David S. Ricketts

Three-dimensional position and orientation measurements using magneto-quasistatic fields and complex image theory [measurements corner]

Traditional wireless position-location systems, operating using propagating waves, suffer reduced performance in non-line-of-sight (NLoS) applications. Traditional systems that use quasistatic fields have instead been limited to short ranges, progressive direction-finding applications, require RF fingerprinting, or do not provide complete immunity to dielectric obstacles (use of electric fields). These limitations impose severe restrictions in applications such as tracking an American football during game play, where position and orientation tracking may be required over long ranges, and when the line-of-sight (LoS) is blocked by groups of people. A technique using magneto-quasistatic fields and complex image theory was recently shown to circumvent these problems, and to enable accurate long-range one-dimensional and two-dimensional measurements. In this work, we present three-dimensional position and orientation measurements using the magneto-quasistatic system and complex image theory over an area of 27.43 m × 27.43 m. Inverting the theoretical expression for the voltage measured at the terminals of the receiving loops to determine three-dimensional position and orientation resulted in mean and median geometric position errors of 0.77 m and 0.71 m, respectively; inclination orientation mean and median errors of 9.67° and 8.24°, respectively; and azimuthal orientation mean and median errors of 2.84° and 2.25°, respectively.

An Efficient, Watt-Level Microwave Rectifier Using an Impedance Compression Network (ICN) With Applications in Outphasing Energy Recovery Systems

We present a transmission line based impedance compression network (ICN) for application in RF-to-dc conversion. We show that the ICN is able to significantly compress the undesired input impedance variation that occurs when there is a large variation of input power. We designed and measured an ICN for a 4 W, 4.6 GHz rectifier and show that the impedance is significantly compressed with the ICN. We then demonstrate the ICN in an outphasing system where we achieve up to a 37% improvement in efficiency at back-off power and an overall more efficient design over large input power variation.

On the efficient wireless power transfer in resonant multi-receiver systems

In this paper we present an analysis of resonant wireless power transfer in systems with multiple receivers. We show that maximum power transfer can be achieved when the source is impedance matched to the set of receivers, i.e. matched to their equivalent impedance as seen by the source. The interaction of the receivers, or coupled modes, simply represent an interdependence of impedances that can be modeled and impedance matched. We explore three methods to achieve impedance matching: frequency tuning, impedance transformation and resonant tuning and show that the later two can achieve the maximum theoretical power transfer for a wide range of coupling between receivers.

Tri-Loop Impedance and Frequency Matching With High-

Resonant wireless power transfer (WPT) using magneto quasi-static fields is an attractive means for delivering power in many applications. Central to the efficient delivery of power in these systems is the use of high-ratio impedance transformers to impedance-match the load to the source via the WPT network. Mini-loop transformers, which use a resonant tank for impedance matching, have become popular as they provide high-ratio impedance matching and low loss. Key to the low loss is the use of high- Q resonators. There are two challenges with this approach. The first is impedance matching the resonators to source/load impedances, and the second is adjusting the center frequency of resonance to exactly match the operating frequency, which can be very difficult with helical coils that use their parasitic capacitance for resonance. In this letter, we introduce a tri-coil impedance-matching network for WPT that provides accurate impedance matching and precise frequency tuning for high- Q coils to overcome these limitations.

Q

Impedance matching to loads that vary with system operating point presents a significant challenge for efficient power transfer. One such load is the RF-to-dc rectifier, whose input resistance varies with rectified power. A resistance compression network (RCN) based on lumped elements was proposed to solve this problem and has demonstrated compressed impedance variation at 48 MHz [3]. At microwave frequencies, however, lumped elements become less viable and the RCN in [3] does not scale well. In this work we present a transmission line based RCN that operates at microwave frequencies. The RCN consists of two microstrip lines with unequal length. We present the theory, the design and an experimental prototype that shows the TRCN is able to compress a large load resistance variation ratio of 20:1 into a ratio of 3:1.

Resonators in Wireless Power Transfer

This paper presents a W-band power amplifier (PA) in 45 nm SOI CMOS. The PA incorporates an 8-way zero-degree combiner to efficiently combine 8 parallel PA units, each of which is a 2-stage cascode PA. At 80 GHz, the PA achieves a saturated output power (Psat) of 21.1 dBm, 10.1 dB peak gain, 5.2% peak PAE, and 12 GHz 3-dB bandwidth, and it consumes 1 mm2 of die area. The Psat of 21.1 dBm is the highest among reported W-band PAs in CMOS technology.

A transmission line based resistance compression network (TRCN) for microwave applications

Maximum power transfer and maximum efficiency are two important design constraints in wireless power transfer applications. Several works have investigated the proper load and impedance match conditions to optimize either efficiency or power transfer. In this paper we show that the optimal load for maximum power transfer and maximum efficiency is the same (a conjugate matched load) when the source resistance is zero. This is important, as many WPT systems have a relatively low, unknown source impedance. Since the optimal load for both efficiency and power is the same as the source impedance approaches zero, the designer can use a bi-conjugate load for a near optimal design for both maximum power and efficiency. As the source impedance becomes significant, the bi-conjugate matched system provides higher power, but at the expense of lower efficiency. Maximum efficiency is achieved with a non-bi-conjugate load, when the source impedance is non-negligible.

A W-band 21.1 dBm power amplifier with an 8-way zero-degree combiner in 45 nm SOI CMOS

We report on the magnetic field and coupling enhancement for increased wireless power transfer (WPT) efficiency using intermediate materials. We examine the physical mechanisms for enhancement using a metamaterial (MM) and magnetic resonant field enhancement (MR-FE) and present an analytical and simulation analysis as well as an experimental study of these enhancement mechanisms. While both increase the mutual coupling, the loss of the contrasting enhancement mechanisms significantly impacts WPT efficiency enhancement. Our analysis shows that the MR-FE approach can have up to a 4-times-higher efficiency over the MM approach due to the lower loss of its field enhancement mechanism.

Optimization of WPT efficiency using a conjugate load in non-impedance matched systems.

We present the theory of high-order modulation for near-field RF identification (RFID) and wireless power transfer (WPT) systems. We show that while related, the design of RFID and WPT systems differ. The theory and calculation of load modulated quadrature amplitude modulation (QAM) and phase shift keying (PSK) is presented. We then present two experimental prototypes. The first demonstrates a 16-QAM RFID link achieving 480 kb/s at a 2.38-MHz carrier (>19.8 fractional bandwidth), significantly higher than the 1% fractional bandwidth of traditional RFID systems. The second experimental prototype demonstrates 4-PSK for WPT applications achieving a data rate 256 kb/s at a 2.38-MHz carrier (a 10.7% fractional bandwidth) with an average efficiency reduction of only 4%.

Magnetic Field Enhancement in Wireless Power With Metamaterials and Magnetic Resonant Couplers

Position and orientation tracking in the magnetoquasistatic region of an electrically small transmitting loop antenna make use of complex image theory (CIT), which is an approximate algebraic model of the fields above a lossy dielectric, semiinfinite half-space. Experimental demonstrations of CIT from a transmitting loop that approximates a horizontal magnetic dipole (HMD) have been reported previously. This work reports the first experimental demonstration of CIT from a vertical magnetic dipole (VMD) in remote sensing and position-tracking applications.