Time-Domain Measurement of the Frequency-Dependent Delay of Broadband Antennas

We present a new time-domain technique for direct measurement of the frequency-dependent delay of broadband antenna structures. By utilizing a series of fixed-bandwidth, variable center frequency waveforms synthesized in a photonics-based electromagnetic pulse shaper, we perform high time resolution (~54 ps over ~10 ns) measurements of several broadband antenna structures. Our results show excellent agreement with the delay extracted via time-domain impulse response measurements, however, compared to conventional impulse response measurements, our method ensures a higher signal-to-noise ratio because it relies on several relatively narrowband measurements as opposed to a single broadband measurement. In addition, the delay uncertainty in our technique (~54 ps) is significantly below that of impulse response measurements (~300 ps) where numerical differentiation of noisy data leads to substantial errors. Our technique is well suited as a diagnostic tool for future variable-frequency fixed-bandwidth systems.     

Design of reconfigurable slot antennas

In this paper the design of a compact, efficient and electronically tunable antenna is presented. A single-fed resonant slot loaded with a series of PIN diode switches constitute the fundamental structure of the antenna. The antenna tuning is realized by changing its effective electrical length, which is controlled by the bias voltages of the solid state shunt switches along the slot antenna. Although the design is based on a resonant configuration, an effective bandwidth of 1.7:1 is obtained through this tuning without requiring a reconfigurable matching network. Four resonant frequencies from 540-890 MHz are selected in this bandwidth and very good matching is achieved for all resonant frequencies. Theoretical and experimental behavior of the antenna parameters is presented and it is demonstrated that the radiation pattern, efficiency and polarization state of the antenna remain essentially unaffected by the frequency tuning     

DMesh: Incorporating Practical Directional Antennas in Multichannel Wireless Mesh Networks

Wireless mesh networks (WMNs) have been proposed as an effective solution for ubiquitous last-mile broadband access. Three key factors that affect the usability of WMNs are high throughput, cost-effectiveness, and ease of deployability. In this paper, we propose DMesh, a WMN architecture that combines spatial separation from directional antennas with frequency separation from orthogonal channels to improve the throughput of WMNs. DMesh achieves this improvement without inhibiting cost-effectiveness and ease of deployability by utilizing practical directional antennas that are widely and cheaply available (e.g., patch and yagi) in contrast to costly and bulky smart beamforming directional antennas. Thus, the key challenge in DMesh is to exploit spatial separation from such practical directional antennas despite their lack of electronic steerability and interference nulling, as well as the presence of significant sidelobes and backlobes. In this paper, we study how such practical directional antennas can improve the throughput of a WMN. Central to our architecture is a distributed, directional channel assignment algorithm for mesh routers that effectively exploits the spatial and frequency separation opportunities in a DMesh network. Simulation results show that DMesh improves the throughput of WMNs by up to 231% and reduces packet delay drastically compared to a multiradio multichannel omni antenna network. A DMesh implementation in our 16-node 802.11b WMN testbed using commercially available practical directional antennas provides transmission control protocol throughput gains ranging from 31% to 57%     

Uncertainty in microscale gas damping: Implications on dynamics of capacitive MEMS switches

Effects of uncertainties in gas damping models, geometry and mechanical properties on the dynamics of micro-electro-mechanical systems (MEMS) capacitive switch are studied. A sample of typical capacitive switches has been fabricated and characterized at Purdue University. High-fidelity simulations of gas damping on planar microbeams are developed and verified under relevant conditions. This and other gas damping models are then applied to study the dynamics of a single closing event for switches with experimentally measured properties. It has been demonstrated that although all damping models considered predict similar damping quality factor and agree well for predictions of closing time, the models differ by a factor of two and more in predicting the impact velocity and acceleration at contact. Implications of parameter uncertainties on the key reliability-related parameters such as the pull-in voltage, closing time and impact velocity are discussed. A notable effect of uncertainty is that the nominal switch, i.e. the switch with the average properties, does not actuate at the mean actuation voltage. Additionally, the device-to-device variability leads to significant differences in dynamics. For example, the mean impact velocity for switches actuated under the 90%-actuation voltage (about 150 V), i.e. the voltage required to actuate 90% of the sample, is about 129 cm/s and increases to 173 cm/s for the 99%-actuation voltage (of about 173 V). Response surfaces of impact velocity and closing time to five input variables were constructed using the Smolyak sparse grid algorithm. The sensitivity analysis showed that impact velocity is most sensitive to the damping coefficient whereas the closing time is most affected by the geometric parameters such as gap and beam thickness.     

Strain rate sensitivity of nanocrystalline Au films at room temperature

The effect of strain rate on the inelastic properties of nanocrystalline Au films was quantified with 0.85 and 1.76 μm free-standing microscale tension specimens tested over eight decades of strain rate, between 6 × 10^?6 and 20 s^?1. The elastic modulus was independent of the strain rate, 66 ± 4.5 GPa, but the inelastic mechanical response was clearly rate sensitive. The yield strength and the ultimate tensile strength increased with the strain rate in the ranges 575–895 MPa and 675–940 MPa, respectively, with the yield strength reaching the tensile strength at strain rates faster than 10^?1 s^?1. The activation volumes for the two film thicknesses were 4.5 and 8.1 b³, at strain rates smaller than 10^?4 s^?1 and 12.5 and 14.6 b³ at strain rates higher than 10^?4 s^?1, while the strain rate sensitivity factor and the ultimate tensile strain increased below 10^?4 s^?1. The latter trends indicated that the strain rate regime 10^?5–10^?4 s^?1 is pivotal in the mechanical response of the particular nanocrystalline Au films. The increased rate sensitivity and the reduced activation volume at slow strain rates were attributed to grain boundary processes that also led to prolonged (5–6 h) and significant primary creep with initial strain rate of the order of 10^?7 s^?1.     

Dispersion Limitations of Ultra-Wideband Wireless Links and Their Compensation Via Photonically Enabled Arbitrary Waveform Generation

In this study, we present compression of ultra-wideband RF waveforms via photonic synthesis of phase pre-compensated waveforms. By exciting a dispersive wireless link (employing Archimedean spiral antennas) with variable-bandwidth excitation waveforms, we first demonstrate that such links exhibit a dispersion-limited output pulse duration. Subsequently, we utilize the RF spectral phase extracted from the impulse response of the link to create signals designed to negate the nonuniform phase response of the spiral antennas utilized in the link. Such waveforms remove the dispersion limitation and enable bandwidth-limited operation. We achieve compression to within a factor of two of the bandwidth limit for signals with bandwidths of 1-10 GHz at a center frequency of ~6 GHz. To our knowledge, this represents the first demonstration of dispersion pre-compensation for signals with fractional bandwidths exceeding 100%. Our technique is reprogrammable and may be extended to larger bandwidths and higher operational frequencies, making it an enabler for future radar and communication systems.     

Tunable MEMS Spiral Inductors With Optimized RF Performance and Integrated Large-Displacement Electrothermal Actuators

We propose a tunable microelectromechanical systems integrated inductor with a large-displacement electrothermal actuator. Based on a transformer configuration, the inductance of a spiral inductor is tuned by controlling the relative position of a magnetically coupled short-circuited loop. The magnetic coupling between the inductors can be changed from 0.17 to 0.8 through an electrothermal actuator that can change their relative position by over 140 mum . For the first time, we investigate the impact of this tuning scheme on the inductance and quality factor and propose optimal designs. While a previous preliminary study has focused on keeping the ratio between the two coupled inductors close to one, we find that optimal performance is a weak function of this ratio. Instead, it primarily depends on the resistive loss of the short-circuited coil. Our theoretical studies are backed by a variety of fabricated and measured tunable inductors that show a ~2:1 inductance tuning ratio over a wide frequency range of approximately 25 GHz. In addition, the maximum and minimum quality factors of the tunable inductor are measured to be 26 and 10, respectively, which agree well with the theoretically expected values.     

A 6-Gb/s Wireless Inter-Chip Data Link Using 43-GHz Transceivers and Bond-Wire Antennas

A 43-GHz wireless inter-chip data link including antennas, transmitters, and receivers is presented. The industry standard bonding wires are exploited to provide high efficiency and low-cost antennas. This type of antennas can provide an efficient horizontal communication which is hard to achieve using conventional on-chip antennas. The system uses binary amplitude shift keying (ASK) modulation to keep the design compact and power efficient. The transmitter includes a differential to single-ended modulator and a two-stage power amplifier (PA). The receiver includes a low-noise amplifier (LNA), pre-amplifiers, envelope detectors (ED), a variable gain amplifier (VGA), and a comparator. The chip is fabricated in 180-nm SiGe BiCMOS technology. With power-efficient transceivers and low-cost high-performance antennas, the implemented inter-chip link achieves bit-error rate (BER) around 10^-8 for 6 Gb/s over a distance of 2 cm. The signal-to-noise ratio (SNR) of the recovered signal is about 24 dB with 18 ps of rms jitter. The transmitter and receiver consume 57 mW and 60 mW, respectively, including buffers. The bit energy efficiency excluding test buffers is 17 pJ/bit. The presented work shows the feasibility of a low power high data rate wireless inter-chip data link and wireless heterogeneous multi-chip networks.     

Low-frequency meandering piezoelectric vibration energy harvester

The design, fabrication, and characterization of a novel low-frequency meandering piezoelectric vibration energy harvester is presented. The energy harvester is designed for sensor node applications where the node targets a width-to-length aspect ratio close to 1:1 while simultaneously achieving a low resonant frequency. The measured power output and normalized power density are 118 μW and 5.02 μW/mm(3)/g(2), respectively, when excited by an acceleration magnitude of 0.2 g at 49.7 Hz. The energy harvester consists of a laser-machined meandering PZT bimorph. Two methods, strain-matched electrode (SME) and strain-matched polarization (SMP), are utilized to mitigate the voltage cancellation caused by having both positive and negative strains in the piezoelectric layer during operation at the meander's first resonant frequency. We have performed finite element analysis and experimentally demonstrated a prototype harvester with a footprint of 27 x 23 mm and a height of 6.5 mm including the tip mass. The device achieves a low resonant frequency while maintaining a form factor suitable for sensor node applications. The meandering design enables energy harvesters to harvest energy from vibration sources with frequencies less than 100 Hz within a compact footprint.     

A Numerical Fatigue Damage Model for Life Scatter of MEMS Devices

This paper presents a fatigue damage model to estimate fatigue lives of microelectromechanical systems (MEMS) devices and account for the effects of topological randomness of material microstructure. For this purpose, the damage mechanics modeling approach is incorporated into a new Voronoi finite-element model (VFEM). The VFEM developed for this investigation is able to consider both intergranular crack initiation (debonding) and propagation stages. The model relates the fatigue life to a damage parameter "D" which is a measure of the gradual material degradation under cyclic loading. The fatigue damage model is then used to investigate the effects of microstructure randomness on the fatigue of MEMS. In this paper, three different types of randomness are considered: (1) randomness in the microstructure due to random shapes and sizes of the material grains; (2) the randomness in the material properties considering a normally (Gaussian) distributed elastic modulus; and (3) the randomness in the material properties considering a normally distributed resistance stress, which is the experimentally determined material property controlling the ability of a material to resist the damage accumulation. Thirty-one numerical models of MEMS specimens are considered under cyclic axial and bending loading conditions. It is observed that the stress-life results obtained are in good agreement with the experimental study. The effects of material inhomogeneity and internal voids are numerically investigated.