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The theoretical design and practical development of RF analog filtering devices with co-integrated lowpass, single $\boldsymbol {/}$ multi-band bandpass, and highpass transfer functions is reported. For this purpose, two different classes of generalized frequency transformation that convert the equivalent normalized lowpass filter prototype into the desired RF filter with several co-designed filtering actions are proposed. They realize a frequency mapping of the reactance of a normalized unitary capacitor into that of a one-port single $/$ multi-resonance cell with added lowpass and highpass filtering capabilities. For these devised frequency transformations, the theoretical foundations and various illustrative filter examples designed at the ideal-circuit-model level are presented. In addition, higher-selectivity filter architectures based on the generation of additional out-of-band transmission zeros (TZs) by means of cross-coupling techniques are shown. Afterwards, the extension of this design methodology to RF multi-functional filtering components, such as input-reflectionless $\boldsymbol {/}$ absorptive filters based on complementary-diplexer circuit networks and two-way filtering power-distribution circuits, is also demonstrated. Furthermore, two design examples of distributed-element and inverterless lumped-element RF filters are provided. Besides, for the distributed-element circuit, a proof-of-concept microstrip prototype is manufactured and measured as experimental validation.

The direct synthesis of a family of digital-filter-based RF analog circuits with transmission zeros (TZs) and constant-group-delay behavior is reported. The adopted RF circuit consists of a transversal filtering network composed of transmission-line segments that are connected through a multi-way power divider and combiner at its input and output terminals, respectively. The synthesis procedure starts by obtaining the coefficients of a finite-impulse-response (FIR) digital filter of interest and finalizes with the determination of the electrical parameters of the RF circuit. In particular, designs corresponding to Dolph-Chebyshev windows are addressed in this brief. For experimental-demonstration purposes, two 5-GHz proof-of-concept microstrip prototypes of a symmetrical-quasi-reflectionless bandpass filter (BPF) with Dolph-Chebyshev-type response and the in-series cascade connection of two replicas of it are designed, manufactured, and tested.

This Special Issue of the IEEE Journal on Emerging and Selected Topics in Circuits and Systems (JETCAS) is dedicated to demonstrating the latest research progress on integrated devices, circuits and systems for the 6G Era. As 5G rolls out worldwide, teams of visionary experts are developing roadmaps and revolutionary applications for the next-generation wireless network: 6G. Indeed, the 6G mobile networks will establish new standards to fulfill the unreachable performance required by the current 5G networks. It is anticipated that 6G technology will be capable of supporting extremely high-performance connectivity with massive numbers of connected devices.

This paper presents an overview of Silicon-based millimeter-wave (mm-wave) passive devices for bandpass and bandstop filtering applications, while also reporting originally-conceived filter developments and future trends. First of all, the state-of-the-art on mm-wave low-loss bandpass filters (BPFs) is covered, and new BPF designs are shown. The engineered BPFs employ a center-tapped ring architecture with shunt-connected capacitors to realize a standard 2nd-order baseline BPF design, which is subsequently scaled to 30-GHz and 60-GHz operational frequencies. To increase the selectivity as well as the stopband rejection levels of this baseline BPF, the in-series cascade connection of the baseline BPF units is used for a higher-order BPF realization. For experimental-validation purposes, a total of four mm-wave BPFs based on these design strategies are implemented, fabricated in 45-nm Silicon-on-Insulator (SOI) complementary-metal-oxide-semiconductor-(CMOS) technology, and tested. Afterward, a review of Silicon-based-integrated bandstop filters (BSFs) operating in the mm-wave region is provided, which includes both reflective-type and reflectionless/absorptive filter realizations for RF-interference-suppression in highly-congested electromagnetic (EM) environments. Finally, future research trends in the Silicon-based-integrated filter area are discussed. They are expected to play a key role in the development of modern radio-frequency (RF) front-ends for emerging beyond 5G and EM-sensing scenarios.

In this work, a family of frequency-variant reactive coupling (FVRC) networks is introduced and discussed as new building blocks for the synthesis of coupled-resonator bandpass filters with real or complex transmission zeros (TZs). The FVRC is a type of nonideal frequency-dependent inverter that has nonzero elements on the diagonal of the impedance matrix, along with a nonlinear frequency-variation profile of its transimpedance parameter. The distinctive feature of these new FVRCs is that they can be modeled with a simple bridged-T network consisting of four reactive elements. Such FVRCs can introduce one pole and up to two TZs—so that they are referred to as double-zero single-pole (DZSP) networks. DZSP networks allow the pole to be located above, below, or in-between the TZs. Depending on the choice of the elements in the bridged-T circuit, the position of the TZs and the pole can be controlled independently. The coupling matrix for filters with DZSP networks can be found by solving an inverse-structured nonlinear eigenvalue problem (ISNEVP). Examples of possible implementations of DZSP coupling networks are provided for both lumped and quasi-lumped-element circuits, as well as for transmission-line-based and waveguide technologies. The application of DZSP coupling networks to the design of microwave bandpass filters with generalized Chebyshev-type characteristics is illustrated by means of the synthesis of three microstrips and two waveguide coupled-resonator bandpass filters. The waveguide filters are verified through electromagnetic (EM) simulations and the microstrip filters are manufactured and characterized for proof-of-concept demonstration purposes, showing a fairly close agreement between EM simulations and measurements.

A high-selectivity hybrid-technology bandpass filter (BPF) at the K band using an inverted microstrip gap waveguide (MGW) approach is reported. The resonant frequencies of the modes TE102 and TE103 (i.e., $f_{102}$ and $f_{103}$ ) of its non-perturbed groove gap waveguide cavity resonator—as example TE resonant modes being contained within the extracted stopband range of the mushroom-type EBG cells—are firstly analyzed. By perturbing the referred cavity with a pin-loaded patch, a second-order BPF block with an adjustable transmission zero (TZ) is implemented. Moreover, by adding a source-load coupling structure among its inverted microstrip feeding sections, one more close-to-passband TZ is created. Next, in order to further increase the filter order and the number of stopband TZs, a stub-loaded microstrip ring resonator is introduced. In this way, by exploiting the perturbed- groove-gap-waveguide-cavity-resonator-based second-order BPF block along with the microstrip ring resonator, a three-pole BPF response is achieved. Furthermore, owing to the multiple signal paths being created in this mixed-technology filter, two additional close-to-passband TZs are produced. Consequently, an overall sharp-rejection third-order BPF with four TZs is realized. The RF operational foundations of the devised mixed-technology BPF are detailed. Besides, for practical-validation purposes, a third-order BPF prototype is developed and tested. The measured BPF circuit exhibits a passband centered at 21.21 GHz with 3-dB fractional bandwidth (FBW) of 4.72%, four out-of-band TZs, and stopband power-attenuation levels above 17.68 dB from 18.65 to 25.7 GHz.

This work presents the design techniques to synthesize a class of compact inline quasi-elliptic-type waveguide cavity bandpass filters based on novel nonlinear frequency-variant couplings (NFVCs). These highly dispersive frequency-variant couplings (FVCs) are realized by means of a pair of partial-height posts that are placed at the junctions between every two cavity resonators. Each NFVC produces a transmission pole in between a pair of independently adjustable transmission zeros (TZs). Although the pole is added to the overall filtering function to augment its order, the TZs can be placed at each side of the filter passband to attain sharp rejection capability and increase the stopband attenuation levels. To synthesize these filters, two coupling-routing-diagram (CRD) approaches for the NFVC are presented that either consider: 1) an arbitrary FVC (AFVC) or 2) two resonating nodes interacting with a zero-susceptance nonresonating node through constant inverters. An equivalent lumped-element circuit model associated with both CRD approaches is provided. It is demonstrated that both CRD models can be exploited for the theoretical synthesis of this type of filter, whereas the equivalent lumped-element circuit model can provide a deeper insight into the systematic dimensioning of the posts. For experimental validation purposes, three design examples are synthesized, and 10-GHz proof-of-concept filter prototypes of two of them are EM-simulated, fabricated, and characterized. The measured results agree well with the simulations and the design theory, thus verifying the concept of inline waveguide cavity filters with TZs using NFVCs.

Various families of flat-in-band-group-delay RF planar bandpass filters (BPFs) with multiple out-of-band transmission zeros (TZs) in their filtering transfer function are presented. As the first approach, a type of BPFs with theoretically-constant group-delay profile and perfect two-port-reflectionless behavior is proposed. The theoretical sufficient conditions to obtain a frequency-constant group delay in a generalized form of transfer function for these BPFs are derived. They are then particularized in transmission-line-based transversal-circuit realizations with flat-in-band-group-delay and quasi-reflectionless characteristics. Afterwards, a class of reflective-type BPFs based on directional power couplers that are arranged in transversal-mode topologies are addressed toward its practical demonstration for the first time. These BPFs, which exhibit flat in-band group delay, are compared with their classic Bessel-type BPF counterparts without TZs. Moreover, discrete-time models of such BPFs are derived, analyzed, and discussed. For experimental-validation purposes, proof-of-concept microstrip prototypes centered at 2.5-GHz of the engineered quasi-absorptive/reflective-type flat-in-band-group-delay BPFs are designed, manufactured, and characterized.