High speed data converter techniques

Moore's law not only applies to the semiconductor technology, it also applies to the Hard Disk Drive (HDD) system in the last 35 years. In order to meet the emerging demands of high performance computing application, HDD will continue to evolve in a very rapid pace. Very high speed analog-to-digital converters are demanded for Hard Disk Drive application.;FLASH architecture provides the highest speed using 2n-1 comparators to perform an n-bit conversion. In the extremes of speed, however, exotic technologies must still be used to achieve conversion rates beyond those obtained with a conventional silicon implementation. In this research, a four-way, time-interleaved flash ADC is demonstrated to achieve conversion speed up to 900MS/s using a 2.5v digital 0.25 micron bulk CMOS process. The maximum conversion rate practical with any technology is extended by the use of an array of well-matched flash ADCs. This technique trades off increased die area for increased speed in nearly one for one relationship but an reduced performance if the ADCs are not well matched in terms of gain, offset, nonlinearities and sampling skew. In the approach considered here, these problems are minimized by use of a simple method that ensures the individual ADC gain, offset and nonlinearities characteristics are inherently almost identical. A simple four phase clock generator is demonstrated which introduces only a small sampling skew. This scheme has been demonstrated in the comparatively simple 6-bit flash ADC case which achieved the highest acquisition speed of 900MS/s. Compared with the prior works, our work achieves higher SNDR at much higher analog input frequency at sampling frequency of 900MS/s. This same scheme may be applied to the first n-bits of a pipeline converter (or other converter method) enabling the same identical performance in the most significant bits.;In the second part of this dissertation, a new calibration principle with Voltage Controlled Resistors (VCR) for matched current sources is proposed. This technique can be used to produce multi copies of current units. Therefore, it is suitable for the calibration of high-resolution digital-to-analog converters that are based on equal current sources.</p

Quadrature transmit array design using single-feed circularly polarized patch antenna for parallel transmission in MR imaging

Quadrature coils are often desired in MR applications because they can improve MR sensitivity and also reduce excitation power. In this work, we propose, for the first time, a quadrature array design strategy for parallel transmission at 298 MHz using single-feed circularly polarized (CP) patch antenna technique. Each array element is a nearly square ring microstrip antenna and is fed at a point on the diagonal of the antenna to generate quadrature magnetic fields. Compared with conventional quadrature coils, the single-feed structure is much simple and compact, making the quadrature coil array design practical. Numerical simulations demonstrate that the decoupling between elements is better than -35 dB for all the elements and the RF fields are homogeneous with deep penetration and quadrature behavior in the area of interest. Bloch equation simulation is also performed to simulate the excitation procedure by using an 8-element quadrature planar patch array to demonstrate its feasibility in parallel transmission at the ultrahigh field of 7 Tesla

Exploring Travel Mobility in Integrated Usage of Dockless Bike-Sharing and the Metro Based on Multisource Data

Dockless bike-sharing (DBS) is a green and flexible travel mode, which has been considered as an effective way to address the first-and-last mile problem. A two-level process is developed to identify the integrated DBS–metro trips. Then, DBS trip data, metro passenger data, socioeconomic data, and built environment data in Shanghai are used to analyze the spatiotemporal characteristics of integrated trips and the correlations between the integrated trips and the explanatory variables. Next, multicollinearity tests and autocorrelation tests are conducted to select the best explanatory variables. Finally, a geographically and temporally weighted regression (GTWR) model is adopted to examine the determinants of integrated trips over space and time. The results show that the integrated trips account for 16.8% of total DBS trips and that departure-transfer trips are greater than arrival-transfer trips. Moreover, the integrated trips are concentrated in the central area of the city. In terms of impact factors, it is found that GDP, government count, and restaurant count are negatively correlated with the number of integrated trips, while house price, entropy of land use, transfer accessibility index, and metro passenger flow show positive relationships. In addition, the results show that the GTWR model outperforms the OLS model and the GWR model

Planar quadrature RF transceiver design using common-mode differential-mode (CMDM) transmission line method for 7T MR imaging.

The use of quadrature RF magnetic fields has been demonstrated to be an efficient method to reduce transmit power and to increase the signal-to-noise (SNR) in magnetic resonance (MR) imaging. The goal of this project was to develop a new method using the common-mode and differential-mode (CMDM) technique for compact, planar, distributed-element quadrature transmit/receive resonators for MR signal excitation and detection and to investigate its performance for MR imaging, particularly, at ultrahigh magnetic fields. A prototype resonator based on CMDM method implemented by using microstrip transmission line was designed and fabricated for 7T imaging. Both the common mode (CM) and the differential mode (DM) of the resonator were tuned and matched at 298MHz independently. Numerical electromagnetic simulation was performed to verify the orthogonal B1 field direction of the two modes of the CMDM resonator. Both workbench tests and MR imaging experiments were carried out to evaluate the performance. The intrinsic decoupling between the two modes of the CMDM resonator was demonstrated by the bench test, showing a better than -36 dB transmission coefficient between the two modes at resonance frequency. The MR images acquired by using each mode and the images combined in quadrature showed that the CM and DM of the proposed resonator provided similar B1 coverage and achieved SNR improvement in the entire region of interest. The simulation and experimental results demonstrate that the proposed CMDM method with distributed-element transmission line technique is a feasible and efficient technique for planar quadrature RF coil design at ultrahigh fields, providing intrinsic decoupling between two quadrature channels and high frequency capability. Due to its simple and compact geometry and easy implementation of decoupling methods, the CMDM quadrature resonator can possibly be a good candidate for design blocks in multichannel RF coil arrays