1. Field of the Invention
The present invention relates to a balanced filter having the function of a balun performing conversion between unbalanced and balanced signals and the function of a filter performing band control, and more particularly to a balanced filter effective in reducing a filter size.
2. Description of the Related Art
Radio communication equipment comprises various RF (radio frequency) devices, such as an antenna, a filter, an RF switch, a power amplifier, an RF-IC, and a balun. Among these parts, resonance devices, such as an antenna and a filter, handle an unbalanced signal on the basis of the ground potential, while an RF-IC for producing and processing an RF signal handles a balanced signal. A balun functioning as an unbalance-balance transformer is therefore used when those two types of parts are connected to each other.
That type of balun is disclosed, for example, in the following Patent Documents:                Patent Document 1: Japanese Unexamined Patent Application Publication No. 2000-134009        Patent Document 2: Japanese Unexamined Patent Application Publication No. 2001-36310        
The baluns disclosed in those Patent Documents are of the type that an unbalanced line and a balanced line are coupled through a coupling line. In the structures of those baluns, as shown in FIG. 3 of Patent Document 2, the unbalanced line and the balanced line are formed on one substrate, and the coupling line is formed on another substrate. The coupling line is laid over both the unbalanced line and the balanced line so that the unbalanced line and the balanced line are coupled to each other.
In a coupling mode of the balun thus constructed, as shown in FIG. 8 and explained in paragraph 0016 of Patent Document 2, “an unbalanced signal inputted from an unbalanced signal terminal 3 is propagated in the order of a first coupling line 101, a second coupling line 102, and a third coupling line 103”.
With the balun structures disclosed in Patent Documents 1 and 2, however, a resulting frequency characteristic is as shown in FIG. 4 of Patent Document 2. Accordingly, the disclosed structures are usable as a balun, but they have a difficulty in ensuring a band characteristic required for the filter.
On the other hand, many balanced filters each comprising a balun and a filter combined into an integral unit have recently been devised with the intent to reduce the size of radio communication equipment. That type of balanced filter is disclosed, for example, in the following Patent Document:                Patent Document 3: Japanese Unexamined Patent Application Publication No. 2003-087008        
The balanced filter disclosed in Patent Document 3 has a structure in which a filter and a balun each designed using a ¼-wavelength resonator are combined on a dielectric substrate. A dielectric layer constituting the filter and a dielectric layer constituting the balun are formed one above the other in an integral structure.
Also, Patent Document 3 discloses a structure in which a DC power supply layer is formed in the balun, for making the balanced filter adaptable for the case where the RF-IC requires a balanced signal superimposed on a DC component. This structure is intended to realize a further reduction of the filter size.
However, the structure in which a balun section and a filter section are separately formed and integrated together has the problem as follows. When the filter function with a high attenuation is demanded, the filter section is required to have a multistage structure. Therefore, satisfactory flexibility in design cannot be ensured in a limited space, and a reduction of the size is very difficult to realize.