Compact RF Research Articles

Compact HOM-damping structure of a beam-accelerating TM020 mode rf cavity

A compact HOM-damping beam-accelerating rf cavity was developed to mitigate coupled-bunch instabilities in electron storage rings. This cavity operates in the TM020 mode, characterized by a high Q-value and a high shunt impedance. The new HOM-damping mechanism leverages the unique field distributions of the TM020 mode, allowing direct embedding of rf absorbing materials into the cavity body. This novel structure underwent validation through rf simulations and measurements using a model cavity resonating at 508.58 MHz in the TM020 mode with a shunt impedance of 6.8 MΩ and designed to have HOM impedances of less than 0.1 MΩ longitudinally and 0.5 MΩ/m transversely. We also fabricated and tested a prototype cavity, showcasing its HOM-damping efficacy and high-power functionality at 120 kW, generating an accelerating voltage of 900 kV. This prototype operated successfully at full power, confirming the viability of both the damping structure and its high-power application.

Research and Design of the RF Cavity for an 11 MeV Superconducting Cyclotron

In contrast to the room temperature cyclotron, the superconducting cyclotron’s high operational magnetic field and small extraction radius lead to a magnet design with a reduced radius. This limits the space available for the RF cavity in the 11 MeV superconducting cyclotron, necessitating a more compact RF cavity design. By using the transmission line theory, the complex structure of the quarter-wavelength coaxial cavity can be represented as a microwave circuit. Through relevant theoretical analytical formulas, equivalent circuit parameters can be derived. The resonant frequency of the RF cavity is then determined using the equivalent circuit method. The optimization of the RF cavity structure was achieved by creating a numerical model and conducting finite element numerical calculations on the high-frequency resonant system. The comparative results between the equivalent circuit and numerical calculations indicate that the frequency error remains within 0.1%, validating the compact RF cavity design. A multiple linear regression analysis facilitates the prediction of resonance frequency across various parameter variables. By analyzing the fitting formula, RF cavity machining error requirements are established, ensuring a prediction error within 1%, thus meeting engineering design criteria.

A Compact RF Front-End SiP With Improved Harmonic Suppression for Dual Polarization Phased Array Radar

A Compact RF Front-End SiP With Improved Harmonic Suppression for Dual Polarization Phased Array Radar

Glass-Core Substrates for RF Heterogeneous Integrated Packages

Glass-core substrates with low-loss organic build-up layers are an attractive alternative to conventional multi-layer organic laminates and ceramic substrates for millimeter wave packages. Under the Department of Defense (DoD) State of the Art (SOTA) Heterogeneous Integrated Packaging (SHIP) RF program, Qorvo and our partners are exploring panel-sized glass-core substrates with fine-pitch multi-layer interconnects on low-loss buildup dielectrics and embedded in-core components for compact RF and mixed-signal microsystems. The suitability of this substrate technology is being evaluated through a series of test vehicles designed to characterize the process capability, RF behavior, and package reliability.

Characterization of plasmas in negative ion sources using a Cs-H Collisional Radiative model

SPIDER, hosted at the Neutral Beam Test Facility (NBTF) in Padova, Italy, is the full scale prototype for the ITER Heating Neutral Beam (HNB) source. Another smaller compact RF ion source, NIO1, hosted by Consorzio RFX, was built to study the production and acceleration of H- ions in continuous operation. Both machines operate with the evaporation of caesium in order to enhance the production of negative ions.A collisional radiative model for caesium-hydrogen plasmas was recently developed. When used in conjunction with measurements from Optical Emission Spectroscopy, Laser Absorption Spectroscopy and electrostatic probes, the model can provide estimates of the plasma parameters with a good spatial resolution thanks to the many lines of sight available.This work presents a characterization of the plasma in SPIDER and NIO1 using this method. In particular, we investigate the influence of the RF power and the magnetic filter field on the plasma properties, and compare the results with those of other source and beam diagnostics.

Compact metamaterial-inspired RF coil based on split-ring resonators to enhance the magnetic field for 1.5T magnetic resonance imaging

In this paper, we propose a metamaterial-inspired RF coil structure based on a multiple split-ring resonator (MSRR) for 1.5T MRI. An additional component that enables to enhance the magnetic field of RF coil is proposed. This novel component is incorporated an array of split-rings and a central feature combined with square and circular rings (SC). By aligning the component with the split-ring resonator (SRR) in a coplanar arrangement, we derive a compact MSRR equipped with the SC structure (SC MSRR). We analyze the effect of different numbers of split rings within the component and the distance from the feed coil on gain of the magnetic field. The results indicate that the proposed metamaterial-inspired structure can effectively improve the near-field properties of the coil. The proposed SC MSRR has enhanced B1− field strength effectively and keeps a lower specific absorption rate (SAR) level in the field of view (FOV), resulting in a significant increase in SAR efficiency and received sensitivity. Compared with the conventional SRR surface coil, the peak B1− field strength can be enhanced by 3.15 times in the case of using SC MSRR, the peak SAR can be reduced by 20.14 % and the peak SAR efficiency can be improved 3.60 times. In addition, the proposed 4-channel head array consisting of SC MSRRs efficiently generates a higher quality B1− field compared to the standard head birdcage coil. The high directivity of the metamaterial-inspired structure allows for good isolation between ports/channels. The high compactness and excellent performance of the SC MSRR prove that it can be an efficient candidate for the surface coil and high density array. This approach provides a new practical way to combine RF coils with metamaterials for MRI applications.

A Compact RF Energy Harvesting System With an Improved Impedance Matching Network

A Compact RF Energy Harvesting System With an Improved Impedance Matching Network

A Compact RF Energy Harvesting Wireless Sensor Node with an Energy Intensity Adaptive Management Algorithm.

This paper presents a compact RF energy harvesting wireless sensor node with the antenna, rectifier, energy management circuits, and load integrated on a single printed circuit board and a total size of 53 mm × 59.77 mm × 4.5 mm. By etching rectangular slots in the radiation patch, the antenna area is reduced by 13.9%. The antenna is tested to have an S11 of -24.9 dB at 2.437 GHz and a maximum gain of 4.8 dBi. The rectifier has a maximum RF-to-DC conversion efficiency of 52.53% at 7 dBm input energy. The proposed WSN can achieve self-powered operation at a distance of 13.4 m from the transmitter source. To enhance the conversion efficiency under different input energy densities, this paper establishes an energy model for two operating modes and proposes an energy-intensity adaptive management algorithm. The experiments demonstrated that the proposed WSN can effectively distinguish between the two operating modes based on input energy intensity and realize efficient energy management.

A Compact Stacked RF Energy Harvester with Multi-Condition Adaptive Energy Management Circuits.

This paper presents a compact stacked RF energy harvester operating in the WiFi band with multi-condition adaptive energy management circuits (MCA-EMCs). The harvester is divided into antennas, impedance matching networks, rectifiers, and MCA-EMCs. The antenna is based on a polytetrafluoroethylene (PTFE) substrate using the microstrip antenna structure and a ring slot in the ground plane to reduce the antenna area by 13.7%. The rectifier, impedance matching network, and MCA-EMC are made on a single FR4 substrate. The rectifier has a maximum conversion efficiency of 33.8% at 5 dBm input. The MCA-EMC has two operating modes to adapt to multiple operating conditions, in which Mode 1 outputs 1.5 V and has a higher energy conversion efficiency of up to 93.56%, and Mode 2 supports a minimum starting input voltage of 0.33 V and multiple output voltages of 2.85-2.45 V and 1.5 V. The proposed RF energy harvester is integrated by multiple-layer stacking with a total size of 53 mm × 43.5 mm × 5.9 mm. The test results show that the proposed RF energy harvester can drive a wall clock (30 cm in diameter) at 10 cm distance and a hygrometer at 122 cm distance with a home router as the transmitting source.

A Hybrid Solar-RF Energy Harvesting System Based on an EM4325-Embedded RFID Tag

This paper presents the deployment of a hybrid energy harvesting system that combines a wireless energy harvesting (EH) system and a 6 V, 170 mA monocrystalline solar energy derived from the Sun’s rays. The hybrid energy harvesting (HEH) system comprises the rectifier, the solar cell panel, the charging circuit, and the EM4325 embedded RFID tag. This study aims to design an efficient EH system capable of increasing the read range of an active RFID tag. The proposed approach integrates a meandered line radio frequency identification (RFID) tag with an EM4325 IC chip as the receiver antenna. A halfwave doubler RF rectifier circuit is connected to the antenna using a 50 Ω SMA connector to convert the captured RF waves into usable electrical power. A solar energy charging module equipped with a Maximum Power Point Tracking (MPPT) system, a rechargeable lithium-ion battery, and a DC-DC converter is configured to manage and store the harvested energy efficiently. The UHF tag antenna operates at 919 MHz, achieving a peak gain of 3.54 dB. The proposed rectenna achieves a maximum measured harvested power conversion efficiency (PCE) of 55.14% for an input power (Pin) of 15 dBm at a distance of 5.10 cm, while the solar cell panel realizes 3.92 W of power. Experimental results demonstrate the hybrid harvester system’s effectiveness, achieving a PCE of 86.49% at an output voltage (VDC) of 5.35 V. The main advantage of this approach is the creation of a compact hybrid RF and solar EH system by combining the solar cell panel with the antenna, thus enabling multi-functionality.

Next Generation RF Modules for 5G, IoT, AR/VR and RFID Applications

The rapid development and deployment of 5G/mm-Wave technologies for communication, sensing and energy harvesting applications have been on the rise. Consequently, the need for low-cost, scalable, agile and compact RF modules has become more prominent than ever. This paper presents a review of recent efforts in utilizing additive manufacturing techniques such as inkjet printing to sustainably accelerate the massive deployment of 5G/mm-Wave. First, a novel flexible and massively scalable multiple-input, multiple-output (MIMO) tile-based phased array enabled by additively manufactured microstrip-to-microstrip transitions is presented. Next, a novel Rotman-Based harmonic mmID tag for Ultra-Long-Range localization is presented. Finally, low-power, low-cost mm-Wave backscattering modules for localization and orientation sensing are demonstrated.

Compact real-time RF spectrum analyzer with 16 GHz instantaneous bandwidth based on photonic frequency-shifting loops

Spectral analysis of broadband RF signals in real time is of primary importance for numerous applications. So far, the instantaneous bandwidth of real-time spectrum analyzers based on conventional digital techniques is limited to a few GHz. This limitation is set by the clock jitter of the analog-to-digital converters, and by the processing capabilities in real time of digital electronics. On the contrary, analog architectures based on microwave photonics are not constrained by such limitations, and offer potentially a very high instantaneous bandwidth. However, they generally suffer from inherent limitations, such as large footprint and high complexity. Here, we propose a much simpler architecture of RF spectrum analyzer based on frequency-shifting loops. It utilizes only compact commercial telecom components, a single CW laser, and slow electronic resources (10 MSa/s). The probability of intercept is 100%, the instantaneous bandwidth reaches 16 GHz, and the spectral and temporal resolutions are respectively equal to 160 MHz and 50 µs. Our system is expected to open new avenues in embedded applications of microwave photonics.

A Compact High-Efficiency RF Rectifier With Widen Bandwidth

In this letter, a compact broadband RF rectifier is proposed and tested for ambient RF energy harvesting (EH). An uncomplicated impedance matching network with multistage transmission lines (quasi T-shaped) is presented. Quasi T-shaped network performs three different functions to complete the broadband matching process. The novel matching network widens working bandwidth using fewer transmission lines without via hole so that it has low loss and high conversion efficiency. After theoretical analysis and simulation, the rectifier is manufactured, and the measured results are consistent with the simulation results. The rectifier shows good broadband performance. The frequency range of rectifier efficiency over 50% is from 0.85 to 2.05 GHz (a bandwidth of 82.7%) at 5 dBm. The rectifier shows a bandwidth of 101.5% (0.8–2.45 GHz) with a power conversion efficiency (PEC) greater than 50% for an 8 dBm input power. In addition, the maximum conversion efficiency of the circuit is as high as 73.1% at the input power of 8 dBm.

Ampere-class bright field emission cathode operated at 100 MV/m

High-current bright sources are needed to power the next generation of compact rf and microwave systems. A major requirement is that such sources could be sustainably operated at high frequencies, well above 1 GHz, and high gradients, well above $100\text{ }\text{ }\mathrm{MV}/\mathrm{m}$. Field emission sources offer simplicity and scalability in a high-frequency era of the injector design, but the output rf cycle charge and high-gradient operation remain a great and largely unaddressed challenge. Here, a field emission cathode based on ultra-nano-crystalline diamond, an efficient planar field emission material, was tested at $100\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ in an $L$-band injector. A very high charge of 38 pC per rf cycle was demonstrated (300 nC per rf pulse corresponding to an rf pulse current of 120 mA). This operating condition revealed a two-dimensional space charge limited emission where the one-dimensional Child-Langmuir limit was surpassed. An injector brightness of ${10}^{14}\text{ }\text{ }\mathrm{A}/(\mathrm{rad}\text{ }\text{ }\mathrm{m}{)}^{2}$ was estimated for the given operating conditions.

CP Antenna with 2 × 4 Hybrid Coupler for Wireless Sensing and Hybrid RF Solar Energy Harvesting.

The main challenge faced by RF energy harvesting systems is to supply relatively small electrical power to wireless sensor devices using microwaves. The solution is to implement a new device in a circularly polarized rectenna with circular polarization sensitivity integrated with a thin-film solar cell. Its dual-feed antennas are connected to a 2 × 4 asymmetric hybrid coupler and a multi-stage voltage doubler rectifier circuit. This configuration has a 2 × 4 asymmetric hybrid coupler used to produce 4 outputs with a 90-degree waveform phase difference. The two ports can independently be connected to the wireless sensor circuit: radiofrequency harvesting of hybrid energy solar and information equipment can be carried out with these two antennas. The Dual-Feed circular patch antenna has a two-port bandwidth of 137 MHz below −15 dB and an axial ratio of less than 3 dB, with a center frequency of 2.4 GHz with directional radiation and a high gain of 8.23 dB. It can be sensitive to arbitrary polarization of the input voltage multiplier waveform to overcome uncertainty in empirical communication environments. A parallel structure is arranged with a thin film solar cell integration from the transmitter with an output voltage of 1.3297 V with a compact composition and RF energy. The importance of adopting a wireless sensor strategy with circular polarization sensitivity and integrated RF solar energy harvesting rather than a single source method makes this research a significant novelty by optimizing the analysis of multiple wireless sensor signal access.

CSIWC RF sensor for micro‐fluidic non‐contact quality assessment of milk

In this work, the design of the SIW-based RF sensor is presented to assess the targeted quality of milk. In the present scenario, mainly two parameters, namely, purity and fat level are considered to assess the milk quality. Customized in-lab preparation of milk adulterant using low-grade vegetable oil, detergent, water, and glucose is carried out to analyze the practical milk adulteration scenario. Moreover, commonly consumed pure cow and buffalo milk samples are also collected to record the corresponding reference RF properties. A systematic approach is followed to develop the complete database related to actual milk samples with known fat percentages. Then accordingly, the proposed RF sensing mechanism is employed to identify the possible deviation in milk quality. The proposed work mainly employs the SIW-based non-contact RF sensing mechanism where the perturbation formulation is used to perform a qualitative assessment. Mathematical processing of the relative shift in the obtained S-parameters, corresponding to the variation in dielectric property of test samples at specified RF frequency, is carried out. Even though the work's main aim is a qualitative assessment of milk quality, dielectric property estimation of considered test samples is also performed to visualize their quantitative RF attributes. A compact, low-cost, non-contact, real-time, and micro-fluidic CSIWC RF sensor with high-quality factor and improved accuracy and sensitivity is designed.

Broadband linearity enhancement method for a 1.3 GHz–2.5 GHz digitally-assisted oscillator in a 55-nm CMOS technology

At the L/S band, the digital array radars with digital beam-forming characteristics require a compact RF module for a plane array. Such an RF module would require a small-sized local clock generation to synchronize with the central distributed clock signal. This paper proposes a 0.04 mm2, broadband local clock generation for the RF module in digital array radar. In such a multi-band digital array radar, the linearity of the clock generator is a bottleneck. A linearity enhanced algorithm for switched-capacitor digitally-assisted oscillators is proposed in this paper. In this linearity enhancement technique, a calibration resistor array is used to improve frequency-tuning characteristics. The digitally-assisted oscillator has a highly linearized frequency tuning curve without demanding much more power and silicon area. A prototype is fabricated using a 55-nm CMOS process. From measurement, it has a frequency tuning range from 1.3 GHz to 2.5 GHz while occupying 225 × 180 μm2. The measured coarse bank tuning maximum DNL is 0.39 LSB, and the maximum INL is 4.64 LSB, which has been reduced to 43% of the other state-of-art work. The maximum power consumption is 15.8 mW from a 1.2 V supply voltage.

Highly Sensitive and Compact Quad-Band Ambient RF Energy Harvester

A highly efficient and compact quad-band energy harvester (QBEH) circuit based on the extended composite right- and left-handed transmission lines (E-CRLH TLs) technique is presented. The design procedure based on E-CRLH TLs at four desired frequency bands is introduced to realize a quad-band matching network. The proposed QBEH operates at four frequency bands: <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${{{\bf f}}_1} = 0.75\,{{\bf GHz}};\,{{{\bf f}}_2} = 1.8\,{{\bf GHz}};\,{{{\bf f}}_3} = 2.4\,{{\bf GHz}}$</tex-math></inline-formula> ; and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${{{\bf f}}_4} = 5.8\,{{\bf GHz}}$</tex-math></inline-formula> . The simulations and experimental results of the proposed QBEH exhibit overall (end to end) efficiency of 55% and 70% while excited at four frequency bands simultaneously with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$ - 20\,{{\bf dBm}}$</tex-math></inline-formula> (10 μW) and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$ - 10\,{{\bf dBm}}$</tex-math></inline-formula> (100 μW) input power, respectively. Due to applying multitone excitation technique and radio frequency (RF) combining method in the QBEH circuit, the sensitivity is improved, and sufficient power is generated to realize a self-sustainable sensor ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> ) using ambient low-level RF signals. A favorable impedance matching over a broad low input power range of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$ - $</tex-math></inline-formula> 50 to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$ - $</tex-math></inline-formula> 10 dBm (0.01 to 100 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">μ</i> W) is achieved, enabling the proposed QBEH to harvest ambient RF energy in urban environments. Moreover, an accurate theoretical analyses based on the Volterra series and Laplace transformation are presented to maximize the output dc current of the rectifier over a wide input power range. Theoretical, simulation and measurement results are in excellent agreement, which validate the design accuracy for the proposed quad-band structure. The proposed new energy harvesting technique has the potential to practically realize a green energy harvesting solution to generate a viable energy source for low-powered sensors and IoT devices, anytime, anywhere.

A Concurrent Triple-band RF Energy Harvesting Circuit for IoT Sensor Networks

In this paper, a concurrent triple-band RF energy harvesting circuit is proposed. The proposed circuit incorporates a triple-band microstrip antenna combining a low-loss diplexer and three compact RF rectifiers. The proposed circuit operates concurrently at three popular frequency bands: GSM-900, GSM-1800, and 2.45 GHz. To improve output voltage and efficiency, three rectifiers are connected in a stacked topology. The simulated and measured results demonstrate that the circuit operates well in concurrent bands. Moreover, the proposed circuit exhibits a low-complexity structure and compactness in size. These advantages make the circuit a strong potential candidate for running low-power devices in IoT sensor networks.

Machine Learning-Inspired Hybrid Precoding for mmWave MU-MIMO Systems with Domestic Switch Network.

Hybrid precoding is an attractive technique in MU-MIMO systems with significantly reduced hardware costs. However, it still requires a complex analog network to connect the RF chains and antennas. In this paper, we develop a novel hybrid precoding structure for the downlink transmission with a compact RF structure. Specifically, the proposed structure relies on domestic connections instead of global connections to link RF chains and antennas. Fixed-degree phase shifters provide candidate signals, and simple on-off switches are used to route the signal to antennas, thus RF adders are no longer required. Baseband zero forcing and block diagonalization are used to cancel interference for single-antenna and multiple-antenna users, respectively. We formulate how to design the RF precoder by optimizing the probability distribution through cross-entropy minimization which originated in machine learning. To optimize the energy efficiency, we use the fractional programming technique and exploit the Dinkelbach method-based framework to optimize the number of active antennas. Simulation results show that proposed algorithms can yield significant advantages under different configurations.