Transversal Filtering Sections Research Articles

Combined signal‐interference/split‐type microwave multi‐band bandpass filters

AbstractTwo classes of RF/microwave planar quad‐band bandpass filters (BPFs) that simultaneously exploit transversal signal‐interference and multi‐passband‐splitting mechanisms are reported. The first approach makes use of a triple‐band bandpass transversal filtering section (TFS) composed of two in‐parallel stepped‐impedance coupled‐line signal‐propagation paths. By inserting an open‐ended stub at the input and output terminals of this TFS, its middle passband is divided into two sub‐bands to attain an overall quad‐passband filtering response. The second configuration employs a dual‐passband TFS based on a coupled‐line directional coupler with open‐ended stubs at its direct and coupled ports. By loading such TFS with a replica of it at the direct and coupled ports of its coupler, the two passbands are split into four sub‐bands to obtain the quad‐band functionality. The RF operational foundations of these quad‐band BPFs, along with single‐ and multi‐stage design examples featuring sharp‐rejection four‐passband filtering responses with transmission zeros (TZs), are detailed. Moreover, for experimental‐demonstration purposes, two microstrip prototypes are developed and tested. Besides, as a further application, a two‐layer balanced‐circuit version with ultra‐broad common‐mode (CM) suppression of the constituent triple‐passband TFS of the first prototype is built and measured.

Filtering Angular Displacement Sensor Based on Transversal Section With Parallel-Coupled-Line Path and U-Shaped Coupled Slotline

An ultra-wideband (UWB) bandpass-filtering microstrip circuit with an embedded in-band transmission zero (TZ) for co-integrated angular displacement RF sensing is reported. It is based on a two-layer structure that exploits a combination of a UWB transversal filtering section (TFS) with a parallel-coupled-line (PCL) path and a U-shaped slotline acting as the stator and a semi-circular slotline acting as the rotor. As the rotor consisting of a semi-circular slotline rotates, the degree of coupling between the slotline of the stator and the rotor is altered in a contactless fashion, thus varying the stator slotline effective lengths and, hence, the in-band TZ location and its rejection depth that enable to perform the rotation-RF-sensing task. The equivalent circuit model of the engineered dual-functionality UWB bandpass filtering/ angular displacement sensing RF device is provided, together with a numerical/parametric evaluation to study its performance. The sensing capability is analyzed for an example circuit built with a UWB bandpass filtering transfer function centered at 1.8 GHz with a fractional bandwidth of 110%. Furthermore, a proof-of-concept microstrip prototype with an angular rotation range of 90° is developed and tested to validate this approach experimentally. The average frequency sensitivity in terms of the variation of the in-band TZ location is 2.27 MHz/°, showing enhanced linearity and contactless operation as added merits compared with prior-art TFS-based rotational angle sensors.

Microstrip Dual-Band Bandpass Filter With Wide Bandwidth Using Paper Substrate

This work presents the design and analysis of a novel dual-wideband bandpass filter (BPF) using the transversal filtering section (TFS), coupled lines (CLs), and stepped impedance stubs (SISs). Initially, a dual-band BPF employing TFS and CLs is designed. To enhance the bandwidth and selectivity, SIS units and quarter wavelength transmission lines are attached at the input and output ports. This novel filter's frequency response has six transmission zeros in the stopband and four transmission poles in each passband. A prototype with center frequencies of 1.17 and 2.4 GHz is realized on a biodegradable substrate and tested. The tested and simulated results are in high congruence, validating the proposed design circuit and theory.

Low-Reflection Signal-Interference Single- and Multipassband Filters With Shunted Lossy Stubs

Signal-interference single- and multiband bandpass-type transversal filtering sections (TFSs) with input- and two-port low-reflection (LR) behavior are reported. These LR TFSs consist of the following three parts: 1) a reflective-type TFS shaped by two in-parallel unequal length transmission line segments; 2) a lossy open-ended stub shunted at the input or both at the input and output nodes of the building reflective-type TFS for the input- and two-port-LR TFS cases, respectively; and 3) input/output power-matching transmission line sections. In this manner, the stopband RF-signal energy that is not transmitted by the reflective-type TFS is dissipated by the resistor(s) of the lossy open-ended stub(s) instead of being reflected back to the source. This allows LR capabilities to be obtained in the overall modified TFSs. The operational principles of the proposed input- and two-port-LR TFSs, along with theoretical design examples and their comparison, are presented. Moreover, multi-TFS-stage LR bandpass filters (BPFs) are shown. They are based on the in-series cascade of several LR TFSs to enlarge the bandwidth and power rejection levels of the stopbands while keeping their LR nature. For experimental validation purposes, microstrip prototypes of a three-TFS-stage BPF with broadened input-LR stopbands and a quintuple-band input-LR TFS are developed and characterized.

An Angular-Displacement Microwave Sensor Using an Unequal-Length-Bi-Path Transversal Filtering Section

A class of angular-displacement microwave sensor based on a microwave signal-interference transversal filtering section (TFS) with unequal-electrical-length paths is reported for the first time. It consists of a bi-path TFS composed of two in-parallel transmission-line segments with distinct lengths, which is modified by adding a rotational open-circuit-ended stub as the rotor to a curved section as the stator of one of its line segments. In this manner, the spectral positions of the transmission zeros, which are produced in its filtering transfer function through destructive signal-energy interactions, are modified with the rotation of this additional stub. This allows single/multi-band angular displacement microwave-sensing capabilities in terms of transmission-zero inter-spacing variation for passbands/stopbands and stopband-peak-level modification to be attained by means of the mechanical rotation of the rotor. The theoretical operational principles of this type of bi-path-TFS-based angular-displacement microwave sensor and design curves for specific values of its electrical parameters are provided. Moreover, for experimental validation purposes, a proof-of-concept prototype of the devised TFS-based microwave sensor approach operating at 961 MHz with 180° dynamic range is prototyped in microstrip technology and measured. A comparative analysis of the developed angular-displacement microwave sensor with the state-of-the-art is also presented.

Lossy Signal-Interference Filters and Applications

Lossy signal-interference transversal filtering sections (TFSs) composed of two in-parallel transmission-line segments with input-output matching lines-reflective-type part-and open-ended stubs with input resistors-lossy part-are presented. First, in their bandstop and dual-band bandpass versions, this class of lossy TFS is applied to the design of filtering networks with input-/two-port-quasi-reflectionless (QR) stopbands, such as wide-band bandstop filters (BSFs) and dual-band bandpass filters (BPFs). Furthermore, multistage circuit architectures made up of equal and dissimilar lossy TFSs for higher rejection and broader stopband realization, respectively, and higher selectivity dual-band BPF synthesis are described. Subsequently, in its bandpass counterpart, the conceived approach of lossy TFS is exploited to develop single-/multistage sharp-rejection bandpass filters (BPFs) with a flattened transmission band. The operational foundations of the engineered types of lossy TFSs for the suggested applications and theoretical design examples are provided. In addition, for experimental-validation purposes, four microstrip filter prototypes are manufactured and characterized, as follows: 2-GHz two-stage symmetrical QR BSFs with similar and frequency-contiguous bandstop TFSs for higher rejection and broader stopband shaping, respectively, a 1.4/2.6-GHz input-QR dual-band bandpass TFS, and a 2-GHz five-stage lossy BPF with enhanced passband amplitude flatness and spectrally enlarged attenuated bands.

Hybrid surface‐acoustic‐wave/microstrip signal‐interference bandpass filters

Radio-frequency (RF) design techniques for the development of signal-interference microwave bandpass filters (BPFs) with hybridised microstrip and surface-acoustic-wave (SAW) elements are reported. They make use of two original types of transversal filtering sections (TFSs) with embedded one-port SAW resonators. The SAW resonators operate either as non-resonating nodes or as an in-band SAW BPF in their bi-path circuit topologies. These new TFS approaches extend the suitability of signal-interference BPFs to small-fractional-bandwidth (FBW) applications, while offering significant advantages in terms of filtering selectivity and occupied circuit size when compared to traditional solutions for narrow-to-moderate/wide-band specifications. Broader FBWs than those attainable through most of the available all-acoustic-wave BPF configurations are also feasible with them. Moreover, a single type of SAW device is employed in the entire BPF structure, which results in a practical benefit regarding robustness to deviations in the prototype fabrication and assembly processes. The proposed filter design concepts are experimentally verified through the fabrication in microstrip technology and characterisation of two multi-TFS-series-cascade-based high-order BPF prototypes with very-narrow and moderate FBWs.

Sharp-Rejection Wide-Band Dual-Band Bandpass Planar Filters With Broadly-Separated Passbands

This letter focuses on the application of signal-interference principles to design planar dual-band bandpass filters (BPFs) with widely-spaced broad passbands. To this aim, two classes of dual-band bandpass transversal filtering sections (TFSs) shaped by directional couplers with double loading stubs are used. Analytical equations for their transmission zeros (TZs), leading to sharp-rejection and high-selectivity performances in their filtering behavior, are given. Besides, design curves to adjust the center frequencies and bandwidths of their dual passbands are provided. Furthermore, two demonstrative microstrip dual-band BPF prototypes are manufactured and characterized.

Low-Pass and Bandpass Filters With Ultra-Broad Stopband Bandwidth Based on Directional Couplers

The design of planar sharp-rejection low-pass and bandpass filters (LPFs and BPFs) featuring a very broad stopband is addressed in this paper. For the LPF case, two new classes of bandstop signal-interference transversal filtering section (TFS) consisting of power directional couplers in reflection mode are proposed. In the BPF application, additional short-circuit stubs are inserted at the TFS input/output accesses. Based on these filtering cells, the devised LPF and BPF approaches are then derived from the cascade of multiple TFSs with frequency-contiguous stopbands. Thus, an overall attenuated band showing an ultra-large spectral width and deep rejection levels through the generation of several transmission zeros (TZs) is synthesized. Furthermore, such unprecedented filtering performances are obtained for much lower circuit complexity than available solutions. The operating principle of the conceived LPF and BPF topologies and guidelines for their realization are detailed. This includes formulas for their analytically designable TZs. For practical validation, three LPF prototypes and one wideband BPF circuit are also fabricated in microstrip technology and tested.

Extended‐stopband microstrip lowpass filter using rat‐race directional couplers

A class of microstrip lowpass filter (LPF) with a sharp-rejection, ultra-broad stopband is reported. It employs rat-race directional couplers properly arranged to operate as bandstop transversal filtering sections (TFSs). The theoretical foundations of this LPF approach are described, and the performance control of its building bandstop TFS is verified. Moreover, a proof-of-concept microstrip LPF prototype with a 3 dB cutoff frequency of 1 GHz and a 20 dB stopband up to 5.86 GHz has been built and tested.

Low-loss W-band CPW bandpass filter using annular CPW branch-line coupler with open load stubs

A W-band coplanar waveguide (CPW) low-loss annular branch-line bandpass filter for planar integrated millimeter-wave circuits is presented. It is designed by using the annular branch-line coupler as a transversal filtering section by loading the coupled ports of the coupler with suitable open load stubs and taking the isolated port as the output node. The proposed W-band bandpass filter shows very good performance, insertion-loss of 0.7 dB with a 14.9% of 3 dB relative bandwidth at center frequency of 94 GHz and return loss is better than −27 dB at the center frequency. The designed and fabricated bandpass filter exhibits the good performance for monolithic integrated millimeter-wave circuit applications. © 2011 Wiley Periodicals, Inc. Microwave Opt Technol Lett 53:2400–2403, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26264

Highly selective bandpass filters using broadside‐coupled coplanar waveguide directional couplers

AbstractIn this work, a new type of wideband coplanar bandpass filter with sharp skirts and large stopband is proposed. The filter exhibits a flat response in the passband, and a steep slope and a high rejection in the stopband. The filter design is based on the use of a broadside‐coupled (BC) coplanar‐waveguide (CPW) directional coupler as a transversal filtering section (TFS). This is performed by conveniently loading two ports of the coupler with shunt stubs, so that the signals in the output port interfere in a way that a bandpass filter response is obtained. The suggested filter scheme has been validated experimentally throughout the design, fabrication, and measurement of a wideband coplanar bandpass filter centered at 2.0 GHz, formed by two TFS connected in cascade. © 2008 Wiley Periodicals, Inc. Microwave Opt Technol Lett 51: 294–298, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24029

High-rejection wideband signal-interference microstrip filters using rat-race couplers

A new type of sharp, high-rejection wideband microstrip filter based on signal-interference techniques is reported. The proposed topology uses, for the first time, rat-race couplers operating as transversal filtering sections (TFSs). Design equations and guidelines for this TFS are provided. Furthermore, to prove its experimental usefulness, a 6.5 GHz ultra-wideband prototype has been synthesised, constructed and measured.