The likely growth in the importance of private land mobile radio in the future presents particular problems in achieving the necessary crystal oscillator reference frequency stability. The high power consumption and slow warm up time of oven controlled crystal oscillators is undesirable for mobile operation, so that frequency changes with temperature must be minimised by other means. This thesis describes a technique for temperature compensation of crystal oscillators which is primarily digital in nature. The system is capable of high stability and offers advantages not present in conventional designs. The use of a digital memory as the compensation law governing element affords great versatility, the same hardware being appropriate in a variety of applications. Automatic calibration of the device is also possible, further improving its performance and reducing the likely cost of production. Of particular importance in the realisation of the scheme is the method employed to adjust the output frequency. The requirement for a digitally controlled very high resolution frequency source of simple construction has led to the development of a new class of digital frequency synthesiser, a detailed discussion of which is included. In order to improve upon conventional methods of thermometry and to maximise the use of digital circuitry a Y cut crystal is used as the temperature sensing element. This crystal is placed in close thermal contact with the primary crystal and its linearly temperature dependent frequency is counted digitally to afford temperature information. The early chapters of the thesis discuss the underlying theory of precision frequency sources, quartz crystals and oscillators, and digital frequency synthesis, leading to a general discussion of the proposed system. Details of the design and performance of a prototype unit are then given, and some techniques of automatic programming of the device are considered. Some relevant mathematical derivations and experimental results are included among a series of appendices.
|Date of Award