1. Field of the Invention
The present invention relates to a filter, and more particularly, to a filter including a plurality of LC parallel resonators.
2. Description of the Related Art
As an invention concerning a known filter, a multilayer band pass filter disclosed in Japanese Unexamined Patent Application Publication No. 2011-71921, for example, is known. FIG. 14 is an exploded perspective view of a multilayer band pass filter 500 disclosed in Japanese Unexamined Patent Application Publication No. 2011-71921.
The multilayer band pass filter 500 includes dielectric layers 502a through 502g and LC parallel resonators 504 and 516. The dielectric layers 502a through 502g are formed in a rectangular shape and are stacked on each other from top to bottom in this order.
The LC parallel resonator 504 includes an inductor electrode 506, via electrodes 508 and 510, a capacitor electrode 512, and a ground electrode 514. The capacitor electrode 512 is disposed on the dielectric layer 502f. The ground electrode 514 is disposed on the dielectric layer 502g. The capacitor electrode 512 and the ground electrode 514 oppose each other with the dielectric layer 502f therebetween so as to form a capacitor.
The inductor electrode 506 is a linear conductor disposed on the dielectric layer 502b and extending in the front-and-rear direction. The via electrode 508 passes through the dielectric layers 502b through 502e in a direction in which the dielectric layers 502b through 502e are stacked. The top end of the via electrode 508 is connected to the rear end of the inductor electrode 506. The bottom end of the via electrode 508 is connected to the capacitor electrode 512. The via electrode 510 passes through the dielectric layers 502b through 502f in a direction in which the dielectric layers 502b through 502f are stacked. The top end of the via electrode 510 is connected to the front end of the inductor electrode 506. The bottom end of the via electrode 510 is connected to the ground electrode 514. With this configuration, the inductor electrode 506 and the via electrodes 508 and 510 form an inductor.
The LC parallel resonator 516 includes an inductor electrode 518, via electrodes 520 and 522, and capacitor electrodes 514 and 524. The capacitor electrode 524 is disposed on the dielectric layer 502f. The ground electrode 514 and the capacitor electrode 524 oppose each other with the dielectric layer 502f therebetween so as to form a capacitor.
The inductor electrode 518 is a linear conductor disposed on the dielectric layer 502b and extending in the front-and-rear direction. The via electrode 520 passes through the dielectric layers 502b through 502e in a direction in which the dielectric layers 502b through 502e are stacked. The top end of the via electrode 520 is connected to the rear end of the inductor electrode 518. The bottom end of the via electrode 520 is connected to the capacitor electrode 524. The via electrode 522 passes through the dielectric layers 502b through 502f in a direction in which the dielectric layers 502b through 502f are stacked. The top end of the via electrode 522 is connected to the front end of the inductor electrode 518. The bottom end of the via electrode 522 is connected to the capacitor electrode 514. With this configuration, the inductor electrode 518 and the via electrodes 520 and 522 form an inductor.
In the multilayer band pass filter 500 configured as described above, the two LC parallel resonators 504 and 516 are disposed side by side in the right-and-left direction. With this arrangement, the LC parallel resonators 504 and 516 are electromagnetically coupled to each other so as to form a band pass filter.
In the multilayer band pass filter 500 disclosed in Japanese Unexamined Patent Application Publication No. 2011-71921, it is difficult to intensify capacitive coupling between the LC parallel resonators 504 and 516. This will be described more specifically. In the multilayer band pass filter 500, the capacitive coupling between the LC parallel resonators 504 and 516 is adjusted in order to obtain a desired transmission characteristic. If it is desired that the pass bandwidth of the multilayer band pass filter 500 will be increased, intensifying of capacitive coupling between the LC parallel resonators 504 and 516 is effective. For intensifying capacitive coupling between the LC parallel resonators 504 and 516, the distance between the LC parallel resonators 504 and 516 is set to be decreased. Then, the capacitance formed between the via-hole electrodes 508 and 520 is increased, and the capacitance formed between the via-hole electrodes 510 and 522 is increased. As a result, a signal of a lower frequency side than the pass band is more likely to pass between the LC parallel resonators 504 and 516, thereby increasing the pass bandwidth of the multilayer band pass filter 500.
However, if the distance between the LC parallel resonators 504 and 516 is excessively decreased, short-circuiting may occur between the via-hole electrodes 508 and 520 and between the via-hole electrodes 510 and 522. Thus, in the multilayer band pass filter 500, it may be difficult to intensify capacitive coupling between the LC parallel resonators 504 and 516 to satisfy a desired frequency characteristic.