Studies in Software-Defined Radio System Implementation

Over the past decade, software-defined radios (SDRs) have an increasingly prevalent aspect of wireless communication systems. Different than traditional hardware radios which implement radio protocols using static electrical circuit, SDRs implement significant aspects of physical radio protocol using software programs running on a host processor. Because they use software to implement most of the radio functionality, SDRs are much more easily modified, edited, and upgraded than their hardware-defined counterparts. Consequently, researchers and developers have been developing previously hardware-defined radio systems within software. Thus, communication standards can be tested under different conditions or swapped out entirely by simply changing some code. Additionally, developers hope to implement more advanced functionality with SDRs such as cognitive radios that can sense the conditions of the environment and change parameters or protocol accordingly. This paper will outline the major aspects of SDRs including their explanation, advantages, and architecture. As SDRs have become more commonplace, many companies and organizations have developed hardware front-ends and software packages to help develop software radios. The most prominent hardware front-ends to date have been the USRP hardware boards. Additionally, many software packages exist for SDR development, including the open source GNU Radio and OSSIE and the closed source Simulink and Labview SDR packages. Using these development tools, researchers have developed many of the most relevant radio standards. This paper will explain the major hardware and software development tools for creating SDRs, and it will explain some of the most important SDR projects that have been implemented to date

Optimized magneto-optical isolator designs inspired by seedlayer-free terbium iron garnets with opposite chirality

Simulations demonstrate that undoped yttrium iron garnet (YIG) seedlayers cause reduced Faraday rotation in silicon-on-insulator (SOI) waveguides with Ce-doped YIG claddings. Undoped seedlayers are required for the crystallization of the magneto-optical Ce:YIG claddings, but they diminish the interaction of the Ce:YIG with the guided modes. Therefore new magneto-optical garnets, terbium iron garnet (TIG) and bismuth-doped TIG (Bi:TIG), are introduced that can be integrated directly on Si and quartz substrates without seedlayers. The Faraday rotations of TIG and Bi:TIG films at 1550nm were measured to be +500 and -500°/cm, respectively. Simulations show that these new garnets have the potential to significantly mitigate the negative impact of the seedlayers under Ce:YIG claddings. The successful growth of TIG and Bi:TIG on low-index fused quartz inspired novel garnet-core waveguide isolator designs, simulated using finite difference time domain (FDTD) methods. These designs use alternating segments of positive and negative Faraday rotation for push-pull quasi phase matching in order to overcome birefringence in waveguides with rectangular cross-sections

A Digital, Constant-Frequency Pulsed Phase-Locked-Loop Instrument for Real-Time, Absolute Ultrasonic Phase Measurements

A digitally controlled instrument for conducting single-frequency and swept-frequency ultrasonic phase measurements has been developed based on a constant-frequency pulsed phase-locked-loop (CFPPLL) design. This instrument uses a pair of direct digital synthesizers to generate an ultrasonically transceived tone-burst and an internal reference wave for phase comparison. Real-time, constant-frequency phase tracking in an interrogated specimen is possible with a resolution of 0.000 38 rad (0.022), and swept-frequency phase measurements can be obtained. Using phase measurements, an absolute thickness in borosilicate glass is presented to show the instruments efficacy, and these results are compared to conventional ultrasonic pulse-echo time-of-flight (ToF) measurements. The newly developed instrument predicted the thickness with a mean error of 0.04 m and a standard deviation of error of 1.35 m. Additionally, the CFPPLL instrument shows a lower measured phase error in the absence of changing temperature and couplant thickness than high-resolution cross-correlation ToF measurements at a similar signal-to-noise ratio. By showing higher accuracy and precision than conventional pulse-echo ToF measurements and lower phase errors than cross-correlation ToF measurements, the new digitally controlled CFPPLL instrument provides high-resolution absolute ultrasonic velocity or path-length measurements in solids or liquids, as well as tracking of material property changes with high sensitivity. The ability to obtain absolute phase measurements allows for many new applications than possible with previous ultrasonic pulsed phase-locked loop instruments. In addition to improved resolution, swept-frequency phase measurements add useful capability in measuring properties of layered structures, such as bonded joints, or materials which exhibit non-linear frequency-dependent behavior, such as dispersive media

Nondestructive Evaluation of Adhesive Bonds via Ultrasonic Phase Measurements

The use of advanced composites utilizing adhesively bonded structures offers advantages in weight and cost for both the aerospace and automotive industries. Conventional nondestructive evaluation (NDE) has proved unable to reliably detect weak bonds or bond deterioration during service life conditions. A new nondestructive technique for quantitatively measuring adhesive bond strength is demonstrated. In this paper, an ultrasonic technique employing constant frequency pulsed phased-locked loop (CFPPLL) circuitry to monitor the phase response of a bonded structure from change in thermal stress is discussed. Theoretical research suggests that the thermal response of a bonded interface relates well with the quality of the adhesive bond. In particular, the effective stiffness of the adhesive-adherent interface may be extracted from the thermal phase response of the structure. The sensitivity of the CFPPLL instrument allows detection of bond pathologies that have been previously difficult-to-detect. Theoretical results with this ultrasonic technique on single epoxy lap joint (SLJ) specimens are presented and discussed. This technique has the potential to advance the use of adhesive bonds - and by association, advanced composite structures - by providing a reliable method to measure adhesive bond strength, thus permitting more complex, lightweight, and safe designs