Extra Transmission Zeros Research Articles

Compact Balanced Dual-Band BPFs Based on Short and Open Stub Loaded Resonators With Wide Common-Mode Suppression

Two balanced microstrip dual-band bandpass filters (BPFs) based on short stub loaded resonator (SSLR) and open stub loaded resonator (OSLR) are presented in this brief, respectively. The center frequencies of the two differential-mode (DM) passbands can be controlled quasi-independently by adjusting the lengths of resonators. Moreover, a stepped impedance microstrip line etched with an interdigital coupling line is utilized to introduce a coupling between the two resonators, which generates two extra controlled transmission zeros (TZs) to obtain better selectivities. Furthermore, balanced microstrip/ slotline (MS) transition structures are employed as feed networks, which can suppress common-mode (CM) inherently while the DM responses are not affected, thereby the design procedure can be simplified significantly. In order to demonstrate the theoretical design, two balanced dual-band BPFs were designed and fabricated. The simulated responses are compared with the measured ones and a good agreement is obtained.

A Compact Dual-Mode Bandpass Filter with High Out-of-Band Suppression Using a Stub-Loaded Resonator Based on the GaAs IPD Process

In this letter, a compact dual-mode bandpass filter (BPF) with an ultra-wide stopband that employs two folded open-loop resonators (FOLRs) and stub-loaded resonators (SLRs) is proposed. The dual-mode resonators are optimized by loading two SLRs onto the folded open-loop resonators, and this process is analyzed using the dual-mode theory. To miniaturize the device size and increase chip performance, the proposed BPF is fabricated by a III–V compound semiconductor-fabrication process using a high-performance GaAs substrate based on the integration passivation device (IPD) fabrication process. A compact dual-mode BPF with low insertion loss and high return loss is designed and fabricated. Two extra transmission zeros (TZs) located in the high-frequency range increase the wide stopband, and the two TZs near the passband result in a higher selectivity. A resonant frequency centered at 7.45 GHz with an insertion loss of −1.21 dB and a measured return loss of higher than −23.53 dB and 3 dB fractional bandwidths of 5.8% are achieved. The stopband can be suppressed up to 20 GHz owing to the two tunable TZs resulting in higher selectivity and wideband rejection. The size of the filter was drastically optimized using a simplified architecture of two FOLRs and SLRs.

Stripline Resonator Loaded Compact SIW Filters With Wide Suppression and Flexible Response

A novel method to design substrate integrated waveguide (SIW) filters with odd-order response is presented in this letter. By combining SIW and stripline technologies, a fully closed, third-order filter consisting of two SIW resonators and one stripline resonator is proposed. The stripline resonator provides a coupling path for the TE101 mode to form a passband, while the couplings for TE102 and TE201 modes are very weak, resulting in the suppression of their propagation. The principle for higher order mode suppression is carefully illustrated. On the basis of this filter, another two filters are designed by introducing different cross couplings, which generate extra transmission zeros (TZs) to enhance the selectivity. These two filters exhibit flexible response with controllable TZs. All these three filters are fabricated and measured. The measured results are in good agreement with the simulation, which validates the proposed design methodology and the idea of incorporating the stripline resonator into the SIW. They demonstrate a low insertion loss, high selectivity, and wide stopband. In addition, the proposed filters are featured by a reduced size due to the applications of the double-layer structure and miniaturized stripline resonators.

A quad‐band bandpass filter using sept‐mode double square‐ring loaded resonator

AbstractA quad‐band bandpass filter (BPF) using a sept‐mode double square‐ring loaded resonator (DSRLR) is presented in this article. Based on the symmetrical geometric construction of the DSRLR, the even‐ and odd‐mode analysis with theoretical equations is employed to analyze the proposed BPF, and current density are obtained. Additionally, multimode splitting and resonant characteristics of the proposed DSRLR are explored. Accordingly, the design procedure is given after a pair of feedlines with dual‐finger structure is added to the I/O ports to introduce extra transmission zeros (TZs). Four TZs—including two created by the dual‐finger feedlines—are produced to separate four passbands that exhibit a sharp skirt and high stopband rejection. Finally, a microstrip quad‐band BPF is fabricated on a Teflon substrate with four measured passbands centered at 2.46, 3.41, 6.65, and 9.09 GHz with insertion losses of 0.68, 0.73, 0.81, and 1.23 dB, respectively. A satisfactory agreement is achieved between the measurement results and full‐wave simulation results.

Dual‐band filtering power divider with unequal power division ratio and low‐loss characteristic

In this article, a dual-band filtering power divider with unequal power-division ability is proposed. Different from conventional equal power dividers constructed by filters or coupled resonators, noncoupled structures are employed in this design. As a result, low-loss characteristic is realized for the proposed power divider. In this proposed structure, the dual-band unequal power allocation is realized by replacing conventional single-band λ/4 transformers with dual-band ones (T-junction structures). Three identical λ/4 stepped impedance resonators are properly attached to all the three ports of the proposed power divider to generate an extra transmission zero between two operational bands. Therefore, a filter-like shaping in its S-parameter results is obtained. A resistor is located between two outputs for output isolation. Mathematical derivations of the overall design procedure are also provided based on the circuit models and transmission line theory. Meanwhile, a resistor for output isolation is also included between two outputs, whose value can be calculated using given equations. For validation, a prototype operating at 0.9 and 2.1 GHz are designed, fabricated, and measured. The isolations between two outputs are 30 and 26 dB while the phase differences are only 2.5°and 4.9° at 0.9 and 2.1 GHz in the measurement, indicating good consistence of outputs. Measured |S21| and |S31| are −(1.76 + 0.3) dB, −(4.77 + 0.2) dB at 0.9 GHz and −(1.76 + 0.6) dB, −(4.77 + 0.5) dB at 2.1 GHz.

A bandwidth‐agile balanced filtering power divider

In this article, a balanced filtering power divider (FPD) that allows for operational agility of the bandwidth (BW) is presented. The differential-mode power dividing and high common-mode (CM) suppression can be realized by microstrip-to-slotline transition. Two slotline open stubs with different lengths are added in shunt to the main slotline for the transition, which can not only introduce transmission poles for extending and controlling transition BW, but also generate two extra transmission zeros (TZs) near to the passband edges, featuring good filtering response. The two transmission poles can be independently tunable by loading varactors to the open ends of slotline stubs and two TZs will be changed accordingly so that the filtering passband BW is electrically tunable. To verify the theoretical prediction, a prototype of tunable balanced FPD is fabricated and measured. The measured results show that the 3-dB fractional bandwidth (FBW) of the passband varies from 5.6% to 12.6%, meaning more than a double tuning range for the FBW, and the CM suppression is better than 40 dB across the frequency band of interest.

W-Band Broadband Waveguide Filter Based on H-Plane Offset Coupling

In this paper, a high-performance waveguide bandpass filter (BPF) operating at W-band is proposed. This fifth-order BPF with a wide fractional bandwidth (FBW) of 20% is realized by adopting strong H-plane offset magnetic coupling, while maintaining a simple and robust structure for the computer numerical control (CNC) milling technology. An extra transmission zero is introduced through the extracted pole method to improve the rejection in the upper out-of-band. The BPF fabricated by aluminum exhibits a low insertion loss of about 0.6 dB in the passband and the 3-dB FBW of 20.2% from 83.6 to 102.3 GHz, which agree well with the simulations. Besides, the effects of tolerance on the filter features are discussed, which indicate that such W-band waveguide filter can be manufactured by the conventional CNC milling technology accurately.

A Compact Bandpass Wilkinson Power Divider With Ultra-Wide Band Harmonic Suppression

A Wilkinson power divider (WPD) with bandpass property and an ultra-wide band harmonic suppression is proposed in this letter. The frequency-selecting coupling structure is embedded to the conventional quarter-wavelength transmission line, which can realize an ultra-wide harmonic suppression with three transmission zeros. Short-ended meander line stubs are shunted at the both output ports for dc block and provide an extra transmission zero for the passband. The proposed WPD operates with a −20 dB bandwidth from 1.74 to 3.71 GHz with an ultra-wide stopband to suppress the dc, $3f_{0}$ , $5f_{0}$ , and $7f_{0}$ harmonic components. The proposed WPD has a compact size of 14.65 mm $\times19.64$ mm ( $0.19\lambda _{g} \times 0.25\lambda _{g})$ .

Compact Balanced Dual-Band Bandpass Filter Based on Modified Coupled-Embedded Resonators

A new compact balanced dual-band bandpass filter based on coupled-embedded resonators with modified ground plane is presented in this work. Common-mode is rejected within the two differential passbands by symmetrically introducing four coupled U-shaped defected ground structures below the resonators. Common-mode rejection is significantly improved when compared with the standard (solid ground plane) filter with similar geometry thanks to the introduction of four extra transmission zeros. Due to the symmetry, the differential mode is not significantly affected by the presence of the U-shaped resonators. Circuit-model data, full-wave simulations and measurements are provided to verify the benefits of the proposed dual-band filter.

Compact Dual-Band Filter Using Open/Short Stub Loaded Stepped Impedance Resonators (OSLSIRs/SSLSIRs)

A novel microstrip dual-band filter with high frequency selectivity and extended rejection band is reported in this letter. The proposed filter employs one pair of open stub loaded stepped impedance resonators (OSLSIRs) functioning at two frequencies of interest. Two short stub loaded stepped impedance resonators (SSLSIRs) are embedded between the coupled OSLSIRs to generate quasi-elliptic response at two diverse frequencies. Contributed by the 0° feed structure, extra transmission zeros are produced to enhance the out-of-band rejection. OSLSIRs/SSLSIRs are chosen to achieve desired in-band performance and misaligned harmonic frequencies simultaneously. To demonstrate the efficacy of the design concept, a dual-band filter centering at 1.63 and 2.73 GHz is implemented. The design methodology and measured results are presented.

Compact SIW dual‐band bandpass filter using novel dual‐resonance quasi‐SIW‐transmission‐line‐structure resonators

A compact substrate integrated waveguide (SIW) dual-band bandpass filter (DBSF) using novel quasi-SIW-transmission-line-structure resonators is proposed in this study. This dual-resonance SIW resonator which works in TE100 mode and TE201 mode can be evolved from a conventional SIW-transmission line or a two-conductor transmission line. Multiple signals transmission paths bring extra transmission zeros (TZs) and improve the selectivity and out-band rejection of the DBSF. This filter works at 4.75 GHz/4.82 GHz, 5.38 GHz/5.51 GHz with 3 dB fractional bandwidths of 3.8 and 3.9%, respectively, and has six TZs. The measured results agree with the simulation results well.

A New Wideband Elliptic Bandstop Filter With Cross Coupling

A novel fifth-order elliptic cross-coupled wideband bandstop filter (BSF) is proposed and designed in this letter. It is devised by introducing additional electric coupling into two nonadjacent quarter-wavelength shunt stubs of a conventional commensurate line BSF. The selectivity and stopband rejection can be effectively enhanced by two extra transmission zeros generated from the coupling. The proposed BSF is designed directly from the normalized lowpass prototype by using the design formulas, which are derived based on the rigorous correspondence between the BSF and its lumped prototype in the $S$ -plane. In order to demonstrate our design, a BSF example is developed, with a measured 27.2 dB stopband over a fractional bandwidth of 141.7%.

Compact tri‐band bandpass filter with stub‐loaded stepped‐impedance resonator

A compact microstrip tri-band bandpass filter (BPF) based on two open stub-loaded multimode λ/4 stepped-impedance resonators (SIRs) with two separate coupling paths is investigated. A source–load coupling configuration is arranged to produce several extra transmission zeros for realising high-isolation performance. The centre frequencies of three passbands can be controlled by tuning the dimensions of two high impedance lines of the proposed open stub-loaded SIRs. The bandwidth of the three passbands could be conventionally controlled by adjusting the coupling strength of two coupling paths. The prototype of the BPF achieved insertion losses of 0.46, 1.05 and 1.4 dB at 0.82, 1.67 and 2.17 GHz, respectively. The measured results are in good agreement with electromagnetic simulation. The circuit only occupies 0.126λ g × 0.108λ g, where λ g is the guided wavelength at a lower passband centre frequency.

Compact Balanced FSIR Bandpass Filter Modified for Enhancing Common-Mode Suppression

A novel balanced bandpass filter (BPF) based on folded stepped impedance resonators (FSIR's) with modified ground plane is presented in this work. By symmetrically introducing a series-LC resonant structure below the FSIR's, common-mode (CM) propagation can be rejected without affecting differential-mode (DM) performance. The filter presents two main advantages with respect to the conventional solid-ground-plane FSIR filter: i) an important improvement of the CM rejection level within the differential passband and ii) an enhanced filter selectivity due to the inclusion of an extra transmission zero in the differential passband. Both the conventional and the novel filters have been modeled as lumped-element circuits that fully account for DM and CM operation. Simulation and measurement results confirm the benefits of the proposed balanced BPF.

Compact stub‐loaded open‐loop BPF with enhanced stopband by introducing extra transmission zeros

Open-loop resonators (OLRs) are loaded with one or two stubs to design a compact bandpass filter (BPF) with a wide stopband by creating extra transmission zeros. By properly selecting the length of the stubs and their positions, extra transmission zeros are created in the stopband before the spurious passband, thus giving wide stopbands. Mathematical analyses are performed to show the effect of open stubs on the OLR. A two-stub-loaded BPF is designed and fabricated, which shows a harmonic suppression of up to 3.2fo by creating five transmission zeros and 44% size reduction as compared with a simple OLR-based filter without stubs. Measured results are in good agreement with the theoretical analysis and simulation results.

DUAL-BAND BANDPASS FILTER WITH WIDE UPPER STOPBAND USING NOVEL STEPPED IMPEDANCE STUB-LOADED QUAD-MODE RESONATOR

This paper presents a wide upper stopband dual-band bandpass filter (BPF) with controllable passband frequencies and bandwidths as well as a high out-of-band rejection level. The proposed filter is realized by utilizing a novel stepped impedance stub-loaded quad-mode resonator. All the four-mode equivalent circuits of the resonator are quarter-wavelength stepped impedance resonators (SIRs), and their fundamental resonance frequencies are used to form the passbands, so the designed filter has a compact circuit size. By controlling the impedance and length ratios of the stubs of the resonator, wide upper stopband performance is obtained. Hook-shape feed-lines and source-load coupling are applied to provide appropriate external coupling and generate three extra transmission zeros, which greatly improve the selectivity of the proposed filter. An experimental filter operating at 1.5 and 3.5 GHz is designed, fabricated, and measured for validation. The measured results have good agreement with the simulated ones.

Full Study of the Parallel-Coupled Stub-Loaded Resonator: Synthesis Method in a Narrow Band With an Extended Optimal Rejection Bandwidth

This paper gives a complete theoretical study in a narrow band for the parallel-coupled filters based on short-circuited stub-loaded resonators. It describes a synthesis method for this filter topology, and a complete resonant mode analysis to fully control the spurious frequencies positions. To enlarge the out-of-band rejection, an original technique, based on simple design steps, is used to address tuning of extra transmission zeros. Harmonic was suppressed to better than 35 dB with a wide stopband of more than six time the fundamental frequency. Design equations and design rules are given. Finally, several three-pole bandpass filters are designed and characterized in stripline and miscrostrip technologies to demonstrate the efficiency of the proposed concepts.

Design of Microstrip Lossy Filter Using an Extended Doublet Topology

A lossy three-pole microstrip bandpass filter using an extended doublet network is proposed in this letter. Nonuniform Q distribution is considered by reducing the Qu of one resonator, resulting in a flatter passband with an enhanced selectivity. Also, the novel feeding network adopted brings about two extra transmission zeros and a wide stopband that further improve the filter performance. Good agreement between the simulated and measured results can be observed to validate the design.

Compact Triple-Band High-Temperature Superconducting Filter Using Multimode Stub-Loaded Resonator for ISM, WiMAX, and WLAN Applications

A compact triple-band high-temperature superconducting (HTS) bandpass filter using multimode stub-loaded resonator is proposed in this paper. The resonant characteristics of the multimode resonator are investigated by the even/odd-mode analysis. To sharpen the passband skirts, a source-load coupling configuration with a nonresonant node is arranged to produce several extra transmission zeros for realizing high-selectivity performances. For demonstration purposes, a triple-band filter operating at 2.45 GHz (ISM band), 3.5 GHz (WiMAX band), and 5.2 GHz (WLAN band) is designed, fabricated, and measured. Measurements are in good agreement with the electromagnetic simulations. The measured minimum insertion losses of the triple-band HTS filter are 0.16, 0.55, and 0.22 dB, whereas the return losses are greater than 16.7, 10.2, and 17.5 dB, respectively. Its overall size is only 8.3 mm <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\times$</tex></formula> 8.6 mm.

Compact wideband lumped balun with out‐of‐band suppression using tail inductor

A novel compact 2.45 GHz wideband lumped balun with out-of-band suppression using a tail inductor has been proposed. By adding an extra transmission zero in even-mode around operation frequency, the tail inductor can improve phase balanced performance and enlarge operational bandwidth of the balun. Besides, the tail inductor together with a band-stop resonator can realise out-of-band transmission zero in even-mode. The measured results illustrate a relative 41% bandwidth, namely from 2.35 to 3.35 GHz. Over the operation frequency, a less than 0.5 dB amplitude imbalance and a less than ± 7° phase error have been demonstrated. The out-of-band suppressions of two output ports at 5.3 GHz are 24.5 and 27.5 dB, respectively.