30   b  is connected to the case  2  as a ground. 
     Consequently, as shown in the equivalent circuit diagrams of FIG.  5  and FIG. 6, a π-type low-pass filter is formed at the input port. Here, C 1  is provided by a portion of the matching capacitance Co′ of the isolator, and therefore does not need to be separately provided, and C 2  is the capacitor  30  provided on the dielectric substrate  18 . 
     In this embodiment, a dielectric film is clasped between the dielectric substrate and the permanent magnet, whereby interference and irregular operation caused by undesirable radiation can be prevented, while reducing the insertion loss of the isolator, consequently obtaining the same effects as the embodiments described earlier. 
     FIG. 7 is an exploded perspective view of a lumped constant isolator according to a third embodiment of the present invention, wherein members identical and corresponding to those of FIG. 1 are designated by identical reference numerals. 
     The lumped constant isolator  1  of the present embodiment is an example in which a dielectric film  25  having low dielectric constant and low loss tangent is clasped between the dielectric substrate  18  and the permanent magnet  6 , the dielectric film  25  being affixed to the lower surface of the permanent magnet  6 , so as to overlie at least the inductor L 1  on the dielectric substrate  18 . 
     In the present embodiment, the dielectric film  25  is provided between the dielectric substrate  18  and the permanent magnet  6 , and in addition, it is affixed to the permanent magnet  6 , whereby the insertion loss of the isolator is reduced as in the previous embodiment, and in addition, the dielectric film  25  can easily be incorporated when the isolator is assembled, improving workability. 
     FIG. 8 is an exploded perspective view of a fourth embodiment of the present invention, wherein members identical and corresponding to those of FIG. 1 are designated by identical reference numerals. 
     The lumped constant isolator  1  of the present invention is an example in which a dielectric film  25  having low dielectric constant and low loss tangent is clasped between the dielectric substrate  18  and the permanent magnet  6 , the dielectric film  25  being affixed to the entire upper surface of the dielectric substrate  18 , or at least a sufficient part of the upper surface to overlie the inductor L 1 . 
     In the present embodiment, the dielectric film  25  is provided between the dielectric substrate  18  and the permanent magnet  6 , and in addition, it is affixed to the dielectric substrate  18 , whereby the insertion loss of the isolator is reduced as in the previous embodiments, and in addition, the dielectric film  25  can easily be incorporated when the isolator is assembled, improving workability. 
     FIG. 9 is diagram explaining a dielectric substrate according to another embodiment of the present invention, wherein members identical and corresponding to those of FIG. 2 are designated by identical reference numerals. 
     In the embodiment, an inductor L 1  is provided, as a circuit element comprised in a low-pass filter, on a first dielectric substrate  31 , and a single-layer second dielectric substrate  32  is provided between the upper surface of the first dielectric substrate  31  and the permanent magnet  6 . 
     According to the present embodiment, a second dielectric substrate  32  is laminated on a first dielectric substrate  31 , which the inductor L 1  is provided on, and therefore the insertion loss of the isolator can be reduced, achieving the same effect as the embodiment described above. Furthermore, the first and second dielectric substrates  31  and  32  can be laminated together, reducing the number of components to less than when a separate dielectric film is used, as mentioned above, thereby further lowering costs. 
     FIG. 10 is a diagram explaining a dielectric substrate according to another embodiment of the present invention, wherein members identical and corresponding to those of FIG. 9 are designated by identical reference numerals. 
     The present embodiment is an example in which an inductor L 1  is provided by patterning on the upper surface of a first dielectric substrate  31 , and a connection electrode  22  and an input electrode  24 , which are connected to the inductor L 1 , are provided by patterning on the upper surface of a second dielectric substrate  32 . 
     In the present embodiment, since the inductor L 1 , the connection electrode  22  and the input electrode  24  are respectively provided on the upper surfaces of the first and second dielectric substrates  31  and  32 , manufacture is easier than when electrode patterns are provided on both surfaces of a single substrate, enabling costs to be lowered further, and making it possible to provide an inexpensive isolator with low loss. 
     FIG. 11 is a diagram explaining a dielectric substrate according to another embodiment of the present invention, wherein members identical and corresponding to those of FIG. 2 are designated by identical reference numerals. 
     In the present embodiment, the inductor L 1  on the upper surface of dielectric substrate  18  is covered with a thick dielectric film  35 , provided using a method such as printing. This dielectric film  35  completely covers the inductor L 1  with the exception of the central portion  36  of the line, which forms a layer of air between the dielectric film  35  and the magnet. 
     In the present embodiment, a dielectric film  35  of low dielectric constant and low tangent is applied over the inductor L 1  on the dielectric substrate  18 , enabling insertion loss of the isolator to be reduced, and achieving the same effects as the above embodiment. Furthermore, since the dielectric film  35  is applied onto the dielectric substrate  18 , an increased number of components, which would lead to higher costs, can be avoided, and the device can be made inexpensive. 
     Furthermore, since the central portion  36  of the inductor L 1  is covered by a dielectric layer comprising air, the same effect is achieved as when the dielectric film  35  is applied over. Alternatively, the dielectric film may be applied to the entire inductor L 1  without leaving the central portion  36  exposed. 
     Each of the above embodiments described an example using a lumped constant isolator, but the present invention can, of course, be applied to a circulator. 
     As has been described above, in the nonreciprocal circuit device, a circuit element is provided by patterning on a dielectric substrate, and a dielectric film or material is sandwiched between the circuit element formed on the dielectric substrate and a magnet, and consequently, the magnet having a high dielectric constant and a high tangent can be kept separate from the circuit element, reducing the insertion loss of the isolator. 
     Furthermore, it is possible to realize an inexpensive low-pass filter having a simple constitution, whereby interference and irregular operation caused by undesirable radiation can be avoided, and the device can be made small-scale at low cost. 
     According to the present invention, the dielectric film or material may be affixed to the magnet, or to the dielectric substrate, whereby the insertion loss of the isolator is reduced as above, and in addition, the dielectric film can be more easily incorporated when assembling the isolator, having the advantage of improving workability. 
     Another embodiment of the invention provides a laminated substrate, there being provided an extra layer between the circuit element on the dielectric substrate and the magnet, whereby the insertion loss of the isolator is reduced as above, and in addition, an increased number of components, which would lead to higher costs, can be avoided, enabling the embodiment to be provided inexpensively. 
     According to another embodiment, a dielectric film covers at least part of the surface of the circuit element on the dielectric substrate, whereby the insertion loss of the isolator is reduced as above, and in addition, an increased number of components, which would lead to higher costs, can be avoided, enabling the invention to be provided inexpensively. 
     According to the present invention, an inductor, a π-type low-pass filter, an LC series bandpass filter, a micro-stripline phase-shift circuit, a stripline phase-shift circuit, a directional coupler, a capacitance coupler and a band-elimination filter, for example, may be the circuit element, and in each case the circuit can be made inexpensive, enabling the device to be made small-scale and at lower cost. 
     Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. Therefore, the present invention is not limited by the specific disclosure herein.