Class-D Amplification has been an absolute improvement in terms of electrical efficiency in audio amplification systems, but it has been during the first stabilization decade of this technology in the market when manufacturers have made public their concern about the incompatibility between distributed audio or PA (Public Address) applications and HiFi or Professional sound applications. High quality sound systems need switching frequencies (>1MHz) whose switching losses do not allow to work at high power (>250W). To the contrary, applications for high power amplification need high electrical efficiency, so the switching frequencies must be set to low values (<400kHz), reducing the signal quality. This incompatibility is called Power-Quality Compromise. The Characteristics of the Multilevel Topologies of DC-AC power electronic converters allow them to work at high power (increasing the working voltage) while maintaining or even increasing the quality of the output signal using the extra levels in the PWM output signal. The application of these topologies in the world of sound amplification could lead to the resolution of the Quality-Power Compromise and so, the main problem of the Class-D amplifiers. In the first place, this Thesis proposes the study of the Multilevel Topologies in order to determine the one who shows better features and feasibility, in terms of implementation as a Switching Audio Amplifier. Having determined the Optimal Topology, it is proposed to study, characterize and model the non-ideal phenomena and those mechanisms who generate Time and Pulse Width errors in the PWM output signals of the Amplifier Topology selected. Using these models, the effects of the errors in the output signals are studied by means of computer simulation, expressing these effects using non normative merit factors. These merit factors, unlike regulations, allow to evaluate quantitatively and qualitatively the degree of involvement of each one of the different error sources intrinsic to the design and topology. From these results it can be developed Estimation Models of the Non Normative Merit Factors, which allow to calculate the values of the Width Error and Distortion, local or total, generated by any source of error in the Amplification Sound Chain. In order to express this information in normative values, Estimation Models of Electric Efficiency, Distortion (THD) and Signal Efficiency are developed from the estimated non normative values of Width Error Distortion. These estimation models provide an information that, once ordered and classified in order to be used as a tool in the different levels of the design process, production and marketing of Switching Amplifier Equipment, allow the optimal component selection in the pre-design phase and the decision-making process to ensure the future validation of the Multilevel Switching Amplifier under market regulatory criteria. The Estimation models of the Width and Distortion Errors and the estimation models of the Electrical Efficiency, Distortion and Signal have been validated through the comparison with experimental results from a laboratory prototype and an industrial manufacturing prototype, designed by a company in the sector using the Optimal Design Tools based on the Estimation Models developed in this Thesis. The experimental results confirm that the normative values measured of the Electrical Efficiency, Distortion and Signal Efficiency are within the acceptable values for the commercialization of the equipment. / L'amplificació en Classe-D ha suposat una millora absoluta en quant a eficiència elèctrica en els sistemes d'amplificació d'àudio, però ha estat durant la primera dècada d'estabilització d'aquesta tecnologia al mercat, quan els fabricants han fet pública la seva preocupació per la incompatibilitat entre aplicacions de so distribuït o PA (Public Addres) i so HiFi o Professional. Els sistemes d'alta qualitat de so, necessiten de freqüències de commutació (>1MHz) per les quals les pèrdues en commutació no permeten treballar a potències elevades (>250W). Per al contrari, per aplicacions d'amplificació de so de potència es necessita d'alta eficiència elèctrica, amb el que les freqüències de commutació han de quedar fixades a valors baixos (<400kHz), reduint la qualitat del senyal. Aquesta incompatibilitat d'aplicacions s'anomena Compromís Qualitat-Potència. Les topologies Multinivell de convertidors electrònics de potència DC-AC, per les seves característiques, permeten treballar a gran potència (augmentant la tensió de treball), mantenint o augmentant la qualitat del senyal de sortida mitjançant els nivells extra en el senyal PWM de sortida. L'aplicació d'aquestes topologies en el mon de l'Amplificació de So, podria suposar la resolució del compromís Qualitat-Potència, i per tant de la principal problemàtica que presenta la Classe-D. Aquesta Tesis proposa en primer lloc l'estudi de les topologies Multinivell per determinar quina és la que millors prestacions i viabilitat mostra en quant a la seva aplicació com a Amplificador d'Àudio Commutat. Un cop determinada la Topologia Òptima, es proposa estudiar, caracteritzar i modelar els fenòmens no ideals i mecanismes generadors d'errors de Temps i d'Amplitud al polsos del senyal PWM de sortida d'un Amplificador amb la Topologia seleccionada. A partir d'aquests models, s'estudia mitjançant simulació els efectes dels errors en els senyals de sortida, expressant aquests efectes mitjançant factors de mèrit no normatius. Aquests factors de mèrit, al contrari que els normatius, permeten avaluar quantitativa i qualitativament el grau d'afectació de cada una de les diferents fonts d'errors intrínseques al disseny i a la topologia. A partir d'aquests resultats es desenvolupen Models d'Estimació dels Factors de Mèrit No Normatius, que permeten calcular el valor d'Error d'Amplitud i Distorsió, local o total, generat per qualsevol font d'error de la Cadena d'Amplificació de So. Per tal d'expressar aquesta informació en valors normatius, es desenvolupen Models d'Estimació d'Eficiències Elèctriques, Distorsió (THD) i Eficiència de Senyal a partir dels valors estimats d'Error d'Amplitud i Distorsió no normatius. Aquests models d'estimació proveeixen d'una informació que, un cop ordenada i classificada per tal de ser utilitzada, com a eina en els diferents nivells del procés de disseny, producció i comercialització d'equips d'amplificació commutada, permet, de forma òptima en la fase de pre-disseny, la selecció de components i la presa de decisions per tal d'assegurar, un cop construït, la futura validació de l'amplificador commutat multinivell, sota criteris normatius de mercat. Els models d'estimació d'Errors d'Amplitud i Distorsió, així com els models d'estimació de les Eficiències Elèctriques, de Distorsió i de Senyal han estat validats mitjançant la seva comparació amb resultats experimentals obtinguts a partir d'un prototipus de laboratori i un prototipus de fabricació industrial, dissenyat per una empresa del sector a partir de l'ús de les Eines de Disseny Òptim basades en els Models d'Estimació, desenvolupades en aquesta Tesis. Els resultats experimentals confirmen que els valors normatius mesurats d'Eficiència Elèctrica, Distorsió i Eficiència de Senyal estan dintre dels valors acceptables per la comercialització de l'equip.

It is widely believed that many drawbacks in today's wireless communication paradigm might be relieved by enabling high carrier frequency transmission, and endowing both the network and the user equipment with some degree of reconfigurability. The urgency of a new framework in wireless digital transmission which should allow for higher bit rate, lower latency and tighter delay constraints, led us to investigate the fundamental building blocks which, at the circuital/device level, will foster a change towards more efficient communication schemes, delivering a more satisfactory end user experience. Specifically, this work deals with the inherently analog devices, found at the core of each transceiver module and capable of providing the carrier signal; these are the oscillators. In particular, two distinct classes of oscillators are regarded central to our contribution. One class is constituted by N-push oscillators, which thanks to coupling effect of N identical core oscillators allow N-fold harmonic generation (and thus high frequency transmission). The second class is constituted by wideband tunable oscillators, whose topology derives from a feedback distributed amplifier and therefore called distributed oscillators; by adequately altering the bias level at each section Distributed Voltage Controlled Oscillators can be implemented (which can scan a wide frequency range). The introductory part of this work, deals with their operation principles in great detail. As microwave oscillators are nonlinear devices, a full nonlinear analysis, synthesis, and optimization is considered for their implementation. Consequently, nonlinear numerical techniques have been reviewed in the second part of the thesis. Particularly, the role of Harmonic Balance simulations and the auxiliary generator/probe method for obtaining the oscillator solutions has been emphasized; the overall research goal of this dissertation is to show that the former techniques are very effective in obtaining detailed information about the periodic steady state behavior for the two class of circuits being investigated. A triple-push oscillator topology has been initially considered. Provided a certain phase distribution is maintained among the oscillating elements, the output power of the third harmonic increases while the lower order harmonics cancel out, which represents the default operating mode. Due to circuit symmetry, to the presence of delay in the coupling network and to unavoidable mismatches, unwanted oscillating modes might coexist with the intended one. A design strategy relying on the Harmonic Balance parametric analysis of the oscillating voltage at a selected node in the coupling network with respect to coupling phase and coupling strength is presented, to the aim of quenching undesired oscillation modes. Moreover the design of a four stage reverse mode distributed voltage controlled oscillator (DVCO) has been described. All the design steps have been reported, from a very idealized, purely behavioral design to a very concrete one, involving details derived from electromagnetic simulations. Harmonic Balance techniques were used to evaluate its tuning function, output power and DC current consumption, which have been completely characterized across the tuning bandwidth. Finally, a method for an optimized design with reduced variations in the output power has been presented. An alternative implementation, targeting wider tuning ranges/ higher oscillation frequencies was introduced. The measurements performed on the fabricated prototypes revealed good agreement with the simulation results, confirming the validity of the approach.

This Thesis proposes the design and the optimization of a hybrid WDM/TDM PON at the L1 (PHY) and L2 (MAC) layers, in terms of minimum deployment cost and enhanced performance for Greenfield NGPON. The particular case of RSOA-based ONUs and ODN using a single-fibre/single-wavelength is deeply analysed. In this WDM/TDM PON relevant parameters are optimized. Special attention has been given at the main noise impairment in this type of networks: the Rayleigh Backscattering effect, which cannot be prevented. To understand its behaviour and mitigate its effects, a novel mathematical model for the Rayleigh Backscattering in burst mode transmission is presented for the first time, and it has been used to optimize the WDM/TDM RSOA based PON. Also, a cost-effective, simple design SCM WDM/TDM PON with rSOA-based ONU, was optimized and implemented. This prototype was successfully tested showing high performance, robustness, versatility and reliability. So, the system is able to give coverage up to 1280 users at 2.5 Gb/s / 1.25 Gb/s downstream/upstream, over 20 Km, and being compatible with the GPON ITU-T recommendation. This precedent has enabled the SARDANA network to extend the design, architecture and capabilities of a WDM/TDM PON for a long reach metro-access network (100 km). A proposal for an agile Transmission Convergence sub-layer is presented as another relevant contribution of this work. It is based on the optimization of the standards GPON and XG-PON (for compatibility), but applied to a long reach metro-access TDM/WDM PON rSOA-based network with higher client count. Finally, a proposal of physical implementation for the SARDANA layer 2 and possible configurations for SARDANA internetworking, with the metro network and core transport network, are presented.

Electro-optical traveling wave modulators (EO-TWM) are basic building blocks of the optical communications industry which is leading a revolution in the way we communicate, work and live. As a result, the demand for high-speed data transmission with low driving voltage is continuously growing up with costs that should be kept below a minimum. Besides communications, a growing number of applications for EO-TWM is continuously emerging with equally stringent requirements. This Thesis is concerned with advances in the eld of systematic design and optimization of EO-TWM for coping inverse of the velocity matching constant has been shown to govern the low-loss limit (LL), while in the velocity matching limit (VM), a constant bandwidth times squared-length rule proportional to the inverse of the squared loss constant has been found more appropriate. In this work we provide insights into the trade-o issue in EO-TWM, and a complete picture of the applicable gures of merit for every operative range. Besides the known LL and VM gures of merit, two intermediate ranges, the quasi-low loss (QLL) and the quasi-velocity matching (QVM), have been identi ed. Also novel closed-forms expressions fully accounting for the e ffects of the skin-e ffect electrode loss and optical-electrical wave velocity mismatch, explicitly relating the operative bandwidth and the electrode length in EO-TWM, have been found. Novel bandwidth and electrode-length charts have been created, which constitute a useful tool for the optimization and design of this modulators. A graphical interface tool called MZM-GIT has been built integrating the analytical optimization and design strategies developed throughout the Thesis. With the aid of the MZM-GIT, several proposals of optimized MZM designs based on practical structures described in literature, and also based on the industry trends, are made and analyzed. with the industrial demands. In EO-TWM, the accumulated electro-optic e ect over the optical wave grows with the co-propagated traveling wave (TW) length, allowing to reduce the required RF driving power. However, in typical electro-optic materials for modulators, among which LiNbO3 stands up, due to the natural mismatch between the velocity of the RF and the optical waves, the modulation bandwidth decreases with the TW length, giving place to a well-known trade-o ff. In typical LiNbO3 substrates, in which this Thesis is focused, this trade-off is seen to mainly depend on the values of the electrical loss constant and the e ective wave velocity mismatch in the TW structure that forms the electrodes, usually a coplanar waveguide (CPW). Special emphasis has historically been placed on the optimized design of the CPW in EO-TWM. In this Thesis the study of closed-form expressions for the propagation parameters of CPW as a function of the geometry, has proven useful for the design and optimization procedures sought. Although some interesting approaches to closed-form formulations have been found in literature, none of them completely ful lls the desired requirements of providing a reliable yet simple description of propagation in CPW, appropriate to systematic and easy to follow design rules for EO-TWM, and therefore new simpli ed closed-form expressions for the CPW transmission parameters have been developed. In a second part of the Thesis, the bandwidth-length trade-off has been examined. To date, two bandwidth-length rules have been proposed: a constant bandwidth-length product proportional to the inverse of the velocity matching constant has been shown to govern the low-loss limit (LL), while in the velocity matching limit (VM), a constant bandwidth times squared-length rule proportional to the inverse of the squared loss constant has been found more appropriate. In this work we provide insights into the trade-off issue in EO-TWM, and a complete picture of the applicable fi gures of merit for every operative range. Besides the known LL and VM gures of merit, two intermediate ranges, the quasi-low loss (QLL) and the quasi-velocity matching (QVM), have been identi ed. Also novel closed-forms expressions fully accounting for the e ffects of the skin-eff ect electrode loss and optical-electrical wave velocity mismatch, explicitly relating the operative bandwidth and the electrode length in EO-TWM, have been found. Novel bandwidth and electrode-length charts have been created, which constitute a useful tool for the optimization and design of this modulators. A graphical interface tool called MZM-GIT has been built integrating the analytical optimization and design strategies developed throughout the Thesis. With the aid of the MZM-GIT, several proposals of optimized MZM designs based on practical structures described in literature, and also based on the industry trends, are made and analyzed.

This thesis is framed in the field of mobile communications and more particularly in handset antennas. The wireless industry is constantly growing, which entails challenging handset antenna specifications. Handset antennas not only have to be multi-band for satisfying the great number of communication services, but also sufficiently small as for fitting in the reduced space imposed by the handset platforms. The appearance of the MIMO (Multiple Input Multiple Output) technology, further exacerbates these challenges. In order to satisfy these requirements, this thesis proposes the use of the ground plane, inherently present in any handset platform, as the main radiator. Electrically small nonresonant elements, called along this thesis as ground plane boosters, are used to transfer energy to this ground plane. The solution removes the need of including a dedicated antenna featured by considerable dimensions, thus releasing space to integrate other antennas, as well as, other handset components, services and functionalities.

Terahertz is the portion of the spectrum that covers a frequency range between 300 GHz - 3 THz. This frequency band has proven its potential for imaging applications thanks to the good compromise between spatial resolution and penetration; however, this push towards high frequencies contains many technological difficulties in all the subsystems involved in the signal generation, transmission and detection. The power budget restrictions and high losses that sources and receivers currently suffer at these frequencies require systems with a high level of integration among all the devices and components of the systems and subsystems. Therefore, the antennas needed for these systems require to be integrated within the same fabrication processes and technologies as the sensing and power converting devices that are used at their terminals. This doctoral thesis has focused on the development of integrated antenna arrays at Terahertz frequencies for imaging applications, for both near-field and focal-plane geometries, with a special emphasis on the technologies and the fabrication capabilities that can be potentially used and are currently available. The current imaging systems require large arrays of antennas in order to achieve the high-speed image acquisition that is required in most THz applications. This fact increases considerably the difficulty and complexity to achieve highly integrated and efficient antennas. This thesis has characterized and analyzed these difficulties and provided solutions to the development of antenna arrays at millimeter and submillimeter wave frequencies. The first part of this thesis has focused on the study of a planar antenna array, called retina, for a specific near-field imaging system based on the Modulated Scatterer Technique (MST) at millimeter and submillimeter-wave frequencies. This system has been selected for its capabilities to perform high-speed imaging and because it does not require a high frequency distribution line network. However, it is hindered by many technological difficulties: the selection of an antenna geometry that achieves high efficiency, the selection of the adequate active element and its integration with the antenna. In this thesis, these challenges have been addressed and studied in-depth, and a design methodology that integrates all the different aspects of the system has been developed. Because planar antennas at millimeter and submillimeter wave frequencies suffer from high losses due to the surface wave modes inside substrate, these losses have been analyzed and quantified for different antennas, and an antenna geometry that reduces significantly this problem has been developed. Different switching technologies currently or potentially available at these high frequencies have been considered in order to study and analyze their capabilities and their integration possibilities: PIN diodes, Schottky diodes and RF-MEMS. These technologies have been studied through the development of three retina prototypes, which have been fabricated using high precision fabrication processes such as laser micromachining and photolithographic. Different measurement set ups were fabricated and assembled to validate the different premises presented. The second part of the thesis is devoted to the study of integrated Focal Plane Arrays (FPA). The development of FPA at submillimeter wave frequencies is highly on demand due to the enormous progress in designing integrated heterodyne receivers. These receivers integrate arrays of submillimeter-wave diode-based mixers and multipliers with Monolithically Integrated Circuit (MIC) amplifiers on the same wafer stack. For this stackable multi-pixel terahertz camera technology to work, a leaky wave antenna with silicon micro-lenses has been developed, which allows wafer level integration compatible with silicon micro-fabrication techniques for bulk array manufacturing and has high directivity in order to illuminate a reflector efficiently. Detailed and thorough design guidelines for this antenna are presented. Two antenna prototypes were built in order to evaluate the two fabrication possibilities: advanced laser micro-fabrication and photolithographic fabrication. A study of the aberrations of the lens has been developed in order to evaluate the performance of the lens profile fabricated. Moreover, a set of radiation pattern measurements of the fabricated prototypes was performed in order to evaluate the performance of the antenna and its possibilities to be used as a FPA.

El objetivo último de este trabajo es el estudio de la degradación de las células solares en el ambiente espacial fuera de la atmósfera, con el fin de conseguir mejores rendimientos durante el mayor tiempo posible de los generadores fotoeléctricos en vehículos espaciales, y tener la capacidad de hacer predicciones a largo plazo del comportamiento de las células.

This research is developed in the context of the Next Generation of Passive Optical Networks (NG-PON) for Fibre to the Home (FTTH) network. In this thesis, several techniques are proposed in order to increase the network capacity, dealing with the main optical impairments present in PON with minimum infrastructure, and with the goals of wavelength transparency and cost effectiveness. The main issues in this work are the photodetection process over chromatic dispersion (CD) and ASE noise impairments, the mitigation strategies for Rayleigh Backscattering (RB) effect and the WDM-PON RSOA-based ONUs, with their limitations of bandwidth and chirp. Regarding photodetection, a novel module which performs the mathematical function SQRT has demonstrated its linearizing capability at partially compensating the square-law of the photon to electron conversion process after reception. Improvements in RoF systems in terms of harmonics level reduction and in digital transmissions with linear FFE equalizers have been achieved. Also, the complexity requirements in MLSE receivers are reduced after the SQRT module. This module has been implemented electronically, first, by using the intrinsic non-linearity of the schottky diodes, on a microstrip circuit. After that, a MMIC chip has been designed and experimentally tested. But the SQRT module is also useful for other applications as downstream cancellation in the upstream signal in bidirectional transmissions, and to reduce the amplitude differences of the different packets in burst mode receivers, relaxing the adjustment requirements for the following limiting amplifiers. The statistics of the received signals in optically amplified (ASE dominant) systems have been also studied. The optimum function, capable of recovering the Gaussian distribution after detection, has been obtained. For systems working at FEC limit, like with MLSE, the improvement over SQRT is small, validating the good performance of the SQRT module. RB effect has been mitigated by using wavelength shifting at the ONU premises. Several wavelength shifter devices have been proposed for the first time, based in 2 and 3 arms capable of translating (in several GHz) the wavelength that comes from the OLT to avoid the overlap between the upstream signal and the RB, while maintaining the signal in the same AWG channel. These devices are less complex than the recently proposed 4-arm structures and require simpler signals than the sawtooth that is necessary for a 1-arm phase modulator. Experimental validation of the WS technique for RB mitigation has been performed, first with a commercial dual arm MZM and after by using a RSOA with a low duty square signal with improvements of 5 to 7dB in tolerance to RB. The new standards that have just been approved (XG-PON, 10G-EPON) propose the upgrading of the network towards symmetrical 10G/10G operation. However, RSOA’s bandwidth and chirp still limit the performance at these bit rates. The first 10Gb/s transmission in real time and with low BER (10-9) using a RSOA as an intensity modulator has been reported by using some of the techniques presented in this thesis: duobinary transmission and detuned optical filtering plus DFE-FFE equalization. Other techniques proposed and experimentally tested are the phase adjustment with a programmable optical filter and the direct detection together with MLSE equalization. / Aquest treball de recerca està desenvolupat en el context de la nova generació de xarxes òptiques passives (NG-PON) de fibra fins a la llar (Fibre to the Home (FTTH)). En aquesta tesi, s’han proposat diverses tècniques que permeten augmentar la capacitat de la xarxa, tractant les principals degradacions presents en les xarxes PON amb la mínima infrestuctura, amb els objectius de mínim cost i transparència front a la longitud d’ona triada per a la transmissió. Els principals temes d’aquest treball són el procés de fotodetecció després d’una transmissió amb dispersió cromàtica (CD) i soroll d’emissió espontània (ASE), les estratègies per combatre l’efecte de Rayleigh Backscattering (RB) en la trasmissió bidireccional amb la mateixa longitud d’ona i les xarxes PON basades en unitats d’usuari (ONU) que utilitzen l’ amplificador semiconductor reflexiu (RSOA) com a transmissor, amb les seves limitacions d’ample de banda i chirp. Pel que fa a la fotodetecció, un dispositiu novedós que realitza la funció matemàtica arrel quadrada (SQRT) ha demostrat la seva capacitat de linearització al compensar parcialment la funció quadrada del procés de conversió de fotó a electró després de recepció. S’ha aconsegit millorar les prestacions en xarxes de ràdio sobre fibra en termes de reducció del nivel d’harmònics i també en transmissió de senyals digitals amb equalitzadors electrònics lineals. La complexitat de l’ equalitzador de màxima verosimilitud, o MLSE, es redueix si s’utilitza el mòdul SQRT. Aquest dispositiu s’ha implementat electrònicament en un circuit microstrip amb dos diodes schottky i també en un xip MMIC . El bloc SQRT també és útil per aplicacions de cancel•lació del senyal de baixada present en el senyal de pujada en les transmissions bidireccionals i per igualar l’amplitud dels paquets en senyals que es reben a ràfegues (burst mode). L’estadística del senyal rebut en sistemes amplificats òpticament (on domina el soroll ASE) també ha estat estudiat. La funció òptima, capaç de recuperar la distribució estadistica Gaussiana després de detecció, ha estat obtinguda. Per sistemes que treballen amb un nivell de errors en el límit FEC, com el MLSE, la millora de la funció òptima és molt petita front la SQRT, demostrant el bon funcionament del mòdul SQRT. L’efecte de RB s’ha minimitzat utilitzant la tècnica de desplaçar la longitud d’ona del senyal de baixada a la ONU. S’han disenyat diversos dispositius electro-òptics capaços de realitzar aquest desplaçament, utilitzant unes configuracions novedoses basades en 2 i 3 branques. L’efecte RB és redueix evitant el sol•lapament espectral però mantenint la longitud d’ona dins el mateix canal del AWG. Aquests dispositius són més senzills que les recentment proposades estructures basades en 4 branques i requereixen senyals més simples que el dent de serra necessari per al modulador de fase d’una branca. La tècnica WS s’ha validat experimentalment usant un MZM comercial i també usant un RSOA, aconseguint-se unes millores de 5 a 7 dB en tolerància a RB. Els nous estàndards que han estat recentment aprovats (XG-PON, 10G-EPON) proposen l’evolució de la xarxa passiva cap a la transmissió simètrica a 10Gb/s. La realitat és, però, que l’ample de banda i el chirp limiten la utilització de RSOAs a aquesta velocitat. La primera transmissió a 10Gb/s en temps real amb baixa tasa d’errors (BER=10-9) usant el RSOA com a modulador d’intensitat a la ONU ha estat presentat utilitzant les tècniques explicades en aquesta tesi: modulació duobinaria i filtrat òptic desplaçat amb equalització electrònica (DFE-FFE). Altres mètodes utilitzats han estat l’adjust de la fase amb un filtre òptic programable i el MLSE.

The advances in the communications systems has brought new requirements and challenges in terms of compactness and efficient systems that looking for increase the signal coverage area. The reconfigurable antennas are a promising solution for the communication systems when looking to improve the channel capacity and/or to extend the signal coverage. The main advantages of the reconfigurable antennas are the capabilities to change their frequency, polarization and radiation beam steering at a low cost. The reconfigurable antennas can be designed to operate in a determined changing environment keeping good electromagnetic characteristics. The design of reconfigurable RF repeaters is a relevant application of this reconfigurable antenna principle. This thesis is devoted to study and propose new repeater architectures in which a set of reconfigurable parasitic elements as part of the repeater are used for reducing the electromagnetic coupling between the Rx and Tx antennas. It is shown that the use of the parasitic elements as a reconfigurable mechanism gives the flexibility to adapt the repeater electromagnetic characteristics to changing environments while keeping a good system performance. The determination of the minimum number of parasitic elements is an important parameter and it is determined by a modal analysis to define the minimum number of parasitic elements able to fulfill specific repeater electromagnetic characteristics. In order to validate the analytical results, different reconfigurable repeater prototypes controlled electronically are manufactured. A reconfigurable repeater prototype that is using eight reconfigurable parasitic elements has been designed for operating at different scatterer environments. The repeater reconfigurable capabilities are studied to evaluate the repeater performance in realistic indoor locations. Finally, in order to obtain a repeater with reconfigurable frequency isolation capabilities between the Rx and Tx antennas over a wide frequency range, a repeater prototype based on a pixeled layer as reconfigurable mechanism has been designed and measured.

Electric conductivity is a good estimator of water quality. It can be measured by using cells with direct contact between the electrodes and the electrolyte, inductive cells or cells with capacitive electrodes. The use of capacitive electrodes is a need when measurements are made from the outside of an insulating container, and it is advisable when electrode degradation is to be avoided. Electrical impedance models for measurements using capacitive electrodes often obviate parasitic capacitances that may affect the measured values. The instruments commonly used for measuring the electrical impedance of materials are insensitive to some of these capacitances, but not to all of them. Some authors have pointed that parasitic capacitances may lead to unpredicted results, suck as inductive effects, but most of the described phenomena are related to direct contact electrodes. Regarding capacitive electrodes there are few descriptions of stray impedances, even fewer analysis and no known solution. The main objective of this work is to define and to verify a method to measure the conductivity of water in non-conductive containers using capacitive electrodes on the outer side of the recipient. The impedance of water (Rx, which contains the information about conductivity, and Cx), the impedance of the electrode (Re and Ce), and the impedance between electrodes (Rhl and Chl) are modelled by a resistance in parallel with a capacitance. The parasitic capacitance Cg between the material under test and erath ground is an element with distributed parameters, and its effects are different depend on the measuring system. This study is focused on those systems that apply a voltage to the material and that measure the output current at the material, in particular on auto-balancing bridges. Cg disminishes the measured current, which is shown as an inductive effect as the calculated impedance increases with frequency. The analysis performed shows the gyration undergone by Cg and the effect is an ¿apparent¿ impedance connected in series with the impedance under test and the impedance of the electrodes. For materials modeled by Rx in parallel with Cx, besides the inductive effect, there are resistive and capacitive effects also due to Cg. This effects due to Cg depend on the ratio Cg/Ce. When direct contact electrodes are used, the impedance of the electrodes and the leakage impedance have no significant effect. Electrodes have influence at the lower frequencies whereas the effects of Cg appear at higher frequencies. In the middle range, there is a flat frequency band where Rx can be easily measured. If Cg is higher than Cx, peaks in the magnitude of the impedance as well as positive phases may be found. The relevant parameter is not the value of Cg but the ratio Cg/Cx. Measurements using four electrodes do not avoid the effects of Cg. In addition to inductive effects and resonances at high frequency, There is, when Cg increases, also an increment of the impedance that is independent from the frequency. The effects of Cg and Chl are more noticeable for capacitive electrodes. The leakage resistance Rhl shunts Re, and when Rhl decreases, the real part of the impedance increases at low frequencies. The effect of the parasitic capacitance between the electrodes Chl is a reduction of the real part of the impedance that is frequency independant. When Cg increases, the real part of the impedance increases, and this increment is not frequency dependant either. If Ce and Chl are known, Cg can be obtained from the measurement at low frequency of the imaginary part of the impedance, since increasing Cg makes the equivalent capacitance between the electrodes to decrease. Once Ce, Chl and Cg are known, Rx can be calculated in the frequency band where the real part of the impedance is flat. In addition, setting Chl and Cg, for instance by shielding the measurement cell, the parameters that depend on Ce, Chl and Cg can be included in an equivalent constant cell. / La conductivitat elèctrica de l’aigua és un bon indicador de la seva qualitat. Es mesura amb cel·les d’elèctrodes de contacte directe entre l’elèctrode i l’electròlit, cel·les inductives o cel·les d’elèctrodes capacitius. La mesura amb elèctrodes capacitius és una necessitat quan es mesura des de l’exterior d’un recipient aïllant, i una conveniència quan es vol evitar la degradació dels elèctrodes. Els models de les mesures d’impedància elèctrica sense contacte amb elèctrodes capacitius solen obviar les capacitats paràsites que podrien afectar les mesures. Els instruments habitualment emprats per mesurar la impedància elèctrica dels materials són insensibles a algunes d’aquestes capacitats paràsites però no a totes. Diversos autors han sospitat que algunes d’elles provoquen resultats imprevistos, com ara efectes inductius, però la majoria dels efectes descrits a la bibliografia s’han observat en mesures amb elèctrodes de contacte directe. Per elèctrodes capacitius, hi ha poques observacions, menys anàlisis, i cap solució coneguda. L’objectiu principal d’aquesta tesi és concebre i verificar un mètode de mesura de la conductivitat elèctrica de l’aigua dins de recipients de parets elèctricament aïllants, mitjançant elèctrodes capacitius situats a l’exterior del recipient. La impedància de l’aigua (Rx, que conté la informació de la conductivitat, i Cx), de l’elèctrode (Re i Ce) i entre elèctrodes (Rhl i Chl) s’han modelitzat com una resistència en paral·lel amb una capacitat. La capacitat paràsita Cg entre el material que es mesura i terra és de paràmetres distribuïts, i té efectes diferents segons els sistema de mesura usat. L’estudi s’ha centrat en els sistemes que apliquen una tensió i mesuren el corrent que surt del material, i en particular en els pseudoponts automàtics. Cg redueix el corrent que es mesura, i això s’interpreta com un efecte inductiu perquè fa que la impedància calculada augmenti amb la freqüència. L’anàlisi mostra que Cg es ¿gira¿ i queda en sèrie amb la impedància del material i dels elèctrodes. Quan es mesura un material modelitzat com Rx en paral·lel amb Cx, a més d’un efecte inductiu hi ha efectes resistius i capacitius, deguts a Cg. Cg canvia la influència de la impedància dels elèctrodes en la mesura segons el valor de la relació Cg/Ce. Mesurant amb elèctrodes de contacte directe, les impedàncies dels elèctrodes i la de fuites no tenen efectes significatius. Els elèctrodes influeixen a baixa freqüència i els efectes de Cg es veuen més a alta freqüència. Entre mig, hi ha una banda plana on es pot mesurar bé la Rx. Si Cg es prou gran respecte a Cx, hi ha pics en el mòdul, i fins i tot fases positives. El paràmetre rellevant no és el valor de Cg sinó la relació Cg/Cx. Les mesures amb quatre elèctrodes no solucionen els efectes de Cg sinó que també s’observen efectes inductius i ressonàncies a alta freqüència, i a més hi ha un augment de la impedància que no depèn de la freqüència quan augmenta Cg. La mesura amb elèctrodes capacitius està més afectada per Cg i Chl que en el cas de contacte directe. La Rhl queda en paral·lel amb Re i si Rhl disminueix, augmenta la part real de la impedància a baixes freqüències. L’efecte de la capacitat paràsita Chl és una disminució de la part real de la impedància, independent de la freqüència. Quan augmenta Cg, augmenta la part real de la impedància; aquest augment també és independent de la freqüència. Si augmenta Cg, augmenta l’efecte de Chl, i fa disminuir l’interval freqüencial de treball. Coneixent el valor de Ce i de Chl es pot obtenir Cg a partir de la mesura de la part imaginària de la impedància a baixa freqüència perquè augmentar Cg fa disminuir la capacitat equivalent entre els elèctrodes de mesura. Conegudes Ce, Chl i Cg es pot calcular Rx en la banda on la part real de la impedància és plana. A més, si fixem Chl i Cg, per exemple apantallant la cel·la de mesura, els factors que depenen de Ce, Chl i Cg es poden incloure a una constant de cel·la equivalent.