Showing papers in "IEEE Transactions on Microwave Theory and Techniques in 2020"

A Review of Technologies and Design Techniques of Millimeter-Wave Power Amplifiers

Abstract: This article reviews the state-of-the-art millimeter-wave (mm-wave) power amplifiers (PAs), focusing on broadband design techniques. An overview of the main solid-state technologies is provided, including Si, gallium arsenide (GaAs), GaN, and other III–V materials, and both field-effect and bipolar transistors. The most popular broadband design techniques are introduced, before critically comparing through the most relevant design examples found in the scientific literature. Given the wide breadth of applications that are foreseen to exploit the mm-wave spectrum, this contribution will represent a valuable guide for designers who need a single reference before adventuring in the challenging task of the mm-wave PA design.

Surface Acoustic Wave Devices Using Lithium Niobate on Silicon Carbide

Abstract: This work demonstrates a group of shear horizontal (SH0) mode resonators and filters using lithium niobate (LiNbO3) thin films on silicon carbide (SiC). The single-crystalline X-cut LiNbO3 thin films on 4H-SiC substrates have been prepared by ion-slicing and wafer-bonding processes. The fabricated resonator has demonstrated a large effective electromechanical coupling (k 2 ) of 26.9% and a high-quality factor (BodeQ) of 1228, hence resulting in a high figure of merit (FoM = k 2 · BodeQ) of 330 at 2.28 GHz. Additionally, these fabricated resonators show scalable resonances from 1.61 to 3.05 GHz and impedance ratios between 53.2 and 74.7 dB. Filters based on demonstrated resonators have been demonstrated at 2.16 and 2.29 GHz with sharp roll-off and spurious-free responses over a wide frequency range. The filter with a center frequency of 2.29 GHz shows a 3-dB fractional bandwidth of 9.9%, an insertion loss of 1.38 dB, an out-of-band rejection of 41.6 dB, and a footprint of 0.75 mm 2 . Besides, the fabricated filters also show a temperature coefficient of frequency of -48.2 ppm/°C and power handling of 25 dBm. Although the power handling is limited by arc discharge and migration-induced damage of the interdigital electrodes and some ripples in insertion loss and group delay responses are still present due to the transverse spurious modes, the demonstrations still show that acoustic devices on the LiNbO3-on-SiC platform have great potential for radio-frequency applications.

Low-Profile Broadband Absorber Based on Multimode Resistor-Embedded Metallic Strips

Abstract: This article proposes a new method to realize a low-profile broadband absorber. The unit cell of the proposed absorber consists of two layers: a lossy layer with four rotationally symmetric bent metallic strips embedded with two chip-resistors, which is modified from a doubly fed dipole antenna, and a metallic ground separated from the lossy layer by an air spacer. Three resonant modes of the metallic strip embedded with two chip-resistors are generated, and the current in the strip passes through the chip-resistors under different modes and it is finally consumed, resulting in energy dissipation. As a result, an ultrawideband absorber is realized. The designed absorber is fabricated and measured, and the measured absorption band with a fractional bandwidth of 127.9% is achieved for at least 10-dB reflectivity reduction under the normal incidence. In addition, the thickness of the designed absorber is only $0.08\lambda _{L}$ , where $\lambda _{L}$ is the wavelength at the lowest operating frequency.

Broadband Millimeter-Wave Textile-Based Flexible Rectenna for Wearable Energy Harvesting

Abstract: Millimeter-wave (mmWave) bands, a key part of future 5G networks, represent a potential channel for RF energy harvesting, where the high-gain antenna arrays offer improved end-to-end efficiency compared with sub-6-GHz networks. This article presents a broadband mmWave rectenna, the first rectenna realized on a flexible textile substrate for wearable applications. The proposed novel antenna’s bandwidth extends from 23 to 40 GHz, with a minimum radiation efficiency of 67% up to 30 GHz, over 3-dB improvement compared with a standard patch. A stable gain of more than 8 dB is achieved based on a textile reflector plane. The antenna is directly connected to a textile-based microstrip voltage doubler rectifier utilizing commercial Schottky diodes. The rectifier is matched to the antenna using a tapered line feed for high-impedance matching, achieving broadband high voltage sensitivity. The rectifier has a peak RF–dc efficiency of 12% and a 9.5-dBm 1-V sensitivity from 23 to 24.25 GHz. The integrated rectenna is demonstrated with more than 1.3-V dc output from 12 dBm of mmWave wireless power across a 28% fractional bandwidth from 20 to 26.5 GHz, a 15% half-power fractional bandwidth, and a peak output of 6.5 V from 20 dBm at 24 GHz.

Efficient Wireless Power Transfer System With a Miniaturized Quad-Band Implantable Antenna for Deep-Body Multitasking Implants

Abstract: Passive operation and battery-charging of deep-body implants can be insured through wireless power transfer (WPT) technologies. However, the power transfer efficiency (PTE) is constrained by device miniaturization and implantation depth. This study proposes a complete WPT system consisting of a patterned WPT transmitter (Tx), an efficient rectifier, and an antenna integrated with the system. The WPT Tx had a size of 6 cm $\times$ 6 cm and was optimized to focus the power on the deep-tissue implants at 1470 MHz. The voltage doubler was optimized at 1470 MHz, had a small size of 5 mm $\times$ 10 mm, and exhibited a high radio frequency (RF)-to-direct current (dc) conversion efficiency of 90% at 2-dBm RF input power. Moreover, the implantable antenna occupies a small volume of 8.43 mm3 and supports quad-band operations: telemetry at 403 and 915 MHz, WPT at the midfield band of 1470 MHz, and control signaling at 2.4 GHz. First, the fabricated prototypes were measured individually in minced pork, in the American Society for Testing and Materials (ASTM) model, and in the saline-filled 3-D head phantom. While operating collectively as an integrated system, the PTE of the system was measured. Additionally, to enhance the PTE of the WPT system, a high-dielectric matching layer ( $\varepsilon _{r} = 78$ ) was used between the WPT Tx and the phantom. Furthermore, to demonstrate the PTE of the WPT system, the voltage doubler was integrated with the implantable antenna, encapsulated in a 3-D-printed capsule endoscope, and its PTE was measured in a saline solution and minced pork. Finally, the compliance of the WPT system with the human safety standards was analyzed and found that the system solely satisfied the safety limits. It is evident from the experimental results that the system can transfer 6.7-mW power to millimeter-sized implants located 5-cm deep in tissues.

A Decoupling and Matching Network Design for Single- and Dual-Band Two-Element Antenna Arrays

Abstract: This article presents a new decoupling and matching network (DMN) design for symmetric, single- and dual-band two-element antenna arrays. The DMNs use parallel-line-based coupling elements. The decoupling between the output ports of the proposed DMN and the reflection coefficients at these ports is obtained using the scattering parameters of the coupled antenna array and those of the DMN. These are clearly expressed as functions of the DMN's design parameters. These design parameters are determined using a simple numerical method to provide high inter-element isolation and good impedance matching for the array. To verify the proposed synthesis procedure, three DMN example designs are considered. These include one for a single-band array and two for dual-band arrays having different frequency ratios. Prototypes of these antennas along with their DMNs were designed, fabricated, and experimentally characterized. In each case, a good agreement between the measured and theoretical results is achieved. Simulation and measurement results show that the proposed DMN design and its associated synthesis procedure can be employed to significantly improve the interelement isolation of single- and dual-band, two-element arrays in various scenarios.

10–60-GHz Electromechanical Resonators Using Thin-Film Lithium Niobate

Abstract: This work presents a new class of microelectromechanical system (MEMS) resonator toward 60 GHz for the fifth-generation (5G) wireless communications. The wide range of the operating frequencies is achieved by resorting to different orders of the antisymmetric Lamb wave modes in a 400-nm-thick Z-cut lithium niobate thin film. The resonance of 55 GHz demonstrated in this work marks the highest operating frequency for piezoelectric electromechanical devices. The fabricated device shows an extracted mechanical $Q$ of 340 and an $f\times Q$ product of $1.87\times 10^{13}$ in a footprint of $2 \times 10^{-3}$ mm2. The performance has shown the strong potential of LiNbO3 antisymmetric mode devices for front-end applications in 5G high-band.

Analysis and Design of Highly Efficient Wideband RF-Input Sequential Load Modulated Balanced Power Amplifier

Abstract: The analysis and design of an RF-input sequential load modulated balanced power amplifier (SLMBA) are presented in this article. Unlike the existing LMBAs, in this new configuration, an over-driven class-B amplifier is used as the carrier amplifier while the balanced PA pair is biased in class-C mode to serve as the peaking amplifier. It is illustrated that the sequential operation greatly extends the high-efficiency power range and enables the proposed SLMBA to achieve high back-off efficiency across a wide bandwidth. An RF-input SLMBA at 3.05–3.55-GHz band using commercial GaN transistors is designed and implemented to validate the proposed architecture. The fabricated SLMBA attains a measured 9.5–10.3-dB gain and 42.3–43.7-dBm saturated power. Drain efficiency of 50.9–64.9/46.8–60.7/43.2–51.4% is achieved at 6-/8-/10-dB output power back-off within the designed bandwidth. By changing the bias condition of the carrier device, higher than 49.1% drain efficiency can be obtained within the 12.8-dB output power range at 3.3 GHz. When driven by a 40-MHz orthogonal frequency-division multiplexing (OFDM) signal with 8-dB peak-to-average power ratio (PAPR), the proposed SLMBA achieves adjacent channel leakage ratio (ACLR) better than −25 dBc with an average efficiency of 63.2% without digital predistortion (DPD). When excited by a ten-carrier 200-MHz OFDM signal with 10-dB PAPR, the average efficiency can reach 48.2% and −43.9-dBc ACLR can be obtained after DPD.

Digital Predistortion of 5G Massive MIMO Wireless Transmitters Based on Indirect Identification of Power Amplifier Behavior With OTA Tests

Abstract: In this article, we present a novel digital predistortion (DPD) architecture for multiple-input–multiple-output (MIMO) transmitters using a real-time single-channel over-the-air (OTA) data acquisition loop. The proposed feedback data acquisition strategy captures OTA signals from a fixed location and indirectly identifies the nonlinear behavior of all power amplifiers (PAs) in the array, as well as their combined signals in the far-field direction. The DPD can, therefore, be effectively constructed without direct measurement at PA output or at user end. The proposed linearization solution can run in real-time and, thus, does not interfere with data transmission in the MIMO transmitters. It can also achieve robust performance when mutual coupling occurs between antenna elements. Simulation and experimental results demonstrate that the proposed scheme can accurately estimate both PA outputs and far-field main beam data. Excellent linearization performance can be achieved with low complexity hardware implementation and reduced computational complexity.

A 37–42-GHz 8 × 8 Phased-Array With 48–51-dBm EIRP, 64–QAM 30-Gb/s Data Rates, and EVM Analysis Versus Channel RMS Errors

Abstract: This article presents a 5G 37–42-GHz $8\times 8$ phased array. The array is based on $2\times 2$ SiGe transmit/receive (TRX) beamformer chips in the SiGe technology with 6 bits of phase control and 8 bits of gain control. A detailed study is presented, showing the effects of array-level amplitude and phase errors on the radiated error vector magnitude (EVM) of 64-element arrays. The array scans to ±60° in the azimuth plane and ±50° in the elevation plane with low sidelobes. The measured peak effective isotropic radiated power (EIRP) is 51 dBm at Psat with a 3-dB bandwidth of 36–41.5 GHz. A 39-GHz communication system is also demonstrated along with a high-pass filter and an integrated upconverter/downconverter and achieves a local oscillator (LO) and image rejection level of 50 dBc, meeting FCC requirements of <−13 dBm/MHz of total leakage power. The array achieves < 5% EVM (−26 dB) using a 64-QAM 200-MHz waveform at an average EIRP of 44 dBm over all scan angles, including the LO and upconverter/downconverter contributions. A 30-Gb/s communication link with 64-QAM modulation is also shown. To our knowledge, this is the first demonstration of a 39-GHz phased-array communication system for 5G applications.

A 22–44-GHz Phased-Array Receive Beamformer in 45-nm CMOS SOI for 5G Applications With 3–3.6-dB NF

Abstract: This article presents a 22–44-GHz phased-array receive beamformer in the GlobalFoundries (GF) 45-nm CMOS SOI. The channel includes a wideband single-ended to differential low-noise amplifier (LNA), a 5-bit vector modulator (VM) phase shifter with a measured rms error of <6°, an attenuator, and a variable gain amplifier (VGA) with 16 dB of gain control. The phased-array channel results in a peak gain of 26.3 dB and a 3-dB bandwidth of 20.5–44 GHz. The measured NF is 3–3.6 dB at 22–44 GHz with an IP1dB of −27.5 to −24.5 dBm and dc power consumption of 112 mW. To the best of our knowledge, this is the first wideband phased-array beamformer that covers the entire millimeter-wave 5G band with high linearity and should be suitable for next-generation 5G systems.

Doppler Cardiogram: A Remote Detection of Human Heart Activities

Abstract: Most medical instruments invented to measure human heart activities, such as electrocardiograms (ECGs), rely on contact electrodes. This causes discomfort and limits application scenarios. Remote acquisition of $\mu \text{V}$ -level bioelectrical cardiac signals through ECG measurement is theoretically challenging. Based on the analysis of magnetic resonance imaging to the volume change of human hearts, we found that a single radar sensor can be used to remotely detect a Doppler cardiogram (DCG) at a distance up to 1 m, by retrieving Doppler signals induced by combined atrial and ventricular motions conducted to the skin of the back chest. This DCG can provide all the timing information of the P-wave, QRS-waves, and T-wave carried in ECGs. The implemented miniature remote sensor could be an ideal portable instrument for patients, such as burn victims, extending application scenarios to personal healthcare, battlefield rescue, and others.

Pseudo-Doherty Load-Modulated Balanced Amplifier With Wide Bandwidth and Extended Power Back-Off Range

Abstract: This article presents a novel architecture of load-modulated balanced amplifier (LMBA) with a unique load-modulation characteristic different from any existing LMBAs and Doherty power amplifiers (DPAs), which is named pseudo-Doherty LMBA (PD-LMBA). Based on a special combination of control amplifier (carrier) and balanced amplifier (peaking) together with proper phase and amplitude controls, an optimal load-modulation behavior can be achieved for PD-LMBA, leading to maximized efficiency over extended power back-off range. More importantly, the efficiency optimization can be achieved with only a static setting of phase offset at a given frequency, which greatly simplifies the complexity for phase control. Furthermore, the cooperations of the carrier and peaking amplifiers in PD-LMBA are fully decoupled, thus lifting the fundamental bandwidth barrier imposed on the Doherty-based active load modulation. Upon theoretical proof of these discoveries, a wideband RF-input PD-LMBA is physically developed using the GaN technology for experimental demonstration. The prototype achieves a highly efficient performance from 1.5 to 2.7 GHz, e.g., 58%–72% of efficiency at 42.5-dBm peak power and 47%–58% at 10-dB output back-off (OBO). When stimulated by a 10-MHz long term evolution (LTE) signal with a 9.5-dB peak-to-average power ratio (PAPR), the developed PD-LMBA achieves an efficiency of 44%–53% over the entire bandwidth at an average output power of around 33 dBm.

Enhancing the Sensitivity of Dielectric Sensors With Multiple Coupled Complementary Split-Ring Resonators

Abstract: This article presents a new concept for sensitivity enhancement of dielectric sensors by loading a microstrip line with multiple coupled resonators The concept is based on the interresonator coupling mechanism, which is represented as a mutual capacitance in an equivalent lumped circuit model This mutual capacitance increases the sensitivity of the sensor (in terms of a shift in the resonance frequency) to detect the presence of dielectric materials (ie, detecting changes in the real-relative permittivity of dielectric laminates) Complementary split-ring resonators (CSRRs) are utilized and coupled to a splitter-combiner microstrip section to design a 2CSRR and a 4CSRR sensor The new sensors provided an appreciable enhancement in the sensitivity when detecting dielectric material The concept is tested using full-wave numerical simulations to detect variations in the dielectric constant of a slab Finally, full experimental validation is provided using fabricated sensors

Design of E- and W-Band Low-Noise Amplifiers in 22-nm CMOS FD-SOI

Abstract: This article presents E- and W-band low-noise amplifiers (LNA) in GlobalFoundries 22-nm CMOS fully depleted silicon-on-insulator (FD-SOI). Both amplifiers employ a three-stage cascode design with gain-boosting transformer loads. Design procedures are presented for E- and W-band LNAs for narrowband and wideband applications. The E-band LNA focuses on a high-gain, low-power implementation, and results in a gain and noise figure (NF) of 20 and 4.6 dB at 77 GHz with a 3-dB bandwidth of 12 GHz, and an input P1dB of −27.4 dBm, for a power consumption of 9 mW. The W-band LNA focuses on wideband applications and results in a peak gain of 18.2 dB with a 3-dB bandwidth of 31 GHz, for a power consumption of 16 mW. The LNAs have a high figure-of-merit (FoM) and show very low-power operation in the 70–100 GHz range. Application areas are in phased arrays for 5G with hundreds or thousands of elements, automotive radars at 77 GHz, and sensors at 94 GHz.

Broadband RF-Input Continuous-Mode Load-Modulated Balanced Power Amplifier With Input Phase Adjustment

Abstract: This article presents the theory and design methodology of broadband RF-input continuous-mode load-modulated balanced power amplifier (CM-LMBA) by introducing the CM output-matching networks in the LMBA architecture. It is illustrated that the CM impedance condition can be achieved by properly adjusting the phase difference between the different PA branches in the proposed CM-LMBA during the entire load modulation process. An RF-input CM-LMBA with 1.45–2.45-GHz bandwidth using commercial GaN transistors is designed and implemented to validate the proposed architecture. The fabricated CM-LMBA attains a measured 11.2–13.4-dB gain and around 40-W saturated power. Power-added efficiency (PAE) of 46.4%–56.5% and 43.2%–50.3% is achieved at 6- and 8-dB output power back-offs throughout the designed band. When driven by a 100-MHz OFDM signal with an 8-dB peak-to-average power ratio (PAPR), the proposed CM-LMBA achieves better than −46-dBc adjacent channel leakage ratio (ACLR) and higher than 45% average PAE after digital predistortion at 1.8 and 2.1 GHz.

2 $\times$ 64-Element Dual-Polarized Dual-Beam Single-Aperture 28-GHz Phased Array With 2 $\times$ 30 Gb/s Links for 5G Polarization MIMO

Abstract: This article presents a 5G 28-32 GHz 2 × 64-element dual-polarized (DP) dual-beam transmit/receive (TRX) phased array. The array is based on a SiGe 2 × 4 TRX dual-beamformer chip with 6 bits of phase and 25 dB of gain control. The chip delivers 11-12 dBm/channel in the transmit-mode and has a noise figure (NF) of 4.8 dB in the receive-mode. Sixteen chips are employed for the construction of a low-cost printed circuit board (PCB) based 2 × 64-element dual-beam array using flip-chip technology. The phased-array has two 1:16 dual Wilkinson networks and microstrip antennas with rotated feeds for cross-polarization cancellation. The array demonstrates a measured effective isotropic radiated power (EIRP) at Psat of 52 dBm for each beam and is capable of scanning ±50° in azimuth and ±25° in elevation with >28-dB cross-polarization rejection. Simultaneous dual-beam operation is demonstrated with near-ideal patterns for each beam. The array demonstrates independent simultaneously transmitted 2 × 16-quadrature amplitude modulation (QAM) and 2 × 64QAM data streams delivering an aggregate maximum data rate of 2 × 20 and 2 × 30 Gb/s, respectively. Also, measurements done over all scan angles at an EIRP of 41 dBm per polarization and 64-QAM waveforms show a data rate of 2 × 4.8 Gb/s with an EVM ≤ -25 dB. To our knowledge, this is the first demonstration of a dual-polarized dual-beam phased array for 5G polarization-based multiple-input-multiple-output (MIMO) systems with 60-Gb/s maximum data rates.

A Highly Sensitive Planar Microwave Sensor for Detecting Direction and Angle of Rotation

Abstract: This article presents a technique based on a modified complementary split-ring resonator (CSRR) to detect angular displacement and direction of rotation with high resolution and sensitivity over a wide dynamic range. The proposed microwave planar sensor takes advantage of the asymmetry of the sensor geometry and measures the angle of rotation in terms of the change in the relative phase of the reflection coefficients. The sensor consists of a movable modified CSRR (the rotor) and a microstrip line with a circular defect in the ground plane (the stator). By selecting the substrate material and the rotor thickness, the sensor can be designed to work at different operating frequencies. A theoretical model of the sensor is proposed and is followed by a detailed numerical analysis involving equivalent circuit simulations, full-wave computations, and measurement results. Using positioning error estimation and air-gap analysis, a technique based on phase-change measurements is found to be better than those based on magnitude measurements alone. The maximum sensitivity for measuring the angular rotation is found to be a 4.3° change in the relative phase of the reflection coefficient per 1° of rotation. The sensor has an angular measurement range from −90° to +90°. The sensor—a stator fabricated on a 0.5-mm-thick Rogers RT5880 substrate and three rotors fabricated on a 1.5-mm-thick Rogers RT5880, a 1-mm-thick FR4, and a 0.5-mm-thick Rogers RT5880—can effectively detect the direction of rotation, measure the angle of rotation and angular velocity with reasonable accuracy.

A 0.096-mm $^{2}~1$ –20-GHz Triple-Path Noise- Canceling Common-Gate Common-Source LNA With Dual Complementary pMOS–nMOS Configuration

Abstract: This article proposes a novel wideband common-gate (CG) common-source (CS) low-noise amplifier (LNA) with a dual complementary pMOS–nMOS configuration to provide a current-reuse output. Triple-path noise-cancellation is effectively revealed to eliminate the thermal noise of the two CG transistors. Simultaneously, partial cancellation of intrinsic third-order distortion of output-stage transistors improves the input third-order intercept point (IIP3). In addition, we embed a resistive feedback in one of the auxiliary CS amplifiers to balance the multiple tradeoffs between noise figure (NF), input matching (S11), and forward gain (S21). Fabricated in 65-nm CMOS, the proposed wideband LNA exhibits an IIP3 of 2.2–6.8 dBm and an NF of 3.3–5.3 dB across a 19-GHz BW while consuming 20.3 mW at 1.6 V. S11 is $\pi $ -type input-matching network. The LNA exhibits a peak $\text{S}_{\vphantom {D_{j}}21}$ of 12.8 dB and occupies a very compact die area of 0.096 mm2.

Variable-Exponent Lebesgue-Space Inversion for Brain Stroke Microwave Imaging

Abstract: This article describes a microwave tomographic approach for the quantitative imaging of brain stroke inside the human head. For the acquisition of the scattered-field information, a prototype of multistatic system is adopted. An array of custom antennas is placed in contact with the head, and a switching matrix is used to measure the scattering parameters for each pair of probes. The collected data are processed by an inversion method based on a variable-exponent Lebesgue-space regularization technique, whose outcome is a map of dielectric properties of a slice of the head. With respect to previous approaches, this kind of inversion procedure performs an adaptive update of the Lebesgue-space exponents on the basis of the results at each inexact-Newton iteration and exploits stepped frequency data. This allows for an automatic setting of the regularization level, which becomes variable and target-dependent inside the whole investigation domain. The proposed approach is validated by means of FDTD synthetic simulations with a realistic 3-D forward scattering model of the human head, as well as by using real experimental cylindrical test phantoms filled with saline and glycerin/water mixtures.

Multitarget Respiration Detection With Adaptive Digital Beamforming Technique Based on SIMO Radar

Abstract: The Doppler radar has been widely used in respiration detection. However, most of the existing microwave respiration detection works are intended for either a single human subject in front of the radar or multiple subjects with known positions. In this article, a system based on single-input–multiple-output (SIMO) continuous-wave (CW) radar equipped with adaptive digital beamforming (ADBF) technique is presented to detect the respiration of multiple human subjects at unknown positions simultaneously. A solution based on the modified Capon (m-Capon) direction-of-arrival (DOA) estimation and linear constraint minimal variance (LCMV) ADBF is proposed to automatically find the angles of human subjects. By forming spatially distributed beams toward the subjects of interest, human respiration can be remotely obtained. Furthermore, for the detection of each person’s respiration, nulls are also generated at the angles of nearby interfering subjects, which results in high target discrimination capability when multiple human subjects are close to each other. The experimental results show good detection accuracy compared with the reference sensor, which verifies the effectiveness of the developed system for multitarget respiration detection.

RF-Harvesting Tightly Coupled Rectenna Array Tee-Shirt With Greater Than Octave Bandwidth

Abstract: This article presents 16and 81-element broadband rectenna arrays screen printed on a cotton tee-shirt for harvesting 4-130-μW/cm 2 power densities between 2 and 5 GHz. A packaged SMS7630-079LF Schottky diode is connected with silver paint and soldered to each element of the array. The diode impedance over frequency as a function of dc load and input power is analyzed using source-pull harmonic-balance simulations. The antenna is a quasi-self-complementary tightly coupled bow-tie array with a period of about λ 0 /6 at the highest frequency. The impedance at the element ports of the array and the array radiation pattern are analyzed with source-pull diode complex impedance port terminations. Full-wave simulations are performed with the cotton fabric on top of specific tissue layer stack-up for a human torso, as well as for a body phantom. Measurements using a water-filled phantom show up to P dc = 32 μW for incident power densities of 4 μW/cm 2 , with a dc load of Rdc = 2 kQ. At higher incident power density levels of 100 μW/cm 2 , up to 32% efficiency is measured. Measured rectified voltage and efficiency for the wearable tee-shirt are in good agreement with phantom measurements for the 81-element array. In addition, the effects of body curvature, air layer between tee-shirt and skin, and washing of the fabric are quantified in either simulation or measurements.

Radiative Near-Field Wireless Power Transfer to Scalp-Implantable Biotelemetric Device

Abstract: Monitoring of elevated intracranial pressure (ICP) is a lifesaving procedure. This article presents an ultrasmall batteryless implantable system for ICP monitoring comprising an off-body power transmitter (Tx) and in-body biotelemetric receiver (Rx). Due to the small implantable antenna (5.6 mm $\times 6$ mm $\times0.2$ mm = 6.72 mm3), the device exhibits dual-band characteristics (i.e., 915 and 1900 MHz) for simultaneous power transmission and data telemetry. The implant is powered wirelessly in the radiative near field (1900 MHz) for enhanced power transfer efficiency (PTE). Moreover, the structure demonstrates the measured peak gain values of −26.8 and −18.8 dBi with the impedance-matched bandwidths of 9.83% and 27.9% at 915 and 1900 MHz, respectively. The wireless PTE of the device was also analyzed in terms of distance variations, and a maximum PTE up to −25.9 dB at 20 mm ( $0.1267\lambda $ ) Tx–Rx separation was achieved. The rectifier attained a maximum power conversion efficiency of 82% at 2-dBm input power. Simulations using the finite-element method and finite difference time domain were performed to evaluate the biotelemetric implantable system. For validation, measurements were conducted in a saline-filled human head phantom, as well as in minced pork. The biotelemetric system yields good agreement between the measured and simulation results.

Supercompact and Ultrawideband Surface Plasmonic Bandpass Filter

Abstract: We propose a supercompact and ultrawideband (UWB) bandpass filter for spoof surface plasmonic polariton (SSPP) mode. By using meander-line technology, the electrical size of the proposed SSPP structure is reduced dramatically, which is only about 1/4 of the traditional rectangular-groove structure with the same geometry. Based on the proposed SSPP structure, we construct a supercompact UWB bandpass filter working in the frequency range of 0.25–4.5 GHz, whose geometric size is only $87.5\times12.8$ mm2. The upper cutoff frequency of the passband can be controlled by changing the meander-line design of the SSPP unit, and the lower cutoff frequency of the passband is determined by the capacitive coupling of the feeding structure. Both the simulation and experimental results show that the UWB SSPP bandpass filter is centered at 2.4 GHz with an extremely wide relative bandwidth of 174%, whose electrical size is only $0.7\lambda \,\,\times 0.1\lambda $ at the central frequency.

Design of Waveguide Filters With Cascaded Singlets Through a Synthesis-Based Approach

Abstract: In this article, we introduce a synthesis-based design approach for waveguide filters, including singlets. As is known, a singlet block comprises one resonator coupled to source and load plus an additional coupling between source and load. In this way, a pole–zero pair is produced, thus allowing the introduction of a transmission zero in the frequency response. Structure making that uses cascaded singlets (especially implemented in waveguide technology) has been successfully used to realize compact and quasi-in-line pseudoelliptic filters. The design of this configuration suffers, however, from a severe drawback; while an isolated singlet can be exactly synthesized, when these blocks are put in cascade, nonresonating nodes (NRNs) are required to allow the cascade connection. Consequently, the resulting topology includes both cross couplings and NRNs, for which no exact synthesis solution is till now available (all the design solutions proposed in the literature are based on optimization). The new design procedure here proposed overcomes this drawback, allowing the synthesis of the equivalent circuit of the cascaded-singlet topology. Once this circuit is obtained, the dimensioning of the singlets can be carried out by exploiting a full-wave simulator according to the procedure described in the article. The proposed design approach allows a very accurate initial dimensioning of the filter, which represents an excellent starting point for final adjustments/optimizations. A detailed design example is presented in the article showing how to practically implement the novel procedure. This new approach is also experimentally validated by a waveguide filter prototype, previously designed by optimization and fabricated. It is shown how this filter can be redesigned with the novel procedure, obtaining very similar physical dimensions in a noticeably shorter time.

Integration of Interdigitated Electrodes in Split-Ring Resonator for Detecting Liquid Mixtures

Abstract: A microwave microfluidic sensor for detecting binary liquid mixtures with a dielectric method at RF/microwave frequencies is presented in this article. The sensor is based on a split-ring resonator (SRR) that is implemented in a microstrip transmission line, with interdigitated electrodes (IDEs) being integrated into the ring for liquid detection. Based on the equivalent circuit of the IDE-SRR device and with a series of finite element simulations, the detection theory is developed, and the device design optimization is investigated. The validation measurements on water–isopropanol liquid mixtures with various concentrations show that the proposed IDE-SRR sensor has higher sensitivity than the previous standard SRR sensor. The IDE-SRR sensor is then used to detect two binary liquids, i.e., water–methanol mixtures and water–tetrahydrofuran mixtures. The measured effective permittivity results of the binary mixtures at RF/microwave frequency range are compared with the existing mixing models for binary dielectric mixtures at zero frequency.

Balanced-to-Doherty Mode-Reconfigurable Power Amplifier With High Efficiency and Linearity Against Load Mismatch

Abstract: A balanced-to-Doherty (B2D) mode-reconfigurable power amplifier (PA) is presented in this article, which is endowed with a unique capability of maintaining high linearity and high efficiency against load mismatch. The Doherty operation of this PA is based on a new Doherty PA (DPA) architecture configured from an ideal balanced amplifier, named quasi-balanced DPA (QB-DPA). This article, for the first time, analytically proves that the QB-DPA is functionally equivalent to a standard DPA. Most importantly, this new discovery enables PA reconfiguration between the Doherty and balanced modes. With the tunability implemented using a silicon-on-insulator (SOI)-based single-pole-double-throw (SPDT) switch, a reconfigurable B2D PA prototype using GaN technology is demonstrated at 3.5 GHz, exhibiting the state-of-the-art linear DPA performance in the nominal 50- $\Omega $ load condition. Specifically, the Doherty mode achieves a continuous-wave measurement efficiency of 70% and 54.5% at the maximum output power of 41.9 dBm and 6-dB power back-off, respectively. In the modulated long-term evolution (LTE) evaluation, the DPA exhibits −37-dB adjacent channel power leakage (ACPR) and 2.36% error vector magnitude (EVM) at the maximum rated power of 34.5 dBm while achieving a 42.4% efficiency. It is experimentally demonstrated that the Doherty (QB-DPA) mode is well resistant to load mismatch with high efficiency across a majority portion of the 2: 1 voltage standing wave ratio (VSWR) circle, while the combination of Doherty and balanced modes can ensure a constantly linear performance of the B2D PA (e.g., 2.2%-5% of EVM) under the entire mismatch condition.

3-D-Printed Modified Butler Matrix Based on Gap Waveguide at W-Band for Monopulse Radar

Abstract: This article presents the design and fabrication of low-loss, -weight, and -cost, modified Butler matrix in groove gap waveguide technology at W-band, implemented by additive manufacturing. The design, simulations, and measurements of each of the elements that conform to the modified Butler matrix are presented. The proposed modified Butler matrix is asymmetric and oriented to an easy assembly with a radial antenna in order to obtain simultaneous sum and difference patterns for a narrowband radar application. The measured individual components of the modified Butler matrix fit very well with the simulations. As for the full modified Butler matrix, measurements provide a 0.7% bandwidth below −20 dB for sum and difference ports centered at 94 GHz. The amplitude and phase imbalance are below 1.5 dB and 20°, respectively. This narrow bandwidth is due to the difference in length between input and output ports due to the asymmetry of the design. The total losses of the modified Butler matrix are below 1 dB.

High-Order Dual-Port Quasi-Absorptive Microstrip Coupled-Line Bandpass Filters

Abstract: In this article, we present the first demonstration of distributed and symmetrical all-band quasi-absorptive filters that can be designed to arbitrarily high orders. The proposed quasi-absorptive filter consists of a bandpass section (reflective-type coupled-line filter) and absorptive sections (a matched resistor in series with a shorted quarter-wavelength transmission line). Through a detailed analysis, we show that the absorptive sections not only eliminate out-of-band reflections but also determine the passband bandwidth (BW). As such, the bandpass section mainly determines the out-of-band roll-off and the order of the filter can be arbitrarily increased without affecting the filter BW by cascading more bandpass sections. A set of 2.45-GHz one-, two-, and three-pole quasi-absorptive microstrip bandpass filters are designed and measured. The filters show simultaneous input and output absorption across both the passband and the stopband. Measurement results agree very well with the simulation and validate the proposed design concept.

Multi-port Active Load Pulling for mm-Wave 5G Power Amplifiers: Bandwidth, Back-Off Efficiency, and VSWR Tolerance

Abstract: The opening of spectral bands in the millimeter-wave (mm-Wave) spectrum from 26 GHz and extending up to the E-band poses new challenges to the power amplifier (PA) design for spectrally agile radios. They are expected to operate with high energy efficiency at peak and back-off levels to process signals with high peak-to-average power ratio (~10 dB), while being able to maintain their performance across a wide range of 5G bands. In addition, the PAs can experience strong load impedance mismatch conditions in a user equipment (UE) that pose additional challenges in handling strong voltage-standing-wave-ratio (VSWR) events. In this article, we present a systematic approach to exploit active load pulling in a multi-port network that synthesizes optimal impedance conditions for 1) broadband peak and back-off operation and 2) mitigating VSWR events at peak power. As proofs of concept, we present two PAs in 65-nm bulk CMOS process. The first chip demonstrates $P_{\text {sat}}$ between 16.3 and 19.3 dBm across 37–73 GHz, with an improvement in the output drain efficiency ( $\eta _{\text {out}}$ ) of up to $3.2\times \!\!/5.8\times $ at 6-/9.6-dB power back-off (PBO) across the frequency range compared to class-A operation. The second chip achieves 26–42-GHz $P_{\text {sat},\, -1 \, {\text{dB}}}$ bandwidth with $P_{\text {sat}} > 19$ dBm and PAE $_{\text {peak}}> 20$ % across all 28–40-GHz bands and with up to $3.35\times $ and $4.84\times $ enhancement in PAE at the PBO levels of 6 and 9.6 dB over class-A operation, respectively. The PA also demonstrates strong tolerance to VSWR events with only 2 dB degradation over a VSWR 4:1 load circle at a frequency of 33 GHz.