The present invention relates to a rotary transformer. More particularly, the present invention relates to a rotary transformer that can be used as a signal transmitting element for a rotary head cylinder in a video tape recorder, a digital audiotape recorder and the like.

In recent years, there has been a severe competition in the price reduction of magnetic recording and reproducing devices such as video tape recorders. Therefore, electronic components used in such devices are required to be inexpensive. A rotary transformer, which is one of the main components, is also required to have a low cost. At the same time, since there is a demand for multifunctional recording and reproducing devices, a rotary transformer applicable in various types of devices is desired.

A conventional rotary transformer will now be described referring to Figures 8, 9 and 10.

Figures 8 and 9 are a plan view and a sectional view of a conventional rotary transformer, respectively. The rotary transformer has a cylindrical inner magnetic core portion 1 made from a sintered ferrite mold that is highly accurately ground and has a high permeability. A plurality of recesses 2 and a plurality of lateral grooves 3 are formed on the outside surface of the inner magnetic core portion 1 horizontally and vertically, respectively. Windings are disposed as a signal coil 4 in each of the recesses 2, and a short ring groove 6 is provided at a predetermined position between the recesses 2. A short ring 5 is disposed in the short ring groove 6 to prevent crosstalk caused between the signal coils 4 in the adjacent recesses 2.

A cylindrical outer magnetic core portion 8 is disposed so as to enclose the inner magnetic core portion 1 with a predetermined gap 7 therebetween. The outer magnetic core portion 8 is made from a sintered ferrite mold that is highly accurately ground and has a high permeability. A plurality of recesses 9 and a plurality of lateral grooves 10 are formed on the inside surface of the outer magnetic core portion 8 horizontally and vertically, respectively. Windings are disposed as a signal coil 11 in each of the recesses 9 so as to oppose each signal coil 4 provided in the recess 2 on the inner magnetic core portion 1.

In this rotary transformer, one of the inner magnetic core portion 1 and the outer magnetic core portion 8 is fixed, and the other is rotated about a common axis, thereby transmitting signals between the signal coils 4 and 11 opposing each other.

The rotary transformer is further provided with a terminal board 13 with a terminal pin 12 at the upper peripheral portion of the inner magnetic core portion 1 as shown in Figure 10. The terminal board 13 is connected to a lead line of the signal coil 4. Similarly, a terminal board 15 with a terminal pin 14 is provided at the lower peripheral portion of the outer magnetic core portion 8. The terminal board 15 is connected to a lead line of the signal coil 11. signals are transferred into and out of the rotary transformer through these terminal boards 13 and 15.

The inner magnetic core portion 1 and the outer magnetic core portion 8 are both formed from the same kind of the specific sintered ferrite having a permeability of approximately 600.

In the production of the rotary transformer having the above-mentioned structure, it is necessary to assemble the inner magnetic core portion 1 and the outer magnetic core portion 8 with the small gap 7 of 50 to 60 µm therebetween. At this point, attention should be paid not to cause any shift of the axis of the magnetic core portions (hereinafter referred to as the "axis shift"). Accordingly, the material for the magnetic core portions must be highly accurately ground and carefully assembled.

The cost for such accurate grinding corresponds to 60 to 70% of the total cost for the materials to be used for the magnetic core portion, and is one of the most significant factors preventing price reduction.

Figure 3 shows the relationship between the magnitude of the axis shift and the inductance increase. The inductance increase is indicated on the basis of the inductance at the time when the axis shift is not caused. As is shown in Figure 3, when an axis shift occurs, characteristic values such as values indicating the inductance, transmission loss and crosstalk are significantly varied. Such variations can bring the inner magnetic core portion 1 into contact with the outer magnetic core portion 8. Since one of the magnetic core portions is rotating at a high speed, the contact results in damage to the rotary transformer itself.

Furthermore, since the same material is used in both inner and outer magnetic core portions 1 and 8, the characteristic values depend upon the structure of the magnetic core portions such as the kind of wirings, the size of the winding groove, etc. This means a plurality of equipment and the like are required to produce a plurality of types of magnetic core portions. Thus, it is difficult to provide magnetic core portions with the same structure but with different characteristic values. In other words, it is difficult to manufacture a magnetic core portion which can be applied in various types of rotary transformers.

The rotary transformer of this invention includes a cylindrical inner magnetic core portion and a cylindrical outer magnetic core portion enclosing the inner magnetic core portion, wherein the inner magnetic core portion and the outer magnetic core portion are rotatable with respect to each other about a common axis: the inner magnetic core portion has windings as a first coil on an outside surface thereof; the outer magnetic core portion has windings as a second coil on an inside surface thereof, the inside surface opposing the outside surface of the inner magnetic core portion with a predetermined distance therebetween; and the outer magnetic core portion is made from a different material from a material for the inner magnetic core portion.

In one embodiment of the invention, the inner magnetic core portion has a first recess for the first coil on the outside surface thereof; and the outer magnetic core portion has a second recess for the second coil on the inside surface thereof.

According to another aspect of the invention, a rotary transformer including a cylindrical inner magnetic core portion and a cylindrical outer magnetic core portion enclosing the inner magnetic core portion is provided. In the rotary transformer, the inner magnetic core portion and the outer magnetic core portion are rotatable with respect to each other about a common axis; the inner magnetic core portion has windings as a first coil on an outside surface thereof; the outer magnetic core portion has windings as a second coil on an inside surface thereof, the inside surface opposing the outside surface of the inner magnetic core portion with a predetermined distance therebetween; and at least one of the inner magnetic core portion and the outer magnetic core portion is made from a resin ferrite.

In one embodiment of the invention, the inner magnetic core portion has a first recess for the first coil on the outside surface thereof; and the outer magnetic core portion has a second recess for the second coil on the inside surface thereof.

In another embodiment of the invention, the inner magnetic core portion is made from a sintered ferrite and the outer magnetic core portion is made from a resin ferrite.

In another embodiment of the invention, the inner magnetic core portion is made from a resin ferrite and the outer magnetic core portion is made from a sintered ferrite.

In another embodiment of the invention, the inner magnetic core portion and the outer magnetic core portion are both made from a resin ferrite.

In another embodiment of the invention, the inner magnetic core portion is made from a first resin ferrite, and the outer magnetic core portion is made from a second resin ferrite that is different from the first resin ferrite.

In another embodiment of the invention, the rotary transformer further includes a cylindrical inner supporting member for supporting the inner magnetic core portion, wherein the inner magnetic core portion is constituted of a plurality of sections provided on the inner supporting member.

In another embodiment of the invention, the rotary transformer further includes a cylindrical outer supporting member for supporting the outer magnetic core portion, wherein the outer magnetic core portion is constituted of a plurality of sections provided on the outer supporting member.

In another embodiment of the invention, the plurality of sections are axially symmetrically disposed with respect to the common axis; and a lead line connected to the first coil is provided in a portion formed between the adjacent sections.

In another embodiment of the invention, the plurality of sections are axially symmetrically disposed with respect to the common axis; and a lead line connected to the second coil is provided in a portion formed between the adjacent sections.

According to another aspect of the invention, a rotary transformer including a disk-shaped upper magnetic core portion and a disk-shaped lower magnetic core portion opposing the upper magnetic core portion is provided. In the rotary transformer, the upper magnetic core portion and the lower magnetic core portion are rotatable with respect to each other about a common axis; the upper magnetic core portion has windings as a first coil on a bottom surface thereof; the lower magnetic core portion has windings as a second coil on a top surface thereof, the top surface opposing the bottom surface of the upper magnetic core portion with a predetermined distance therebetween; and the lower magnetic core portion is made from a material that is different from a material for the upper magnetic core portion.

In one embodiment of the invention, the upper magnetic core portion has a first recess for the first coil on the bottom surface thereof; and the lower magnetic core portion has a second recess for the second coil on the top surface thereof.

According to another aspect of the invention, a rotary transformer including a disk-shaped upper magnetic core portion and a disk-shaped lower magnetic core portion opposing the upper magnetic core portion is provided. In the rotary transformer, the upper magnetic core portion and the lower magnetic core portion are rotatable with respect to each other about a common axis; the upper magnetic core portion has windings as a first coil on a bottom surface thereof; the lower magnetic core portion has windings as a second coil on a top surface thereof, the top surface opposing the bottom surface of the upper magnetic core portion with a predetermined distance therebetween; and at least one of the lower magnetic core portion and the upper magnetic core portion is made from a resin ferrite.

In one embodiment of the invention, the upper magnetic core portion has a first recess for the first coil on the bottom surface thereof; and the lower magnetic core portion has a second recess for the second coil on the top surface thereof.

In another embodiment of the invention, the upper magnetic core portion is made from a sintered ferrite, and the lower magnetic core portion is made from a resin ferrite.

In another embodiment of the invention, the upper magnetic core portion is made from a resin ferrite, and the lower magnetic core portion is made from a sintered ferrite.

In another embodiment of the invention, the upper magnetic core portion and the lower magnetic core portion are both made from a resin ferrite.

In another embodiment of the invention, the upper magnetic core portion is made from a first resin ferrite, and the lower magnetic core portion is made from a second resin ferrite that is different from the first resin ferrite.

Thus, the invention described herein makes possible the advantages of (1) providing an inexpensive rotary transformer which does not require an accurate grinding process and in which characteristic values are unlikely to be varied due to an axis shift; and (2) providing a rotary transformer applicable in various types of recording and reproducing devices.

These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.

Figure 1 is a sectional view of a rotary transformer according to Example 1 of this invention.

Figure 2 is a sectional view of a rotary transformer according to Example 2 of this invention.

Figure 3 is a graph showing the relationship between the inductance increase and the magnitude of the axis shift in the rotary transformer of Figure 1 and in a conventional rotary transformer.

Figure 4 is a magnetic circuit diagram showing signal transmission in the rotary transformer of Figure 1.

Figure 5 is a plan view of a rotary transformer according to Example 3 of this invention.

Figure 6 is a sectional view of the rotary transformer of Figure 5.

Figures 7A and 7B are sectional views of a rotary transformer according to Example 4 of this invention.

Figure 8 is a plan view of a conventional rotary transformer.

Figure 9 is a sectional view of the rotary transformer of Figure 8.

Figure 10 is a sectional view of another conventional rotary transformer.

The rotary transformer of the present invention will now be described by way of examples referring to the accompanying drawings.

As is shown in Figure 1, the rotary transformer of this example comprises an axially symmetric cylindrical inner magnetic core portion 16 and an axially symmetric cylindrical outer magnetic core portion 21 enclosing the inner magnetic core portion 16. The inner magnetic core portion 16 has, for example, an inside diameter of 12.0 mm, an outside diameter of 15.0 mm and an axial length of 12.0 mm. The outer magnetic core portion 21 has, for example, an inside diameter of 15.1 mm, an outside diameter of 18.0 mm and an axial length of 12.0 mm. The term "cylindrical" magnetic core portion used herein widely covers an axially symmetric magnetic member having an axially symmetric bore therein. Specifically, the shape of a magnetic member in section along a symmetrical axis is not limited to a rectangle. The sectional shape may alternatively be a triangle, a trapezoid, and a combination thereof.

The inner magnetic core portion 16 has two recesses 17 on the outside surface thereof. Windings are disposed as a first signal coil 18 in each of the recesses 17. The inner magnetic core portion 16 also has two lateral grooves (not shown) extending along the symmetric axis on the outside surface thereof. These lateral grooves are provided for lead lines of the first signal coils 18. The inner magnetic core portion 16 further has a short ring groove 20 between the recesses 17 on the outside surface. A short ring 19 is provided in the short ring groove 20 so as to prevent crosstalk between the first signal coils 18 in the adjacent recesses 17.

The outer magnetic core portion 21 has two recesses 24 on the inside surface thereof. Windings are disposed as a second signal coil 23 in each of the recesses 24. The outer magnetic core portion 21 also has two lateral grooves (not shown) extending along the symmetric axis on the inside surface thereof. These lateral grooves are provided for lead lines of the second signal coils 23.

The recesses 17 and 24 are formed in such positions as to have each of the first signal coils 18 oppose each of the second signal coils 23. The recesses 17 and 24 preferably have a sufficient depth to completely contain the signal coils 18 and 23, respectively, and the depth is, for example, 0.3 mm. The width of the recesses 17 and 24 is, for example, 1.0 mm. The short ring groove 20 is, for example, 0.5 mm wide and 0.4 mm deep.

The inner magnetic core portion 16 and the outer magnetic core portion 21 are rotatable with respect to each other about a common axis, and are disposed so as to oppose each other with a predetermined distance of, for example, 50 to 60 µm between the outside surface of the inner magnetic core portion 16 and the inside surface of the outer magnetic core portion 21. Thus, a gap 22 is formed between the inner magnetic core portion 16 and the outer magnetic core portion 21. While the inner magnetic core portion 16 and the outer magnetic core portion 21 are rotating with respect to each other at a high speed, signals are transmitted between the first signal coils 18 and the second signal coils 23.

The outer magnetic core portion 21 is made from a sintered ferrite, and has a permeability of approximately 400 to 800. The inner magnetic core portion 16 is made by molding a resin ferrite containing 90 wt% of powdery sintered ferrite, 9.5 wt% of a polyphenylene sulfide resin and 0.5 wt% of a stabilizer. The term "a resin ferrite" used herein indicates a material containing a powdery sintered ferrite and a resin. The "sintered ferrite" is obtained by molding the "resin ferrite" and then sintering it. The inner magnetic core portion 16 made from such a resin ferrite has a permeability of approximately 13, which is remarkably small as compared with that of the sintered ferrite used for the outer magnetic core portion 21. In order to compensate the small permeability and to keep characteristic values of the signal transmitting system in the rotary transformer at a high level, only two signal coils, that is, signal coils 18 and 23, are used in this example. However, the number of the signal coils is not limited to two but can be three or more.

Figure 4 is a magnetic circuit diagram showing signal transmission in the rotary transformer of Figure 1. When the inner magnetic core portion 16 is made from a material having a different permeability from that of the material for the outer magnetic core portion 21 as in this example, the magnetic circuit formed by the signal transmission from the first signal coils 18 is identical to the magnetic circuit formed by the signal transmission from the second signal coils 23. Therefore, signals are transmitted from the first signal coils 18 to the second signal coils 23 in the same manner as from the second signal coils 23 to the first signal coils 18. As a result, transmission characteristics of the signals are not affected by their transmitting directions.

As described above, the outer magnetic core portion 21 of this example is made from a sintered ferrite as in a conventional rotary transformer, but the inner magnetic core portion 16 is made from a resin ferrite. Although a resin ferrite generally has a low permeability of approximately 5 to 18 as compared with a sintered ferrite, it has a high workability. Therefore, a magnetic core portion made by molding such a resin ferrite makes an accurate grinding process unnecessary, resulting in reducing the material cost by 60 to 70%, i.e., the cost for the grinding. Moreover, in this example, the inner magnetic core portion 16 is molded from the resin ferrite without conducting a sintering process after the molding. In a conventionally used method, this sintering step is indispensable and causes a size reduction in the magnetic core portions. Therefore, the size reduction of the inner magnetic core portion 16 is prevented in this example. Additionally, damages such as cracks and chips are unlikely to be caused in the magnetic core portion made from the resin ferrite.

For the above-mentioned reasons, the rotary transformer of this example presents a higher yield and a lower production cost as compared with the conventional rotary transformer. For example, the yield, which is conventionally about 93%, is improved to be about 98% in this example. Further, the rotary transformer of this example can be more compact because of the workability of the resin ferrite.

The rotary transformer of this example further has the following advantage: Since the inner magnetic core portion molded from the resin ferrite has a lower permeability, the reluctance (as shown in Figure 4) caused by the axis shift in the rotary transformer is less varied. Therefore, even if an axis shift is caused in combining the inner magnetic core portion 16 and the outer magnetic core portion 21, such an axis shift does not invite the variations in the characteristic values of the rotary transformer. As is apparent from Figure 3, even if the axis shift is significantly large in the rotary transformer of this example, the inductance does not increase as much as in the conventional rotary transformer. This effect which is attained by lowering the permeability of one of the magnetic core portions can also be attained when using a sintered ferrite with a comparatively low permeability of, for example, about 30 for one of the magnetic core portions.

In this example, the inner magnetic core portion 16 is made from a resin ferrite. However, when the outer magnetic core portion 21 is made from a resin ferrite and the inner magnetic core portion 16 is made from a sintered ferrite to the contrary, the production yield can be improved and the production cost can be decreased as a whole.

The inner magnetic core portion 16 and the outer magnetic core portion 21 which are made from different magnetic materials can further provide the following advantage: By appropriately selecting the magnetic materials for the magnetic core portions, various characteristic values of the rotary transformer can be presented without changing the structure thereof. Therefore, the same apparatus can be used in producing various magnetic core portions used in different types of the rotary transformers. Thus, a number of types of the rotary transformers can be produced at a low cost. In order to achieve the above-mentioned advantage, the inner magnetic core portion 16 and the outer magnetic core portion 21 can be made from two kinds of sintered ferrites each having a different permeability. In such a case, however, the advantage owing to the workability of the resin ferrite can not be achieved. Nevertheless, the variation in the reluctance caused by the axis shift can be sufficiently prevented by using a material with a lower permeability in one magnetic core portion than in the other, as described above. A sintered ferrite with a low permeability of approximately 15 to 50 is preferably used to produce such a rotary transformer.

Alternatively, the inner magnetic core portion 16 and the outer magnetic core portion 21 can be molded from different resin ferrites. In this case, the workability of resin ferrite can be maximumly taken advantage of. In order to specifically achieve the advantage of the workability of the resin ferrite, the inner magnetic core portion 16 and the outer magnetic core portion 21 can be molded from the same resin ferrite.

The components and the contents thereof in the sintered ferrite are not limited to those used in this example. Any mixture containing powdery sintered ferrite and a small amount of a resin component can be used as the resin ferrite for the rotary transformer of this invention.

Preferably, the content of the powdery sintered ferrite is within the range of 60 to 95 wt%. Examples of the usable resin include, in addition to the polyphenylene sulfide resin, thermoplastic resins such as nylon and polypropylene and thermosetting resins such as epoxy.

The rotary transformer of this example has a cylindrical inner magnetic core portion 16 and a cylindrical outer magnetic core portion 21 enclosing the inner magnetic core portion 16 as is shown in Figure 2. The inner magnetic core portion 16 has, for example, an inside diameter of 6.0 mm, an outsider diameter of 8.0 mm and an axial length of 10.0 mm. The outer magnetic core portion 21 has, for example, an inside diameter of 8.14 mm, an outside diameter of 11.0 mm and an axial length of 10.0 mm.

The inner magnetic core portion 16 has four recesses 17 on the outside surface thereof. Windings are disposed as a first signal coil 18 in each of the four recesses 17. The inner magnetic core portion 16 also has two lateral grooves (not shown) extending along the symmetric axis. The lateral grooves are provided for lead lines of the first signal coils 18. A short ring groove 20 is further provided between the two recesses 17 in the middle. A short ring 19 is provided in the short ring groove 20 so as to prevent crosstalk between the signal coils 18 in the adjacent recesses 17.

The outer magnetic core portion 21 has four recesses 24 on the inside surface thereof. Windings are disposed as a second signal coil 23 in each of the four recesses 24. The outer magnetic core portion 21 also has two lateral grooves (not shown) extending along the symmetric axis. The lateral grooves are provided for lead lines of the second signal coils 23.

The recesses 17 and 24 are formed in such positions as to have each of the first signal coils 18 oppose each of the second signal coils 23. The recesses 17 and 24 preferably have a sufficient depth to completely contain the signal coils 18 and 23, respectively, and the depth is, for example, 0.2 mm. The width of the recesses 17 and 24 is, for example, 0.5 mm. The short ring groove 20 is, for example, 0.3 mm wide and 0.25 mm deep.

The inner magnetic core portion 16 and the outer magnetic core portion 21 are rotatable with respect to each other about a common axis, and are disposed so as to oppose each other with a predetermined distance of, for example, 70 µm between the outside surface of the inner magnetic core portion 16 and the inside surface of the outer magnetic core portion 21. Thus, a gap 22 is formed between the inner magnetic core portion 16 and the outer magnetic core portion 21.

In the rotary transformer of this example, both the outer magnetic core portion 21 and the inner magnetic core portion 16 are molded from a resin ferrite containing 91 wt% of powdery sintered ferrite, 8.5 wt% of a polyphenylene sulfide resin and 0.5 wt% of a stabilizer. The inner magnetic core portion 16 and the outer magnetic core portion 21 have a permeability of approximately 16.

In this manner, both the inner magnetic core portion 16 and the outer magnetic core portion 21 are made from the same resin ferrite in this example. Therefore, damages such as cracks and chips are unlikely to be caused in the magnetic core portion during the production, thereby improving the production yield as compared with the conventional rotary transformer. Further, this technique is advantageous to produce a compact rotary transformer because both the inner and outer magnetic core portions are made from resin ferrite having an excellent workability. Additionally, the variation in the reluctance caused by the axis shift can be reduced because the magnetic core portion made from a resin ferrite has a low permeability. Therefore, even when an axis shift is caused in combining the inner magnetic core portion 16 and the outer magnetic core portion 21, the variation in the characteristic values of the rotary transformer due to the axis shift can be prevented.

The rotary transformer of this example has a symmetric inner magnetic core portion 16 and a symmetric outer magnetic core portion 21 enclosing the inner magnetic core portion 16 as is shown in Figures 5 and 6. The inner magnetic core portion 16 has, for example, an inside diameter of 10.0 mm, an outside diameter of 12.0 mm and an axial length of 13.0 mm. The outer magnetic core portion 21 has, for example, an inside diameter of 12.1 mm, an outside diameter of 14.1 mm and an axial length of 13.0 mm.

The inner magnetic core portion 16 is divided into two sections 16a and 16b each having a thickness of 1.0 mm. The sections 16a and 16b are in the axially symmetric shape. The sections 16a and 16b are provided around a cylindrical inner support 29. Two lateral grooves 30 are formed between the sections 16a and 16b. The lateral groove 30 has, for example, a width of 1.2 mm and a depth of 1.0 mm, which corresponds to the thickness of the sections 16a and 16b.

Each of the sections 16a and 16b has two lateral grooves 31 extending along the symmetric axis on the outside surface thereof. The lateral grooves 31 are provided for lead lines of a first signal coils 18. The inner magnetic core portion 16 constituted of the sections 16a and 16b has four recesses 17 on the outside surface thereof. Windings are disposed as the first signal coil 18 in each of the recesses 17. The inner magnetic core portion 16 further has a short ring groove 20 between the two recesses 17 in the middle. A short ring 19 is provided in the short ring groove 20 so as to prevent crosstalk between the first signal coils 18 in the adjacent recesses 17.

The outer magnetic core portion 21 is divided into two sections 21a and 21b each having a thickness of 1.0 mm. The sections 21a and 21b are in the axially symmetric shape. The sections 21a and 21b are provided around a cylindrical outer support 32. Two lateral grooves 33 are formed between the sections 21a and 21b. The lateral groove 33 has, for example, a width of 1.5 mm and a depth of 1.0 mm, which corresponds to the thickness of the sections 21a and 21b.

Each of the sections 21a and 21b has two lateral grooves 34 extending along the symmetric axis on the inside surface thereof. The lateral grooves 34 are provided for lead lines of a second signal coils 23. The outer magnetic core portion 21 constituted of the sections 21a and 21b has four recesses 24 on the inside surface thereof. Windings are disposed as the second signal coil 23 in each of the recesses 24. The inner support 29 and the outer support 32 are preferably made from a metal such as aluminum or a synthetic resin such as a polyphenylene sulfide resin.

The recesses 17 and 24 are formed in such positions as to have each of the first signal coils 18 oppose each of the second signal coils 23. The recesses 17 and 24 preferably have a sufficient depth to completely contain the signal coils 18 and 23, respectively, and the depth is, for example, 0.22 mm. The width of the recesses 17 and 24 is, for example, 0.9 mm. The short ring groove 20 is, for example, 0.35 mm wide and 0.3 mm deep.

The inner magnetic core portion 16 and the outer magnetic core portion 21 are rotatable with respect to each other about a common axis, and are disposed so as to oppose each other with a predetermined distance of, for example, 50 µm between the outside surface of the inner magnetic core portion 16 and the inside surface of the outer magnetic core portion 21. Thus, a gap 22 is formed between the inner magnetic core portion 16 and the outer magnetic core portion 21.

In this example, all the sections 16a, 16b, 21a and 21b of the inner magnetic core portion 16 and the outer magnetic core portion 21 are molded from the resin ferrite including 90.5 wt% of powdery sintered ferrite, 9.0 wt% of a polyphenylene sulfide resin and 0.5 wt% of a stabilizer. The inner and outer magnetic core portions 16 and 21 have a permeability of approximately 14.

As a result of this structure, the same effects as attained in Example 2 can be attained also in this example. Further, since the magnetic core portions are constituted of a plurality of sections, the molding is simplified as compared with the molding in the cylindrical shape as in Examples 1 and 2. It goes without saying that the number of the sections in the magnetic core portion is not limited to two.

As described above, all the sections of the magnetic core portions 16 and 21 are molded from the resin ferrite in this example. However, such a modification can be made that either the sections 16a and 16b of the inner magnetic core portion 16 or the sections 21a and 21b of the outer magnetic core portion 21 are molded from the resin ferrite, and the rest are made from a sintered ferrite. Alternatively, the section 16a of the inner magnetic core portion 16 and the section 21a of the outer magnetic core portion 21 are molded from the resin ferrite, and the section 16b of the inner magnetic core portion 16 and the section 21b of the outer magnetic core portion 21 are made from the sintered ferrite.

A plane type rotary transformer will be described in this example referring to Figures 7A and 7B. This rotary transformer has an upper magnetic core portion 116 and a lower magnetic core portion 121 opposing each other, both in the shape of an axially symmetric disk.

The upper magnetic core portion 116 has two recesses 117 on the bottom surface thereof. Windings are disposed as a first signal coil 118 in each of the recesses 117. A short ring groove 120 is provided between the recesses 117. A short ring 119 is provided in the short ring groove 120 so as to prevent crosstalk between the first signal coils 118.

The lower magnetic core portion 121 has two recesses 124 on the upper surface thereof. Windings are disposed as a second signal coil 123 in each of the two recesses 124.

The recesses 117 and 124 are formed in such positions as to have each of the first signal coils 118 oppose each of the second signal coils 123. The recesses 117 and 124 preferably have a sufficient depth to completely contain the signal coils 118 and 123, respectively, and the depth is, for example, 0.2 mm. The width of the recesses 117 and 124 is, for example, 0.5 mm. The short ring groove 120 is, for example, 0.3 mm wide and 0.25 mm deep.

The upper magnetic core portion 116 and the lower magnetic core portion 121 are rotatable with respect to each other about a common axis, and are disposed so as to oppose each other with a predetermined distance of, for example, 70 µm therebetween. Thus, a gap 122 is formed between the upper magnetic core portion 116 and the lower magnetic core portion 121.

As in the above-described examples, at least one of the upper and lower magnetic core portions 116 and 121 is molded from a resin ferrite. Alternatively, the upper and lower magnetic core portions 116 and 121 can be made from different sintered ferrites each having a different permeability.

The rotary transformer of this example has the following advantages: Since one of the upper and lower magnetic core portions 116 and 121 is formed from a material with a lower permeability, the characteristic values of the rotary transformer are not significantly varied even when the upper and lower magnetic core portions 116 and 121 are shifted as is shown in Figure 7B. This type of rotary transformer can also attain the same effects of the rotary transformers of the above-described examples.

Additionally, it is not necessary to provide a terminal plate and the like to the rotary transformer of this invention unlike the conventional rotary transformer. Thus, the structure can be simplified.

Various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the scope and spirit of this invention. Accordingly, it is not intended that the scope of the claims appended hereto be limited to the description as set forth herein, but rather that the claims be broadly construed.