An RF-DC power conversion system is designed to efficiently convert far-field RF energy to DC voltages at very low received power and voltages. Passive rectifier circuits are designed in a 0.25 mum CMOS technology using floating gate transistors as rectifying diodes. The 36-stage rectifier can rectify input voltages as low as 50 mV with a voltage gain of 6.4 and operates with received power as low as 5.5 muW(22.6 dBm). Optimized for far field, the circuit operates at a distance of 44 m from a 4 W EIRP source. The high voltage range achieved at low load current make it ideal for use in passively powered sensor networks.

Efficient Far-Field Radio Frequency Energy Harvesting for Passively Powered Sensor Networks

Neural implants have emerged over the last decade as highly effective solutions for the treatment of dysfunctions and disorders of the nervous system. These implants establish a direct, often bidirectional, interface to the nervous system, both sensing neural signals and providing therapeutic treatments. As a result of the technological progress and successful clinical demonstrations, completely implantable solutions have become a reality and are now commercially available for the treatment of various functional disorders. Central to this development is the wireless power transfer (WPT) that has enabled implantable medical devices (IMDs) to function for extended durations in mobile subjects. In this review, we present the theory, link design, and challenges, along with their probable solutions for the traditional near-field resonant inductively coupled WPT, capacitively coupled short-ranged WPT, and more recently developed ultrasonic, mid-field, and far-field coupled WPT technologies for implantable applications. A comparison of various power transfer methods based on their power budgets and WPT range follows. Power requirements of specific implants like cochlear, retinal, cortical, and peripheral are also considered and currently available IMD solutions are discussed. Patient's safety concerns with respect to electrical, biological, physical, electromagnetic interference, and cyber security from an implanted neurotech device are also explored in this review. Finally, we discuss and anticipate future developments that will enhance the capabilities of current-day wirelessly powered implants and make them more efficient and integrable with other electronic components in IMDs.

Wireless Power Transfer Strategies for Implantable Bioelectronics

This paper presents an efficient energy harvester for RF-powered sensor networks. The circuit is based on an improved multi-stage rectifier, which exploits a fully passive threshold self-compensation scheme to overcome the limitation due to the input dead zone. A CAD-oriented design methodology is also proposed, which is aimed at maximizing the overall power conversion efficiency of the harvester through an optimum trade-off among matching losses, power reflection and rectifier efficiency. According to the proposed methodology, a 915-MHz harvester comprising an integrated input matching network and a 17-stage self-compensated rectifier has been designed and fabricated in a 90-nm CMOS technology. The rectifier exhibits a remarkably low input power threshold, as it is able to deliver a 1-V dc output voltage to a capacitive load with a very small input power of -24 dBm (4 μW). When driving a 1-MΩ load, the device can supply a 1.2-V output with an input power of -18.8 dBm (13.1 μW). The achieved results exceed the performance of previously reported RF multi-stage rectifiers in standard analog CMOS technology.

A 90-nm CMOS Threshold-Compensated RF Energy Harvester

This expanded and thoroughly revised edition of Thomas H. Lee's acclaimed guide to the design of gigahertz RF integrated circuits features a completely new chapter on the principles of wireless systems. The chapters on low-noise amplifiers, oscillators and phase noise have been significantly expanded as well. The chapter on architectures now contains several examples of complete chip designs that bring together all the various theoretical and practical elements involved in producing a prototype chip. First Edition Hb (1998): 0-521-63061-4 First Edition Pb (1998); 0-521-63922-0

The Design of CMOS Radio-Frequency Integrated Circuits: RF CIRCUITS THROUGH THE AGES

A first proof-of-concept mm-sized implantable device using ultrasonic power transfer and a hybrid bi-directional data communication link is presented. Ultrasonic power transfer enables miniaturization of the implant and operation deep inside the body, while still achieving safe and high power levels (100 $\mu$ W to a few mWs) required for most implant applications. The current implant prototype measures 4 mm $\times $ 7.8 mm and is comprised of a piezoelectric receiver, an IC designed in 65 nm CMOS process and an off-chip antenna. The IC can support a maximum DC load of 100 $\mu$ W for an incident acoustic intensity that is $\sim $ 5% of the FDA diagnostic limit. This demonstrates the feasibility of providing further higher available DC power, potentially opening up new implant applications. The proposed hybrid bi-directional data link consists of ultrasonic downlink and RF uplink. Falling edge of the ultrasound input is detected as downlink data. The implant transmits an ultra-wideband (UWB) pulse sequence as uplink data, demonstrating capability of implementing an energy-efficient M-ary PPM transmitter in the future.

A mm-Sized Implantable Medical Device (IMD) With Ultrasonic Power Transfer and a Hybrid Bi-Directional Data Link

This paper presents a RF to DC conversion model for multi-stage rectifiers in UHF RFID transponders. An equation relating the RF power available from the antenna to the DC output voltage produced by a multi-stage rectifier is presented. The proposed model includes effects of the nonlinear forward voltage drop in diodes and impedance matching conditions of the antenna to rectifier interface. Fundamental frequency impedance approximation is used to analyze the resistance of rectifying diodes; parasitic resistive loss components are also included in the analysis of rectifier input resistance. The closed form equation shows insights into design parameter tradeoffs, such as power available from the antenna, antenna radiation resistance, the number of diodes, DC load current, parasitic resistive loss components, diode and capacitor sizes, and frequency of operation. Therefore, it enables the optimization of rectifier parameters for impedance matching with a low-cost printed antenna and shunt tuning inductor, in order to improve the RF to DC conversion efficiency and the operational distance of UHF RFID transponders. Three diode doublers and three multistage rectifiers were fabricated in a 130 nm CMOS process with custom no-mask added Schottky diodes. Measurements of the test IC are in good agreement with the proposed model.

A RF to DC Voltage Conversion Model for Multi-Stage Rectifiers in UHF RFID Transponders

The paper presents analysis of the input impedance, as well as the input capacitance and output resistance of diode doublers and multistage rectifiers. Tradeoffs between device sizes and the number of rectification stages are presented with an emphasis on low cost impedance matching. As a result, it is possible to achieve higher efficiency in RF to DC conversion with low cost impedance matching by using just a strap inductor between the antenna and the rectifier IC. Four diode doublers and three multistage rectifiers have been designed and fabricated using a 0.13/spl mu/m CMOS process with EEPROM. Measurement results are in agreement with the analysis.

Design of multistage rectifiers with low-cost impedance matching for passive RFID tags

With climate change, spring heatwaves have become frequent in the Mediterranean region. High temperatures combined with wind and low humidity are problematic for subtropical crops adapted to high humidity and mild climate. Avocado is a valuable crop-nutritionally and economically-and many new orchards are planted in Mediterranean areas. Spring heatwaves increase avocado fruitlets dropping, severely decreasing yields. Addressing and solving the problem are necessary to maintain the crop's profitability. This study presents a sprinkler-based canopy cooling method that uses the existing pressurized irrigation system. The study aimed to test the system's performance during spring heatwaves, after the flowering season, in avocado orchards cultivated in a semi-arid region. The experiments examined the effect of various sprinkler types with varying flow rates and installation methods: sprayers, sprinklers and pulsing sprinklers, on foliage temperature, stem water potential, salt accumulation in the leaf, fruitlet survival and yield. The system reduced leaf temperatures by approximately 10 °C, significantly decreasing the trees' drought stress and increasing yields by 8-12%. Using low-quality water is possible, but requires adjustments to avoid salt damage to the leaves. The system can mitigate heat stress, and provides a relatively simple solution for handling spring heatwaves. The evaporative cooling system is modeled for semi-desert and desert conditions; the dry, windy climate contributes to the method's effectiveness. 

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S. Lazar

Papers

A RF to DC Voltage Conversion Model for Multi-Stage Rectifiers in UHF RFID Transponders

Design of multistage rectifiers with low-cost impedance matching for passive RFID tags

Canopy-cooling systems applied on avocado trees to mitigate heatwaves damages

Foliar application of boron to improve ‘Hass’ avocado productivity