An integrated RF balanced-filter with enhanced rejection characteristics

In this paper, a new and simple 2.4-GHz third-order integrated RF balanced-filter for LTCC-based applications is proposed. This device is derived from the basic center-tapped transformer circuit and works as a balun as well as a bandpass filter at the same time. According to the center-tapped transformer representation, three coupled resonant tanks are used to perform the balun-type operation. Thus, with only minor modifications, a balun having specific bandpass filtering characteristics can be designed by the use of traditional coupled-resonator filter theory. In our case, a third-order Chebyshev filtering balun is designed and implemented. Furthermore, a transmission zero at 1.9-GHz is introduced to its transmission response by means of the extracted-pole technique. Experimental measurements show good agreement with computer simulations.