Tunable Transmission Zeros Research Articles

Compact Varactor-Tuned Microstrip High-Pass Filter With a Quasi-Elliptic Function Response

This paper presents a novel circuit topology of a tunable high-pass filter with a quasi-elliptic function response. A detailed theoretical analysis for the performance operating mechanism is demonstrated. With the assistance of mutually inductive coupling introduced, a pair of tunable transmission zeros (TZs) is synthesized at the lower passband edges and significantly improves the filter selectivity. Experiments are carried out by using the liquid crystal polymer bonded multilayer printed circuit board technology to validate the design. It is found that the implemented filter achieves a wide continuous tuning range covering from 1030 to 2150 MHz with low insertion loss, and which indeed are limited by the Schottky diodes used. It is also illustrated that two measured TZs are properly controlled at each tuning stage, which not only offers a sharp cutoff frequency response with a rejection level greater than 22 dB, but also provides an approximately unchanged response shape. Good agreement between the measured and simulated results can finally be observed.

Compact Single-/Dual-Wideband BPF Using Stubs Loaded SIR (SsLSIR)

This letter presents a new stubs loaded stepped impedance resonator (SsLSIR) to design single-/dual-wideband bandpass filter (BPF). The proposed SsLSIR exhibits multiple tunable transmission zeros (TZs) and transmission poles (TPs), which can be arranged to build up single/dual-wideband BPF. The proposed single-band BPF has a central frequency of 1.91 GHz with -3 dB fractional bandwidth (FBW) of 52.4% and the dual-band BPF has a central frequency of 1.92/5.44 GHz with -3 dB FBW of 66.7%/28.3%. The higher-order spurious frequencies of the single-band BPF are tuned and canceled by the TZs, resulting in a wide stopband with -20 dB spurious suppression from 2.19 to 9.19 GHz. The TZs between two passbands of the dual-band BPF provide a wideband and high band-to-band isolation. Two filters have the merits of low insertion loss, simple physical topology and compact size.

Microwave bandpass filters with tunable center frequencies and reconfigurable transmission zeros

This article presents the design of a microwave bandpass filter with tunable center frequencies and reconfigurable transmission zeros. The tuning of the center frequencies is realized by loading varactors at the ends of the λ/2 resonator. The reconfiguration of transmission zero is implemented through varactors in combination with tapped λ/4 stubs. The equivalent circuit models of the tunable filters are provided. The transmission zeros can be set to be at the lower side, the upper side, or both sides of the passband. The design concept is demonstrated by measurement results for both one transmission zero case and two transmission zeros case. The tuning frequency ranges are 680–890 MHz and 680–1020 MHz for one zero case and two zeros case, respectively. © 2013 Wiley Periodicals, Inc. Microwave Opt Technol Lett 55:1526–1531, 2013; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.27628

Microstrip bandpass filters with series‐LC resonator loaded coupled lines

AbstractIn this article, series‐LC resonator loaded coupled lines are proposed to implement microstrip bandpass filters. The proposed loaded coupled line can generate three tunable transmission zeros. Conditions for the transmission zeros of the series‐LC resonator loaded coupled lines are analyzed and discussed. To demonstrate the applications, two second‐order bandpass filters using the proposed loaded coupled‐lines are designed with wide stopband rejections. The measured results show a good agreement with the simulations. © 2013 Wiley Periodicals, Inc. Microwave Opt Technol Lett 55:1273–1276, 2013; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.27542

A compact and high isolation dual-mode dual-band bandpass filter with tunable transmission zeros

A new dual-mode dual-band bandpass filter (BPF) is presented by utilizing a coupled-line with open-circuited quarter-wavelength stub. Classical even- and odd-mode analysis shows that the new dual-band structure has four tunable transmission poles and two tunable and three fixed transmission zeros. The fixed transmission zero at the design frequency divides four transmission poles into two groups, resulting in a dual-band BPF with two transmission poles in each passband. Two tunable transmission zeros can be located either between two passbands to provide a wide and high isolation band or outside two passbands to improve the selectivity of sidebands. As examples, two dual-mode dual-band BPFs, filter A with two transmission zeros between two passbands as well as filter B with two transmission zeros outside two passbands, are designed, fabricated, and measured, of which two passbands are located at the center frequencies of 2.4 and 5.8 GHz for WLAN application, respectively. The simulated and measured results coincide well, validating the design theory.

Design of coaxial quasi-elliptic filter using mixed electric and magnetic coupling

A coaxial quasi-elliptic bandpass filter (BPF) with controllable mixed electric and magnetic coupling is proposed in this paper. Adding a ridged transmission line section between the posts of inner resonator cavities is applied to enlarge the magnetic coupling between the adjacent resonators. On the other hand, the metallic caps on the top of the inner posts increase the capacitance loading of the coaxial resonators, thus not only decreasing its resonant frequencies and reducing the filter size, but also enhancing the electric coupling between the cavities. The proposed three-order BPF has two independently tunable transmission zeros (TZs). By simply adjusting the heights of the ridges and the distances between metallic gaps in a mixed coupled filter, the magnetic and electric coupling coefficients can be changed and two TZs can be tuned in the lower stopband, upper stopband, or both. The filter carries many advantages, such as compact size, low insertion loss, and high rejection level. A three-order mixed coupling filter has been simulated, fabricated, and measured. The measured results show good agreement with the simulation response showing the validity of the presented method.

Analytical design of a microstrip UWB BPF with sharp rejection

This paper presents a new Ultra-WideBand (UWB) BandPass Filter (BPF) using half-wavelength (λ/2) Stepped-Impedance Stub-Loaded Resonator (SISLR). Analytical equations derived by the even-odd mode analysis show the new filter has two tunable transmission zeros at both sides of the passband to provide a sharp rejection and seven transmission poles inside the passband to achieve U.S. UWB performance. For verification, a UWB BPF is designed, fabricated and measured. The measured results show that the fabricated filter has a -3 dB fractional bandwidth from 3.0 GHz to 10.9 GHz and its insertion loss less than 0.9 dB over the whole passband. Furthermore, the new filter exhibits a simple topology, sharp rejection, and deep stopband suppression.

Compact UWB Bandpass Filter With a Notched Band Using Radial Stub Loaded Resonator

In this letter, a novel compact ultra-wideband (UWB) microstrip bandpass filter (BPF) with a notched band is presented by using the proposed radial stub loaded resonator (RSLR). The RSLR, on the one hand, is able to control two tunable transmission zeros separately and on the other hand, has the ability to provide a notched band. In addition, the vertical dimension of the UWB BPF can be significantly reduced by using RSLR, as compared with the UWB BPF based on the conventional stub-loaded resonator (SISLR). For validation, two UWB BPFs, one without the notched band and the other with a notched band at 8.0 GHz for rejecting satellite-communication signal, are developed and fabricated. Measured results show two fabricated filters have the merits of low insertion losses, sharp rejections and flat group delays. Moreover, two filters exhibit the simple topologies and compact sizes.

On-Chip Single-to-Balanced Multicoupled Line Bandpass Filters With Good Selectivity

In this work, on-chip single-to-balanced bandpass filters with good selectivity are proposed. The functions of bandpass filter and balun are integrated using a simple multicoupled line structure along with capacitive loadings. The use of inductor is avoided such that the proposed single-to-balanced bandpass filters can achieve very compact size when implemented on chip. They can be designed with the desired bandpass response and reference impedances through the proposed equivalent-circuit models and design equations. In addition, by introducing two cross-coupled capacitors, one tunable transmission zero can be achieved in either the lower or upper stopband to improve the selectivity. Specifically, two third-order single-to-balanced bandpass filter designs in commercial pHEMT process on 4-mil GaAs substrate with different transmission zero locations are presented. The circuit size is less than 1.02 mm × 0.86 mm for a center frequency of 5.5 GHz and a bandwidth of about 18.18%. The corresponding electrical size is only 0.054λg×0.045λg at the center frequency. Good selectivity and balanced performance are demonstrated.

UMTS diplexer design using the antiparallel‐coupled lines loaded with a stepped impedance resonator

AbstractWe present a novel UMTS diplexer design using the filters of antiparallel‐coupled lines structure loaded with a stepped impedance microstrip resonator. The diplexer is composed of two filters. And a microstrip line of quarter‐wavelength is used to links each filter to the common port. The composing filters exhibit two tunable transmission zeros which are used to implement the mid‐passband zero implantation for improving the isolation between the transmitting (Tx) and the receiving (Rx) ports. A pair of spurlines is used in the coupled lines of the Tx‐filter to further increase the isolation. An experiment is conducted to assure the diplexer's performance and to verify the circuit design. © 2011 Wiley Periodicals, Inc. Microwave Opt Technol Lett 53:2988–2990, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26421

Design of Dual-Band CT Filter with Sourceload Coupling

A dual-band filter consisting of two cascaded triplet (CT) units is presented in this paper. Two sets of λ /4 resonators are employed to construct the CT unit. The utilization of λ /4 resonators not only makes the filter compact, but also allows the inductive cross coupling to be realized conveniently. This results in a transmission zero of finite frequency on the high side of each passband. The bandwidth of each passband can be tuned independently by adjusting the coupling via hole. The source-load coupling is also proposed to introduce two tunable transmission zeros, which can improve the selectivity of the filter. A trial dual-band filter is designed and measured, and the filter centered at 2.45 and 5.25 GHz occupies an area of only 33 × 15 mm2. Measured results show that a rejection level of 20 dB up to more than four times the first passband frequency can be obtained.

Compact and Sharp Rejection Microstrip UWB BPF with Dual Narrow Notched Bands

This paper presents a new ultra-wideband bandpass (UWB) bandpass filter (BPF) based on the half-wavelength (λ/2) stepped-impedance stub-loaded resonator (SISLR). Classical even- and odd-mode analysis shows that the new filter has multiple transmission poles and two tunable transmission zeros on both sides of the filter passband. In order to reject the interfered signals, the technique of generating a notched band inside the UWB passband is studied. As examples, two UWB BPFs, one without the notched band and the other with dual narrow notched bands at 5.8/8.0 GHz, are designed, fabricated and measured. The notched band at 5.8 GHz is realized by an embedded open-circuited stub (EOCS) while the notched band at 8.0 GHz is implemented by a folded stepped-impedance resonator (FSIR). The simulations and measurements agree well with each other. Two filters show the simple topologies, compact sizes, low insertion losses and sharp rejections.

Harmonic‐suppressing bandpass filter based on coupled triangular open‐loop stepped‐impedance resonators

AbstractThis work develops two harmonic‐suppressing bandpass filters. One has a direct‐coupled‐resonator structure and the other has a cross‐coupled‐resonator structure. The triangular open‐loop stepped‐impedance resonator is used to synthesize filters because it can create two tunable transmission zeros to suppress multi‐spurious responses in the upper stopband. The coupling coefficients of typical coupled pairs of resonators and the external quality factors of two basic feeding configurations are exploited in filter synthesis. A three‐pole Chebyshev filter and a four‐pole selective‐response filter are designed and fabricated herein. The first spurious peaks of these two experimental filters occur at five times and four times the operating frequency, respectively. © 2009 Wiley Periodicals, Inc. Microwave Opt Technol Lett 52: 187–191, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24866

Compact Ultra-Wideband Conductor-Backed Coplanar Waveguide Bandpass Filter With a Dual Band-Notched Response

This work presents a novel ultra-wideband (UWB) bandpass filter (BPF) based on a conductor-backed coplanar waveguide structure with tunable transmission zeros. The symmetrical UWB BPF, which consists of the broadside-coupled transitions and the stub resonators in double-layer configuration, achieves a UWB bandwidth with transmission zeros. For characterizing this structure, the equivalent-circuit model is established to realize a four-pole response with two transmission zeros located close to the passband edges. To eliminate the interference of the coexisting wireless local area network (WLAN) within the UWB spectrum, two slotlines are symmetrically arranged on the ground plane of UWB BPF to generate the band-notched frequencies at 5.2 and 5.8 GHz simultaneously. The proposed UWB BPFs have the advantages of compact size, low insertion loss, good selectivity, and flat group delay. All results obtained from the equivalent-circuit model and the full-wave simulation are verified by measurements.

Compact double-mode cross-coupled microstrip bandpass filter with tunable transmission zeros

A compact double-mode cross-coupled microstrip bandpass filter with an asymmetric frequency characteristic and three transmission zeros is presented. The bandpass filter consists of a cross-coupled gap and a double-mode resonator of a half-wavelength line with a shunt-stepped-impedance open stub. In comparison to the conventional square loop dual-mode resonator, the proposed double-mode resonator achieves a 59% size reduction. The parametric studies on the electrical length of the open stub and the capacitance of the cross-coupling are provided in order to gain a better insight into the transmission poles and zeros. Then, a bandpass filter is designed, fabricated and tested to validate the design concept. From 2.3 to 2.5 GHz, the return loss is better than 10 dB and the insertion loss is <1.5 dB. There are three transmission zeros in the upper stopband at 2.8, 3.4 and 4.9 GHz.

Microstrip Open-Loop Resonator With Multispurious Suppression

A compact open-loop resonator with multispurious suppression is proposed. When excited symmetrically, the resonator shows two tunable transmission zeros. By adjusting the open-end gap capacitance, one of the zeros is placed near the passband and the other is tuned to collocate with the leading two degenerated higher order resonances, so that the circuit has a sharp transition as well as a wide upper stopband. The experimental circuit shows the first spurious peak occurs at four times the passband frequency (4f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sub> ), and the measurement shows good agreement with the theoretic prediction.

Using high permittivity ceramic substrates to design a bandpass filter with open stub

AbstractThe advantage of using high permittivity ceramic substrates is to miniaturize the sizes of microstrip bandpass filters. The selectivity and stopband rejection of the designed filters can be improved significantly by utilizing open stubs. The designed filter with appropriate open stub can create tunable transmission zero. These factors will reduce the size of the filter and increase its performance. The responses of the filters using Al2O3 (εr = 9.8, tanδ = 0.0001) and 0.96(Mg0.95Zn0.05) TiO3‐0.04SrTiO3 (εr = 20.7, tanδ = 0.00007) ceramic substrates are designed at a center frequency of 2.45 GHz. Finally, the open stub and high permittivity ceramic substrates are applied to the design of a direct‐coupled hairpin filter. The compact size, low‐loss, sharp response, and performance of the filter are presented in this article. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 771–773, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22252

Bandpass filters with reconfigurable transmission zeros using varactor-tuned tapped stubs

In this letter, bandpass filters with one or two tunable transmission zeros are proposed. The reconfigurable transmission zeros are implemented through varactors in combination with tapped quarter-wavelength stubs. Based on mixed mode simulations including EM simulation and circuit simulation, the proposed filters were designed and fabricated on low-cost FR4 board. The measurement results verified the design concepts.

Design of a New Bandpass Filter Using Anti-Parallel Coupled Asymmetric SIRs

In this paper a newly designed internally-coupled asymmetric stepped-impedance resonator (SIR) bandpass filter (BPF) is proposed. The asymmetric SIR structure not only can effectively reduce the circuit size but also can provide two flexibly tunable transmission zeros near the lower and upper passband edges. The first transmission zero is due to the series resonance of the quarter-wavelength open stepped-impedance stub, and the second one is produced by anti-parallel coupling between adjacent SIRs. The proposed BPF was fabricated and simulated using the commercial software HFSS, and agreement between the measured and simulated results was observed. A 0.9-dB insertion loss and a shape factor of 3.6 were achieved in the passband, thus indicating that the proposed filter structure is of practical value.

Miniaturized multilayer hairpin bandpass filter with tunable transmission zeros

AbstractA miniaturized multilayer hairpin bandpass filter using tapped‐line I/O with tunable transmission zeros is designed and implemented. This structure provides additional cross‐coupling between two layers, as well as the coupling effect on the same layer. The designed filter demonstrates superior passband characteristics with a central frequency f0= 2.39 GHz, insertion loss S21= −1.45 dB and a return loss S11= −32 dB. Moreover, by combining with two layers and tapped‐line I/O structure, the filter produces two tunable transmission zeros at 2.8 GHz with −48 dB and 3.2 GHz with −43‐dB rejection that the typical elliptic function response can be easily. Experimental results of the fabricated filter measured by network analyzer agree well with the simulation results. © 2004 Wiley Periodicals, Inc. Microwave Opt Technol Lett 41: 53–55, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.20044