We report a self-biased, low-profile circulator operating in the Ka-band fabricated by an entirely low-temperature (70 °C) screen-printing approach on printed circuit board (PCB). The magnetic material for the circulator is a barium hexaferrite BaFe12O19 (BaM)/polydimethylsiloxane (PDMS) nanocomposite that exhibits a zero-bias ferromagnetic resonance frequency ( $f_{\mathrm {FMR}}$ ) at 46.6 GHz. Using this material, a microstrip-based circulator operating at 35 GHz is designed, fabricated, and characterized. A combination of mechanical milling, screen printing, photolithography, and electroplating is used for fabricating the circulator, in which the circulator disk (diameter of 2 mm and thickness of $250~\mu \text{m}$ ) is completely embedded in the PCB, realizing a packaging compatible low-profile architecture with the total device area of 33 mm2. The measured isolation (IS) and insertion loss (IL) of the fabricated circulator at 35 GHz is 3.9 and 8.7 dB, respectively. When the additional magnetic bias is applied to the circulator using external permanent magnets, the IS and IL performance is improved to be 7.4 and 8.4 dB, respectively. The impact of the loss factors associated with dielectric loss and surface roughness on the device performance is analyzed using High Frequency Structural Simulator (HFSS).

35-GHz Barium Hexaferrite/PDMS Composite-Based Millimeter-Wave Circulators for 5G Applications

This article reports on the design and practical development of RF co-designed MMIC components that exhibit the function of a bandpass filter and an RF isolator. They are based on cascaded non-reciprocal frequency-selective stages (NFSs), one-pole/two-transmission zero (TZ) multi-resonant stages, and impedance inverters. The combination of these components results in a two-port network that exhibits a nonreciprocal quasi-elliptic bandpass filter (NBPF) response in the forward direction and full RF signal cancellation in the reverse one. The operating principles of the NBPF are presented through various circuit-based design examples of low- and high-order configurations. For proof-of-concept demonstration purposes, NBPF prototypes were designed on an MMIC GaAs process and were experimentally validated at $X$ -band. They include a two-pole/two-TZ NBPF and a three-pole/four-TZ NBPF.

X -Band Quasi-Elliptic Non-Reciprocal Bandpass Filters (NBPFs)

This article reports on the RF design and practical development of RF co-designed bandpass filters/isolators (BPFIs). They are based on series-cascaded nonreciprocal resonant stages, microwave resonators, and multiresonant cells. The nonreciprocal stages are shaped by an in-parallel cascaded transistor-based path and a transmission line (TL) that result in a zero-phase resonance in the forward direction and high isolation (IS) in the reversed one. The operating principles of the nonreciprocal resonant stage are presented through circuit analysis and coupled-resonator diagrams (CRDs) that facilitate their design with coupled-resonator-based filter design methods. Various CRD-synthesized examples of low- and high-order transfer functions with and without transmission zeros are shown. They demonstrate the applicability of the proposed method for alternative-type of filtering responses with and without gain in the forward direction and high levels of IS in the reversed one. For proof-of-concept demonstration purposes, five prototypes were designed, manufactured, and measured at $S$ -band.

RF Co-Designed Bandpass Filters/Isolators Using Nonreciprocal Resonant Stages and Microwave Resonators

This article presents a study of the impact of different external magnetic bias field (MBF) distributions on Y-junction microstrip ferrite circulator performance. By using magnetostatic, full-wave, and circuit commercial simulation tools for a circular ferrite disk, several permanent magnet (PM) configurations are investigated for unsaturated and nonuniform bias conditions. Eigenmode simulations are used to analyze the circulator bandwidth over a range of MBF intensities. We show in simulations and experiments that different bias conditions can result in an operating frequency range from 1.5 to 7GHz using the same 9.7-mm-diameter commercial ferrite disk resonator.

Microstrip Ferrite Circulator Design With Control of Magnetization Distribution

In this paper, a simple optical vector quadrature de-multiplexer (QD) scheme is proposed for de-aggregating input 10 Gbaud 16/32/64 quadrature amplitude modulation (QAM) signals into two pulse amplitude modulation (PAM) streams. The proposed QD is based on a single bi-directional degenerate phase-sensitive amplifier (PSA), where the wavelength and the polarization status of the extracted in-phase and quadrature components stay the same as the input signal. Since the proposed QD is realized using PSA based on semiconductor optical amplifier (SOA), it is also possible to realize an on-chip QD system, providing an integrated platform for optical signal processing. Through numerical simulation, the transfer characteristics of the proposed QD system show that the SOA-based PSA has a high phase-sensitive gain extinction ratio of 44.1 dB. The constellations, error vector magnitudes (EVMs), and bit-error rates (BERs) of the data streams after the de-aggregation are numerically investigated to verify the proposed QD scheme. For the 10 Gbaud 16/32/64 QAM signals with an input optical signal-to-noise ratio (OSNR) of 25 dB, the de-aggregated PAM4/PAM6/PAM8 signals show 6.1, 5.6, and 8.2 dB receiver OSNR improvements at the BER of 10−3, respectively. Also, to optimize the performance of the subsystem, the BER performance dependence on the phase difference between two arms in the subsystem is also examined. The simulation results reveal that the proposed QD can accomplish the function of optical vector de-aggregation well for the high-level QAM signals. The proposed QD can be applied to information de-aggregation, format conversion, and direct detection for optical vector signals, which may have great potential values for the flexible optical networks.

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On-Chip Optical Vector Quadrature De-Multiplexer Proposal for QAM De-Aggregation by Single Bi-Directional SOA-Based Phase-Sensitive Amplifier

This paper reports on a new class of miniaturized RF circulators with bandpass filtering capabilities. They are based on capacitively-loaded ferrite disk resonators that occupy a 64% smaller area than conventional RF circulator architectures. Frequency selectivity is achieved by introducing four transmission zeros (TZs) below and above the circulator passband. The spectral locations of the TZs are controlled by i) the position of the capacitive load on the ferrite-based disk and by ii) incorporating resonant-type external coupling elements to its input, output and isolated ports. In this manner, non-reciprocal quasi-elliptic type power transmission responses can be obtained, which eliminate the need for additional RF filtering elements in the RF front-end. For proof-of-concept validation, two microstrip prototypes are designed, built and measured at 2.5 GHz, achieving 2.2 and 1.4 dB minimum insertion loss, and 33.4 and 30 dB maximum isolation in the pass-band and are presented in this manuscript for the first time.

Frequency Selective Ferrite Circulators with Quasi-Elliptic Transmission Response

A modeling approach for the design of microstrip ferrite circulators is presented and validated on several examples using the same commercially-available ferrite disk. A baseline narrowband 4.2S-GHz microstrip circulator is first demonstrated with a commercial 4.97-mm radius ferrite disk operated in saturation and below the ferromagnetic resonance. The non-uniform DC magnetic field distributions of a cylindrical permanent magnet is taken into accout by spatial discretization of the ferrite properties in full-wave simulations. Several design parameters are shown to affect the frequency response; the ferrite thickness relative the microstrip substrate thickness shifts the operating frequency, while external matching networks can increase the fractional bandwidth from 10% up 40%. Another degree of freedom is the applied DC magnetic field, which can be reduced to set the ferrite operation below the ferromagnetic resonance with significant miniaturization of the overall device, as demonstrated with a 1.6-GHz circulator designed with the same 4.97-mm radius ferrite disk, resulting in an almost factor of 3 reduction in linear electrical size.

Design-Oriented Modelling of Microstrip Ferrite Circulators

This letter reports on a new type of miniaturized RF co-designed bandpass filter/circulator (BPFC). Miniaturization is achieved by: 1) capacitively loading a multi-mode ferrite-based disk resonator; 2) combining the function of an RF circulator and a bandpass filter (BPF) within the volume of a single microwave component; and 3) by tuning its transfer function. The proposed BPFC exhibits a quasi-elliptic power transmission response in the forward direction—passband in-between four transmission zeros (TZs)—and an all-stop response in the reverse one. Reconfigurability is achieved by tuning the capacitive loads of the ferrite disk and the mixed electromagnetic coupling elements in the RF input/output. For proof-of-concept validation purposes, an S-band tunable prototype was designed, built, and measured. It demonstrated multiple levels of tuning. These include the frequency tuning of 1.27:1, the bandwidth tuning of 1.73:1, the out-of-band isolation tuning, and intrinsic switching-off.

Andrea Ashley

Papers

X -Band Quasi-Elliptic Non-Reciprocal Bandpass Filters (NBPFs)

RF Co-Designed Bandpass Filters/Isolators Using Nonreciprocal Resonant Stages and Microwave Resonators

Frequency Selective Ferrite Circulators with Quasi-Elliptic Transmission Response

Design-Oriented Modelling of Microstrip Ferrite Circulators

RF Co-Designed Bandpass Filter/Circulator With Tunable Center Frequency, Bandwidth, and Out-of-Band Isolation