Chip Type Directional Coupler

1. Field of the Invention

The present invention relates to a chip type directional coupler employing striplines.

2. Description of the Background Art

In order to manufacture a waveguide circuit, which has been the mainstream of microwave circuits, highly precise machining is required. Therefore, such a waveguide circuit is unsuitable for mass production, high-priced, large-sized and weighty. In a communication device handling frequencies exceeding the UHF level, further, it is difficult to form passive elements such as coils and capacitors into a lumped parameter circuit, since these elements have extremely small values. In a radio set or a BS receiver, therefore, a high frequency circuit is generally formed by a distributed parameter circuit. Microstriplines and striplines, which can be readily implemented in small sizes are employed for forming such a distributed parameter circuit.

A directional coupler is a circuit element which is adapted to draw an output being proportional to only unidirectional power from microwave power flowing through a transmission line without reference to reverse power. FIG. 5 shows a conventional quarter-wavelength coupled-line directional coupler, which is formed by using stripline electrodes 40a and 41a. Referring to FIG. 5, stripline electrodes 40a and 41a partially approach each other in grade separation over a length of .lambda./4, where .lambda. represents a wavelength.

Due to the coupling mode of the portions approaching each other in grade separation over the aforementioned length of .lambda./4. A high frequency signal which is applied to the principal line through a port 1 appears on another port 2, while several-tenth power thereof simultaneously appears on a port 3 of the secondary line. It is possible to arbitrarily set the output level to the port 3 by changing the space between the striplines 40a and 41a. When the space between the striplines 40a and 41a is so set that the output level to the port 3 is half the input level to the port 1, for example, this directional coupler serves as a distributor which equally distributes the input received in the port 1 to the ports 2 and 3 while a signal at the port 3 regularly delays that at the port 2 by a signal phase angle of 90.degree. in this case. Referring to FIG. 5, the stripline electrodes 40a and 41a are shielded by ground electrodes 40b and 41b, shown in two-dot chain lines, which are arranged to hold the stripline electrodes 40a and 41a from upper and lower directions while being insulated from the same.

A well-known directional coupler of such a broadside coupling type is formed by providing striplines on both surfaces of a resin substrate, arranging resin substrates on upper and lower portions thereof and compression-bonding the same through jointing materials, further holding this substance from upper and lower directions by ground plane metal plates, and connecting these layers with each other by screws of the like.

In a digital portable telephone or the like, such a directional coupler is applied to a 90.degree. phase converter or distributor of a phase modulation circuit by utilizing such a 90.degree. phase converting function or distributing function, as a directional coupler 50 shown in FIG. 6. When an output signal of a local oscillator 51 is inputted through a port 1 (corresponding to the port 1 in FIG. 5) of the directional coupler 50, ports 2 and 3 output signals of the same level which are 90.degree. out of phase with each other, and these signals are inputted in mixers 52a and 52b as seen in FIG. 6 respectively. The mixers 52a and 52b as seen in FIG. 6 phase-modulate the local signals by 180.degree. with I and Q pulsing signals respectively. When the modulation outputs are combined with each other in a combiner, the local signals are subjected to four type of phase modulations of 0.degree., 90.degree., 180.degree. and 270.degree..

However, the aforementioned portable telephone has an important subject of miniaturization, and hence further miniaturization is required also for a 90.degree. shifter comprised of the directional coupler. The stripline electrode requires a length of .lambda./4, e.g., 7.5 cm at 1 GHz with a dielectric constant of 1, as hereinabove described. In order to couple linear stripline electrodes having such lengths a substrate having a relatively large volume is required. In the broadside coupling type directional coupler shown in FIG. 5, the coupled lines are positioned along the vertical direction. If the coupled lines are superposed with a plurality of substrates and screwed, therefore, miniaturization of the directional coupler is limited and the cost is increased.

In consideration of the aforementioned circumstances, an object of the present invention is to provide a further miniaturized chip type directional coupler.

A chip type directional coupler according to the present invention comprises a laminated structure having two dielectric substrates, each provided with a quarter-wavelength stripline electrode formed in a meandering shape on one major surface thereof, which are so stacked as to electromagnetically couple the stripline electrodes with each other; and two substrates, each provided with a ground electrode on at least one major surface thereof, which are directly or indirectly stacked on both sides of the two dielectric substrates for hold the same, and a plurality of external electrodes provided on side surfaces of the laminated structure. Respective end portions of the stripline electrodes and the ground electrodes are electrically connected to different ones of the external electrodes.

The term "meander" described above is used herein to cover not only a shape combining curves such as the shape of snake but also a shape combining line segments such as a zigzag shape resembling lightning.

According to the aforementioned structure, the quarter-wavelength stripline electrode portions are formed nonlinearly in a meandering shape so that whole portions can be formed on smaller substrates, as compared with a structure having linear electrode portions, whereby the chip type directional coupler can be miniaturized. Further, the ground electrodes are adapted to vertically hold or enclose the stripline electrodes for shielding the same from upper and lower directions, whereby an electromagnetic shielding structure can be implemented by the laminated structure with no requirement for a metal case etc. Further, the chip type directional coupler can be surface-mounted on a substrate, since the external electrodes are formed on its side surfaces.

The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description off the present invention when taken in conjunction with the accompanying drawings.

FIG. 1 is a perspective view showing a chip type directional coupler according to an embodiment of the present invention;

FIG. 2 is an exploded perspective view illustrating respective substrates in the chip type directional coupler shown in FIG. 1;

FIG. 3 is a perspective view showing respective substrates employed for mass-producing chip type directional couplers;

FIG. 4A is a perspective view illustrating a laminated substrate formed by the substrates shown in FIG. 3, FIG. 4B is a perspective view showing a state of the laminated substrate provided with through holes, and FIG. 4C is an enlarged perspective view showing one of a plurality of chip type directional couplers obtained by cutting the laminated substrate shown in FIG. 4B along prescribed cutting lines after injecting a metal into the through holes;

FIG. 5 is a perspective view showing a conventional broadside coupling type directional coupler; and

FIG. 6 is a block diagram showing a quadrature modulation circuit employing a directional coupler.

An embodiment of the present invention is now described with reference to FIGS. 1 to 4C.

FIG. 1 is a perspective view showing the appearance of a chip type directional coupler 1. This chip type directional coupler 1 has a laminated structure which is formed by stacking a first ground electrode substrate 2, a first stripline electrode substrate 3, a second stripline electrode substrate 4, a second ground electrode substrate 5, and a protective substrate 5. This laminated structure is provided on its side surfaces with external electrodes C, D and E for ground electrodes, a principal line and a secondary line respectively. The substrates 2 to 6 are formed of ceramic green sheets, in practice. The ceramic green sheets are first provided with respective electrode films on major surfaces thereof, and then stacked with each other. The as-obtained green laminate is provided with the external electrodes C, D and E on its side surfaces, and thereafter sintered to form the coupler 1. In practice, therefore, no separation lines appear between the layers of the respective substrates 2 to 6 shown in FIG. 1. The external electrodes C, D and E may be formed by applying conductive paste to the laminate and baking the same, or by performing plating or evaporation after firing the laminate of the ceramic sheets.

As understood from FIG. 2 showing an exploded perspective view of the directional coupler 1 shown in FIG. 1, the first ground electrode substrate 2 is formed by a square ceramic substrate 2a and a ground electrode 2b provided on one major surface thereof. The ground electrode 2b is sized to be capable of covering a U-shaped quarter-wavelength stripline electrode portion 3f as described later. This ground electrode 2b is not formed over the entire major surface of the ceramic substrate 2a. In other words, the ground electrode 2b is not formed on a peripheral edge portion of the substrate 2a, to be prevented from electrical connection with external electrodes 2d and 2e as described below. The ceramic substrate 2a is provided on its side surfaces with external electrodes 2c, 2d and 2e. The ground electrode 2b is extended toward the external electrodes 2c, while the external electrodes 2d and 2e are not electrically connected with the ground electrode 2b, as described above.

The first stripline electrode substrate 3 is formed by a square ceramic substrate 3a and a stripline electrode 3b (secondary line) provided on one major surface thereof. This stripline electrode 3b is formed by a substantially U-shaped quarter-wavelength stripline electrode portion 3f, lead electrode portions 3g which are connected to external electrode portions 3e as described later, and intermediate portions 3h connecting the lead electrode portions 3g with both end portions of the quarter-wavelength stripline electrode portion 3f. The intermediate portions 3h are adapted by tapering (not shown), to impedance-match the stripline electrode portion 3f with the lead electrode portions 3g. Further, external electrode portions 3c, 3d and 3e corresponding to the aforementioned external electrode portions 2c, 2d and 2e are positioned on the side surfaces of the ceramic substrate 3a respectively. The quarter-wavelength stripline electrode portion 3f is formed to define a large U shape in the range of the ground electrode 2b as close as possible along its peripheral edge portion.

The second stripline electrode substrate 4, which is substantially similar in structure to the first stripline electrode substrate 3, has a square ceramic substrate 4a, a stripline electrode 4b (principal line), a U-shaped quarter-wavelength stripline electrode portion 4f, lead electrode portions 4g, and external electrode portions 4c, 4d and 4e. The lead electrode portions 4g are bent in a direction for avoiding grade separation with the lead electrode portions 3g, and connected with the external electrode portions 4d.

The second ground electrode substrate 5, which is identical in structure to the second ground electrode substrate 2, has a square ceramic substrate 5a, a ground electrode 5b, and external electrode portions 5c, 5d 5e.

The protective substrate 6 is formed by a square ceramic substrate 6a, which is provided on its side surfaces with external electrode portions 6c, 6d and 6e corresponding to the external electrode portions 2c, 2d and 2e respectively. The external electrode portions of the respective substrates 2 to 6 are stacked and compression-bonded to each other. Therefore, the external electrodes C for the ground electrodes are defined by the external electrode portions 2c, 3c, 4c, 5c and 6c, the external electrodes D for the principal line are defined by the external electrode portions 2d, 3d 4d, 5d and 6d, and the external electrodes E for the secondary line are defined by the external electrode portions 2e, 3e, 4e, 5e and 6e respectively, as shown in FIG. 1.

According to the aforementioned structure, the quarter-wavelength stripline electrode portions 3f and 4f of the first and second stripline electrode substrates 3 and 4 which are held between the first and second ground electrode substrates 2 and 5 approach each other in grade separation, thereby forming the directional coupler 1.

The quarter-wavelength stripline electrode portions 3f and 4f are so nonlinearly formed that the same can be provided on smaller substrates as compared with linear ones, whereby the chip type directional coupler 1 can be miniaturized. Further, the ground electrodes 2a and 5a are arranged to hold the two stripline electrodes 3b and 4b for shielding the same from upper and lower directions, whereby an electromagnetic shielding structure can be implemented by the laminated structure with no requirement for a metal case etc. In addition, the external electrodes C, D and E are formed on the side surfaces of the laminate, to enable surface mounting of the directional coupler 1 on a substrate.

A method of manufacturing the aforementioned chip type directional coupler 1 is now briefly described. First, two green sheets provided with strip conductors are vertically superposed with each other and then held by green sheets printed with ground electrodes from upper and lower directions. Then a green sheet for serving as a protective substrate is stacked on this substance, and the as-formed laminate is integrally fired after application of respective external electrode materials. Such external electrodes may alternatively be formed after the firing step, as a matter of course.

While the directional substrates may be prepared from either resin or ceramic substrates, ceramic can suppress power loss of the principal line since the same has smaller dielectric loss than glass epoxy resin etc. as described below, and is excellent in heat radiation for attaining further miniaturization.

Such chip type directional couplers can be mass-produced in the following manufacturing method: As shown in FIG. 3, a substrate 12 made of a green sheet and provided with a plurality of ground electrodes, a substrate 13 made of a green sheet and provided with a plurality of stripline electrodes (secondary lines), a substrate 14 made of a green sheet and provided with a plurality of stripline electrodes (principal lines), a substrate 15 made of a green sheet and printed with a plurality of ground electrodes and a pair of substrate 16 made of green sheets for defining protective substrates are stacked to obtain a mother laminated substrate 20 shown in FIG. 4A. Then, through holes h are formed in portions for defining external electrodes and other pairs of portions (the pairs of portions are adapted to improve junction strength between the respective substrates) as shown in FIG. 4B, and a metal for defining electrodes is injected into the through holes h. Thereafter the mother laminated substrate 20 is cut along prescribed cutting lines, and each cut piece fired obtain a chip type directional coupler 1 having external electrodes C, D and E on its side surfaces, as shown in FIG. 4C.

While the degree of coupling and the characteristic impedance in a quarter-wavelength stripline electrode portion depends on the dielectric constant, film thickness, line width and line spacing, such line spacing can be easily adjusted by interposing a non-printed sheet of a prescribed width between substrates since the inventive chip type directional coupler has a laminated structure.

Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope the present invention being limited only by the terms of the appended claims.