Passive RF Research Articles

Characterization of Additively Produced RF-Structures on Low-Cost PCB Materials Based on Inkjet Technology for Industrial Applications up to 80 GHz

Usually, the printed circuit board industry has to use special subtractive manufacturing methods, if small line and space dimensions down to 50 μm are desired. This drastically raises production effort and costs. In this paper a method is shown, how the inkjet technology can be used to produce passive RF structures with minimum line and space dimensions of 25 μm. For the first time, this is possible over complete panels of low-cost PCB substrates, which have a relatively high surface roughness compared to ceramic or glass carriers. To ensure process reliability, printed lines must also be plated with an additional electroless copper layer to overcome the surface roughness with conductive layers and to lower insertion loss in the two-digit GHz range. Additionally, a surface impedance model is developed, which allows broadband first time right simulations. Finally, additively produced, passive RF filters are compared to their subtractive counterparts to show their benefits regarding production costs and electrical performance up to 80 GHz.

Designand test of a compact beam current monitor based on a passive RF cavity for a proton therapy linear accelerator.

In a medical accelerator, real-time monitoring systems of the beam and dose delivered to the patient are mandatory. In this work, we present a compact current profile detector that has been designed and tested in the framework of the TOP-IMPLART (Intensity Modulated Proton Linear Accelerator for RadioTerapy) project. This project foresees the realization of a proton linear accelerator, currently under construction at ENEA Frascati, for proton therapy applications. The linac produces a pulsed proton beam with 3 µs duration at 50Hz repetition rate with a pulse current between 0.5 and 50 μA. A large dynamic range and spatial constraints make the use of usual noninterceptive beam diagnostics unfeasible. Therefore, the use of a beam current monitor based on a passive RF cavity working in the TM010 mode has been proposed. This paper reports the electromagnetic designof the device guided by a simplified analytical model. A prototype of such a device has been realized, characterized, and tested on the linac with a 35MeV beam varying the beam current. The test results in air and in vacuum, together with the signal detection systems used, are presented.

Passive High Power RF Comb Filters Using Epitaxial GaN/NbN/SiC HBARs.

This report presents the first demonstration of passive RF comb filters made using epitaxial GaN/NbN/SiC high overtone bulk acoustic resonators (epi-HBARs). The two-port device is fabricated on electronic-grade GaN, electrically transduced, and acoustically coupled. The multi-mode epi-HBAR comb filter demonstrated here has 158 sharp filter passbands periodically distributed between 1 and 4 GHz (L-S-bands) with a free spectral range (FSR) of 17 MHz. The individual passbands of the epi-HBAR comb filter demonstrate transmission bandwidths (BWs) up to 800 kHz, f × Q values of up to 7×1014 Hz, and an average [Formula: see text] figure of merit of 41.2 at room temperature. The GaN/NbN/SiC epi-HBAR comb filter is capable of operating at high RF power levels, with linear and distortion-free performance seen up to at least 1 W of continuous wave (CW) power and up to at least 10 W of pulsed power. The compact epi-HBAR comb filters can be co-fabricated with GaN-based electronics and could potentially replace larger, off-chip or discrete-component comb filters. They can be used for spectrum sensing and as signal processing elements for remote sensing and pulsed radar.

Pulsed Harmonic Doppler Radar and Passive RF Tags for Tracking the Dynamic Motion of Held Objects

A new method for detecting microwave harmonic Doppler signatures of held objects in cluttered environments is presented. Based on a passive harmonic tag combined with a pulsed microwave radar, the motion of tagged objects held by a person can be detected in high-clutter environments. In this letter, we present a new derivation of the dynamic harmonic signal response of a harmonic tag in motion. Based on a truncated Bessel function expansion, the model predicts the time-frequency signatures of sinusoidally moving objects with high accuracy. We present a pulsed harmonic radar transmitting at 2.51 GHz and receiving at 5.02 GHz, which is capable of measuring the harmonic micro-Doppler response of the tag with better signal-to-noise since the clutter response only exists at 2.51 GHz. The pulse waveform provides detection of the harmonic tag at larger distances than continuous-wave systems by increasing the peak power of the signal incident on the tag, while the average power is fixed with a low level. Measurements of a held tag at a distance of 1 m in a highly cluttered environment are presented, showing a strong match to the simulated results.

Second-Order Analytic Extension of Eigenvalues for Fast Frequency Sweep Analysis of RF Circuits

A second-order analytic extension of eigenvalue (AEE) method is presented and investigated for efficiently computing the Z- parameters of passive RF circuits over a wide frequency band with full-wave accuracy. The Z- parameters of an RF circuit are first extracted based on a full-wave simulation on sampling frequencies and then decomposed into eigenmodes, whose eigenvalues are analytically extended to all other frequencies within the frequency band of interest based on functional equations constructed from second-order series and parallel RLC circuits. An eigenvector-eigenvalue identity is adopted to compute the frequency-dependent eigenvectors from the eigenvalues of the submatrices, which are used in the expansion of the Z- parameters. A comparison with full-wave solutions is given for the second-order AEE where four frequencies are employed to accurately approximate the Z- parameters and predict the frequency response over the entire band of interest that goes to much higher frequencies than the first-order AEE. With this, the previously developed first-order AEE for a quasi-static analysis is successfully extended to much higher frequencies. Numerical examples are provided to validate the accuracy and demonstrate the capability of the second-order AEE. It is found that the second-order AEE is very accurate for modeling RF circuits with electrical sizes up to one wavelength that possibly contains resonances, as compared to the first-order AEE, which is applicable only to RF circuits with electrical sizes smaller than one-tenth to one-fifth of a wavelength that contains no resonance.

Cable Current Detection With Passive RF Sensing Tags

Cable current detection plays a vital role in power grids toward improving transmission efficiency, relieving power congestion, and ensuring grid reliability. However, the sensors in existing current detection systems either are wired with measurement instruments or are wireless yet require batteries, which is inconvenient for monitoring overhead powerlines. Inspired by RF sensing tags, which are widely used for applications such as temperature and humidity measurements, in this article, we propose a distinctive cable current detection system that can avoid the constraint of wires and batteries. By detecting the ambient magnetic field produced by ac current flowing through a cable, the proposed system realizes a minimal measurement range of 5–17.5 A at 50/60 Hz with a maximum wireless distance of 5.2 m. This prototype can detect cable current with an extendable measurement range, providing a promising strategy for passive and wireless current detection.

Two‐region model for harmonic radar transponders

Harmonic radar transponders are passive RF tags that produce a harmonic response when illuminated with an RF signal. With a Schottky diode as the main component, the tag current is usually modelled as an exponential function of antenna voltage, resulting in an approximate polynomial function for small signals. The authors introduce the Lambert function that accounts for the resistive load and leads to a two-region tag model: a power series representation for small signals and an asymptotic linear large-signal model. Furthermore, they provide the relationship between the modulated bandpass signals at the input and output of the tag in the two regions by expressing the tag signals with complex envelopes. The derived expressions offer deeper insight of the transponder's impact on the operation of harmonic radar systems.

Characterization and Production of PCB Structures With Increased Ratio of Electromagnetic Field in Air

Producing passive printed circuit board (PCB) structures in the millimeter-wave range includes some fundamental challenges, such as low manufacturing costs and high fabrication accuracy. These aspects, in general, have to be combined with low signal attenuation, precise simulation models, and a reliable producibility. This article shows a new method to manufacture different kinds of passive RF waveguides with reduced attenuation and decreased electrical length on low-cost PCB substrates. First, the complex material parameters of the used substrate and the used copper foil are extracted from 10 to 100 GHz to have accurate simulation models in CST Microwave Studio. This is necessary because the parameters provided by the manufacturer are often no longer generally valid in this frequency range. In addition, PCB materials have a significantly higher surface roughness compared with conventional materials for millimeter-wave applications. Therefore, the correct determination and the correct modeling of surface roughness is particularly important. Next, laminate is removed partially on the outer layer of the PCB to increase the electromagnetic field ratio in air. The undesired substrate is removed by a CO2 laser drilling machine by setting narrow drills sequentially. The RF performance of these waveguides is compared with those manufactured in conventional PCB technology according to attenuation and producibility.

Additively Manufactured RF Devices for 5G, IoT, RFID, WSN, and Smart City Applications

With the development of inkjet-/3D-/4D-printing additive manufacturing technologies, flexible 3D substrate with complex structures can be patterned with dielectric, conductive and semi-conductive materials to realize novel RF designs. This paper provides a review of state-of-the-art additively manufactured passive RF devices including antennas and frequency selective surfaces (FSS), couplers, where origami-inspired structure enables unprecedented capabilities of on-demand continuous frequency tunability and deployability. This paper also discusses additively manufactured active RF modules and systems such as inkjet printed RF energy harvester system with high sensitivity and efficiency for Internet of Things (IoT), smart cities and wireless sensor networks (WSN) applications, inkjet-printed RF front ends, and inkjet-printed mm-wave backscatter modules.

Guest Editorial: Special Issue on IEEE RFID 2019 Conference

With the proliferation of the Internet of Things, there is an increased demand to enhance every day physical objects with sensors and communication abilities so that they can have digital counterparts (“digital twins”) in a connected world. While many solutions based on protocols such as ZigBee, Bluetooth, ANT, LoRa, LPWAN, NB-IoT, HaLow, etc. do exist, RFID technology presents a compelling alternative. Radio Frequency Identification (RFID) is a generic term for a variety of active and passive RF technologies. RFID transponders have varying degrees of sophistication and their design is often a tradeoff between functionality and cost. At one end of the spectrum, there are active RFID tags that offer on-board battery and memory, sensor integration, active RF transmitters and hence communication range up to kilometers albeit at increased costs. At the other end, passive and semi-passive RFID tags rely on a simple load modulation to backscatter ID and data back to the reader over several meters distance. Passive RFID tags, in particular, lend themselves well for applications needing large-scale, low-cost, automatic identification and are a good option for imparting digital identity to a host of physical assets. Industry alliances such as the NFC forum (for HF RFID) and the RAIN RFID alliance (for UHF RFID) have been formed to motivate and promote these efforts.

A Fully Passive RF Front End With 13-dB Gain Exploiting Implicit Capacitive Stacking in a Bottom-Plate N-Path Filter/Mixer

A low-power interferer-robust mixer-first receiver front end that uses a novel capacitive stacking technique in a bottom-plate N-path filter/mixer is proposed. Capacitive stacking is achieved by reading out the voltage from the bottom plate of N-path capacitors instead of their top plate, which provides a $2\times $ voltage gain after downconversion. A step-up transformer is used to improve the out-of-band (OOB) linearity performance of small switches in the N-path mixer, thereby reducing the power consumption of switch drivers. This article explains the concept of implicit capacitive stacking and analyzes its transfer characteristics. A prototype chip, fabricated in 22-nm fully depleted silicon on insulator (FDSOI) technology, achieves a voltage gain of 13 dB and OOB IIP3/IIP2 of +25/+66 dBm with 5-dB noise figure while consuming only 600 $\mu \text{W}$ of power at $f_{\mathrm{ LO}}=1$ GHz. Thanks to the transformer, the prototype can operate in the input frequency range of 0.6–1.2 GHz with more than 10-dB voltage gain and 5–9-dB noise figure. Thus, it opens up the possibility of low-power software-defined radios.

A Reconfigurable Passive Switched-Capacitor TX RF Front End With −57 dB ACLR2

This letter presents a wireless transmitter (TX) front end in which the functionalities of digital-to-analog conversion, baseband filtering, and signal upconversion are implemented by a passive switched-capacitor (PSC) network. The result is a versatile architecture that fully benefits from CMOS scaling. The front end was integrated in a 65-nm CMOS technology and can support various channel bandwidths simply by adjusting the switching frequency for the PSC network. Thanks to its third-order reconfigurable filter, it maintains a thermal noise floor better than −156 dBc/Hz at a power dissipation of 45 mW. The measured prototype achieves an ACLR2 of −57 dB and an EVM of −31 dB for a 20-MHz 64-QAM OFDM and a 20-MHz 16-QAM signal, respectively.

Frugal Energy Harvesting: Microwave Energy Radiated Into the Environment From Wireless Networks

In today's interconnected world, small sensor nodes are becoming increasingly important. There are countless applications using devices of minimal size in biomedical equipment, either for remote diagnosis or as wearable sensors in clothing. Equipping all these devices with batteries is logistically and economically difficult. Instead, the most sensible approach is for the sensor nodes to collect their energy from the environment. There are various possibilities, such as solar energy, kinetic energy from movements, and thermal energy. However, in this article, we will look at another possibility: microwave energy that is radiated into the environment from wireless networks. To use this microwave energy, we need a wireless energy harvester (WEH). There are numerous approaches to design such a WEH depending on the use case; for example, a WEH design could be flexible [1] or broadband [2]. Information on passive RF identification systems can be found, for example, in [3] and [4]. For such systems, energy harvesting from incident radio waves is a key aspect. In this article, we explain our harvester design presented for the June 2019 IEEE Microwave Theory and Techniques Society (MTT-S) International Microwave Symposium (IMS) Student Design Competition (SDC), Wearable/Frugal Microwave Energy Harvesting Design; our design was driven by being both small and economical.

Analysis and Design of a Full-Duplex Two-Element MIMO Circulator-Receiver With High TX Power Handling Exploiting MIMO RF and Shared-Delay Baseband Self-Interference Cancellation

In this article, we present how full-duplex (FD) operation can be integrated with multi-input multi-output (MIMO) operation using RF and baseband (BB) self-interference cancellation (SIC). MIMO operation increases the SIC requirements by a factor of $N^{2}$ , resulting in severe area, power consumption, and noise penalty. We present the detailed analysis, design, and implementation of RF/BB cancellers that address this challenge, as well as a bootstrapping technique to enhance the power handling of integrated circulator-receivers (circ.-RX). A 65-nm CMOS scalable two-element MIMO FD circ.-RX array with integrated $N$ -path-filter-based nonmagnetic circulators is described that: 1) exploits bootstrapping in the circulator $N$ -path filters to enhance Transmitter (TX) power handling by 8 dB and 2) features area- and power-efficient passive RF and active BB wideband MIMO cancellation with shared-delay elements to address the O( $N^{2}$ ) cancellation challenge. The prototype exhibits: 1) up to 35-/45-dB average SIC across 40-/20-MHz BW; 2) more than 42-/53-dB average crosstalk (CT)-SIC across 40-/20-MHz BW with <2.1-dB degradation in RX NF; and 3) overall TX power handling of +14 dBm enabled by clock bootstrapping.

A 22.3-nW, 4.55 cm2 Temperature-Robust Wake-Up Receiver Achieving a Sensitivity of −69.5 dBm at 9 GHz

This article presents a miniaturized wake-up receiver (WuRX) that is temperature-compensated yet still consumes only 22.3 nW (7.3 nW excluding the temperature-compensation blocks) while operating at 9 GHz ( ${X}$ -band). By moving the carrier frequency to 9 GHz and designing a high impedance, passive envelope detector (ED), the transformer size is reduced to 0.02 cm2 while still achieving 13.5 dB of passive RF voltage gain. To further reduce the area, a global common-mode feedback (CMFB) technique is utilized across the ED and baseband (BB) amplifier that eliminates the need for off-chip ac-coupling components. Multiple temperature-compensation techniques are proposed to maintain constant bandwidth of the signal path and constant clock frequency. The WuRX was implemented in 65- (RF) and 180-nm (BB) CMOS and achieves −69.5- and −64-dBm sensitivity with and without an antenna, respectively. Importantly, the sensitivity is demonstrated to vary by only 3 dB from −10 to 40 °C. This article demonstrates state-of-the-art performance for WuRXs operating at >1 GHz while achieving the smallest area and best temperature insensitivity.

An RF backscatterer system for real-time multi-sensing applications

In this paper, a power-efficient passive analog frequency modulation based RF system is proposed for real-time communication with multiple sensor elements for internet-of-things (IoT) based smart applications. The proposed RF tag with integrated sensors uses an onboard ultra low power oscillator to generate a single frequency modulation signal unique to each of the sensor elements. The generated single tone frequency is modulated to generate the backscattered RF signal, which is immune to the interference present in the propagating environment. The maximum power required for continuous operation of the sensing oscillator is 30 μW, which is harvested from the received RF signal. The designed multi-sensor RF tag has a sensitivity of −6 dBm and has the capacity to integrate up to six different sensors within the available single channel RFID bandwidth of 300 kHz. The wireless communication distance of 10 ft is achieved due to the low power consumption of the multi-sensor RF tag. A passive RF tag with two example sensing elements; temperature and luminescence are demonstrated. The temperature and luminescence change from 25 °C to 60 °C and from 0 to 20 footcandle (lumen/ft2) is represented by 75–100 kHz and 125–150 kHz frequency range, respectively.

Rectification in Spin-Orbit Materials Using Low-Energy-Barrier Magnets

The coupling of spin-orbit materials to high energy barrier ($\sim$40-60 $k_BT$) nano-magnets has attracted growing interest for exciting new physics and various spintronic applications. We predict that a coupling between the spin-momentum locking (SML) observed in spin-orbit materials and low-energy barrier magnets (LBM) should exhibit a unique multi-terminal rectification for arbitrarily small amplitude channel currents. The basic idea is to measure the charge current induced spin accumulation in the SML channel in the form of a magnetization dependent voltage using an LBM, either with an in-plane or perpendicular anisotropy (IMA or PMA). The LBM feels an instantaneous spin-orbit torque due to the accumulated spins in the channel which causes the average magnetization to follow the current, leading to the non-linear rectification. We discuss the frequency band of this multi-terminal rectification which can be understood in terms of the angular momentum conservation in the LBM. For a fixed spin-current from the SML channel, the frequency band is same for LBMs with IMA and PMA, as long as they have the same total magnetic moment in a given volume. The proposed all-metallic structure could find application as highly sensitive passive rf detectors and as energy harvesters from weak ambient sources where standard technologies may not operate.

Modeling and Analysis of the Nonideality of LO Pulse Overlap for Multi-Phase Passive Mixer First RF Frontend

Multi-phase passive mixer (PM) first RF frontend has regained great concern for its satisfactory noise performance comparable to LNA-first frontend. Moreover, the re-configurable impedance, bi-directional signal transparency and bi-directional impedance translation performance enable a splendid application future for the multi-phase PM-first RF frontend. However, it still suffers from quite a few nonidealities, among which the adjacent Local Oscillator (LO) pulse overlap is the remaining nonideality lacking deep research, and how the LO pulse overlap decays the impedance matching and noise performance of multi-phase PM-first RF frontend remains unclear. In this work, an accurate model is, for the first time, established and analyzed to reveal how the LO pulse overlap decays the impedance matching and noise performance of multi-phase PM-first RF frontend. The analytical and simulation results demonstrate that the impedance matching will be drastically deteriorated because of LO overlap, and when LO pulse overlap exceeds 2% the impedance matching will be collapsed, while the noise figure (NF) is deteriorated by 3.1 dB when LO overlap ranges from 0% to 2%. Moreover, even a chocking inductor technique and a technique by introducing a synchronous phase-shifted signal have been proposed to suppress the LO overlap, the gain and noise performance can be deteriorated or the power and chip area cost are big. To address this question, a LO pulse overlap suppression technique is proposed furthermore by introducing an overlap safeguard factor (OLSF) in this work. The additional impedance smaller than 2[Formula: see text][Formula: see text] in the main signal path makes the OLSF scheme beneficial for noise and gain improvement, and the proposed OLSF scheme is power, area (0.04[Formula: see text]mm[Formula: see text] and cost efficient compared with existing LO overlap suppression techniques.

Guest Editorial Special Issue on IEEE RFID 2018 Conference

Radio Frequency Identification (RFID) is a generic term for a variety of active and passive RF technologies that wirelessly convey object identification information. RFID technologies encompass multiple frequency bands and different mechanisms of information transfer: Low Frequency (LF) and High Frequency (HF) RFID tags typically use inductive coupling, Ultra-High Frequency (UHF) and microwave RFID typically use electromagnetic wave propagation although they often operate in near-field application scenarios as well. RFID transponders have varying degrees of sophistication and their design is often a tradeoff between functionality and cost. At one end of the spectrum, there are active RFID tags that offer on-board battery and memory, sensor integration, active RF transmitter and hence communication range up to kilometers albeit at increased costs. At the other end, passive RFID tags rely on a simple load modulation to backscatter ID back to the reader. The tiny ICs of those passive tags are very complex: they contain more transistors than the Intel 80386 processor in the 1985 IBM PC-AT (that processor had about 275,000 transistors) but have lower power consumption than a honeybee’s brain (which consumes about 10 microwatts) and thus can be RF-powered. These tags are range limited to the order of several meters but, at a price point of several cents per tag, are well suited for large-scale, low-cost automatic identification.

Data Verification in Integrated RFID Systems

Radio frequency identification (RFID) is widely used as a tagging system to facilitate physical transactions in the real world. Thanks to the availability of inexpensive passive RF tags, RFID technology is now the catalyst of Internet of Things, i.e., every object can be uniquely identified in an Internet-like way. In the future, many individual RFID systems are likely to be integrated into a few exascale RFID systems. In an integrated RFID system, service providers (SPs) that offer RFID-based data service and clients that use the data service are different organizations. As a consequence, quality of data in terms of authenticity is of significant concern. In this paper, we first formulate a data verification problem in RFID systems and build a model of integrated RFID systems where multiple SPs and clients exist. Then, we propose two data verification protocols to ensure data generated by SPs associated with a particular tag and has not been modified. In addition, we implement our system as a prototype. The computer simulations, analyses, and testbeds based on the prototype all demonstrate that the proposed verifiable integrated RFID system achieves a high level of security and performance.