Enhanced Stopband Characteristics Research Articles

High Isolation Microstrip Diplexer with Enhanced Stopband Characteristics for GSM and WLAN Application

ABSTRACTIn this article, a high‐isolation microstrip diplexer with enhanced stopband characteristics is presented for GSM and Wireless Local Area Network (WLAN) application. The proposed diplexer consists of two compact bandpass filters (BPFs) using short‐circuit centered stepped impedance resonators (SCSIRs), operated at 1.8 and 2.45 GHz, respectively. Due to the intrinsic characteristics of the SCSIRs, multiple transmission zeros can be generated for high selectivity and wide upper‐stopband. By locating the transmission zero of one channel at the passband of the other channel, and utilizing good impedance matching between two BPFs, high output isolation can be achieved. A microstrip diplexer prototype is designed and fabricated. The upper‐stopband of two channels in experiment is extended up to 10.2 GHz with an insertion loss larger than 28 dB. Meanwhile, the measured isolation greater than 48 dB in GSM passband and more than 52 dB in WLAN passband is achieved between two channels. A satisfactory agreement between the simulated and measured results validates the proposed configuration. The size without the 50‐Ω feed lines is only in which is the guided wavelength at 1.8 GHz. © 2013 Wiley Periodicals, Inc. Microwave Opt Technol Lett 55:2990–2993, 2013

Design of compact bandpass filter with enhanced stopband characteristics

In this paper, a compact third-order bandpass filter with enhanced stopband characteristics is proposed. The filter consists of three stepped impedance resonators, where one is embedded into the other two resonators for compact size. Multiple transmission zeros (TZs) can be realized near the passband by introducing frequency-dependent coupling between the nonadjacent resonators for enhanced frequency selectivity. Besides, defected ground structure is employed to suppress the harmonics over a broad bandwidth. A new filter centered at 1.51 GHz has been analyzed, simulated, and fabricated to verify the validity of the proposed method. The measured results show that there are three finite TZs near the passband, which are located at 1.1 GHz with 62.1 dB rejection, 1.76 GHz with 34.2 dB rejection, and 2.48 GHz with 37.2 dB rejection, respectively. And, the spurious frequencies of the upper stopband are suppressed below 20 dB from 1.7 to 25 GHz.

Compact tunable bandpass filter with wide tuning range and enhanced stopband characteristics

A compact tunable bandpass filter (BPF) with wide tuning range and enhanced stopband characteristics based on low-temperature co-fired ceramic technique is proposed. The wide tuning range is realised by properly introducing an axial coupling structure. Three transmission zeros are obtained near the passband for enhanced frequency selectivity by introducing mixed electric/magnetic (E/M) coupling between the source/load and the taped inductor. Back-to-back varactor diodes are adopted to improve linearity characteristics. A tunable BPF with a 120% tuning range from 370 to 850 MHz has been designed and fabricated to verify the proposed design concept. The measured results are in good agreement with the electromagnetic simulation. The circuit occupies only 4.0 × 3.0 × 1.6 mm3.

Tunable Combline Bandpass Filter Using Cross-Coupled Stepped-Impedance Resonators with Enhanced Characteristics

This paper proposes a tunable combline bandpass filter with high selectivity, constant bandwidth, and good stopband performances. A filter with these characteristics is obtained by applying cross-coupling to the conventional combline bandpass filter using stepped-impedance resonators(SIRs). For high selectivity and constant bandwidth, cross-coupling is utilized and the SIR configuration is used for enhanced stopband performances. The proposed combline tunable bandpass filter with 5% of fractional bandwidth at 1.6 GHz was fabricated and tested. The measured results showed 11.6% tunability with constant bandwidth, high selectivity and enhanced stopband characteristics.

A compact bandpass filter with enhanced stopband characteristics by an asymmetric cross-shape defected ground structure

In this paper, we present an effective technique to enhance stopband bandwidth for a bandpass filter by using an asymmetric, cross-shape defected ground structure (CSDGS). A single CSDGS can provide a higher attenuation rate near passband and broader stopband bandwidth. Moreover, with the asymmetric CSDGS, two different resonant frequencies can be excited and controlled independently, resulting in synthesizing a passband. The synthesized bandpass filter can be modeled by parallel LC resonant circuits in connection with T-networks. The bandwidth and insertion loss of this bandpass filter at 4.2 GHz is 28.6% and -1.44 dB, respectively, and 20 dB rejection in the stopband is up to four times the central frequency. All the synthesized bandpass filters have been measured and are in good agreement with simulated results.

Multilayer LTCC bandpass filter design with enhanced stopband characteristics

Lumped-element bandpass filter structures with enhanced stopband rejection targeted for low temperature co-fired ceramic (LTCC) implementation are proposed. Design equation formulations that take into account the effect of the transmission zeros are presented. For demonstration, RF filters operating at 2.4 GHz are fabricated and characterized. Both simulation and measurement results are shown for comparison.