Presented is a vertically and horizontally folded half-wavelength resonator in a three-dimension (3D) environment and its application to a balanced bandpass filter (BPF). Benefiting from the multilayered resonator based on the transmission line theory and low temperature co-fired ceramic (LTCC) technology, the proposed balanced BPF can obtain compact size and good performance. For demonstration, a LTCC balanced BPF centred at 2.45GHz has been designed using the proposed resonator, and the size of the circuit is 6.0 × 5.0 × 1.6 mm 3 .Introduction: With the rapid development of modern wireless communication systems, increasing demands for balanced circuits are arising. The balanced bandpass filter (BPF), as an essential passive component, usually occupies a very large area of the printed circuit board (PCB). As a result, it is very important to reduce the filter's size. In the developed balanced BPFs, planar topologies are in the majority [1,2]. They all have good performance, but they may suffer from bulky size, which would be an obstacle in practical applications. Recently, low temperature co-fired ceramic (LTCC) technology has attracted much attention as it can reduce the circuit's size significantly. Accordingly, many lumped-element or semi-lumped-element LTCC BPFs with compact size and good performance have been demonstrated [3,4]. However, as the frequency increases, the parasitic effect and unwanted coupling may influence the original characteristic of the lumped element. Therefore, the values of the lumped element cannot be accurately predicted and controlled. The distributed resonator and filter do not have the same problem except the relatively large sizes [5,6]. Hence, size reduction is a challenge in designing the LTCC distributed resonator and filter.In this Letter, a vertically and horizontally folded half-wavelength (λ g / 2, λ g is the guided wavelength at the centre frequency) resonator based on the transmission line theory is proposed. Accordingly, a balanced BPF constructed by a pair of the multilayered resonators has been designed and fabricated in LTCC technology. In this way, the proposed distributed-element LTCC balanced BPF can obtain the same performance and compact size as the LTCC balanced BPF based on lumped elements [4].

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Presented is a novel multilayer dual-mode dual-band bandpass filter (BPF) using two identical microstrip cross-slotted patch resonators placed back-to-back. The lengths of the slots on the patch resonator are designed to be different, which can be used to control the two degenerate modes individually so as to build two passbands of the proposed BPF. Two separated coupling paths for the two modes can be realised by the etched slits on the common ground plane of the two microstrip resonators. As a result, the coupling coefficients between the two resonators in each passband can be controlled independently. Furthermore, a new source-load coupling scheme is incorporated in this design to enhance the stopband rejection and then improve the band-to-band isolation significantly.

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In this paper, a novel defected ground structure (DGS) is proposed for lowpass filter (LPF) applications. The proposed structure exhibits wideband attenuation without increasing the circuit size, as compared with the conventional DGS. An equivalent circuit for the proposed DGS is described, and its equivalent-circuit parameters are analyzed and extracted. Benefitting from the quasi-elliptic function of the DGS, a demonstration LPF using one DGS unit and a compensated microstrip line is designed, and an attenuation pole near to the passband edge is realized to enhance roll-off of the stopband. To improve rejection level of the stopband, three cascaded DGS units are utilized for the LPF design. The fabricated LPF shows more than 20 dB attenuation from 2.1 GHz to over 8.0 GHz, as well as sharp rolloff. The simulated and measured results are presented, showing good agreement.