Volume 71, Issue 7-8 -

In this paper, a novel microwave continuously tunable band-pass filter, based on nematic liquid crystals (LCs), is proposed. It uses liquid crystal (LC) as the electro-optic material to mainly realize frequency shift at microwave band by changing the dielectric anisotropy, when applying the bias voltage. According to simulation results, it achieves 840 MHz offset. Comparing to the existing tunable filter, it has many advantages, such as continuously tunable, miniaturization, low processing costs, low tuning voltage, etc. Thus, it has shown great potentials in frequency domain and practical applications in modern communication.

A new microstrip coupled-line balun topology and its application to the balun bandpass filter (BPF) with a triple mode response are proposed in this paper. The involved balun structure is composed of two back-to-back quarter-wavelength ( λ /4) asymmetrical coupled-line sections. Detailed design formulas based on the asymmetrical coupled-line theory are given to validate the feasibility of the balun. Besides, to obtain filtering performance simultaneously, the balun is then effectively integrated with a pair of triple mode resonators. To demonstrate the design concept of the balun BPF, a prototype operating at 2.4 GHz with the fractional bandwidth (FBW) of about 19.2 % is designed, fabricated, and measured. Results indicate that between the two balanced outputs, the amplitude imbalance is less than 0.3 dB and the phase difference is within 180°±5° inside the whole passband. Both simulated and experimental results are in good agreement.

A new CPW-fed bandpass filter based on zeroth order resonant (ZOR) technique is presented in this paper. Proposed filter structure is designed on a CPW single layer where via is not required, hence reduces fabrication complexity. The property of metamaterial of ZOR has been utilized to reduce the filter size. The proposed structure is symmetrically CPW-fed and contains tuning-fork stub, which connects patch to CPW ground plane. The metamaterial properties are characterized by plotting dispersion diagram of proposed structure. The experimental result of proposed filter design shows an insertion loss of 0.51 dB, return loss of 22.5 dB with fractional bandwidth 61.5 % at centre frequency 2.60 GHz. The size of the filter is 0.45 λg×0.36 λg (λg is the guided wavelength at centre frequency).

A compact quad-band band-pass filter (BPF) based on stub loaded resonators (SLRs) with defected microstrip structure (DMS) is analyzed and designed in this paper. The proposed resonator is created by embedding DMS into the SLR and can achieve four narrow passbands. By employing the pseudointerdigital coupling structure between the two resonators, transmission zeros among each passband are generated to improve the passband selectivity and a high isolation is achieved. In order to validate its practicability, a prototype of a quad-band BPF centred at 1.57, 2.5, 4.3 and 5.2 GHz is designed and fabricated. The proposed filter is more compact due to the slow-wave characteristic of DMS. The simulated and measured results are in good agreement with each other. In addition, the DMS idea can be extended to the design of other microstrip passive devices.

This paper presents an alternative concept of metal insert to design compact, wideband, waveguide bandpass filter. Three planar circular fractals, integrated with Split Ring Resonators, have been placed on the transverse plane of a standard WR–90 waveguide at quarter wavelength distance to form the wideband, tri-pole bandpass filter. The proposed filter has been simulated using Ansoft HFSS (version 14) and using equiripple Chebyshev approximation its lumped element equivalent circuit has been obtained. The filter was also fabricated and its frequency response has been measured. The measured result shows a 3-dB fractional bandwidth of 14.08 % at resonant frequency 10.15 GHz. The total length of the filter is 20.33 mm. Detailed design procedure has been presented along with necessary diagram. A table has been provided to compare the performance of the proposed filter with those already available in literatures. The table shows that the proposed filter is compact, has higher bandwidth and better frequency response compared to the rests.

A compact dual-band bandpass filter (BPF) based on a half-mode substrate integrated waveguide (HMSIW) and double-side complementary split-ring resonators (DS-CSRR) is investigated in this paper. The proposed DS-CSRR consists of two complementary split-ring resonators (CSRRs) with different sizes. One is etched on the top surface of HMSIW, the other is on the ground. By changing the parameters of DS-CSRR, both the center frequencies and the bandwidths can be controlled independently, respectively. In order to validate its practicability, a dual-band BPF centered at 3.5 GHz/5.2 GHz is designed and demonstrated. The simulated and measured results are in good agreement with each other.

This article presents a high selectivity wideband bandpass filter (BPF) adopting stub loaded resonator. Hereinto, aim passband is determined by BPF without stub embedded, which is only composed of four half-wavelength open-loop resonators. Based on typical tapped-line coupling, two same stubs are located at physical middle points of two resonators connected with I/O feed lines, respectively. Due to embedded point at middle of loaded resonator, the stub with two open-end branches has no influence on original half-wavelength resonant frequency, and aim passband keeps unchanged. Because of different even-mode resonant frequencies between loaded and unloaded resonators, no new passband is constructed. With the help of embedded stubs, original transmission zero (TZ) near low-edge of aim passband is shifted towards passband, and a new TZ is introduced near high-edge. High selectivity and good passband characteristics are obtained optimizing sizes of stubs, I/O tapped position and top open-end length of loaded resonator.

This paper presents the design of a compact narrow-band bandpass filter using the cylindrical dielectric resonator (CDR) coupled with a microstrip line. The TE 11δ mode of the dielectric resonator is used to obtain the narrow passband performance while, spurious mode suppression can be improved by etching the slot on the ground plane of the filter due to mode separation with the desired mode. Presented narrow bandpass filter performances are: center frequency 6.15 GHz, fractional bandwidth 0.7 %, and insertion loss (IL) 0.16 dB. To validate the above concept a second order narrow-band bandpass filter is designed, fabricated and measured. There is good agreement between simulated and measured results.

In this paper, a triple stop-band filter with a ratioed periodical defected microstrip structure is proposed for wireless communication applications. The proposed ratioed periodical defected microstrip structures are spiral slots, which are embedded into a 50 Ω microstrip line to obtain multiple stop-bands. The performance of the proposed triple stop-band filter is investigated numerically and experimentally. Moreover, the equivalent circuit model of the proposed filter is also established and discussed. The results are given to verify that the proposed triple stop-band filter has three stop bands at 3.3 GHz, 5.2 GHz, 6.8 GHz to reject the unwanted signals, which is promising for integrating into UWB communication systems to efficiently prevent the potential interferences from unexpected narrowband signals such as WiMAX, WLAN and RFID communication systems.

A new microstrip lowpass filter with sharp roll-off and high suppression level in stop-band is presented. To achieve the lowpass filter with high performance, coupled T-shaped, elliptical and radial resonators are adopted. New formulas for calculating LC equivalent circuit of elliptical and radial resonators are derived based on T-shaped resonators. Transfer functions of the resonators are calculated using the LC equivalent circuits. The designed filter with 3 dB cut-off frequency at 1.67 GHz has been fabricated and measured. Results demonstrate that a relative stop-band bandwidth (RSB) of 157 % (referred to a suppression degree of 30 dB) is obtained while achieving a sharp roll-off about 255.2 dB/GHz. The features of lowpass filter exhibit that the figure of merit is about 45372. Also good agreements are achieved between measured and simulated S-parameters of the fabricated and designed filter.

In this paper, a compact via-free low pass filter is designed based on composite right/left handed (CRLH) concept. The structure uses open ended concept. Rectangular slots are etched on signal transmission line (TL) to suppress the spurious band without altering the performance and size of filter. The filter is designed for low pass frequency band with cut-off frequency of 3.5 GHz. The proposed metamaterial structure has several prominent advantages in term of selectivity up to 34 dB/GHz and compactness with average insertion loss less than 0.4 dB. It has multiple applications in wireless communication (such as GSM900, global navigation satellite system (1.559–1.610 GHz), GSM1800, WLAN/WiFi (2.4–2.49 GHz) and WiMAX (2.5–2.69 GHz)). The design parameters have been measured and compared with the simulated results and found excellent agreement. The electrical size of proposed filter is 0.14λ 0 × 0.11λ 0 (where λ 0 is free space wavelength at zeroth order resonance (ZOR) frequency 2.7 GHz).