A balun-BPF using coupled lines with capacitive loads

In this letter, a balanced-to-unbalanced (balun) bandpass filter (BPF) with capacitive loaded coupled lines is presented. The bandpass filter is realized by two half-wavelength resonators which are coupled at their ends. Thus, it adds extra transmission zeros to the filter, which can effectively reject strong interference in the stopband. By using the symmetric and skew-symmetric feed structures, two transmissions zeros are created to the filter response, and the filter presents good out-of-band performance. Through theoretically analyses in combination with electromagnetic simulation, a balun-filter with extra transmission zeros is realized. The century frequency of the designed filter is 2.4GHz with 4% bandwidth, which is widely used in Wireless Personal Area Network (WPAN). The experimental measurement agrees well with the simulation result.