Synthesis and design of tunable bandpass filters with constant absolute bandwidth using varactor‐loaded microstrip resonator

This article presents two new types of tunable filters with constant absolute bandwidth using varactor‐loaded microstrip resonators. First, the second‐ and third‐order Butterworth tunable filters are designed based on the parallel coupled‐line J inverters. Second, a fourth‐order Chebyshev tunable filter is designed based on the alternative J/K inverters, in this design, two adjacent resonators are coupled with each other through a short‐circuited transmission line as the K inverter. The proposed two topologies can be easily extended to high‐order tunable filter. Three tunable bandpass filters with J and alternative J/K inverters, respectively, are built with a tuning range from ～1.8 to ～2.3 GHz. The measured second‐order filter has a 3‐dB bandwidth of 160 ± 6 MHz and an insertion loss of 2.4–3.8 dB. The third‐order filter shows a 3‐dB bandwidth of 197 ± 5 MHz and an insertion loss of 3.8–4.8 dB. The fourth‐order filter shows a 3‐dB bandwidth of 440 ± 5 MHz and an insertion loss of 2.1–2.6 dB. For all the designed filters, the measured results are found in excellent agreement with the predicted and simulated results. © 2014 Wiley Periodicals, Inc. Int J RF and Microwave CAE 24:681–689, 2014.     

A novel E‐plane substrate inserted bandpass filter with high selectivity and compact size

In this article, a novel E‐plane substrate inserted waveguide bandpass filter with high selectivity and compact size is proposed in Ka‐band. By integrating an extra resonator between two metal septa, the E‐plane waveguide filter is achieved with two transmission zeros at both sides of the passband which contribute to the high‐skirt selectivity. One sample is fabricated, whose total length is just 5 mm, namely, less than 0.5 and the minimum insertion loss is only about 0.3 dB. Good agreements between simulated and measured results are obtained. © 2013 Wiley Periodicals, Inc. Int J RF and Microwave CAE 24:451–456, 2014.     

A widely tunable bandpass filter adopting novel fully tunable resonators

A widely tunable bandpass filter adopting a novel fully tunable resonator is proposed in this letter. The fully tunable resonator is mainly consisted of an open ring, a frequency tuning structure, and a coupling tuning structure. Adopting the fully resonators, the center frequency, the input/output external Q value, and the coupling coefficients can be tuned at the same time. And the tuning range of a tunable filter adopting the novel fully tunable resonators can be expanded. The fully tunable resonators can be combined to achieve more excellent suppression performance. A widely tunable bandpass filter consisted of five fully tunable resonators with 86.5% frequency tuning range is designed, fabricated, and measured. The simulated results are in good agreement with the measured ones.     

梳状线可调带通滤波器的设计与仿真

针对现有的微带可调滤波器中心频率调节范围较窄的问题,依据梳状滤波器的原理和变容二极管的特性,设计出一种梳状线可调带通滤波器,经ADS仿真可得其中心频率可调范围达到0．49～5．08GHz、相对带宽仅为6．1％～9．4％,该梳状线可调带通滤波器具有中心频率调节范围很宽、结构简单、易于实现、易于扩展等优点,在工程上具有一定的推广应用价值。     

Reconfigurable bandpass filter with constant absolute bandwidth based on neural network

In this paper, a novel design method for reconfigurable bandpass filter (BPF) with constant absolute bandwidth (ABW) based on neural network is proposed. A lumped-element circuit is used to design the reconfigurable BPF, which consists five series LC tanks. Three series tanks can provide three transmission poles, while two parallel tanks can introduce two same transmission zeros (TZs). This method allows designing BPFs with tunable center frequency and constant ABW. A set of inductance and capacitance values for the center frequency and ABW requirements are optimized by using neural network. Then, optimize the capacitance values to adjust the center frequency with fixed inductance values by the three series tanks and the TZ by the parallel tanks. In order to verify this design method, three varactor diodes are employed to achieve the reconfigurable BPFs with the constant ABW under different work states. Experiments show that this method can optimize multiple LC circuit parameters of BPFs, which meet design requirements, with the same ABW and different center frequencies effectively and accurately     

A balanced tri‐band bandpass filter with high selectivity and controllable bandwidths

In this article, a balanced tri‐band bandpass filter (BPF) with high selectivity and controllable bandwidths is proposed. Two differential‐mode (DM) passbands are formed by applying stepped impedance resonators into the design. Uniform impedance resonators are introduced to realize the third DM passband. Moreover, frequencies and bandwidths of DM passbands can be independently controlled by the lengths of resonators and the gaps between them. In addition, good DM transmission can be realized while high common‐mode suppression is achieved intrinsically without affecting the DM parts, thereby simplifying the design procedure significantly. In order to validate the practicability, a balanced tri‐band BPF operating at 2.45 GHz, 3.5 GHz, and 4.45 GHz is fabricated and designed. A good agreement between the simulated and measured results is observed.     

A balanced dual‐band bandpass filter with independently tunable differential‐mode frequencies

A balanced dual‐band bandpass filter (BPF) with independently tunable differential‐mode (DM) frequencies is proposed in this letter. The proposed BPF is composed of complementary split‐ring resonators (CSRRs) etched on the ground and varactors loaded on the resonators. A balanced stepped‐impedance microstrip‐slotline transition structure is introduced to transfer the DM signals successfully and block the common‐mode (CM) signals transmission. Good DM transmission and CM suppression can be achieved. Moreover, by changing the reverse bias voltages of the varactors loaded on coupling CSRRs, two DM resonant frequencies of the proposed balanced BPF can be tuned independently. To verify the feasibility of the design method, a balanced BPF with DM frequency ranging from 0.80 GHz to 1.12 GHz and 1.55 GHz to 2.05 GHz is fabricated and measured. Good agreement between the simulation and measurement results demonstrate the validity of the design.     

Design of high selectivity tri‐band bandpass filter with wide upper stopband

This article presents a dual‐plane structure high selectivity tri‐band bandpass filter (BPF) which consists of a pair of T‐shaped microstrip feed lines with capacitive source‐load coupling as well as spur lines embedded, and three resonators, i.e., a dual‐mode stub‐loaded stepped impedance resonator and two nested dual‐mode defected ground structure resonators. Using the intrinsic characteristics of the resonators and feed lines, nine transmission zeros near the passband edges and in the stopband can be generated to achieve high selectivity. An experimental tri‐band BPF located at 2.4/5.7 GHz [wireless local area networks (WLAN) application] and 3.5 GHz [worldwide interoperability for microwave access (WiMAX) application] has been simulated and fabricated. Good agreement between the simulated and measured results validates the design approach. © 2012 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2013.     

Design of novel compact dual‐band filtering power divider using stepped‐impedance resonators with high selectivity

A novel compact dual‐band power divider with filtering responses is presented in this article. The proposed circuit utilizes coupled quarter‐wavelength stepped‐impedance resonators. By controlling these resonators, dual‐band operation can be realized. Furthermore, a resistor is connected between the two open ends of the input feed line to obtain good isolation at two bands. Source load coupling is utilized to enhance the selectivity. To verify the proposed idea, a filtering power divider with the operating frequencies of 2.4 and 5.8 GHz is implemented. Good agreement between the simulated and measured results validates the proposed idea. The total size of the circuit is 0.23λ_g × 0.28λ_g, where λ_g is the guide wavelength of 2.4 GHz. © 2016 Wiley Periodicals, Inc. Int J RF and Microwave CAE 26:262–267, 2016.     

Variable bandwidth,wide tunable frequency,voltage tuned filter

This article discusses the development of an electronically tuned filter capable of a wide tunable frequency range and simultaneous 3-dB bandwidth variations at any frequency within its tuning range. Varactor-tunable filters are designed using high-dielectric, soft-substrate material for printed resonators as well as also high-Q ceramic resonators, and their test data are compared. Greater than 50% tuning range with low insertion loss at a center frequency in the L and S frequency bands is demonstrated with a 4:1 change in 3-dB bandwidth—30 to 120 MHz for printed resonators and 14 to 46 MHz for ceramic resonators. The concept of tuning a filter's 3-dB bandwidth with voltage is demonstrated and the effect of the bandwidth tuning elements on the tunable filter performance is discussed. © 2003 Wiley Periodicals, Inc. Int J RF and Microwave CAE 14, 64–72, 2004.     

A novel symmetrical folding miniaturized substrate integrated waveguide tunable bandpass filter

In order to improve the utilization of spectrum resources, dynamic frequency allocation technology has become a method of spectrum saving. The research of electric tunable filter has received much attention in recent years. Substrate integrated waveguide (SIW) is a new technology of microwave integration in the field. The innovation of this article is to combine the SIW miniaturization technology with tunable technology. In this article, a miniaturized symmetrically folded substrate integrated waveguide (SFSIW) filter is designed. Based on the miniaturization of SIW filter, the center frequency can be adjusted electrically and the measured data show that the tunable filter can achieve eight states under the condition that the bandwidth below ?3 dB remains approximately constant 300 MHz. The center frequency varies from 3.23 GHz to 3.9 GHz. The range of the filter's insertion loss is 1.2 dB to 2.04 dB, and the return loss is higher than 20.9 dB. This filter has practical value in the electronic countermeasures of military communication.     

Widely separated dual‐band half‐mode SIW bandpass filter

A novel half‐mode substrate integrated waveguide (HMSIW) based dual‐band bandpass filter (DBBPF) is proposed. Back to back connected two defected ground structure (DGS) resonators on the top layer of HMSIW cavity constitute the passband with two transmission zeros (TZs) at a lower frequency. The higher modes TE₃₀₁ and TE₃₀₂ of HMSIW cavity give the passband response at higher frequency using the mode shifting technique with slot perturbation. The source‐load coupling has been used to create finite frequency TZs to improve the selectivity of the second passband. Therefore, the proposed filter gives two widely separated passbands, center frequencies (CFs) at 5.83 and 18.1 GHz with an attenuation of greater than 10 dB between the passbands. The synthesized filter is fabricated using a low‐cost single layer PCB process, and the measured S‐parameters are almost mimic the EM‐simulation results.     

A folded dual‐mode bandpass filter with circular waveguide

In this article, a folded circular waveguide dual‐mode filter without tuning screws is designed for the fifth‐generation (5G) mobile communication system. The folded filter is composed of two stacked circular cavities operated at 3.5 GHz. Each cavity has two resonant modes, which can generate and control two transmission zeroes at specific frequencies. Through a coupling iris, the two single‐cavity filters are connected together, and can control four poles, which helps to expand the 3dB fractional bandwidth to 11.4%. The measured insertion losses are around 0.5 dB in the passband (from 3.4–3.6 GHz). The experiment results show an excellent agreement with the simulation results. Such folded filters have the advantages of very low insertion loss, compact size, high frequency selectivity, and low cost.     

A highly selective balanced wideband bandpass filter based on nested split‐ring resonators

A balanced wideband bandpass filter (BPF) with a high frequency selectivity, controllable bandwidth, and good common‐mode (CM) suppression based on nested split‐ring resonators (SRRs) is proposed in this article. The proposed nested SRRs are applied to form three transmission poles (TPs) that can achieve a wide differential‐mode (DM) passband centered at 3.0 GHz. Meanwhile, two transmission zeros (TZs) are generated to realize a high frequency selectivity of the DM passband. Moreover, TPs and TZs can be quasi‐independently controlled by changing the physical lengths of SRRs and the gaps between them, which can greatly improve the flexibility and practicality of the design. The proposed balanced BPF is fed by balanced microstrip‐slotline (BMS) transition structures. For the CM signals, the BMS transition structures can achieve a good wideband CM suppression without affecting the DM ones, thereby simplifying the design procedure. In order to validate its practicability, a balanced wideband BPF is fabricated and a good agreement between the simulated and measured results is obtained.     

Compact wideband microstrip bandpass filter with wide upper stopband based on coupled‐stub loaded resonator

A novel wideband microstrip bandpass filter (BPF) based on a coupled‐stub loaded resonator (CSLR) is presented in this article. The CSLR is constructed by attaching one short‐circuited parallel coupled microstrip line (PCML) in shunt to a high impedance microstrip line. The filter bandwidth can be conveniently controlled via reasonable adjusting of the impedance of PCML. Moreover, new defected microstrip structures (DMSs) introduced in the PCML functions as a means of adjusting the positions of transmission zeros, created by the PCML. The resonant mode and transmission zero chart are given, indicating that the higher modes could be suppressed by the transmission zeros. Finally, to validate the proposed method, two wideband BPF filters with and without DMSs centered at 3 GHz with 3 dB fractional bandwidth of 87% are designed and fabricated. The measured results show that both the return losses are better than 15.8 dB, while the BPF with DMSs has a ?19.4 dB isolation wideband from 1.57 to 4.23 . The measured results are in excellent agreement with full‐wave electromagnetic simulation results. © 2014 Wiley Periodicals, Inc. Int J RF and Microwave CAE 25:122–128, 2015.     

Design of wideband bandpass filter with high and wide stopband rejection using asymmetrical‐loaded stepped‐impedance resonator

This study presents a wideband bandpass filter (WBBPF) with wide and high stopband suppression by loading a stepped‐impedance resonator (SIR). The prototype of WBBPF is composed of an inverted π‐shaped resonator with T‐shaped resonator and open stub loaded, centrally. Odd‐/even‐mode analysis technique is employed to characterize the resonant properties of this prototype. Then, a SIR is loaded to this filter, asymmetrically, to improve the out‐of‐band performance. For experimental validation, a WBBPF is designed, fabricated, and tested. The measurement results show that the center frequency of WBBPF is located at 5.095 GHz, and the 3‐dB fraction bandwidth is about 71%. Plus, the out‐of‐band suppression with 30‐dB rejection level can be extended to 18.17 GHz.     

Dual‐wideband differential bandpass filter with intrinsic common‐mode suppression based on hybrid transmission structure

Based on the microstrip‐line/slot‐line hybrid transmission structure, a novel method of realizing dual‐wideband differential bandpass filter (DDBPF) is proposed and demonstrated in this paper. By virtue of hybrid transmission structure, the proposed DDBPF has intrinsic common mode (CM) rejection. Moreover, the dual‐wideband transmission performance of differential mode (DM) signal can be approached by using two dual‐mode stub‐loaded stepped‐impedance resonators. The two passbands of the proposed DDBPF are respectively centered at 2.76 GHz and 4.12 GHz which fractional bandwidths are 20.7% and 11.4%. The DM harmonic suppression can extend to 43.5 GHz with rejection level of 13 dB. The CM attenuation of DDBPF can reach 60 dB within DM passband. Furthermore, the CM suppression with attenuation level of 20 dB is ranging from 0.5 GHz to 43.5 GHz. The measurement results are in good agreement with the simulation results, which validates the effectiveness of proposed design methodology.     

CAD of high‐εr microstrip bandpass filters

A simple design procedure is described for miniaturized microstrip bandpass filters on high‐ε_r substrates. Costs and design times are reduced by replacing experimental measurements with EM simulations. The procedure is confirmed through the design of a miniaturized microstrip hairpin‐coupled bandpass filter at 1.8 GHz on BaO? PbO? Nd₂O₃? TiO₂ (ε_r=90.9) ceramic substrate. © 2002 Wiley Periodicals, Inc. Int J RF and Microwave CAE 12: 229–235, 2002.     

A tunable third‐order bandpass filter based on combining dual‐mode square‐shaped substrate integrated waveguide resonator with triangular‐shaped resonator

A frequency reconfigurable third‐order bandpass filter based on two substrate integrated waveguide (SIW) cavities is presented in this article. The purposed filter consists of a dual‐mode square‐shaped resonator and a triangular‐shaped resonator. In the square‐shaped cavity, four lumped capacitors are loaded as electrical tuning elements in the area where the electric fields of diagonal TE₂₀₁ and TE₁₀₂ modes are strongest. And an another capacitor is loaded at the suitable region of the triangular‐shaped cavity. Square‐shaped cavity introduces two transmission zeros and the triangular‐shaped cavity can suppress out‐of‐band spurious modes. The method that combines the resonators with different shapes and multiple modes into an organic whole cannot only achieve synchronous tuning but also have complementary advantages and improve out‐of‐band rejection. To verify its practicality, a SIW reconfigurable bandpass filter is simulated when the capacitance value varies from 0 to 1.4 pF and measured at 0.7, 0.8, and 0.9 pF, respectively. Measured results show that when the center frequency is tuned from 3.42 to 3.52 GHz, the proposed filter exhibits good tuning performance with insertion loss of less than 2.5 dB and return loss of better than 10 dB, which is suitable for fifth‐generation communication system.     

Design and fabrication of a continuous tunable microstrip lowpass filter with wide stopband and sharp response

Tunable microstrip lowpass filters (LPFs) with a good performance are used in most telecommunication systems. In this article, a third‐order circuit with stepped impedance resonator is used to design LPF, and suppression cells are used to improve the performance of the filter in stopbands up to 20 GHz. The ?3 dB cut‐off frequency can be controlled in the range of 0.35 to 1 GHz with the tuning range of 96%. Rejection stopbands of 30 dB (type‐I) and 20 dB (type‐II) can be extended to more than 20 GHz. For the passband, the insertion loss variations are in the range of 0.35 to 2.12 dB. The proposed tunable LPF has a sharp roll‐off rate (42‐96 dB/GHz). The varactor‐tuned microstrip LPF is designed, simulated, fabricated, and measured. The results of simulation and measurement for the proposed LPF are in good agreement. The size of the proposed tunable LPF is small and the filter dimensions are equal to 0.267λg × 0.13λg.