Directional coupler and wireless communication device

#The present invention relates to a directional coupler and a wireless communication device, and more particularly, to techniques for providing a flat frequency characteristic for the degree of coupling in the directional coupler.

#A directional coupler (hereinafter simply referred to as the “coupler”), which enables extraction of part of power propagated on a transmission line, has become an indispensable component for designing a transmitter circuit of a variety of wireless communication devices such as portable telephones, wireless LAN communication device, communication devices of Bluetooth standard, and the like.

#Specifically, a coupler forms part of an adjuster means for controlling a transmission signal to maintain at a certain level. The adjuster means comprises a power amplifier (hereinafter referred to as the “PA”) capable of controlling a gain; a coupler for detecting the level of a transmission signal; and an automatic output control circuit (hereinafter referred to as the “APC circuit”). An inputted transmission signal is amplified by the PA, and subsequently outputted through the coupler. The coupler provides the APC circuit with a monitor signal which is at a level corresponding to the level of the transmission signal outputted from the PA. The APC circuit controls the gain of the PA such that the output of the PA remains constant in accordance with the level of the monitor signal (i.e., the level of the transmission signal). Such feedback control of the PA facilitates stabilization of transmission output.

#FIG. 1 is a diagram showing a coupler according to a first embodiment of the present invention;

#FIG. 2 is a graph representing the frequency characteristic for the degree of coupling of the coupler according to the first embodiment in comparison with that of a conventional coupler;

#FIG. 3 is a graph representing the frequency characteristic for isolation of the coupler according to the first embodiment, in comparison with that of a conventional coupler;

#[First Embodiment]

#As shown in FIG. 1, a coupler 11 according to a first embodiment of the present invention comprises a primary line 12 for transmitting a transmission signal, and a secondary line 13 disposed in close proximity to the primary line 12 for electromagnetic coupling therewith. The primary line 12 has an input port P1 at one end thereof, and an output port P2 at the other end thereof. The secondary line 13, in turn, has a coupling port P3 at one end thereof, and an isolation port P4 at the other end thereof. In the following description, the input port may be sometimes referred to as “P1,” the output port as “P2,” the coupling port as “P3,” and the isolation port as “P4,” respectively.

#Also, as described above, the degree of coupling can be made flat if the length of the primary line and secondary line is set to approximately one quarter wavelength of a used frequency band. However, since this designing results in extremely long primary line and secondary line, both the primary line and secondary line in the first embodiment have a length shorter than one quarter wavelength of a used frequency band.