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The dissertation covers the physical, design, simulation and measurement aspects of extremely high Q precision oscillators for space applications. Existing design approaches and methods are evaluated, and it is shown that only physically complete models can deliver a good agreement between theory and experiments. A prototype was built showing very good phase noise and stability.
The design of high Q oscillators, using Crystals at lower frequencies, (and dielectric resonators at much higher frequencies), has long been considered a black art. This may be due to the fact that a systematic approach with optimized design guideline for crystal oscillators could not be found after extensive literature search.
In this dissertation, after analyzing the first crystal oscillator by W.G. Cady (1921), other high performance crystal oscillators will be discussed, analyzed and calculated.
A single transistor crystal oscillator design as used by HP (Hewlett Packard) in one of their designs, the HP10811A is considered in this thesis for mathematical analysis and CAD (Computer aided Design) simulation. This was also measured on state-of-the-art signal source analyzer. After validation, this design is scaled to 100MHz, the frequency of interest for this dissertation.
Though most designers use a single transistor based oscillator circuit, it is not an optimized design because of limited control over key design parameters such as loop gain, dc current etc.
This dissertation is an attempt to overcome the limitation due to the single transistor circuit and to give a step by step procedure, explaining the significance of a two transistor design with thorough analysis and design simulation results. This two stage transistor circuit is also not yet a best solution in terms of phase noise performance and output power, and some add-on circuitry will be needed for an optimized performance.
An important contribution of this work is to show that since the voltage gain is the ratio of the collector resistor and the emitter resistor, the performance is practically independent of the VHF transistor and gives better control over various parameters of the oscillator, in order to optimize the design.
A grounded-base amplifier is then introduced and added for improving the isolation and the output power. Unlike most oscillators, that take the output from the collector, a novel concept introduced by Rohde [14], is incorporated here, where the crystal is used as a filter that is then connected to the grounded base amplifier, a technique which many companies have been using. This dissertation will show that this technique increases the output power without significantly affecting the phase noise. Such a validation is needed for better understanding and as per my knowledge, has not been done so far.
For the oscillator, the tuning diode sensitivity and flicker noise contribution are also taken into consideration, by calibrating the mathematics and its validation is shown.
Crystal resonators of the type AT and stress-compensated (SC) cut devices will be considered as they give the best performance. The one port Colpitts type oscillator is considered first and the two port two transistor design later. Both will need a post amplifier/buffer stage.
A complete step by step design procedure for an optimized 100MHz crystal oscillator is then presented.
For completeness, CAD Simulation and Experimental results are provided for 10 MHz, 128 MHz and 155 MHz VCO circuits.
In modern information technology, increasingly powerful electronic circuits are required for the targeted generation of complex signals with well-defined amplitudes and phases. In circuits of this type, oscillators frequently form the central element because of its phase noise and stability, which essentially determines the achievable precision in the signal generation. Further requirements are derived from the electronic definability of the signal properties and the operational behavior of the oscillators. Conventional oscillator circuit models autonomous circuits, mainly consist of a passive frequency-selective or phase-selective network and an active amplifier element, which together produce an oscillatory circuit via a suitable feedback. At first glance, the circuit topology seems to be quite simple, and can often be explained quite visibly. However, when it comes to describing in particular the very important phase noise dynamics and stability of oscillators, it very soon becomes apparent that highly complex structures are involved. A fundamental difficulty in the theoretical description arises due to the non-linear behavior of oscillators, the understanding of which is crucial for a reliable description of jitter and oscillator phase noise. The resonant condition of oscillators arises due to the fact that the noise in the oscillator circuit is always present in the system, which is amplified in a frequency-selective manner to the extent that a stable oscillation arises at most at a fixed frequency because of non-linear limitation of the amplification. The frequency selectivity arises due to the frequency selectivity or phase selectivity of the passive feedback path. The non-linear limitation of the amplification in the oscillator normally results in a very reliable control of the amplitude noise of the output oscillation. It is well understood that any particular oscillator’s phase noise could be improved by increasing the generated signal amplitude or increasing the quality factor of the resonant network. Increasing the signal level is limited by the utilized supply voltage or the break down limits of transistors and cannot be increased further to improve the phase noise. Accordingly, the remaining phase noise, which can normally be minimized via resonating circuits with pronounced phase selectivity and therefore a high quality factor resonator, is of great importance for oscillators. Traditional high Q-factor resonators (ceramic resonator, surface acoustic wave, bulk acoustic wave, dielectric resonator, YIG resonator, Whispering gallery mode resonator, Optoelectronic resonator, etc.) are usually 3-dimensional structures and bulky for both handheld and test-measurement equipments and does not offer integration using current foundry technology. The current and later generation wireless communication market is pushing the need for miniaturization to its limits. Printed coupled transmission line resonator is a promising alternative due to its ease of integration and compatibility with planar fabrication processes but limited by its large physical size and low quality factor, making it a challenging choice to design low phase-noise oscillators. This problem is more prominent in integrated circuits (ICs) where high degrees of thin conductor losses reduce the quality factor by orders of magnitude compared to hybrid circuit technologies.
This thesis describes the design and investigation of a variation of printed resonators using Möbius slow-wave and Metamaterial structures for the applications in oscillator circuits. A novel Möbius slow-wave mode-coupled structure offers additional degrees of freedom (higher Q-factor and multi-band characteristics for a given physical size) as compared to conventional transmission line printed resonators. A design study has been carried out to optimize the phase noise performance by using the novel resonant structures (mode-coupled, slow-wave, Möbius strips, evanescent mode, negative index material-Metamaterial) in conjunction with mode locking and injection locking for improving the overall performances, beyond the limits imposed by conventional limitations. The thesis also covers a broad spectrum of research on DRO and OEO ranging from practical aspects of circuit implementation and measurement, including the modeling of optical fiber delay line used as a thermally stable high Q-factor resonator structure. This thesis is research work carried out from 2004-2014, organized in 11 chapters, theoretical and experimental results documented by a range of specific measurement results and substantiated by over 200 scientific publications over dozen patents. The Metamaterial Möbius technology discussed in this thesis can open new era in the field of imaging, sensors, cloaking, energy harvesting and energy efficient microwave circuit and system Solutions.
Voltage controlled oscillators are commonly realized using varactor diodes in order to tune the resonator frequency. The drawback in practical implementation is the relatively high cost of the diodes, and the large space required for the inductors. Replacing the inductors by active tunable inductors would solve these issues and open a path for cost-effective small integrated oscillator designs. In the past, active-inductor approaches were not competitive due to the high phase noise inherent to the active inductor. This work investigates active inductor oscillators, and presents a novel approach that yields low phase-noise VCOs. The theoretical investigations are verified by measurements of demonstrator circuits.