Finite Transmission Zeros Research Articles

Dual- and triple-band quasi-elliptic bandpass filters based on single-cavity multi-mode hybrid cavities

In this paper, a novel construction method for a single hybrid cavity is proposed for the first time, which integrates a quarter-mode substrate integrated waveguide structure into a patch cavity (QMSIWP), and a novel single-cavity multi-mode resonator is realized through the incorporation of cross-slot lines. A detailed analysis of its characteristics, combined with a specialized feed structure, leads to the design of multiple dual-band quasi-elliptic bandpass filters (BPFs) with varying frequency ratios and a triple-band quasi-elliptic BPF. The finite frequency transmission zeros (FTZs) and bandwidths (BWs) can be controlled by adjusting various parameters. For the demonstration, a triple-band quasi-elliptic BPF is fabricated and measured to validate the performance and design method of the proposed QMSIWP hybrid cavity and feed structure. The measured results agree with the synthesized and simulated results, confirming the proposed method's feasibility and universality. The proposed filter exhibits advantages such as compact size, high selectivity, and excellent group delay, which expands its application prospects and potential value.

Design of HTS linear phase bandpass filter based on the inline topology

Abstract Inline topology filters have the advantages of a simple structure and fewer coupling coefficients. Based on the inline topology, this paper constructs a phase compensation topology of the filter, which can independently and controllably introduce two types of transmission zeros (TZs), and is convenient to realize the Chebyshev-like function response and compensate group delay. The finite frequency transmission zero is directly introduced through the source and load (S/L), and the real zero point is introduced and adjusted through the cross-coupling unit of the main path in this inline phase compensation topology. The two types of TZs can be introduced and regulated independently, so the side-band suppression and group delay compensation can be freely adjusted. A High-temperature superconducting (HTS) filter with inline is designed and processed with a typical structure based on application requirements. The filter introduces two pairs of symmetrical real TZs to improve sideband rejection and a pair of real TZs to control group delays to realize the linear phase. In addition, it has a small size and straightforward construction, and the results of the full-wave analysis method and the theoretical analysis approach match well with the measured results of the frequency response characteristics.

A design method for filter to achieve arbitrary distribution of multiple transmission zeros without cross-coupling

This paper proposes a method for introducing transmission zeros (TZs) in filters using dual-mode resonators. By loading a dual-mode resonator as a second-order extracted-pole unit (EPU) at the input or output of the filter, a pair of finite TZs can be introduced without cross-coupling. This approach reduces the size of the filter while generating two finite transmission zeros compared to conventional methods. Since the dual-mode resonator is characterized by its separable odd and even modes, and by adjusting the resonant frequency of the EPU, it can achieve an arbitrary distribution of TZs. It is worth noting that the EPU does not affect the insertion loss of the filter because it operates mainly outside the passband. Furthermore, this work delves into the EPU’s mechanism for generating TZs and the method of adjusting TZ position by designing filters of second, fourth, sixth, and eighth orders, providing a solid foundation for the creation of higher-order filter circuits.

Design of planar metal stripline filter and diplexer with frequency‐dependent couplings

AbstractIn this letter, frequency‐dependent coupling (FDC) is introduced into the design of metal stripline filters and diplexers for the first time. FDC is a popular research method to introduce the finite‐position transmission zeros in the synthesis and design of filters in recent years. Finite transmission zeros are critical for filters to enhance the rejection ratios. To verify the realizability and effectiveness of FDC, a filter centered at 3.52 GHz and a diplexer operating at 2.55 and 2.65 GHz have been designed and fabricated. The planar filter and diplexer are compact in structure with multiple transmission zeros, whose resonators and cavity are integrated and formed, the cavity and cover plate are soldered together. We designed, assembled and tested the filter and diplexer, the synthesis results, the simulation results, and the tested results are well matched with each other. Compared with the dielectric waveguide filter, the planar stripline filter has better insertion loss, better harmonic suppression, and higher reliability, which is expected to be widely used in 5G base station filters.

A compact fifth-order SIW BPF based on TSV technology with high selectivity

In this paper, a fifth-order Substrate Integrated Waveguide (SIW) filter using Through-Silicon Via (TSV) technology is proposed. The slots on the top and bottom redistribution layers (RDL) of the filter provide negative coupling and a finite transmission zero (FTZ). This effectively improves stopband attenuation. Asynchronous tuning is applied to the coupling path of this component. The center frequency of the filter is 3.6 GHz (fractional bandwidth is 3.50 %). The minimum insertion loss is 1.58 dB, the minimum return loss is 23.6 dB, and the overall size is 1.855 × 1.316 mm2. The simulation results show that the proposed SIW filter can achieve better in-band characteristics and selectivity with the similar size or smaller size as compared to the SIW filter operating at the similar frequency.

Highly Selective and Compact Filtering Antennas Using Dual-Mode SIW Resonators

A novel class of compact and highly selective filtering antennas controlling two finite position transmission zeros (TZ) are presented. The filtering antennas are composed of a substrate integrated waveguide (SIW) filtering doublet and a wideband cavity backed slot antenna (CBSA). Design guidelines are presented to implement bandpass filtering antennas with TZ either: above, below or on opposite sides of the passband. The TZ location is highly flexible relative to the passband and is entirely determined by the input feed offset and via position. Stand-alone devices demonstrating the three via arrangements are presented around 16 GHz. All devices demonstrate a flat passband gain around 5 dB with a single element, direct control over the TZ position, high passband roll-off and out-of-band attenuation, and low cross-polarization. Compared with the state-of-art SIW filtering antennas, the fabricated devices demonstrate the highest combined bandwidth, passband roll-off and out-of-band attenuation. Additionally, the device with TZ on either side of the passband is sequentially rotated in a 2x2 topology to form a right-handed circularly polarized filtering array with a flat passband gain around 9 dB, retaining the high selectivity.

Three element resonator lowpass filter

AbstractThe conventional lowpass prototype filter with the finite transmission zeros close‐to‐band edge often yields negative circuit elements that lead to a practical difficulty of physical realizations. This paper presents an alternative topology of the lowpass prototype filter named three element resonator lowpass filter, which is composed of series inductances and shunt three element resonators. For one‐section three element resonator lowpass filter, the closed‐form solutions of the non‐negative circuit elements are deduced based on its characteristic function recovered from a prescribed transmission zero. For multi‐section one, the associated numerical synthesis procedure using gradient‐based optimization is presented. In physical realization, a much more practical configuration of three element resonator is proven by the equivalent circuit transformation. A synthesis example of lowpass prototype filter with 12 pairs of transmission zeros illustrates the effectiveness of the numerical synthesis method. Also, a design example of five‐section three element resonator lowpass filter, which has a passband up to 1775 MHz, is implemented by the distributed elements based on its prototype circuit of the numerical synthesis. The measured results of the fabricated lowpass filter experimentally validate the prototype filter.

Terahertz monolithic integrated coplanar band‐pass filter based on slotline mode

AbstractIn this letter, a terahertz monolithic integrated planar band‐pass filter based on coplanar waveguide (CPW) resonators has been proposed. By adopting the feeding line in form of a slotline, the need for air‐bridges in traditional CPW filters is eliminated, achieving good integrability and reducing fabrication difficulty. To further improve the filtering performance, the defected ground structure is used in the design, introducing a finite transmission zero at the lower sideband. The filter is manufactured on the semi‐insulation silicon carbide (SiC) substrate, to fit the existing advanced GaN‐on‐SiC monolithic integration system. Measurement reveals the fractional band‐width of 5.7% at the center frequency of 279 GHz, with an insertion loss of 5.1 dB. With a volume of 500 × 400 × 50 μm (0.45 × 0.36 × 0.045 λ0), the proposed filter improves the performance of the planar filter operating at the terahertz band effectively.

Synthesis and design of asymmetric cul-de-sac structure HTS filter with transmission zero introduced in Pairs

An asymmetric prototype of a cul-de-sac topology filter to introduce finite frequency transmission zeros (FTZs) in pairs with the least cross-coupling is synthesized and designed in this paper, which is suitable for the design of the high temperature superconducting (HTS) narrowband filter with high selectivity sideband suppression and symmetrical frequency response about the central frequency, especially. The resonator in the cul-de-sac structure filter can adopt the same frequency resonator without self-coupling so that the cul-de-sac structure filter can have fewer coupling coefficients and fewer resonator types. These characteristics of cul-de-sac structure filters are convenient for the design of high-order HTS filters with temperature shifts. A typical 7-order HTS filter with a central frequency of 4000 MHz, 5% relative bandwidth, five coupling coefficients, one cross-coupling, and one pair of FTZs is designed and fabricated on a 0.5 mm-thick MgO wafer with double-sided YBa2Cu3O7 (YBCO) thin films employing this cul-de-sac filter theory. The analyzed frequency response characteristic obtained with full-wave tools of this filter agrees well with the synthesis results and measured results at 77 K (Kelvin temperature).

Wideband Chebyshev Bandpass Filter With Extended Upper Stopband Using Shunt Short-Ended Slotline Stubs

A wideband bandpass filter (BPF) with in-band Chebyshev equal-ripple responses and extended upper stopband is proposed, which is composed of composite microstrip, slotline, and coplanar waveguide (CPW) structure. According to the odd-mode and even-mode equivalent circuits, there are four in-band transmission poles constructing the ultrabroad bandwidth can be directly deduced. In addition, there are two sets of lowpass filter (LPF) embedding in the CPW connecting line to greatly widen the upper stopband without increasing circuit size. In order not to affect original passband Chebyshev equal-ripple responses, the LPFs are synthesized using the low-pass prototype network. Furthermore, there are two finite frequency transmission zeros (TZs) which generated by cross-coupling effects located at lower and upper side of passband to effectively improve the shirt selectivity. Finally, for experimental verification, the proposed BPF prototype is manufactured and tested. Experimental results are consistent with predicted theoretical responses, which verify the validity and feasibility of proposed approach.

New Triple-Resonance Configuration Using Stubbed Waveguide Dual-Mode Cavities

This letter presents new configurations of stubbed waveguide cavities for the realization of pseudo-elliptic and self-equalized filters. The basic structure consists of a main TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">102</sub> /TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">201</sub> dual-mode cavity that is loaded with a pair of waveguide stubs. The two stubs are located at the top and bottom walls of the cavity while always being parallel and in-line to each other. Such a two-stub arrangement can be seen as a TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">110</sub> mode cavity, which is embedded into the main dual-mode cavity. By properly setting the polarization of the stubs with respect to the main cavity as well as finely adjusting the difference between the lengths of the stubs, triple-resonance structures with selected topologies and controllable coupling mechanisms can be implemented. The resulting third-order filtering functions comprise three poles and two zeros, the latter being either group delay equalization zeros or finite frequency transmission zeros depending on the polarization of the stubs. The experimental results of a fifth-order filter with two finite frequency transmission zeros using one stubbed waveguide cavity between two standard cavities validate the new configuration.

Tri-Layered Stacked Substrate Integrated Waveguide Bandpass Filter Using Non-Resonant Nodes Excitation

Tri-layer stacked substrate integrated waveguide (SIW) bandpass filters (BPFs) using non-resonant nodes based on single-and dual-mode cavities are presented. Two identical single-mode (TM110) cavities in the top and bottom layer are independently coupled to a dual-mode (TM120/TM210) cavity in the middle layer by two slots. Based on carefully designed coupling topology, we can realize SIW BPFs with one or two flexible finite transmission zeros (FTZs). TM110 and TM220 modes in the dual-mode cavity are further used to excite non-resonant nodes by changing the position of slots, and additional FTZs in the upper and lower stopband can be obtained. In order to verify the proposed strategy, two samples with a center frequency of 10 GHz have been designed and manufactured.

A high‐performance empty substrate integrated waveguide filter with mixed electric and magnetic coupling

AbstractAn empty substrate integrated waveguide (ESIW) bandpass filter with enhanced electric and magnetic coupling is proposed in this letter. ESIW uses the air as dielectric in order to reduce the dielectric loss caused by usual medium materials. However, the existence of air cavities requires at least three layers of structure, which greatly increases the difficulty of designing complex circuits with high‐performance and multiple requirements. The proposed mixed coupling structure creates the finite transmission zeros based on ESIW technology for the first time, which makes it possible to realize complex high‐performance circuits with ESIW technology. Taking advantage of the multilayer structure of the ESIW, the proposed cross‐coupling is conveniently realized with higher frequency selectivity and lower insertion loss while maintaining the substrate of three layers and the self‐encapsulation characteristics of ESIW. A fourth‐order filter with an H‐shaped slot line on the top metal plane of two adjacent air cavities is produced. A good agreement between the measurements and simulations indicates that the proposed filter achieves an insertion loss as low as 0.7 dB and the 6.1% bandwidth at 9.85‐GHz X band. It demonstrates the improvement of insertion loss and frequency selectivity, which lays a foundation for the design of complex microwave devices.

Wideband MIMO Antenna Isolation Enhancement Using 4th-Order Cross-Coupled Decoupling Circuit

This article presents a new fourth-order stub-based decoupling circuit for isolation improvement in a wideband tightly coupled multiple-input–multiple-output (MIMO) antenna array. The proposed decoupling circuit uses a fourth-order coupling-resonator filter effect and is designed using the structure’s admittance parameter. In the decoupling circuit, the pair of dual-band second-order stub-based filters is used for the desired band’s lower and upper frequencies. Finite transmission zeros provide cross-coupling, which helps maintain the admittance slope stopband between these bands and reduces coupling among MIMO elements. The proposed fourth-order decoupling circuit concept can be utilized for different wideband MIMO antenna applications to redesign the targeted band and cross-coupling scenario properly. In addition, to demonstrate the proposed decoupling circuit concept’s influence in achieving good wideband isolation in a compact dimension of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$45\times 40\times1.6$ </tex-math></inline-formula> mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> , octagonal- and circular-shaped patch MIMO antennas are analyzed. To analyze the impact of the proposed decoupling circuit of the dimension <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$11.3\times11$ </tex-math></inline-formula> mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> in the design, edge-to-edge separation between MIMO elements is kept at a minimum of 0.2 mm ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.0035\lambda _{0 \thinspace }$ </tex-math></inline-formula> at 5.4 GHz, WLAN band). This demonstrates that the decoupling circuits are antenna-independent and can be applied to other MIMO antennas. The measured S-parameters of the proposed wideband MIMO antenna present high isolation (>20 dB) between the antenna elements. Moreover, the proposed solutions’ radiation behavior and diversity performance are evaluated. The MIMO antenna element measured radiation efficiency is increased from 50% to 75%. It demonstrates good properties that make it a suitable contender for advanced scientific applications.

Balanced multiple‐layer dual‐mode substrate integrated waveguide filters with controllable finite‐transmission zeros

This paper proposes three balanced multiple-layer dual-mode substrate integrated waveguide (SIW) bandpass filters. The first balanced filter has two substrate layers, and two pairs of balanced feeding ports are located on the upper substrate layer. The lower and upper dual-mode SIW cavities are coupled by two crossed slots in the middle metal layer. The second balanced filter has two substrate layers, and two pairs of balanced feeding ports locate parallelly on the upper and lower substrate layers, respectively. The third balanced filter has three substrate layers, and two pairs of balanced feeding ports locate vertically on the upper and lower substrate layers, respectively. The proposed three balanced filters can produce three, two and one finite-transmission zeros (FTZs), respectively. Characteristics of the proposed multiple-layer SIW filters with controllable FTZs' locations are analysed in detail. For the demonstration, some design examples with the centre frequency of 7.5 GHz and bandwidth of 240 MHz are presented, and three design examples are fabricated to verify the performance of the proposed balanced filters. The measured results agree well with simulated ones.

Compact triple‐mode bandpass filters based on single perturbed substrate integrated waveguide cavity using coaxial feed

AbstractA compact triple‐mode bandpass filter (BPF) based on a modified rectangular substrate integrated waveguide (SIW) cavity is presented. The perturbation of the dominant mode (TE101) and the third resonance mode (TE102) are realized by two metalized vias in order to keep the first three resonant modes evenly distributed in the passband. Using this strategy, we can control both bandwidth and finite‐transmission zero (FTZ). The bandwidth of BPF can be controlled by simply changing the diameter of the metalized vias. Coaxial excitation is used to achieve a parallel coupling with small external quality factor, resulting in a wideband response. The proposed triple‐mode SIW BPF also has an FTZ, which is decided by the angle between two feeds. In order to verify the proposed strategy, two samples with a center frequency of 5.5 GHz have been designed and manufactured.

High-Selective Bandpass Filters Based on New Dual-Mode Rectangular Strip Patch Resonators

The resonant characteristic of rectangular strip patch resonator (RSPR) is studied in detail, and a new dual-mode resonator based on TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">100</sub> and TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">020</sub> modes is proposed to design bandpass filters (BPFs). A two-pole BPF with two finite transmission zeros (FTZs) based on one nondegenerate dual-mode RSPR is presented and analyzed. And then, a four-pole BPF with four FTZs based on two cascaded nondegenerate dual-mode RSPRs is proposed. The two dual-mode RSPRs are connected directly by a quarter wavelength strip-line. Finally, a four-pole BPF with the center frequency of 5.8 GHz is designed, fabricated, and measured. The measured insertion loss in center frequency, 1-dB bandwidth, and return loss are 1.45 dB, 206 MHz, and 12.85 dB, respectively. Measured <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$Q$ </tex-math></inline-formula> -factor is about 315.

The Extracted-Zero: A Practical Solution for Transmission Zeros in Wideband Filters

This letter presents a practical solution to realize finite frequency transmission zeros with independent location for wideband evanescent-mode filters. The method consists of using offline resonators located at the input and/or output of the filter. In contrast with the extracted-pole technique commonly employed for narrowband filters to generate a pole-zero pair, each offline resonator is used here in such a way so that the sole transmission zero is actively used in the filtering function. The latter characteristic is what makes this technique practical for wideband filters as it prevents the presence of unfeasibly strong coupling coefficients, as well as avoiding impractical nonresonating node elements. The offline resonator, which we named extracted-zero, can be coupled to the first or last resonator of a wideband bandpass filter, and it is conveniently realized using the same technology as any other resonator of the filter. The extracted-zero can be seen as a first-order notch filter which is directly embedded into the bandpass filter (instead of being cascaded afterward). The feasibility of this solution is validated by the experimental results of a challenging 225-450-MHz bandpass filter (66.6% fractional bandwidth) which employs 17 inline resonators plus 2 extracted-zeros so as to obtain 50 dB rejection below 195 MHz.

Dual mode filter with tunable cross Iris

ABSTRACT The aim of this work is to analyze the effect of tuning screws located at the cross iris of a dual-mode vacuum filled cylindrical waveguide filter. Dual-mode waveguide filters with and without cross iris screws are designed and manufactured. Their simulation and measurement results are compared. It is clear that the bandwidth of the filter (and also Finite Transmission Zero (FTZ) locations) can be adjusted with cross iris screws. So, design and mechanical production tolerances can be easily compensated depending on the technical requirements.

Balanced substrate integrated waveguide filters with controllable finite transmission zeros

Balanced substrate integrated waveguide filters with controllable finite transmission zeros