Thin Film Ferromagnetic Resonance Tuned Filter

FIG. 1 is a diagrammatic illustration showing the basic constitution of a thin film ferromagnetic resonance tuned filter according to the present invention;

FIG. 2 is a sectional view of a thin film ferromagnetic resonance tuned filter, in a preferred embodiment, according to the present invention;

FIG. 3 is an exploded perspective view of a filter assembly incorporated into the tuned filter of FIG. 2;

FIG. 4 is a graph showing the distribution of magnetic field on signal transmission lines;

FIG. 5 is a graph showing the frequency dependency of k-1/Qu and 1/Qe.sub.eff ;

FIGS. 6 to 8 are graphs showing the filter characteristics of the thin film ferromagnetic resonance tuned filters of the present invention;

FIG. 9 is a diagrammatic illustration showing the basic constitution of a known YIG thin film tuned filter; and

FIGS. 10 and 11 are graphs showing the filter characteristics of the YIG thin film tuned filters of FIG. 9;

FIGS. 12 and 13 are enlarged diagrammatic plan views of the essential portions of filter assemblies employed in further embodiments of the filter according to the present invention, respectively;

FIG. 14 is an enlarged schematic exploded perspective view of a filter assembly according to the present invention;

FIG. 15 is an enlarged schematic sectional view of a filter according to the present invention;

FIG. 16 is a diagrammatic illustration of the distribution of a magnetic field in the coupling part of the signal transmission line coupled with an YIG thin film;

FIGS. 17 to 19 are graphs showing the isolation characteristics of filters.

According to the present invention, the coupling of ferromagnetic resonance thin films, for example, YIG thin films, and input and output signal transmission lines is increased in a low frequency region and is decreased in a high frequency region to vary the coupling of the ferromagnetic resonance thin films with the input and output signal transmission lines with the frequency-dependent variation of the coupling coefficient k so that a state nearly the same as critical coupling is established over a wider frequency range and overcoupling is established at the opposite ends of a variable frequency band.

That is, in an YIG thin film tuned filter of the present invention, as shown in FIG. 1 illustrating the basic constitution thereof, comprising ferromagnetic YIG thin films Y.sub.1 and Y.sub.2 serving as magnetic resonators, an input signal transmission line L.sub.1 and an output signal transmission line L.sub.2 respectively coupled with the YIG thin films Y.sub.1 and Y.sub.2, and a magnetic circuit, not shown, for applying a DC magnetic field to the YIG thin films Y.sub.1 and Y.sub.2, the distances from the respective points of coupling of the input signal transmission line L.sub.1 and the output signal transmission line L.sub.2 with the YIG thin films Y.sub.1 and Y.sub.2 to the corresponding grounded ends P.sub.1 and P.sub.2 are one-tenth or above to less than one-fourth the wavelength of a wave at the upper limit frequency of a tuning frequency band. In FIG. 1, indicated at L.sub.1 2 is a coupling transmission line extending across the YIG thin films Y.sub.1 and Y.sub.2.

Thus, according to the present invention, the distances from the respective points (center of YIG thin films Y.sub.1 and Y.sub.2) of coupling of the input signal transmission line L.sub.1 and the output signal transmission line L.sub.2 with the YIG thin films Y.sub.1 and Y.sub.2 to the corresponding grounded ends P.sub.1 and P.sub.2 are determined selectively at values in the range of one-tenth or above to less than one-fourth the wavelength of a wave at the upper limit frequency of a tuning frequency band. Accordingly, the coupling of the YIG thin films and the signal transmission lines is increased with the decrease of frequency and is decreased with the increase of frequency. That is, standing waves are generated in the transmission line grounded at one end thereof, and the distribution of the intensity of a high-frequency magnetic field generated by the signal transmission line is the highest at the grounded end and decreases cosine-functionally with distance from the grounded end to zero at a position at a distance corresponding to a quarter of the wavelength from the grounded end as indicated by a curve 41 in FIG. 4, while the distribution of the intensity of a low-frequency magnetic field generated by the signal transmission line is flat as indicated by a curve 42 in FIG. 4. Accordingly, in a magnetic resonance thin film tuning filter of the present invention, the degree of magnetic coupling of the YIG magnetic thin films and the signal transmission lines is high in the low frequency region of a variable frequency band and is low in the high frequency region of the same.

Suppose that the filter is in the state of critical coupling when the frequency is f.sub.0. Then, from expression (4),

On the other hand, k-1/Qu (log (K-1/Qu)) varies with frequency f (log f) along a straight line 51 as shown in FIG. 5. However, when the distance l from the point of coupling of the YIG thin films with the corresponding signal transmission lines to the respective grounded ends is less than a quarter of the wavelength, 1/Qe.sup.eff varies with frequency f in three ways as represented by curves 52a, 52b and 52c in FIG. 5 depending on the value of the distance l.

Suppose that the variation of 1/Qe.sup.eff is represented by the curve 52b. Then, the following equation (8) is satisfied

Therefore,

Substituting expression (7) into expression (9), we obtain

From expression (10), .beta..sub.0 l=0.761. Therefore,

where .lambda..sub.0 is wavelength for frequency f.sub.0. The variation of 1/Qe.sup.eff with frequency when the distance l is larger than .lambda..sub.0 /8.26 is represented by the curve 52a, and by the curve 52c when the distance l is smaller than .lambda..sub.0 /8.26. As obvious from FIG. 5, when 1/Qe.sup.eff varies along the curves 52a or 52b, the frequency f.sub.0 can be the upper limit frequency of the tuning frequency band. However, since a critical coupling mode appears again at a frequency f.sub.1 higher than the frequency f.sub.0 when 1/Qe.sup.eff varies along the curve 52c, the frequency f.sub.0 cannot be the upper limit frequency of the tuning frequency band of the filter. Accordingly, the effective range of the distance l is defined by

However, in practical application, the frequency band can sufficiently be expanded when

An YIG thin film tuning filter, in a preferred embodiment, according to the present invention will be described hereinafter with reference to FIGS. 2 and 3. A disk-shaped YIG thin film Y.sub.1 and a disk-shaped YIG thin film Y.sub.2 are formed by forming an YIG thin film over the entire surface of a nonmagnetic substrate 31, for example, GGG (Gallium Gadolinium Garnet) substrate by a liquid-phase epitaxial growth process, and etching to form the YIG thin film through a photolithographic process. The nonmagnetic substrate 31 carrying the two YIG thin films Y.sub.1 and Y.sub.2 is placed on a lower conductor 32. A cavity is formed in a predetermined area in the lower conductor 32 to form an air gap 36 in an area corresponding to the YIG thin films Y.sub.1 and Y.sub.2 as shown in FIG. 2. A dielectric substrate 33, for example, a quartz substrate, is placed on the nonmagnetic substrate 31. Parallel input signal transmission line L.sub.1 and an output signal transmission line L.sub.2 are formed on one surface of the dielectric substrate 33 facing the YIG thin films Y.sub.1 and Y.sub.2 so as to extend across the YIG thin films Y.sub.1 and Y.sub.2, respectively. A connecting transmission line L.sub.1 2 is formed on the other side of the dielectric substrate 33 transversely with respect to the direction of extension of the signal transmission lines L.sub.1 and L.sub.2 so as to extend over the YIG thin films Y.sub.1 and Y.sub.2. An upper conductor 34 is placed on the dielectric substrate 33 carrying the input signal transmission line L.sub.1 and the output signal transmission line L.sub.2 so as to hold the dielectric substrate 33 and the nonmagnetic substrate 31 carrying the YIG thin films Y.sub.1 and Y.sub.2 between the upper conductor 34 and the lower conductor 32 and so that the opposite side edges of the upper conductor 34 are disposed opposite to the opposite side edges of the lower conductor 32, respectively. A cavity is formed in a predetermined area in the inner surface of the upper conductor 34 to form an air gap 37 in an area corresponding to the YIG thin films Y.sub.1 and Y.sub.2, the input side of the input signal transmission line L.sub.1 and the output side of the output signal transmission line L.sub.2.

Grounding terminals e.sub.12a and e.sub.12b contacting the upper conductor 34 are formed at the Opposite ends of the connecting transmission line L.sub.12 formed on the dielectric substrate 33 held between the lower conductor 32 and the upper conductor 34. Grounding terminals e.sub.1 and e.sub.2 are formed at the grounding end, namely, an end opposite the input end, of the input signal transmission line L.sub.1 and at the grounding end, namely, an end opposite the output end, of the output signal transmission line L.sub.2, respectively, so as to contact the lower conductor 32.

Thus, a filter assembly 35 comprising the YIG thin films Y.sub.1 and Y.sub.2, input and output signal transmission lines L.sub.1 and L.sub.2 coupled with the YIG thin films Y.sub.1 and Y.sub.2, and connecting transmission line L.sub.12 which are provided between the upper and lower conductors 32 and 34 is constructed.

As illustrated in FIG. 2, the filter assembly 35 is disposed within a magnetic gap 22 of a magnetic circuit 21. The magnetic circuit 21 is constructed, for example, by disposing a pair of bell-shaped magnetic cores 24A and 24B respectively having central cores 23A and 23B opposite to each other so as to form a magnetic gap 22 between the central cores 23A and 23B. A coil 25 is wound at least on either the central core 23A or the central core 23B. A DC current is supplied to the coil 25 to apply a desired DC magnetic field to the filter assembly 35 disposed within the magnetic gap 22.

The intensity of the magnetic field is varied by varying the intensity of the DC current applied to the coil 25 to vary the tuning frequency.

The relative arrangement of the YIG thin films Y.sub.1 and Y.sub.2, and the input and output transmission lines L.sub.1 and L.sub.2 are determined so that the distance l from the coupling point of the YIG thin film Y.sub.1 and the input transmission line L.sub.1 to the grounding terminal e.sub.1, and the distance l from the coupling point of the YIG thin film Y.sub.2 and the output signal transmission line L.sub.2 to the grounding terminal e.sub.2 are 1/10 or above and less than 1/4 the wavelength of a wave of the upper limit frequency.

When the signal transmission lines are formed on the GGG substrate as mentioned above to increase the effective dielectric constant of the signal transmission lines, clearances between the coupling point of the input signal transmission line L: and the YIG thin film Y.sub.1, and the grounded end e.sub.1 and between the coupling point of the output signal transmission line L.sub.2 and the YIG thin film Y.sub.2, namely, the actual distances, can be reduced.

FIG. 6 shows filter characteristics obtained through a simulation test of a filter where the distance 1, namely, the distance from the coupling point of the input signal transmission line L.sub.1 and the YIG thin film Y.sub.1 to the grounded end e.sub.1 and the distance from the coupling point of the output signal transmission line L.sub.2 and the YIG thin film Y.sub.2 to the grounded end e.sub.2, corresponds to a clearance of 12.3 mm. In FIG. 6, curves 60RL, 60IL and 60BW represents the variation of reflection loss, insertion loss and 3 dB bandwidth, respectively, with frequency. In this case, the reflection loss is 6 dB or greater when the variable frequency band is from 0.5 GHz to 4.9 GHz, which is far greater than that of the known filter shown in FIGS. 10 and 11.

FIG. 7 shows filter characteristics obtained through a simulation test of a filter where the distance l corresponds to a clearance of 15.2 mm, in which curves 70RL, 70IL and 70BW represent the variation of reflection loss, insertion loss and 3 dB bandwidth, respectively, with frequency. In this case, a variable frequency band to provide a reflection loss of 10 dB or greater is 0.68 GHz to 3.76 GHz, namely, 2.4 octaves, and that to provide a reflection loss of 6 dB or greater is 0.5 GHz to 3.9 GHz, namely, 3 octaves. FIG. 8 shows the experimental filter characteristics of the filter of the present invention, corresponding to the filter characteristics of the known filter shown in FIG. 10. The filter characteristics determined through simulation agree well with the experimental filter characteristics shown in FIG. 8. The filter characteristics of FIG. 6 and those of FIG. 7 (FIG. 8) are obtained when the distance l corresponds to clearances of 12.3 mm and 15.2 mm, respectively. The clearances 12.3 mm and 15.2 mm correspond to 1/5 the wavelengths, namely, 80% of 1/4 the wavelengths, at upper limit frequencies 4.9 GHz and 3.9 GHz, respectively.

As obvious from the comparison of the filter characteristics of filters of the present invention shown in FIGS. 6 and 7 (FIG. 8) and those of the conventional filter shown in FIG. 10 (FIG. 11), in the filter of the present invention, the 3 dB bandwidth varies with the variation of the center frequency within a narrow range and is maintained substantially at a fixed value.

As apparent from the foregoing description, according to the present invention, the degree of coupling of the YIG thin films with the input and output signal transmission lines is increased in a low-frequency range and is decreased in a high-frequency range by selectively determining the distance from the coupling point of the YIG thin films and the input and output signal transmission lines to the respective grounded ends of the input and output signal transmission lines. Accordingly, as obvious from the relation between the curves 51 and 52c in FIG. 5, critical coupling occurs at frequencies f.sub.0 and f.sub.1 corresponding to two points A and B of intersection of the curves 51 and 52c, and hence a state nearly the same as critical coupling is established in a wide frequency range, so that the variable frequency band is expanded greatly. Furthermore, since overcoupling occurs at the opposite ends of the variable frequency band, the filter has satisfactory spurious characteristics over the entire variable frequency band. That is, the main mode of the YIG thin film filter is a uniform mode and the high-order magnetostatic mode is a spurious mode, and Qe.sup.u <Qe.sup.s (Qe.sup.u is the external Q of the uniform mode and Qe.sup.s is the external Q of the spurious mode). Therefore, when the uniform mode tends to be the state of undercoupling, the spurious mode tends to approach the state of critical coupling, which relatively enhances the spurious response. Accordingly, as mentioned above, in the conventional YIG thin film filter, the uniform mode becomes a state of undercoupling in a high-frequency range, which deteriorates the spurious characteristics in the high-frequency range. On the contrary, in the YIG thin film filter of the present invention, a state nearly the same as the state of critical coupling appears in most part of the variable frequency band and a state of overcoupling appears at the opposite ends of the variable frequency band. Thus, the YIG thin film tuning filter of the present invention has satisfactory spurious characteristics over the entire variable frequency band.

Furthermore, according to the present invention, the range of variation of the 3 dB bandwidth with the variation of the resonance frequency is narrow, and hence the YIG thin film tuning filter of the present invention has a fixed 3 dB bandwidth over the entire variable frequency band, which is advantageous to application of the YIG thin film tuning filter to a system.

Further embodiments will be explained hereinafter in which isolation characteristics of the above constructed filter is improved. As seen from the comparative observation of FIGS. 1 and 9, the enhancement of direct high-frequency magnetic coupling between the parallel transmission lines due to substantial increase in the respective lengths of the parallel portions of the input and output signal transmission lines by the provision of the extensions L.sub.1E and L.sub.2E is one of the causes of the deterioration of isolation characteristics. It is another cause of the deterioration of the isolation characteristics that the intensity of a high-frequency electric field is enhanced near the coupling parts of the input and output signal transmission lines coupled with the YIG thin films, namely, the magnetic resonance thin films, particularly at the upper limit frequency of a variable frequency band due to the provision of the extensions L.sub.1E and L.sub.2E. This increases capacitive coupling with the COupling transmission line L.sub.12.

FIG. 12 shows a plan view of the essential portion of the filter assembly. The filter assembly comprises ferrimagnetic thin films, that is, YIG thin films Y.sub.1 and Y.sub.2, serving as magnetic resonator, an input signal transmission line L.sub.1 and an output signal transmission line L.sub.2 respectively coupled with the YIG thin films Y.sub.1 and Y.sub.2, and a magnetic circuit, not shown, for applying a DC magnetic field to the YIG thin films Y.sub.1 and Y.sub.2, in which, as previously described with reference to FIG. 1, extensions L.sub.1E and L.sub.2E are extended from the ends of the input signal transmission line L.sub.1 and the output signal transmission line L.sub.2, respectively, so that the respective distances from the respective coupling parts of the input and output signal transmission lines to the grounded ends of the extensions are one-tenth or above and less than one-fourth, more specifically, nearly one-fourth the wavelength of a wave at the upper limit frequency of a tuning frequency band.

In this thin film ferromagnetic resonance tuned filter, at least the extensions L.sub.1E and L.sub.2E located on the same side as the coupling parts of the input signal transmission line L.sub.1 and the output signal transmission line L.sub.2 coupled with the YIG thin films Y.sub.1 and Y.sub.2, and the other ends of the input signal transmission line L.sub.1 and the output signal transmission line L.sub.2 respectively opposite the extensions L.sub.1E and L.sub.2E are bent, curved or gradually outwardly expanded to form interval increasing parts L.sub.w spaced apart from each other by a distance greater than the distance D between the respective coupling parts of the input signal transmission line L.sub.1 and the output signal transmission line L.sub.2.

Furthermore, for example, as illustrated in FIG. 13, the coupling part of the input signal transmission line L.sub.1 coupled with the YIG thin film Y.sub.1 and/or the coupling part of the output signal transmission line L.sub.2 coupled with the YIG thin film Y.sub.2 is split into two longitudinal parts to form a split section L.sub.1D and/or a split section L.sub.2D. In FIG. 13, parts corresponding to those shown in FIG. 12 are denoted by the same reference characters and the description thereof will be omitted to avoid duplication.

In such construction, interval increasing parts L.sub.w are formed in the extensions L.sub.1E and L.sub.2E, respectively, so that the distance between the extension L.sub.1E and the other end of the signal transmission line L.sub.2 and the distance between the extension L.sub.2E and the other end of the signal transmission line L.sub.1 are greater than the distance between the rest of the portions of the signal transmission lines L.sub.1 and L.sub.2. Accordingly, the deterioration of high-frequency magnetic field isolation between the signal transmission lines L.sub.1 and L.sub.2 attributable to the provision of the extensions L.sub.1E and L.sub.2E is avoided.

Still further, when the split sections L.sub.1D and L.sub.2D are formed in the respective coupling parts of the signal transmission lines L.sub.1 and L.sub.2 coupled with the YIG thin films Y.sub.1 and Y.sub.2, respectively, magnetic flux is applied uniformly over the entire surfaces of the YIG thin films Y.sub.1 and Y.sub.2 with the impedances of the signal transmission lines L.sub.1 and L.sub.2 held constant, as illustrated by a magnetic flux distribution chart indicated by thin lines in FIG. 16. Accordingly, the enhancement of the capacitive coupling of the YIG thin films Y.sub.1 and Y.sub.2 with another transmission line, for example, the coupling line extending across the YIG thin films Y.sub.1 and Y.sub.2, due to the local enhancement of field strength is avoided, and thereby the isolation characteristics are improved.

An YIG thin film tuning filter, in a preferred embodiment, according to the present invention will be described with reference to FIGS. 12, 14 and 15. A disk-shaped YIG thin film Y.sub.1 and a disk-shaped YIG thin film Y.sub.2 are formed by forming an YIG thin film over the entire surface of a nonmagnetic substrate 31, for example, a GGG (Gallium, Gadolinium Garnet) substrate by a liquid-phase epitaxial growth process, and etching to form the YIG thin film disk through a photolithographic process. The nonmagnetic substrate 31 carrying the two YIG thin films Y.sub.1 and Y.sub.2 is placed on a lower conductor 32. A cavity is formed in a predetermined area in the lower conductor 32 to form an air gap 36 opposite to the YIG thin films Y.sub.1 and Y.sub.2 as shown in FIG. 15. A dielectric substrate 33, for example, a GGG substrate, is placed on the non-magnetic substrate 31. Parallel input signal transmission line L.sub.1 and an output signal transmission line L.sub.2 are formed on one surface of the dielectric substrate 33 facing the YIG thin films Y.sub.1 and Y.sub.2 so as to extend across the YIG thin films Y.sub.1 and Y.sub.2, respectively. A coupling transmission line L.sub.12 is formed on the other side of the dielectric substrate 33 transversely with respect to the direction of extension of the signal transmission lines L.sub.1 and L.sub.2 so as to extend over the YIG thin films Y.sub.1 and Y.sub.2. An upper conductor 34 is placed on the dielectric substrate 33 carrying the input signal transmission line L.sub.1 and the output signal transmission line L.sub.2 so as to hold the dielectric substrate 33 and the nonmagnetic substrate 31 carrying the YIG thin films Y.sub.1 and Y.sub.2 between the upper conductor 34 and the lower conductor 32 and so that the opposite side edges of the upper conductor 34 are disposed opposite to the opposite side edges of the lower conductor 32, respectively. A cavity is formed in a predetermined area in the inner surface of the upper conductor 34 to form an air gap 37 in an area corresponding to the YIG thin films Y.sub.1 and Y.sub.2, the input side of the input signal transmission line L.sub.1 and the output side of the output signal transmission line L.sub.2.

Grounding terminals e.sub.12a and e.sub.12b contacting the upper conductor 34 are formed at he opposite ends of the coupling transmission line L.sub.12 formed on the dielectric substrate 33 held between the lower conductor 32 and the upper conductor 34. Grounding terminals e.sub.1 and e.sub.2 are formed at the grounding end, namely, an end opposite the input end, of the input signal transmission line L.sub.1 and at the grounding end, namely, an end opposite the output end, of the output signal transmission line L.sub.2, respectively, so as to contact the lower conductor 32.

Thus, a filter assembly 35 comprising the YIG thin films Y.sub.1 and Y.sub.2, input and output signal transmission lines L.sub.1 and L.sub.2 coupled with the YIG thin films Y.sub.1 and Y.sub.2, and coupling transmission line L.sub.12 which are provided between the upper and lower conductors 32 and 34 is constructed.

As illustrated in FIG. 15, the filter assembly 34 is disposed within a magnetic gap 22 of a magnetic circuit 21. The magnetic circuit 21 is constructed, for example, by disposing a pair of bell-shaped magnetic cores 24A and 24B respectively having central cores 23A and 23B opposite to each other so as to form a magnetic gap 22 between the central cores 23A and 23B. A coil 25 is wound at least on either the central core 23A of the central core 23B. A DC current is supplied to the coil 25 to apply a desired DC magnetic field to the filter assembly 35 disposed within the magnetic gap 22.

The intensity of the magnetic field is varied by varying the intensity of the DC current applied to the coil 25 to vary the tuning frequency.

Extensions L.sub.1E and L.sub.2E are extended respectively from the input signal transmission line L.sub.1 and the output signal transmission line L.sub.2 so that the distances from the respective coupling parts of the signal transmission lines L.sub.1 and L.sub.2 coupled with the YIG thin films Y.sub.1 and Y.sub.2 to the respective grounded ends e.sub.1 and e.sub.2 are one-tenth or above and less than one-fourth the wavelength of a wave at the upper limit frequency.

In this embodiment, the extension L.sub.1E extending from one end of the input signal transmission line L.sub.1, and the extension L.sub.2E extending from one end of the output signal transmission line L.sub.2 extend in opposite directions with respect to each other, and the extensions L.sub.1E and L.sub.2E are bent outward, namely, away from each other, in an L-shape to form interval increasing parts L.sub.w therein, respectively, so that the distance between the extension L.sub.1E and the other end of the output signal transmission line L.sub.2, and the distance between the extension L.sub.2E and the other end of the input signal transmission line L.sub.1 are greater than the distance between the rest of the parts of the input signal transmission line L.sub.1 and the output signal transmission line L.sub.2. The respective outer corners of the bends of the interval increasing parts L.sub.w are cut diagonally to prevent reflection by the corners of the interval increasing parts L.sub.w.

The lines L.sub.1, L.sub.2 and L.sub.12 each may be split, for example, into two parts at the respective coupling parts coupled with the YIG thin films Y.sub.1 and Y.sub.2 to form split sections L.sub.1D, L.sub.2D and L.sub.12D in the lines L.sub.1, L.sub.2 and L.sub.12, respectively.

Although the extensions L.sub.1E and L.sub.2E respectively extending from the input signal transmission line L.sub.1 and the output signal transmission line L.sub.2 of this embodiment are bent in an L-shape to form the interval increasing parts L.sub.w, the extensions L.sub.1E and L.sub.2E may be formed in an oblique pattern SO that the extensions L.sub.1E and L.sub.2E extend gradually away from the output signal transmission line L.sub.2 and the input signal transmission line L.sub.1, respectively. Furthermore, although the interval increasing parts L.sub.w are formed in the extensions L.sub.1E and L.sub.2E in this embodiment, it is also possible to form the interval increasing parts L.sub.w in both the extensions L.sub.1E and L.sub.2E and the corresponding ends of the output signal transmission line L.sub.1 and the input signal transmission line L.sub.2, or to form interval increasing parts L.sub.w only in the ends of the input signal transmission line L.sub.1 and the output signal transmission line L.sub.2 respectively located opposite to the extensions L.sub.2E and L.sub.1E. Still further, in this embodiment, the respective grounded ends of the input signal transmission line L.sub.1 and the output signal transmission line L.sub.2 are opposite to each other with respect to the coupling parts of the signal transmission lines L.sub.1 and L.sub.2 coupled with the YIG thin films Y.sub.1 and Y.sub.2. However, if the grounded ends of the input signal transmission line L.sub.1 and the output signal transmission line L.sub.2 are on the same side with respect to the coupling parts, the extensions L.sub.1E and L.sub.2E are located opposite to each other. In such a case, the interval increasing part L.sub.w is formed only in either the extension L.sub.1E or L.sub.2E, or the interval increasing parts L.sub.w are formed in both the extensions L.sub.1E and L.sub.2E.

Furthermore, the coupling transmission line L.sub.12 of this embodiment may be substituted by a third YIG thin film to be magnetically coupled with the YIG disk Y.sub.1 and Y.sub.2.

In the above embodiments, although the YIG thin film filter is provided with the extensions L.sub.1E and L.sub.2E, the elongation of the lengths of the respective coincident portions of the input signal transmission line L.sub.1 and the output signal transmission line L.sub.2 is avoided, and the deterioration of the isolation characteristics due to the local concentration of electric field on the coupling part of the input signal transmission line L.sub.1 coupled with the YIG thin film Y.sub.1 and on the coupling pat of the output signal transmission line L.sub.2 coupled with the YIG thin film Y.sub.2 is avoided. FIGS. 17 and 18 show the isolation characteristics of filters according to the present invention respectively employing the filter assemblies respectively shown in FIGS. 12 and 13. FIG. 19 shows the isolation characteristics of an YIG thin film filter similar in construction to the YIG thin film filter shown in FIG. 12, except that extensions L.sub.1E and L.sub.2E are extended straight from the YIG thin films Y.sub.1 and Y.sub.2 without forming any interval increasing part L.sub.w therein. Although all these YIG thin film filters are variable frequency filters having a variable frequency bandwidth of three octaves in the range of 0.5 GHz to 4.0 GHz, as obvious from FIG. 19, the isolation is 40 dB at the upper limit frequency of 4 GHz, and is in the range of 45 to 50 dB in the frequency band below the upper limit frequency when the extensions L.sub.1E and L.sub.2E are straight. However, as obvious from FIGS. 17 and 18, the isolation of the YIG thin film filter employing the filter assembly of FIG. 12 is on the order of 40 dB at the upper limit frequency and is 60 dB or greater in the frequency band below the upper limit frequency, and the isolation of the YIG thin film filter employing the filter assembly of FIG. 13 is 50 dB or greater at the upper limit frequency of 4 GHz and is in the range of 65 to 70 dB in the almost entire range of the variable frequency band. Thus, the present invention improves the isolation characteristics.

Furthermore, as mentioned above, when the extensions L.sub.1E and L.sub.2E are straight, the area of the filter element, namely, the dielectric substrate 33, is as large as 10.times.11.6 mm.sup.2, whereas the area of the filter element is as small as 12.times.5 mm.sup.2 when the extensions L.sub.1E and L.sub.2E are bent in an L-shape. Thus, the present invention provides a compact thin film ferromagnetic resonance thin film tuning filter.

Still further, when the transmission lines L.sub.1 , L.sub.2 and L.sub.12 are split into longitudinal sections, uniform magnetic field is applied to the magnetic resonance YIG thin films Y.sub.1 and Y.sub.2 and thereby the spurious characteristics is improved.