Speed, power and reliability of analogue integrated circuits (IC) exhibit temperature dependency through the modulation of one or several of the following variables: band gap energy of the semiconductor, mobility, carrier diffusion, current density, threshold voltage, interconnect resistance, and variability in passive components. Some of the adverse effects of temperature variations are observed in current and voltage reference circuits, and frequency drift in oscillators. Thermal instability of a voltage-controlled oscillator (VCO) is a critical design factor for radio frequency ICs, such as transceiver circuits in communication networks, data link protocols, medical wireless sensor networks and microelectromechanical resonators. For example, frequency drift in a transceiver system results in severe inter-symbol interference in a digital communications system. Minimum transconductance required to sustain oscillation is specified by Barkhausen’s stability criterion. However it is common practice to design oscillators with much more transconductance enabling self-startup. As temperature is increased, several of the variables mentioned induce additional transconductance to the oscillator. This in turn translates to a negative frequency drift. Conventional approaches in temperature compensation involve temperature-insensitive biasing proportional-to-absolute temperature, modifying the control voltage terminal of the VCO using an appropriately generated voltage. Improved frequency stability is reported when compensation voltage closely follows the frequency drift profile of the VCO. However, several published articles link the close association between oscillation amplitude and oscillation frequency. To the knowledge of this author, few published journal articles have focused on amplitude control techniques to reduce frequency drift. This dissertation focuses on reducing the frequency drift resulting from temperature variations based on amplitude control. A corresponding hypothesis is formulated, where the research outcome proposes improved frequency stability in response to temperature variations. In order to validate this principle, a temperature compensated VCO is designed in schematic and in layout, verified using a simulation program with integrated circuit emphasis tool using the corresponding process design kit provided by the foundry, and prototyped using standard complementary metal oxide semiconductor technology. Periodic steady state (PSS) analysis is performed using the open loop VCO with temperature as the parametric variable in five equal intervals from 0 – 125 °C. A consistent negative frequency shift is observed in every temperature interval (≈ 11 MHz), with an overall frequency drift of 57 MHz. However similar PSS analysis performed using a VCO in the temperature stabilised loop demonstrates a reduced negative frequency drift of 3.8 MHz in the first temperature interval. During the remaining temperature intervals the closed loop action of the amplitude control loop overcompensates for the negative frequency drift, resulting in an overall frequency spread of 4.8 MHz. The negative frequency drift in the first temperature interval of 0 to 25 °C is due to the fact that amplitude control is not fully effective, as the oscillation amplitude is still building up. Using the temperature stabilised loop, the overall frequency stability has improved to 16 parts per million (ppm)/°C from an uncompensated value of 189 ppm/°C. The results obtained are critically evaluated and conclusions are drawn. Temperature stabilised VCOs are applicable in applications or technologies such as high speed-universal serial bus, serial advanced technology attachment where frequency stability requirements are less stringent. The implications of this study for the existing body of knowledge are that better temperature compensation can be obtained if any of the conventional compensation schemes is preceded by amplitude control. / Dissertation (MEng)--University of Pretoria, 2013. / Electrical, Electronic and Computer Engineering / unrestricted

As electronics are getting both smaller and more advanced, the need to verify and validate remains and the means are getting more complex the more functions and components are added. Traditionally, in-circuit tests (ICTs) are performed by probing dedicated test points on the Printed Circuit Board Assembly (PCBA) in a test sequence that is unique to each product. But as the density of components increases, the choice between component and test point must be considered. Instead of decreasing the reliability during verification by having to remove less system-critical test points, this thesis suggests the use of a near-field probe (NFP) based around a Giant Magneto-Resistance (GMR) sensor to possibly replace the need for a physical test point by instead performing contactless testing. The use of a GMR sensor allows for bandwidth from 0 Hz up to the MHz range, whereas commercial NFPs are based on a different technique and are operational from the MHz range and up. The goal of this project was to improve the non-linearity of typically 15% present in the AAH002-02 model from NVE by the use of an analogue closed-loop magnetic feedback circuit. The project successfully improved the linearity to 99.8% by the use of an instrumentation amplifier, a subtractor and a push-pull amplifier in conjunction with a 3x30 turn planar coil embedded in a PCB, located beneath the sensor Integrated Circuit (IC). The resulting linearity was verified by a Helmholtz coil where a uniform magnetic field was produced with linearly increased field strength, and calculated using the R2 value from a linear regression analysis on the acquired data. In the future, the data acquired from this kind of NFP could be used together with a Machine Learning (ML) model to remove the manual labour required when constructing these product-unique test sequences. / Dagens elektronik blir både mindre och mer avancerad, men behovet av verifiering och validering av dessa kvarstår och metoderna för detta ökar i komplexitet ju fler funktioner och komponenter som läggs till. Dagens kretskortstester genomförs genom att sondera dedikerade testpunkter strategiskt utplacerade på kretskortet enligt en testsekvens som är unikt skapad för varje produkt. Men med att densiteten av komponenter ökar måste valet mellan komponent och testpunkt tas i beaktning. Instället för att minska tillförlitligheten vid validering genom att ta bort mindre kritiska testpunkter föreslår denna avhandling användandet av en närfältssond baserad runt en Giant Magneto-Resistance (GMR)-sensor för att möjligen ersätta behovet av en fysisk testpunkt genom att istället genomföra kontaktlös testning. Användandet av en GMR-sensor tillåter en bandbredd från 0 Hz upp till MHzområdet, där kommersiella närfältssonder är baserade på annan teknik och är funktionsdugliga från MHz-området och uppåt. Målet med detta projekt var att förbättra olinjäriteten på typiskt 15% som är närvarande hos en sensor av modell AAH002-02 från NVE genom en analog magnetisk återkopplingskrets med sluten slinga. Projektet lyckades förbättra linjäriteten till 99.8% genom användandet av en intrumentförstärkare, en subtraherare och en push-pull-förstärkare i samverkan med en plan spole på 3x30 varv inbyggd i ett mönsterkort placerd under sensorns integrerade krets. Den resulterande linjäriteten validerades med hjälp av en Helmholtz-spole där ett uniformt magnetfält producerades med linjärt ökande fältstyrka och beräknades genom R2 -värdet från en linjär regression-analys på den inhämtade datan. I framtiden kan datan som inhämtats från den här sortens närfältssond kunna användas tillsammans med en maskininlärningsmodell för att ersätta det manuella arbetet som idag krävs för att konstruera dessa produktunika testsekvenser