RF Design Research Articles

RF design and frequency sensitivity analysis of a 1500 MHz passive third harmonic superconducting cavity for the SILF

To reduce Touschek scattering in the beam and extend the lifetime of the beam in the Shenzhen Innovation Light-source Facility (a fourth-generation light source), a 1500 MHz passive third harmonic system will be used. The present work is devoted to the RF design and frequency sensitivity analysis of the 1500 MHz passive third harmonic superconducting cavity, optimizing the RF parameters of the cavity, and giving a frequency scheme for the cavity in fabrication. The reliability of the cavity during operation is taken as the main goal for the optimization of the geometrical parameters, and its structural and RF parameters are obtained with E peak/E acc of 2.17, B peak/E acc of 5.12 mT/(MV/m) and G · R/Q of 25052 at 2 K. A pair of fluted beam pipes are used to propagate the higher order modes (HOMs) of the harmonic cavity, allowing a smooth transition of the first dipole mode to the absorber located at room temperature area. For the frequency sensitivity analysis, the frequency variations of the cavity in fabrication are obtained by multi-physics coupling simulation in the paper, which gives the target frequency of the cavity between welding and preloading. After electron beam welding (EBW), the resonant frequency of the cavity should be maintained at 1500.096 MHz, and the pre-tuning target frequency is 1497.456 MHz. The slow tuning range is ±400 kHz, the fast-tuning range is 600 Hz, and the maximum allowable tuning force is 15 kN.

PACOSYT: A Passive Component Synthesis Tool Based on Machine Learning and Tailored Modeling Strategies Towards Optimal RF and mm-Wave Circuit Designs

In this paper, the application of regression-based supervised machine learning (ML) methods to the modeling of integrated inductors and transformers is examined. Different ML techniques are used and compared to improve accuracy. However, it is demonstrated that none of the ML techniques considered provided good results unless a smart modeling strategy, tailored to the specific design problem, is used. Taking advantage of these modeling strategies, high accuracy can be obtained when compared to full-wave electromagnetic (EM) simulations (less than 2% error) and experimental measurements (less than 5% error). The most accurate model, obtained by the appropriate combination of an ML technique and modeling strategy, has been integrated into a tool called PACOSYT. The tool uses optimization algorithms to allow the designer to obtain an inductor/transformer with optimal performances in just seconds while keeping the accuracy of EM simulations. Furthermore, the tool provides the passive component S parameter description file for seamless use in circuit simulations. The tool can be used standalone or integrated with design frameworks, like Cadence Virtuoso or AIDASoft, a framework for circuit optimization. To illustrate the different usages of the tool, several passive devices are synthesized, and hundreds of millimeter-wave power amplifiers are synthesized using AIDASoft together with PACOSYT. The tool has been developed using open-source Python frameworks and does not use any closed-source licenses. PACOSYT, which also allows other designers to create their models for different technologies, is made publicly available.

High-Order Quasi-Elliptic-Type Single-Ended and Balanced Wideband Bandpass Filters Using Microstrip-to-Microstrip Vertical Transitions

High-order single-ended and balanced wideband bandpass filters (BPFs) with quasi-elliptic-type (QET) responses are reported. Firstly, a fifth-order wideband BPF based on a two-layer microstrip-to-microstrip vertical transition with two shunt open-circuit-ended two-section microstrip stubs at the input and output ports, respectively, is designed. Compared to a typical third-order vertical transition, two more transmission poles (TPs) along with a pair of close-to-passband transmission zeros (TZs) are attained. Next, an application to design a ninth-order QET balanced wideband BPF using a vertical transition is carried out. Owing to the used slotline resonator, an intrinsic common-mode (CM) suppression capability is obtained for it. The RF operational and design principles of these BPF devices are detailed by means of their relevant transmission-line-(TL)-based equivalent circuits. For experimental-validation purposes, two microstrip prototypes corresponding to a 3-GHz fifth-order wideband BPF and a 2-GHz ninth-order balanced wideband BPF are developed and tested. The measured single-ended and balanced BPF circuits feature sharp-rejection QET responses with two close-to-passband TZs and stopband-power-attenuation levels above 27.3 dB and 43.66 dB, respectively. The measured CM suppression levels for the balanced BPF device are higher than 34.59 dB from DC to 5 GHz.

Preface

4th National Conference on Communication Systems (NCOCS-2022) December 23rd, 2022 ABOUT THE CONFERENCE National Conference on Communication Systems (NCOCS-2022) is fourth in the series of conferences intended to be held every year at NITPY, aimed to disseminate knowledge on the latest technological developments and cutting-edge research in wireless communication, signal processing, antenna and RF design, optical communication, internet of things and sensor networks & electronics among academicians, researchers, scientists, industry personnel, and entrepreneurs across India. This conference will provide the premier interdisciplinary forum relevant with Electronics and Communication Engineering to discuss the recent technologies, innovations, recent research findings, challenges, solutions, and perspectives of the future directions of Communication Engineering through keynote lectures and paper presentations. Young researchers are encouraged to participate and utilize the opportunity to network with others who seek to develop their research abilities and experience. OBJECTIVE & SCOPE OF THE CONFERENCE The objective of the NCOCS 2022 is to provide a platform to perceive, share, and exchange the recent trends related to various aspects of Communication Engineering. The main aim of this conference is to bring together researchers, young faculties and practitioners working in multi-disciplinary fields to discuss and deliberate on challenges, opportunities and strategies involved for next-generation communication systems. The conference will provide a forum to academic researchers and practitioners to discuss critical issues concerning latest technologies towards 6G and mmWave communications. Conference Chairperson Dr. M. Surendar Assistant Professor, Department of ECE,National Institute of Technology Puducherry, Karaikal – 609 609, IndiaConference Website: https://sites.google.com/view/ncocs-2022/home?authuser=0List of Conference Committee is available in this Pdf.

Multiband Magnetless Isolators and Circulators With Reconfigurable Bandpass Filtering Capabilities

This article reports on the RF design and practical validation of new classes of magnetless multiband isolators and circulators with reconfigurable bandpass filtering capabilities. They are based on transversal frequency-selective signal paths shaped by frequency-tunable and spatiotemporally modulated resonators. Each path creates a passband that can be independently controlled in terms of frequency and direction of propagation. Thus, the overall network may exhibit 1-to- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$K$ </tex-math></inline-formula> frequency-reconfigurable non-reciprocal passbands. The operating principles and schematic of the magnetless and reconfigurable filtering isolator concept are first demonstrated through circuit-based simulations of two- and three-path transversal filtering networks (i.e., two- and three-band networks). The independent modulation parameters of the bands are determined through detailed parametric analyses. Next, the concept is extended to the design of magnetless, multiband filtering circulators. The operating and design principles of the circulator are expounded through a circuit schematic and an ideally simulated dual-band example. For practical demonstration purposes, three lumped-element (LE) prototypes, two isolators, and one circulator, shaped by two or three RF signal paths (i.e., two and three passbands), were implemented at very high frequency (VHF) band. They exhibited a maximum in-band isolation (IS) up to 50 dB. Moreover, a frequency tuning of up to 1.22:1 and change of directionality are achieved.

RF Design and Measurements of a C-Band Prototype Structure for an Ultra-High Dose-Rate Medical Linac

In this paper, we illustrate the RF design and measurements of a C-band prototype structure for an Ultra High Dose Rate medical linac. (1) Background: FLASH Radiotherapy (RT) is a revolutionary new technique for cancer cure. It releases ultra-high radiation dose rates (above 100 Gy/s) in microsecond short pulses. In order to obtain a high dose in a very short time, accelerators with high-intensity currents (the order of 100 mA peak currents) have to be developed. In this contest, Sapienza University, in collaboration with SIT-Sordina IORT Technology spa, is developing a new C-band linac to achieve the FLASH regime. (2) Methods: We performed the RF electromagnetic design of the prototype of the C band linac using CST STUDIO Suite Code and the RF low power RF test at Sapienza University of Rome. The measurements of the field in the cavity have been done with the bead-pull technique. (3) Results: This device is a nine-cell structure operating on the π/2 mode at 5.712 GHz (C-band). We report and discuss the test measurement results on a full-scale copper prototype, showing good agreement with CST RF simulations. A tuning procedure has been implemented in order to ensure proper operating frequency and to reach a field profile flatness of the order of a few percent. (4) Conclusions: The prototype of a C-band linac for FLASH applications was successfully tested with low RF power at Sapienza University. The fabrication and ad hoc tuning procedures have been optimized and discussed in the paper.

Quasi-Alvarez drift-tube linac structures for heavy ion therapy accelerator facilities

Next generation heavy ion therapy and research facilities require efficient accelerating structures. Particularly, at low beam energies, right after the standard scheme of the ion source, low-energy beam transfer, and radio-frequency quadrupole (RFQ), several options for accelerating structures are available including the classic drift-tube linac (DTL), the interdigital H-mode DTL (IH-DTL), and superconducting quarter-wave resonators. These structures need to integrate the beam acceleration with the focusing channel, nowadays typically provided by permanent-magnet quadrupoles (PMQs). The frequency of operation needs to be in line with that of the RFQ structure, and it has been chosen at 750 MHz for practical considerations for the Next Ion Medical Machine Study (NIMMS) that is the application focus of this manuscript. While classic DTL structures at low ion beam energies do not provide enough space for PMQs at that frequency within a single $\ensuremath{\beta}\ensuremath{\lambda}$ period, IH-DTL structures do not provide the regular focusing channel with consequences on the beam quality. For these reasons, quasi-Alvarez drift-tube linac (QA-DTL) structures are reevaluated in this manuscript as they might fill this gap. They have not received much attention in the literature so far and therefore their design is described in detail. The design procedure presented here may serve as a blueprint for DTL design in general. In addition to the overall rf design, axial field stabilization with a new technique and multiphysics studies of the rf structure are described. A cost estimation completes the NIMMS QA-DTL study.

MMIC GaAs Isolators and Quasi-Circulators With Co-Designed RF Filtering Functionality

This paper reports on the RF design and practical development of GaAs MMIC multi-functional RF isolators and quasi-circulators with embedded filtering functionality. They are based on cascaded arrays of non-reciprocal resonators (NRRs), passive resonators, and impedance inverters. A unique transistor-based architecture is used for the realization of the NRR that exhibits an one-pole type bandpass response in the forward direction and RF signal cancellation in the reversed one. By controlling the number and type of resonators within cascaded resonator arrays, different levels of gain, out-of-band selectivity and in-band isolation can be obtained. The operating principles of the multi-functional isolator/filtering and quasi-circulator/filtering concepts are presented through synthesized and linear circuit-simulated examples. Practical design aspects using a commercially available GaAs MMIC process are also discussed. For proof-of-concept demonstration purposes, four prototypes were designed and experimentally validated at X-band. They include: i) a NRR, ii) a two-resonator based frequency selective isolator (FSI), iii) a five-resonator based FSI, and iv) a two-pole/two-TZ frequency selective quasi-circulator (FSQC).

CSRR DGS-Based Bandpass Negative Group Delay Circuit Design

The unfamiliar negative group delay (NGD) circuit is the less familiar function for most of RF and microwave design engineers. Among the existing types, the bandpass (BP) NGD type circuits are the most convenient for the wireless communication microwave technology. Therefore, it is particularly important to explore different microwave circuit topologies operating as BP-NGD function. An innovative design of BP-NGD topology constituted by defected ground structure (DGS) with complementary split ring resonator (CSRR) is developed in the present paper. The DGS-based BP-NGD structure design method is introduced in function of the CSRR geometrical elements followed by S-parameter parametric analyses. As proof-of concept (POC), the design method of the proposed BP-NGD passive fully distributed circuit is described. The effectiveness of the BP-NGD structure and the test feasibility are investigated by implementing two different prototypes represented by single- and double-wing DGS passive circuits. It is observed that significant BP-NGD function performances were validated by well-correlated simulations and measurements showing -1.9 ns NGD value around the center frequency, 2.46 GHz over 31 MHz NGD bandwidth. In addition, the tested BP-NGD prototypes present insertion loss better than 4 dB and reflection loss better than 16.7 dB. Because of its potential integration, the investigated BP-NGD circuit is potentially useful for the communication system performance improvement for example via delay effect reduction in the RF and microwave devices.

Multipacting study for fundamental power coupler of a superconducting cavity

The research work of Hefei Advanced Light Facility (HALF) is in progress. A 499.8 MHz superconducting cavity will be used in the HALF RF system. The preliminary RF design and thermal analysis for the fundamental power coupler (FPC) of HALF 499.8 MHz superconducting cavity have been completed. Multipacting (MP) is one of the major causes of the breakdown in many high power input couplers and RF cavities, thus, MP simulation for our FPC is necessary. In this paper, the MP simulation for the FPC was performed by using the CST. The MP performance of the FPC was studied. The formation process of the MP in the FPC was investigated and the physical mechanism of the delay of the MP establishment was analyzed. The simulation results show that MP will occur when the power level is higher than 175 kW, and the MP location is near the ceramic. Therefore, MP suppression is required in the FPC. In order to suppress the MP, the effect of the biasing DC electric voltage to MP in the FPC was analyzed in this paper. The simulation results indicates that the biasing DC electric voltage not only can suppress the MP, but also can excites new MP in the coupler. Thus, it is crucial to select the appropriate biasing DC electric voltage for the MP suppression in the coupler. The results show that the MP in the FPC can be suppressed very well by using 2 kV or 3 kV biasing DC electric voltage.

75 Years of RF Design: Highlights and Paradigm Shifts

75 Years of RF Design: Highlights and Paradigm Shifts

Parasitic-Aware Simulation-Based Optimization Design Tool for Current Steering VGAs

Designing millimeter-wave variable gain amplifiers (VGAs) is very challenging owing to the parasitic effects of the interconnects of both active and passive devices. An automated parasitic-aware optimization RF design tool is proposed in this paper to address this challenge. The proposed tool considers the parasitic effects prior to layout. It employs a knowledge-aware optimization technique. The augmentation between parasitic-aware and knowledge-aware techniques speeds up the design process and leads to a design as close to the final design after finalizing the layout. The proposed tool gives limitless and guaranteed converged solutions in a wide range of RF frequencies. A four-bits current steering VGA design is used as a validation of the tool. The tool is tested on three different frequencies using the 65 nm-technology node. The three tested frequencies (7, 10, and 13 GHz) show a root mean square gain error at approximately 0.1 dB and a phase variation at approximately 3.5° within a 16-dB gain control range. To our knowledge, it is the first reported automated design tool for a current steering VGA.

An Evaluation of Surface-Active Agent Hexadecyltrimethylammonium Bromide with Vertical Self-Alignment Properties to Align Liquid Crystals for Various Cell Gap Conditions

We evaluated hexadecyltrimethylammonium bromide (HTAB) for liquid crystals (LCs) in layered ITO cells with various cell gap conditions. HTAB is a surfactant that can self-align vertically on the surface of indium tin oxide (ITO) substrates and induce homeotropic alignment of the LC molecules. For implementing RF devices with HTAB and LCs, we should consider limitations caused by the design conditions which are different from conventional liquid crystal displays such as cell gap. We quantified the concentration of HTAB ([HTAB]) that is necessary to form and maintain a sufficiently dense vertical alignment of 5CB (4-Cyano-4′-pentylbiphenyl). The required [HTAB] for full-homeotropic alignment was increased to the cell gap until it was too large to support the transfer of the surface alignment to the LC molecules, due to the weak anchoring nature of HTAB. We also showed the phase-change characteristic of the LC mixture related to [HTAB] for the design of RF devices driven by light or heat. This study may help to guide the development of new approaches to designing efficient RF devices that use LCs.

Non-Reciprocal Balanced Bandpass Filters With Quasi-Elliptic Response

This brief reports on the RF design and practical development of a non-reciprocal balanced bandpass filter (BPF) that exhibits a highly-selective quasi-elliptic response in the forward direction of propagation that is shaped by four transmission poles and two transmission zeros (TZs). By modulating some of the filter’s resonators with phase-progressed AC signals, a non-reciprocal response is obtained in the differential mode. Its common-mode is also highly suppressed due to the incorporation of a balanced network that results in two additional TZs and resistive loss that are unique to the common-mode. The filter order can be increased by cascading additional resonators. For validation purposes, a microstrip prototype centered at 725 MHz was designed, manufactured, and measured. It showed a high isolation in the differential-mode reverse transmission of up to 62.1 dB. Moreover, the common-mode was suppressed by over 45 dB in a bandwidth greater than one octave.

Universal and dynamic ridge filter for pencil beam scanning particle therapy: a novel concept for ultra-fast treatment delivery

Objective. In pencil beam scanning particle therapy, a short treatment delivery time is paramount for the efficient treatment of moving targets with motion mitigation techniques (such as breath-hold, rescanning, and gating). Energy and spot position change time are limiting factors in reducing treatment time. In this study, we designed a universal and dynamic energy modulator (ridge filter, RF) to broaden the Bragg peak, to reduce the number of energies and spots required to cover the target volume, thus lowering the treatment time. Approach. Our RF unit comprises two identical RFs placed just before the isocenter. Both RFs move relative to each other, changing the Bragg peak’s characteristics dynamically. We simulated different Bragg peak shapes with the RF in Monte Carlo simulation code (TOPAS) and validated them experimentally. We then delivered single-field plans with 1 Gy/fraction to different geometrical targets in water, to measure the dose delivery time using the RF and compare it with the clinical settings. Main results. Aligning the RFs in different positions produces different broadening in the Bragg peak; we achieved a maximum broadening of 2.5 cm. With RF we reduced the number of energies in a field by more than 60%, and the dose delivery time by 50%, for all geometrical targets investigated, without compromising the dose distribution transverse and distal fall-off. Significance. Our novel universal and dynamic RF allows for the adaptation of the Bragg peak broadening for a spot and/or energy layer based on the requirement of dose shaping in the target volume. It significantly reduces the number of energy layers and spots to cover the target volume, and thus the treatment time. This RF design is ideal for ultra-fast treatment delivery within a single breath-hold (5–10 s), efficient delivery of motion mitigation techniques, and small animal irradiation with ultra-high dose rates (FLASH).

Progress of the ECHo SDR Readout Hardware for Multiplexed MMCs

The electron capture in ^{163}Holmium (ECHo) experiment seeks to achieve sub-eV sensitivity of the electron neutrino mass through calorimetric decay spectroscopy of ^{163}Ho in large arrays of cryogenic magnetic microcalorimeters (MMCs). Microwave SQUID multiplexing serves to efficiently increase the number of readout channels, thus calorimeters per array and ultimately per cryostat. A corresponding frequency multiplexing room temperature software-defined radio (SDR) system is in development to enable the readout of this increased number of MMCs per cable. The SDR consists of a custom FPGA platform that provides signal generation and analysis capabilities, as well as tailored signal conversion and analog conditioning front end electronics that enable the room-temperature-to-cryogenic interface. Ultimately, the system will read out 400 multiplexer channels with double pixel detectors through a bandwidth of 4 GHz (IEEE C band). As high-resolution data converters are limited in sample rate, the C-band is split into five sub-bands using a two-stage mixing method. In this contribution, a prototype of the heterodyne RF design is presented. It comprises one of the five 800 MHz sub-bands for a target frequency range between 4 and 8 GHz. Furthermore, the second version of the A/D converter stage is presented, capable of generating and digitizing up to five complex basebands using 1 GSs^{-1} converters, the reference clocks and a flux-ramp signal. We will show first results of their single and combined characterization in the lab. The current state of the prototype hardware enables preliminary measurements, only limited in bandwidth and with slightly higher noise. Potential improvements could be derived and will be implemented in the full bandwidth, 5-sub-band RF PCB design.

Integrated-Transformer-Based Impedance Matching Method: Impedance Matching With Transformers

Impedance matching is a critical aspect of any RF or millimeter-wave (mm-wave) design. Although historically addressed with transmission lines or discrete elements, it can also be implemented with integrated transformers as their quality factors now reach decent values (approximately 10–20) at high frequencies (used at 77 GHz in this article) due to the thicker upper metal layers present in most integrated stacks.

Design of a radio frequency quadrupole for a high intensity heavy-ion accelerator facility

We report a design work of a continuous-wave (cw) radio frequency quadrupole (RFQ) for the injector linac of the High Intensity heavy-ion Accelerator Facility project. The rf design and multiphysics analysis were conducted to enhance the long-term stability and reliability of the cavity. To minimize the peak modified Poynting vector and peak surface electric field associated with the rf breakdown rate, the cross section and the vanes segmentation of the RFQ cavity were optimized. Pi-mode stabilizing loops (PISLs) were adopted for the cavity to increase the mode separation between the quadrupole and the neighboring dipole modes, intending to reduce the emittance growth and beam loss caused by the field asymmetry of the quadrupole mode. Due to the lack of a universal criterion of acceptable mode separation, the field asymmetry of the quadrupole mode was directly used as the design criterion for the PISL. Tuners were designed to tune the cavity frequency and longitudinal field flatness. Thermal distributions on tuners with and without rf sealing were investigated, and we found that rf sealing was indispensable in decreasing the tuner's temperature. A multiphysics analysis was carried out to maintain the cavity frequency while running with different rf power, in an attempt to avoid frequently adjusting the cooling water temperature when accelerating different ions. The analysis also verified that a large wall thickness could make the cavity possess a low stress and a minor deformation.

Numerical Investigation of Signal Launch Imperfections for Edge Mount RF Connectors

In this paper, common practice RF design guidelines for SMA edge mount connectors are investigated in terms of numerical simulations and VNA measurements. These guidelines are used in a variety of applications for coaxial-to-planar interfaces but often do not provide information regarding the physical origins of increased insertion and transmission losses. The presented results in this work focus on different RF PCB design features and their impact on electromagnetic field distributions in the launching zone. The presented investigations should raise awareness on the issue of electromagnetic field resonances occurring in the RF frequency range and assist PCB design engineers to identify potential issues occurring at an coaxial-to-planar interface. The investigated PCB features facilitate a high performance RF PCB design up to a frequency of 26 GHz.

Broadband high power rf window design for the BNL Electron Ion Collider

Brookhaven National Laboratory (BNL) and Thomas Jefferson National Accelerator Facility are collaborating on the design and construction of the next Electron Ion Collider (EIC) to be built at BNL. The EIC is a unique high-energy, high-luminosity, polarized electron-proton/ion collider. The EIC accelerator complex needs about 10 new rf and SRF systems with frequencies spanning 24 MHz to 1773 MHz, requiring more than 60 new high-power fundamental power couplers (FPCs). These couplers will operate in either pulsed mode or continuous wave mode with peak traveling wave power ranging from 10 s to 380 kW. Here we present our design for a 1-MW broadband rf window suitable for EIC rf and SRF systems with operating frequencies up to 591 MHz. This design takes advantage of the numerous synergies between the various rf and SRF systems to make it broadly applicable. The rf window design criteria are based on the requirement for the 591-MHz electron storage ring (ESR) SRF cavities, as it will operate at both the highest traveling wave power and the highest peak power over the EIC $\mathrm{rf}/\mathrm{SRF}$ complex. The results presented will detail the FPC power requirement, rf window choices, design criteria, and multiphysics performance in the most critical application, the ESR SRF cavity, and how the rf window design applies to other EIC $\mathrm{rf}/\mathrm{SRF}$ systems.