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.     

Dual‐band differential bandpass filter using stepped impedance resonators

In this article, a novel dual‐band differential bandpass filter using (SIRs) is designed. To demonstrate the design ideas, the differential and common mode equivalent half circuits are built and studied. Two resistors are connected between the two ends of the SIRs to consume the power in common mode. A capacitor is connected between the Ground and Center of the SIR to adjust the spurious frequencies, also strength the coupling of the two SIRs. The theoretical analysis shows the second band can be obtained by the proper impedance ratios of the resonances and the capacitor connected to the resonator. Two through ground vias (TGVs) connecting the top and bottom sides of the SIR filter, are used to realize the common mode rejection. To investigate the proposed filter in detail, a set of design equations are derived based on the circuit theory and transmission line theory. A phototype dual‐band differential filter operating at 1.5 and 2.75 GHz has been realized to validate the proposed concept and theory. © 2015 Wiley Periodicals, Inc. Int J RF and Microwave CAE 25:468–473, 2015.     

A bandpass filter using asymmetric stepped‐impedance resonators and symmetric T‐shape interdigital capacitor structures with wide out‐of‐band rejection

In this study, a novel stepped impedance resonator (SIR) is proposed. This SIR is composed of two stepped impedance transmission‐lines and an interdigital capacitor structure. The proposed resonator has a high ratio of the first spurious frequency f_s to the fundamental frequency f₀ and is suitable to design wide stopband filters. An equivalent model is used to analyze the resonant properties of the resonator. The design guidelines of the proposed resonator are summarized. Moreover, the coupling properties of the resonator are simulated and analyzed. Finally, a small high‐temperature superconducting bandpass filter is designed and fabricated using the proposed SIRs. The stopband of the filter is extended up to 4.0 f₀ and 3.5 f₀ with 30 and 60 dB out‐of‐band rejection levels, respectively.     

Dual‐band bandpass filters with multiple transmission zeros using λ/4 stepped‐impedance resonators

Two novel dual‐band microstrip bandpass filters (BPFs) with multiple transmission zeros are proposed in this article. The dual‐band BPFs with second‐order bandpass responses are due to two λ/4 stepped‐impedance resonators (SIRs). Two passbands (center frequency ratio f _s/f₀ is 2.36) are realized based on the asymmetric SIRs. The transmission zeros near the passbands can be adjusted conveniently using the stopband transmission characteristic of the open/shorted coupled lines. Two planar microstrip dual‐band BPFs (ε _r = 2.65, h = 0.5 mm) with four and six transmission zeros are designed and fabricated. High selectivity and good in‐band performances can be achieved in the proposed filters.     

Analysis of four‐stage stepped‐impedance resonators and their application to quad‐band microstrip bandpass filter

Four‐stage stepped‐impedance resonator (FSSIR) is proposed and its resonant characteristics are analyzed in detail. The formulas of the first four resonances are deduced and the optimization techniques are presented on the basis of the impedance ratios. A quad‐band bandpass filter with third‐order filtering response in each passband is synthesized and designed as a demonstration of the application of the proposed FSSIR. Thanks to the cross‐coupling topology and skew‐symmetrical feeding configuration, multiple transmission zeros have been generated out of the passbands. Additionally, the frequency and the couplings of each passband can be flexibly controlled, respectively.     

Design of a moderate‐wideband superconducting filter with modified stub structure

This paper presents a design method for moderate‐wideband filters with modified λ/2 stub resonators mid‐tapped by λ/4 connecting lines which act as admittance inverters. The modified tapped‐stubs avoid low characteristic impedances of the stubs which results in a very wide strip‐width and a large circuit size. An example filter at 2250 MHz with a fractional bandwidth (FBW) of 45% is successfully designed with this method and fabricated. It has a compact size of 26.5 mm × 10.5 mm on a YBCO/LaAlO₃ superconducting wafer. The measurements show good response and match well with the simulations.     

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.     

Effect of stepped‐impedance resonators on rectangular metamaterial unit cells

In this article, effect of the stepped‐impedance resonator (SIR) miniaturization technique on rectangular metamaterial unit cells is investigated and the influence of the method's structural parameters on the characteristic impedance and the distance between higher resonance modes is discussed. According to the results, the ideal unit cell for this method should be thick enough and have an impedance ratio greater than one. Furthermore, the SIR technique is applied on a conventional two‐turn spiral metamaterial unit cell and a new compact spiral unit cell is introduced. The effect of this method on the unit cell parameters is investigated and the new unit cell is compared to the conventional spiral. According to the results, a miniaturization factor of 0.75 can be achieved with the new unit cell. To validate the results, a two dimensional array of the unit cell is fabricated and its S‐parameters are measured using the free space method. © 2015 Wiley Periodicals, Inc. Int J RF and Microwave CAE 25:582–590, 2015.     

A balanced dual‐band bandpass filter with a wide stopband and controllable differential‐mode frequencies and fractional bandwidths

In this article, a balanced microstrip dual‐band bandpass filter (BPF) is designed. The proposed filter is achieved by employing a microstrip U‐shape half‐wavelength resonator, a folded stub‐loaded resonator and balanced microstrip/slotline transition structures. The center frequencies and the fractional bandwidths of the two differential‐mode (DM) passbands can be controlled independently by changing the physical lengths of the two resonators and the gaps between each resonator, respectively. The balanced microstrip/slotline transition structures can achieve a wideband common‐mode (CM) suppression. Meanwhile, the DM passbands are independent from the CM responses, which significantly simplify the design procedure. In addition, a wide DM stopband is also realized. In order to validate the design strategies, a balanced dual‐band BPF centered at 2.57 and 3.41 GHz was fabricated and a good agreement between the simulated and measured results is observed.     

Compact quad‐band band‐pass filters using stepped‐impedance coupled‐line resonators

In this article, compact quad‐band band‐pass filters are realized by using stepped‐impedance coupled‐line quad‐mode resonators (SICLQMRs). The compactness of the quad‐mode resonator relies in its folded structure without extra space between the parallel lines. Unlike stepped‐impedance resonators, SICLQMRs provide more design freedoms for controlling the four resonating frequencies since the even‐ and odd‐mode equivalents can be separately assigned with characteristic impedances. Internal and external couplings are also parallel couplings, resulting in very compact dimensions of the filters. Simulated and measured S parameters are compared with good agreement, demonstrating the feasibility of the design.     

Compact dual‐ and triband bandpass filter using frequency selecting coupling structure loaded stepped‐impedance resonator

This article presents two novel resonators, that is, frequency selecting coupling structure loaded stepped‐impedance resonator (FSCSLSIR) and π‐section loaded FSCSLSIR. The resonator behaviors and guidelines are given to design FSCSLSIR dual‐band bandpass filter (BPF) and π‐section loaded FSCSLSIR triband BPF. The proposed dual‐ and triband BPF have very compact sizes of 0.13 λ_gd × 0.06 λ_gd and 0.115 λ_gt × 0.074 λ_gt, respectively. Moreover, good return loss, low insertion loss, and high band‐to‐band isolation can be observed, and the proposed FSCSLSIR dual‐band BPF has an ultrawide stopband from 5.79 to 36 GHz. The experimental results are in good agreement with the simulations. © 2014 Wiley Periodicals, Inc. Int J RF and Microwave CAE 25:427–435, 2015.     

Novel slotted stepped‐impedance resonator and its application to compact and high performance bandpass filter and diplexer design

By etching slots in the low‐impedance section of the conventional stepped‐impedance resonator, a novel slotted stepped‐impedance resonator (SSIR) is proposed. As two examples, a fourth‐order bandpass filter (BPF) operating at 1 GHz with a size of 0.078 λ_g × 0.062 λ_g and a miniaturized diplexer operating at 0.9/1.57 GHz with a size of 0.054 λ₀ × 0.086 λ₀ are designed based on the proposed SSIR. The fabricated BPF exhibits a high selectivity and a wide ?30 dB rejection upper stopband from 1.13 f₀ to 6.52 f₀, while the fabricated diplexer has up to ?60 dB output isolation. © 2012 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2013.     

Ultra‐wide stopband low‐pass filter using symmetrical cascaded modified hairpin resonators

An elliptical function low‐pass filter (LPF) with ultra wide stopband and sharp cutoff frequency is proposed. This filter is composed of symmetrical cascaded modified hairpin resonators and U‐shaped resonators. The transition band is from 1 to 1.21 GHz with ?3 and ?20 dB, respectively. For this filter, the return loss is better than 17 dB in 80% of passband width, where the insertion loss is less than 0.3 dB. The band‐stop rejection is greater than 20 dB from 1.21 to 26.35 GHz and 40 dB from 1.35 to 12.5 GHz. To validate the design and analysis, the proposed LPF has been designed and fabricated on a 20 mil thick RO4003 substrate with a relative dielectric constant 3.38 and loss tangent of 0.0021. The filter is evaluated by experiment and simulation with a good agreement. © 2013 Wiley Periodicals, Inc. Int J RF and Microwave CAE 24:314–321, 2014.     

High‐order dual‐band superconducting filter with independently controllable passbands and wide stopband

A stepped‐impedance‐stub loaded stepped‐impedance resonator (SISLSIR) is proposed to design a dual‐band bandpass filter. The even‐ and odd‐mode frequencies and the coupling strength of the proposed resonators can be independently designed and adjusted. A dual‐feedline structure is used to meet the required external couplings of the 2 passbands. Thus, both the center frequencies and the bandwidths of the 2 passbands can be independently controlled. A 6‐pole dual‐band filter with the passbands of 3300～3600 MHz and 4800～5000 MHz is successfully designed using the proposed method and fabricated with YBCO/MgO high‐temperature superconducting (HTS) wafer. The measured results of the filter exhibit high performance and match well with the simulations. The measured insertion losses are less than 0.2/0.3 dB, and the return losses are greater than 15/14 dB for the lower/upper passbands, respectively. The out‐of‐band rejection is greater than 68 dB up to 12 GHz.     

Dual‐band band pass filter using stub‐loaded open‐loop resonators with wide controllable bandwidths

Two dual‐band band pass filters (BPF) using stub‐loaded open‐loop (SLOL) resonator are presented in this article. A novel coupling tuning method by changing the relative coupling position of the resonators is proposed to control the bandwidth of each passband in a wide range. Transmission zeros are created to improve the selectivity by source‐load coupling. Because of the large ratio of two bandwidths, a novel dual‐band matching method is proposed to match the different load impedances at two passband frequencies to the same source impedance. Hence, relax the fabrication requirement of gap. The proposed dual‐band band pass filter is designed and fabricated. The measured 3 dB fractional bandwidths (FBWs) of two 2.45/5.25 GHz dual‐band BPFs are 6.5%/14.5% and 9.8%/5.5%, respectively. The results are in good agreement with the simulation. © 2013 Wiley Periodicals, Inc. Int J RF and Microwave CAE 24:367–374, 2014.     

New multi‐standard dual‐wideband and quad‐wideband asymmetric step impedance resonator filters with wide stop band restriction

New multi‐standard wide band filters with compact sizes are designed for wireless communication devices. The proposed structures realize dual‐wideband and quad‐wideband characteristics by using a new skew‐symmetrical coupled pair of asymmetric stepped impedance resonators, combined with other structures. The first and second dual‐wideband filters realize fractional bandwidths (FBW) of 43.2%/31.9% at the central frequencies (CF) of 1.875/1.63 GHz, and second bandwidths of 580 MHz/1.75 GHz at CF of 5.52/4.46 GHz, respectively. The proposed quad‐band filter realizes its first/second/third/fourth pass bands at CF 2.13/5.25/7.685/9.31 GHz with FBW of 46.0%/11.4%/4.6% and 5.4%, respectively. The wide pass bands are attributed to the mutual coupling of the modified ASIR resonators and their bandwidths are controllable by tuning relative parameters while the wide stop band performance is optimized by the novel interdigital cross coupled line structure and parallel uncoupled microstrip line structure. Moreover, the quad band is generated by introducing the novel defected rectangle structure. These multi‐standard filters are simulated, fabricated and measured, and measured results agree well with both simulated results and theory predictions. The good in‐band and out‐of‐band performances, the miniaturized sizes and simple structures of the proposed filters make them very promising for applications in future multi‐standard wireless communication.     

Dual‐band filter with high selectivity and wide stopband rejection

This article presents a highly selective dual‐passband filter based on stepped‐impedance‐resonator (SIR) and mixed electromagnetic coupling. First, the surface area of the filter is effectively reduced by the triangular topology. Second, four controllable transmission zeros are introduced by source‐load coupling feed and mixed electromagnetic, which increases the selectivity of the filter. Third, a perturbation structure is added to independently control the resonance points of each passband. Finally, the improved defect ground structure (DGS) is integrated to obtain wide stopband rejection. The measured S‐parameters are well agreement with the simulated results, which show that the center frequencies of the two passbands are 2.4 GHz and 5.2 GHz; and the passband insertion losses are 0.85 dB and 1.6 dB; and the relative bandwidths are 14.6% and 5.7%, respectively. Besides, the structure is with six transmission zeros, and 20 dB suppression for the third harmonic and the fourth harmonic are achieved. Compared with the traditional SIR double‐passband filter, this filter has many advantages, such as simple design, small size, small insertion loss, controllable frequency, high selectivity, and high spurious suppression.     

Novel compact dual‐narrow/wideband branch‐line couplers using T‐Shaped stepped‐impedance‐stub lines

This article presents a compact model to reduce the physical size and increase the frequency ratio between the second and first resonance frequencies of a dual‐function stepped‐impedance‐stub (SIS) line, which was subsequently employed in the realization of dual‐band branch‐line couplers. The proposed model comprises of a loaded spiral T‐shaped SIS that reduces the size of a conventional SIS line as well as improving its frequency ratio. The proposed model behaves exactly similar to the recently developed dual‐band resonators with the advantage of size reduction of ～35% as well as having a wide range of realizable frequency ratios between 1.4 and 3.7 compared to 1.7–2.7 and 1.8–2.3 for the conventional SIS and T‐shaped transmission‐lines, respectively. Dual‐narrowband and wideband branch‐line couplers were developed based on the spiral T‐shaped SIS lines. The dual‐wideband device's bandwidth was enhanced by 2.7% accompanied by a size reduction of 58.6% in comparison with the conventional dual‐wideband couplers operating at the same frequencies. The theoretical results were verified by measurement. © 2011 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2011.     

Compact metamaterial unit‐cell based on stepped‐impedance resonator technique and its application to miniaturize substrate integrated waveguide filter and diplexer

In this article, a new miniaturized metamaterial unit‐cell using stepped‐impedance resonator technique is proposed. The proposed unit‐cell is used to miniaturize the physical size of the conventional complementary split‐ring resonators (CSRRs). In the proposed unit‐cell which is called complementary G‐shaped resonator (CGR), the slot line in the conventional circular CSRR is replaced with the stepped‐impedance slot line. As well as, by carving two trapezoidal shapes inside the inner ring, the resonance frequency of the proposed CGR unit‐cell has been more decreased. Compared to the conventional circular CSRR structure, the electrical size of the proposed CGR is decreased and miniaturization is occurred. To investigate the performance of the proposed CGR unit‐cell in the size reduction, two substrate integrated waveguide filters and a diplexer are designed. To validate the proposed miniaturization technique, the designed filters and diplexer loaded by the CGR unit‐cell are fabricated and measured. The measured results are in a good agreement with the simulated ones. The results shows that, a miniaturization factor about 0.69 is achieved.     

Microstrip stepped‐impedance ring all‐pass filter and wideband 90° phase shifter

A new all‐pass filter (APF) is proposed. The APF is based on a symmetrical ring, consisting of four sections of transmission line, which are identical in electrical length, different in characteristic impedance. Two input/output ports are connected orthogonally to the ring. The APF is analyzed by using the odd‐even model, and the all‐pass condition is then theoretically obtained. Meeting the condition, the circuit is all‐pass in frequency, but nonlinear in transmission phase. The nonlinear transmission phase with frequency may be adjusted by changing the lines' impedances, while remaining the all‐pass property. Then the APF is used to design a wideband 90° differential phase shifter with adjustable bandwidth. Samples are designed, fabricated and measured. Good agreements are achieved among the theoretic, numerical and experimental results. © 2013 Wiley Periodicals, Inc. Int J RF and Microwave CAE 24:191–195, 2014.