Compact integrated combline band‐pass filter with interdigital capacitive loads using LTCC technology

Abstract

COMPACT INTEGRATED COMBLINE BAND-PASS FILTER WITH INTERDIGITAL CAPACITIVE LOADS USING LTCC TECHNOLOGY. A lumped-element band-pass filter using embedded passives was proposed, but it has a relatively large size and a complicated structure [3]. The use of low-temperature co-fired ceramic (LTCC) material can be a good solution because it has outstanding mechanical and electrical properties and offers the possibility of integration of various passive elements. To achieve a compact embedded band-pass filter, this paper presents a new tapped-line combline filter with multilayer vertical interdigital capacitors. The combline structure has capacitive loads at the end of the resonator in order to reduce the physical length while extending the electrical length of the resonator and a tappedline arrangement offers size reduction and ease of interface with other circuit components. Moreover, a vertical interdigital capacitor further reduces the overall size of the filter than the use of conventional metal-insulator-metal (MIM)-type capacitor [4]. The proposed filter was fully integrated into a general LTCC substrate and has a very simple structure and a small size. DESIGNA combline filter consists of mutually coupled resonators which reduce physical length by less than a quarter-wavelength by loading capacitive loads. The proposed filter has an interdigital capacitor at the end of the resonator for realizing small size by employing the slow-wave effect. The basic design method of the combline filter (see Fig. 1) was fully described in [5]. To reduce the overall size, a shorter electrical length of the resonator was chosen, while a large value of loading capacitance was employed. The three-dimensional (3D) view of the proposed filter is shown in Figure 2(a). It uses the minimum number of poles (two), and a relatively large capacitance is obtained by employing multilayer vertical interdigital capacitors. By using interdigital-type capacitors, the 2D area required for large capacitance is easily reduced. The capacitor was modeled by extracted S-parameter data using a commercial EM simulation tool. The effective capacitance C eff was calculated by the following equation:whereThe design was accomplished using the above equations and tuned using a conventional EM simulation tool. MEASUREMENTFigure 2(b) shows a cross-sectional view of the integrated filter. The filter was fabricated using a conventional LTCC technology with Dupont 951 powder and 6142B silver paste. Figure 3 shows a photograph of this filter integrated in an LTCC substrate. The characteristics of the filter were measured using a Wiltron 360B network analyzer and Cascade microtech air coplanar-type 500-m pitch ground-signal-ground (G-S-G) probe tip. A TRL calibration method was used to calibrate the system. The fabricated filter has 1.8-dB insertion loss, 37.6-dB return loss, and 280-MHz bandwidth at the center frequency of 5.09 GHz, as shown in Figure 4. The overall size of the filter is 2.7 ϫ 2.03 ϫ 0.4 mm without input and output pads for the measurement. The mea...