Novel Balanced Microstrip Coupled-Line Bandpass Filters

Novel balanced coupled-line bandpass filter based on the four-line microstrip structure is proposed. By introducing an additional shunt inductance to the conventional coupled-line filter, along with the capacitively coupled gap between coupled-line sections, two attenuation poles at upper and lower stopbands may be achieved without increasing the circuit area. The proposed filter exhibits good stopband rejection with no repeated passband at twice the center frequency as compared to the conventional coupled-line filters. Specifically, a balanced microstrip bandpass filter centered at 2.13GHz with 8.5% bandwidth is implemented. INTRODUCTION Bandpass filters are essential building blocks in communication system designs. It can reduce the harmonic and spurious emissions for transmitters, and may improve the rejection of interferences for receivers. Various works on bandpass filters were reported in the literature, and the coupled-line bandpass filters based on quarter-wavelength ( /4) coupled-line sections were widely used for their easy realization in planar circuits and simple design procedure [1]-[2]. To implement a bandpass filter with sharp and deep rejection outside the passband, many improvements to the conventional coupled-line bandpass filters were reported. In [3]-[5], the /4 open stubs and/or capacitively coupled gap were introduced in the conventional coupled-line bandpass filter structure to create attenuation poles at stopband, such that the selectivity of the bandpass filter can be improved. On the other hand, previous works on bandpass filter designs for balanced circuit configurations are rather limited. A balanced circuit topology may reduce the level of even harmonic spurious responses, which are crucial for direct conversion transceivers. In this work, a novel balanced bandpass filter based on the four-line coupled microstrip structure is proposed. The proposed filter may achieve two attenuation poles at upper and lower stopbands without increasing the circuit area, and has no repeated passband at twice the center frequency as conventional designs [1]-[3], so that good stopband rejection can be achieved. FILTER STRUCTURE Shown in Fig. 1(a) is the proposed balanced microstrip bandpass filter structure, with its equivalent-circuit model in Fig. 1(b). It is based on two cascaded /4 coupled-line sections with an additional shunt inductance L in-between. Here the symmetric four-line microstrip structure as shown in Fig. 2 is adopted for the coupled-line sections to achieve the balanced circuit topology. Basically, there are four fundamental (quasi-TEM) modes for this four-line microstrip. Under differential excitation, only the two modes, namely odd mode and even mode, as shown in Fig. 2(a) and 2(b) will be excited. The capacitor C in the equivalent-circuit model [Fig. 1(b)] represents the gap-coupled capacitance between two coupled-line sections. An additional shunt inductance L realized by a metal strip is served as a K-inverter such that the resulted filter may be equivalent to a 2nd-order /4 LC coupled bandpass filter. In addition, the inclusion of shunt inductance L along with the gap capacitance C may introduce two additional attenuation poles in the upper and lower stopbands. Note that no via hole is required to realize the shunt inductor due to the balanced configuration, hence fabrication of the proposed filter is easy. The physics for the creation of additional attenuation poles may be illustrated in Fig. 3. Here Path 1 represents the original signal path for conventional coupled-line filter, while the gap capacitance C provides a second cross-coupled path (Path 2) along which the signal will cancel the one traveling along Path 1 at the frequencies of attenuation poles. Simulated frequency responses for this proposed filter along Path 1 and Path 2 are shown in Fig. 4. Here the signals from these two paths have the same magnitude and are nearly 180 out-of-phase at two frequencies, such that they will cancel out each other at these two frequencies. Therefore the overall filter response, also shown in Fig. 4, has two attenuation poles. Comparing to the filters [4]-[5] utilizing the similar mechanism to create one attenuation pole at upper stopband, the inclusion of L in the proposed filter structure introduces additional phase change along Path 1 such that two attenuation poles may be achieved. The locations of the attenuation poles are determined by the coupled-line impedance, L value, and C value. The former two are determined by the Jand K-inverter values in filter design, and one may suitably choose the C value through changing the gap width d to adjust the attenuation poles to the