Ferromagnetic resonators.

The invention relates to ferromagnetic resonators and to filter devices utilising ferromagnetic resonance.

Our US Patent US-A-4 547 754 discloses a filter device utilising a ferrimagnetic thin film of yttrium iron garnet (YIG) formed on a gadolinium-gallium garnet (GGG) substrate by a liquid phase epitaxial (LPE) growth process. Filter devices of this type using YIG thin film elements attract attention for use as microwave integrated circuit (MIC) filters because of the high Q values of their resonance characteristics in the microwave frequency band, compact structure, and suitability for mass production by a selective patterning process by LPE and lithography.

An MIC band-pass filter using a YIG thin film may be constructed generally as shown in Fig. 1 of the accompanying drawings, for example. A dielectric substrate 1 made of alumina or the like has a first main surface coated with a ground conductor 2 and has a second main surface coated with first and second microstrip lines disposed in a parallel arrangement to form input and output transmission lines 3 and 4. As shown in US-A-4 547 754, both ends of each of the strip lines 3 and 4 have heretofore been connected to the ground conductor 2 by respective connnecting conductors. Ends 3a and 4a of the input and output lines 3 and 4 are connected to input and output circuits respectively. Adjacent the second main surface of the substrate 1 are first and second magnetic resonance elements, in the form of YIG thin film elements 7 and 8, which are electromagnetically coupled with the respective microstrip lines 3 and 4. The YIG thin film elements 7 and 8 are produced by forming a YIG thin film on a main surface of a GGG substrate 9 by the above-mentioned thin film forming technique and patterning the film into circular lands by a selective etching technique, for example photolithography. Extending between the first and second YIG thin film elements 7 and 8 is a third microstrip line 10 for providing electromagnetic coupling between the elements. The coupling transmission line 10 is formed on a second main surface of the substrate 9, with both ends of the transmission line 10 being connected to the ground conductor 2 by connecting conductors 11 and 12.

MIC filter devices constructed as described in US-A-4 547 754 are restricted to relatively low centre frequencies of at most several GHz due to two major reasons that will now be explained. The first reason is that the YIG thin film elements need to be placed at positions where the magnetic field is maximum for the purpose of magnetic coupling with each microstrip line. However, this condition is not met for relatively high centre frequencies. In particular, the magnetic field is maximum at the grounding end of the microstrip line and minimum at a position λg/4 (where Xg is the propagation wavelength) away from the maximum position. Therefore, each YIG thin film element needs to be disposed as near to the grounding end of the microstrip line as possible for good coupling at relatively high centre frequencies. The propagation wavelength λg is expressed in terms of the effective dielectric constant ε_eff, determined from the dielectric constants of the dielectric substrate 1 and GGG substrate 9 and the shape of the microstrip lines, as

<maths id="math0001" num=""><img file="EP0196918A2_D0001.tif" /></maths>Accordingly, the propagation wavelength Xg is reduced to 1/(ε_eff)^½ of the free space wavelength λo. On the other hand, each YIG thin film element needs a finite volume for substantial magnetic coupling with the associated microstrip line - for example, for a thickness of 20 to 30 micrometres, the element diameter should be around 2mm - and at a high frequency of several GHz, even if the YIG element is disposed at the grounding end of the microstrip line the distance between this position and the YIG element centre is comparable with Xg/4, resulting virtually in the disposition of the YIG thin film elements at locations of weaker magnetic field, and accordingly resonant high-frequency coupling efficiency between the YIG thin film elements and the microstrip lines is reduced for relatively high resonant frequencies, and the insertion loss between the filter input and the filter output at the resonance frequency (which should be low) becomes relatively high. The second of the above-mentioned two reasons is that the intersections of the input and output microstrip lines and the microstrip line for linking the YIG thin film elements are not located at the grounding end portions where the electric field is minimal, but instead the distance between the intersections and the respective grounding ends approaches Xg/4, at which the electric field is maximal as the operating frequency goes higher, which causes the capacitive coupling to increase, so that the Isolation characteristics deteriorate significantly at higher frequencies. Figs. 2 and 3 of the accompanying drawings show the insertion loss (in dB) of the filter device of US-A-4 547 754 as a function of the operating frequency (in GHz), and it is apparent that the input/output coupling undesirably increases at frequencies above 4.5 GHz. That is, the device propagates the input signal irrespective of the resonance of the YIG thin film elements, and does not function as a filter.

With the intention of overcoming the above-mentioned deficiencies, we have proposed in Japanese Patent Application 59-187079 a filter device in which the microstrip lines each have one of their ends open with YIG thin film elements 7 and 8 being disposed at positions distant from open ends by an odd multiple of Xg/4.

A filter of this construction can have a high centre frequency above several GHz as shown in Fig. 4 of the accompanying drawings, which is a plot of isolation (in dB) against frequency (in GHz), but it is suitable only for a fixed band or narrow bandwidth variable filter because of the narrow bandwidth of the high-frequency coupling efficiency and isolation characteristics: a broad band variable filter cannot be realised. Fig. 4 shows as a measurement result the isolation characteristics of this filter device for different input frequencies and indicates that an effective filtering function with an isolation of 40 dB or more is accomplished in a narrow band of about three gigahertz between 11.75 and 14.75 GHz.

According to a first aspect of the invention there is provided a ferromagnetic resonator comprising: <ul id="ul0001" list-style="none"><li>a non-magnetic substrate,</li><li>a ferrimagnetic thin film element formed on a major surface of the non-magnetic substrate,</li><li>a strip line disposed at another major surface of the non-magnetic substrate and electromagnetically coupled to the ferrimagnetic thin film element,</li><li>a conductive wall connected or connectable to ground potential, the wall facing the strip line and being spaced at a predetermined distance therefrom,</li><li>an end of the strip line being connected to the conductive wall, and bias magnetic field means for applying a dc magnetic field to the ferrimagnetic thin film element perpendicular to the major surface thereof.</li></ul>

According to a second aspect of the present invention there is provided a ferromagnetic resonator comprising a non-magnetic substrate, a ferrimagnetic thin film element formed on a major surface of said non-magnetic substrate, a strip line electromagnetically coupled to said ferrimagnetic thin film element, a conductive wall of ground potential facing said strip line, and spaced at a predetermined distance therefrom, an end of said strip line being connected to said conductive wall of ground potential, and bias magnetic field means applying a dc magnetic field to said ferrimagnetic thin film perpendicular to said major surface thereof.

Ferromagnetic resonator embodying the present invention and described hereinbelow are operable at high frequency; and are suitable for use as variable filter devices having a wide frequency band.

Filter devices embodying the invention are suitable for use in microwave integrated circuits (MI(s).

The invention will now be further described, by way of illustrative and non-limiting example, with reference to the accompanying drawings, in which: <ul id="ul0002" list-style="none"><li>Fig. 1 is a perspective view of a filter device utilising ferromagnetic resonance and having input and output microstrip lines constructed as described in US Patent US-A-4 547 754;</li><li>Figs. 2 and 3 are characteristic graphs showing insertion loss as a function of input frequency for the filter device of US Patent US-A-4 547 754;</li><li>Fig. 4 is a plot of the isolation provided by the above-described modified filter device wherein YIG discs are provided at positions distant from open ends by an odd multiple of λg/4 as a function of input frequency;</li><li>Figs. 5, 6 and 7 are structural views of a ferromagnetic resonator embodying the present invention;</li><li>Figs. 8 and 9 are characteristic graphs for the resonator of Figs. 5, 6 and 7; and</li><li>Figs. 10, 11, 12 and 13 are structural views used to explain other embodiments of this invention.</li></ul>

Ferromagnetic resonators embodying the invention and described in detail below are of a "short" type having microstrip lines grounded at the ends, and are constructed with the intention of lowering the effective dielectric constant E eff of its transmission system down to almost unity by the utilisation of a so-called suspended substrate strip line configuration or an inverted microstrip line configuration. Fig. 5 shows the structural arrangement of embodiments of this invention, which comprise a device main body 25 including a non-magnetic substrate (for example, a GGG substrate) 21, ferrimagnetic thin film elements (for example, YIG magnetic thin film elements) 22 formed on one main surface of the non-magnetic substrate 21, and strip lines 23 electromagnetically coupled with the ferrimagnetic thin film elements 22, and is further provided with conductive walls 24 which confront the strip lines 23 with a certain spacing formed therebetween and which ground one end of each of the strip lines, and a means 26 for applying a dc bias magnetic field to the ferrimagnetic thin film elements (that is, the YIG magnetic thin film elements) 22, so that transmission lines are formed in the structure of a suspended substrate strip line configuration or an inverted microstrip line configuration.

A first embodiment of this invention will now be described in more detail with reference to Figs. 5, 6 and 7, which show a cross-sectional view, a plan view of the main body 25 and a partially-exploded perspective view of the device, respectively. This embodiment employs the suspended substrate strip line structure, and the conductive walls 24 are constructed to form a shielding case which encloses the device main body 25. The device main body 25 includes a GGG non-magnetic substrate 21, and its one main surface has first and second YIG magnetic thin film elements 22A and 22B with a certain spacing from each other and a third YIG magnetic thin film element 22C disposed between the YIG elements 22A and 22B for providing the magnetic coupling for them. The magnetic thin film elements 22A, 22B and 22C may have a groove in the periphery on one main surface of the magnetic thin film or may have a smaller thickness in the central portion than the peripheral portion so as to suppress a spurious response, as disclosed in the aforementioned US Patent US-A-4 547 754. On the main surface of the GGG non-magnetic substrate 21 opposite to that where the YIG magnetic thin film elements 22A, 22B and 22C are formed, there is formed a pattern of conductive material providing a conductor 27. The conductor 27 has sections providing first and second microstrip lines, namely an input strip line 23A and an output strip line 23B disposed parallel to each other and extending across the first and second YIG magnetic thin film elements 22A and 22B, respectively, a central ground pattern 23C located between and parallel to the strip lines 23A and 23B and extending across the third central YIG magnetic thin film element 22C and connected at its opposite ends with the strip lines 23A and 23B, and grounding ends 27A and 27B engaging the grounded surface of part 24B and connecting the strip line 23A to one end of the central ground pattern 23C and the strip line 23B to the other end of the central ground pattern 23C.

The ground conductive walls 24, which are at ground potential, comprise a first conductive wall section 24A and a second conductive wall section 24B as shown in the partly exploded perspective view of Fig. 7. The first conductive wall section 24A has ledges 28A and 28B for supporting the GGG non-magnetic substrate 21 at the ends of the substrate 21 adjacent to the YIG magnetic thin film elements 22A and 22B. The ledges 28A and 28B are separated by an interposed recess 29. By being placed on the ledges 28A and 28B, the GGG non-magnetic substrate 21 confronts the inner surface of the conductive wall section 24A with a certain spacing dl being provided by the recess 29. The second conductive wall section 24B has recesses 30A and 30B in portions confronting the first and second microstrip lines 23A and 23B, that is, the locations of the first and second YIG magnetic thin film elements 22A and 22B (not shown in Fig. 7). The structure is dimensioned such that when the conductive wall sections 24A and 24B are put together with the substrate 21 interleaved therebetween, a protruding section 31 between the recesses 30A and 30B comes into contact with the central ground pattern 23C of the conductive pattern of the conductor 27 so as to establish an electrical connection therebetween, while at the same time the protruding section 31 and the central ground pattern 23C in combination provide isolation between the input and output lines 23A and 23B, and the recesses 30A and 30B provide a certain spacing d2 between the GGG non-magnetic substrate 21 and the confronting inner surfaces of the conductive wall section 24B.

The dc bias magnetic field application means 26 is constructed in such a way that a pair of cores 32a and 32b have central magnetic poles 32al and 32bl thereof confronting each other and disposed at opposite sides of the device main body 25, with windings 43a and 43b being placed on the respective central magnetic poles 31al and 31bl, so that a dc bias magnetic field is created between the poles.

According to the resonator structure described above, the transmission lines are constructed to form a so-called suspended substrate strip line structure, which allows a smaller effective dielectric constant ε_eff despite the use of the GGG non-magnetic substrate 21. Typically, using a GGG substrate of 0.4mm in thickness for the non-magnetic substrate 21, with spacings dl and d2 of 0.6mm each being provided between the upper and lower surfaces of the GGG substrate 21 and the conductive walls 24, an effective dielectric constant of 2.2 is achieved for a 50-ohm strip line which is 1.25mm in width. When the YIG magnetic thin film elements 22A and 22B are placed near the grounding ends 27A and 27B of the strip lines 23A and 23B to meet the condition that L is less than or equal to Xg/4, where L denotes the distance from the centre of each YIG element to the respective grounding end, this condition is satisfied up to a frequency as high as 25 GHz for the YIG magnetic thin film elements 22. Accordingly, this structure retains the efficiency of coupling between the input and output lines and the YIG magnetic thin film elements, that is, the YIG resonator, up to such a high frequency, whereby a broadband variable filter operative at high frequencies can be realised.

The filter device described in connection with Fig. 1 uses a GGG substrate 9 of high dielectric constant E_r = 13, and therefore, even by the combinational use of a dielectric substrate 1 having a small dielectric constant, the effective dielectric constant ε_eff cannot be made sufficiently small. For example, using an alumina sheet of 1.27mm in thickness (ε_r = 10) as a dielectric substrate 1 and a GGG substrate 9 which is 0.4mm in thickness, the effective dielectric constant ε_eff of the microstrip lines with a 50-ohm characteristic impedance is 8.6 for the line on the alumina substrate 2 and 7.3 for the line on the GGG substrate 9. In another example using a quartz substrate (E = 3.8) which is 0.5mm in thickness as a dielectric substrate 1 and a GGG substrate 9 which is 0.4mm in thickness, the effective dielectric constant ε_eff of the 50-ohm microstrip lines is 4.9 for the line on the quartz substrate and 5.1 for the line on the GGG substrate.

The filter structure embodying the present invention, using direct coupling for the YIG resonator, that is, the YIG magnetic thin film elements, enables perfect isolation up to extremely high frequencies owing to the absence of a strip line for linking the resonator elements, and because of the high-frequency isolation between the input and output strip lines provided by the protruding section 31 between the recesses 30A and 30B in the conductive walls 24 and the central ground pattern 23C of the conductive pattern 27, and also the isolation provided by the conductive walls 24 surrounding the device main body 25.

Fig. 8 is a graph showing the insertion loss plotted against frequency for the filter device described with reference to Figs. 5 to 7, and indicates an isolation of 40 dB or more up to a frequency as high as 17 GHz. Fig. 9 shows the characteristics of the filter with a dc bias magnetic field being applied so that the centre frequency is set to 3 GHz. The centre frequency can be varied by adjustment of the magnetic field.

Although the foregoing embodiment employs direct coupling for the YIG resonator, the present invention is not limited to this. Alternatively, for example, first and second YIG magnetic thin film elements 22A and 22B can be formed on one main surface of a GGG substrate 21, as shown in Figs. 10 and 11, so that the elements are coupled by a third microstrip line 33 in the same manner as described in connection with Fig. 5. In this case, the third microstrip line 33 can be formed on a base 24 of polyester film, for example, so that the third microstrip line 33 on the polyester film confronts the first and second YIG magnetic thin film elements 22A and 22B on the GGG non-magnetic substrate 21. The third microstrip line 33 may be provided at both of its ends with grounding ends 33A and 33B, which are interposed together with the non-magnetic substrate 21 between the first and second conductive wall sections 24A and 24B and which ends 33A and 33B are in contact with the first conductive wall section 24A of the conductive walls 24 of ground potential. The remaining arrangement of Figs. 10 and 11 is common to that of Figs. 5 and 6, and like parts are designated by the same references, so that the explanation thereof need not be repeated.

This modified arrangement also meets the condition that the YIG magnetic thin film elements are placed in the vicinity of the grounding ends of the strip lines for frequencies up to as high as 25 GHz, and high- efficiency coupling between the strip lines and YIG magnetic thin film elements can be retained. The distance from each of two intersections between the first and second microstrip lines 22A and 22B and the third microstrip line 33 to the grounding end becomes equal to Xg/4 at a frequency of 12.5 GHz and, although the frequency with satisfactory isolation is not so high as compared with the arrangement shown in Figs. 5 to 7, a significant improvement is achieved when compared with a conventional filter device.

Although the foregoing embodiment is constructed so that the YIG magnetic thin film elements 22 (22A and 22B) are on one surface of the GGG non-magnetic substrate 21 and the conductive pattern 27 such as the first and second strip lines is on the other surface, in an alternative arrangement the conductive pattern 27 is formed on a film made of polyester or the like provided separately from the non-magnetic substrate 21, and then the film with the formation of conductive pattern is placed over the GGG non-magnetic substrate 21.

Although the above-described embodiments employ a suspended substrate strip line structure, the present invention can be applied equally to the inverted microstrip line structure. Figs. 12 and 13 show a cross-sectional view and a plan view of the device for the latter case. In Figures 12 and 13, components identical to those shown in Figs. 5 and 6 are designated by common references and an explanation thereof is not repeated. The arrangement of Figures 12 and 13 has part of the conductive walls 24, that is, the conductive wall section 24A, removed, and an open wall structure is formed.

In this structure, with a spacing of 0.4mm produced by the recesses 30A and 30B in the ground potential conductive walls 24 with respect to the surface of the GGG non-magnetic substrate 21 on the side of the microstrip line, the 50-ohm line has a width of 1.26 mm and an effective dielectric constant f- ef of as small as 1.9. Also in this case, however, when the cores 32a and 32b of the bias magnetic field source are made of material having a shielding effect, the overall structure becomes virtually identical to the suspended substrate microstrip line structure.

The YIG magnetic thin film elements 22A, 22B and 22C formed on a main surface of the GGG non-magnetic substrate can be produced concurrently by growing an YIG thin film epitaxially on the entire main surface and thereafter patterning the film into the lands by photolithography, so that this embodiment is suitable for volume production.

As described above, the present invention enables a drastic reduction in the effective dielectric constant ε_eff of the transmission lines, whereby a filter with a high centre frequency of the order of GHz can be achieved despite the "short" type structure.

It is also possible to construct a broadband variable filter having a variable centre frequency from a low frequency to a high frequency of the order of GHz through the provision of a variable bias magnetic field source.