1. Field of the Invention
The present invention generally relates to a high-frequency coupler for deriving a portion of a high-frequency signal such as a microwave signal or a millimeter wave signal from a main transmission line, and more particularly, to a high-frequency coupler constructed with microstrip lines.
2. Description of the Prior Art
In radio equipment, a high-frequency coupler deriving a portion of power of a high-frequency signal is commonly used for a power monitor and an automatic level control (ALC), etc., at an output end of a high-power amplifier of a transmitter. In the ALC, to maintain an output signal of the amplifier at a constant level, a level of an input signal to the high-power amplifier is controlled by a level of the amplified high-frequency signal which is monitored by the high-frequency coupler. More specifically, in a case that the high-frequency coupler is connected to a front end of the transmitter, to prevent an effect of a reflection from an antenna, a directional coupler, which does not couple to a signal of an opposite direction, is generally used.
Such a high-frequency coupler is also applied to a portable-type radio equipment used in mobile communication, and a miniaturization of the coupler is strongly desired in that field. In general, the high-frequency coupler may be constructed with well-known microstrip lines shown in FIG. 1 and FIG. 2.
FIG. 1 shows a pattern structure of a conventional branch-line-hybrid-type directional coupler. A signal applied to an input line 1 is transmitted through a main transmission line 2 and is output from an output line 3. In this case, a portion of the signal is also applied to an output line 6 of a sub-transmission line 5 through parallel lines 7, 8. A ratio of a coupling power Pc in the output line 6 to an input power Pin in the input line 1 (10.times.log(Pc/Pin)) is referred to as a coupling ratio, which depends on a material of a base, a width of the pattern, and a frequency. However, the coupling ratio obtained by the coupler shown in FIG. 1 is typically a few dB, for example, approximately -3 dB. It is very difficult to obtain a low coupling ratio of, for example, -20 dB for a restricted-size design.
FIG. 2 shows a pattern structure of a conventional quadrature-hybrid-type directional coupler. A portion of a signal applied to the input line 1 of the main transmission line 2 is coupled to the sub-transmission line 5 by distributed coupling of an edge of a microstrip line, and is supplied to the output line 4. The coupling ratio in this directional coupler also depends on the material of the base, the width of the pattern, and the frequency. And, in the directional coupler shown in FIG. 2, a directionality can be obtained by equalizing phase speeds in quadrature modes of an odd mode and an even mode of a transmission signal. However, since it is difficult to achieve the equalization, there is a problem that a desired directionality cannot be obtained.
In mobile communications using such as a pager, an automobile/portable telephone, and a wireless phone, a usage frequency band is from a hundreds-of-MHz band to a 2-GHz band, which is a lower frequency band than the millimeter wave band. More specifically, in the automobile and portable telephones, an 800-MHz band is mainly used, and a .lambda.g/4 line length at this frequency is approximately 90 mm in a vacuum. When the conventional microstrip line technologies are applied in such a low-frequency band, the high-frequency coupler may be increased in size. Recently, for a cost reduction of the portable-type radio equipment, a conventional glass epoxy resin is used for the base material. When using this base material, the high-frequency coupler may have a big size of approximately 50 mm.
Further, since the .lambda.g/4 line is constructed as the coupling pattern in the sub-transmission line, the coupling pattern may operate as an antenna, and thus, the sub-transmission line may be easily influenced by outside sources. In a case of the portable-type radio equipment, a variety of circuits are integrated in a very small area, therefore, the circuits may be easily influenced by a variety of electromagnetic waves.
Recently, serving as a high-frequency coupler which is reduced in size and is influenced little by outside sources, a multilayered-type chip coupler shown in FIG. 3 is being widely used. The chip coupler is small in size and is superior in electrical performance. However, this chip coupler is very expensive for a component used in the portable-type equipment.
Therefore, it is desired to develop a small low-cost high-frequency coupler using the microstrip line technologies. When the high-frequency coupler is constructed using the microstrip line technologies, it is possible to manufacture the high-frequency coupler in the same process as that of high-frequency circuits such as high-frequency amplifiers and high-frequency filters. Therefore, the chip coupler may be unnecessary, and this achieves an overall cost reduction of the high-frequency circuits.
At present, some practical high-frequency couplers using the microstrip line technologies have been proposed. In the following, descriptions will be given of the proposed prior-art high-frequency couplers, by referring to FIG. 4 to FIG. 7.
FIG. 4 shows a pattern structure of an improved prior-art high-frequency coupler. The coupler is disclosed in Japanese Laid-Open Patent Publication No.1-30321 as a "super-high-frequency coupler". In FIG. 4, a part of a signal supplied to the main transmission line 2 is coupled to the sub-transmission line 5 through a coupling part 9 and is output from an output line 6. The sub-transmission line 5 is connected to a ground 12 through two resistances 10, 11 to obtain a good impedance matching for the output line 6. In the coupler shown in FIG. 4, the sub-transmission line 5 is coupled to the main transmission line 2 in a point form, and the coupler may be small in size. The coupling ratio of the coupler is adjustable by a distance between the main transmission line 2 and the coupling part of the sub-transmission line 5. However, this coupler does not have directionality.
FIG. 5 shows a pattern structure of another improved prior-art high-frequency coupler. The coupler is disclosed in Japanese Laid-Open Patent Publication No.4-4763 as a "directional coupler". In FIG. 5, a portion of a signal supplied to the main transmission line 2 is connected to sub-transmission lines 15, 16 through coupling patterns 13, 14, and coupled signals are output from the output line 6. The coupling patterns 13, 14 are respectively connected to ground patterns 19, 20 through termination resistors 17, 18. Each of the ground patterns 19, 20 is connected to ground via a through hole. The termination resistors 17, 18 are provided to stabilize an operation of the directional coupler. This coupler may have a sufficient directionality by adjusting a difference between transmission line lengths between the set of the coupling pattern 13 and the sub-transmission line 15 and the set of the coupling pattern 14 and the sub-transmission line 16 to .lambda.g/4. In this coupler, since widths of the sub-transmission lines 15, 16 may be thinned, an influence from outside sources can be reduced. And also, in this coupler, the coupling ratio is adjustable by a distance between the main transmission line 2 and the coupling patterns 13, 14.
FIG. 6 shows a pattern structure of another improved prior-art high-frequency coupler. The coupler is disclosed in Japanese Laid-Open Patent Application No.5-14019 as a "directional coupler". In FIG. 6, in a dielectric board 30, the sub-transmission line 5 is multilayered on the main transmission line 2 with a dielectric therebetween. For a coupling operation, a part of the signal supplied to the main transmission line 2 is coupled to the sub-transmission line 5 through the dielectric, and is supplied to the output line 6. In the above reference, it is described that such a configuration may be useful over an applicable frequency range. And the reference shows that electromagnetic shielding by using these lines as an overcoat may reduce an influence from the outside sources.
FIG. 7 shows a pattern structure of another improved prior-art high-frequency coupler. The coupler is disclosed in Japanese Laid-Open Patent Application No.5-83015 as a "directional coupler". In FIG. 7, a dielectric base 40 is sandwiched between the main transmission line 2 and the sub-transmission line 5, and they are sandwiched between external bases 41, 42, each having a ground side. By the electromagnetic shielding of the coupling part, an influence from outside sources may be reduced.
However, the above-mentioned prior-art high-frequency couplers have the following problems.
In the high-frequency couplers shown in FIGS. 5, 6, and 7, extreme miniaturization for the portable-type equipment may be difficult. When trying to miniaturize the high-frequency coupler shown in FIG. 5, a pitch between the coupling patterns 13, 14 needs to be less than .lambda.g/4. In this case, to obtain the good directionality, the widths of the sub-transmission lines 15, 16 further have to be increased. Therefore, it is difficult to reduce an overall size of the high-frequency coupler. As for each of the high-frequency couplers shown in FIGS. 6, 7, since the size of the coupler is determined by .lambda.g/4, the reduction in size may cause a large degradation of the electrical performance.
Furthermore, in the high-frequency couplers shown in FIGS. 4, 5, since the sub-transmission line operates as the antenna, the couplers are easily influenced by outside electromagnetic waves. Though the coupler shown in FIG. 5 may be improved by thinning the width of the sub-transmission line, a degree of the influence cannot be reduced to a sufficient low value to be applied to the portable-type radio equipment.