Delay-line stabilized microwave oscillator with frequency control

Abstract

The technique whereby the frequency of an oscillator is stabilized using an external feedback network incorporating a delay-line is extended in this thesis to provide digital control of the frequency of the oscillator. Novel circuit techniques have been developed and analysed to permit the implementation of the proposed frequency control technique in a compact format, suitable for implementation in either hybrid or monolithic microwave integrated circuits. Two new microstrip circuits have been introduced, namely the three-port ring discriminator and the single PIN diode phase shifter. Frequency control of an oscillator is achieved by digitally controlling the time delay through phase shifters in the feedback path of the oscillator's stabilization circuit. A new microstrip phase shifting circuit has been developed which has the advantage of requiring only one active switching element to achieve each digital bit of phase change. The circuit has been analysed in detail and results obtained at X-band to support the theoretical predictions. As part of the analysis of this new component, and in order to permit a greater degree of precision in the design of the phase changes, some new microstrip design techniques have been introduced. These have led to a more exact design for coupled line phase shifters and to an equivalent circuit to represent the excess phase in microstrip DC breaks. Delay-line stabilization of an oscillator requires the use of a phase sensitive network, or frequency discriminator. Whilst this could be realised on microstrip using conventional circuitry, by interconnecting two hybrid rings, a new circuit component, namely the three-port ring discriminator, was developed to provide a simpler, more compact solution. A rigorous analysis of the new circuit is presented, and the behaviour verified through measurements over the frequency range 8-12GHz. The new single PIN diode phase shifter has been incorporated in the delay path of a three-port ring discriminator, and used to control the frequency of an oscillator. Results are presented for circuits at X-band which show the degree of frequency stabilization that has been obtained, together with the reduction in oscillator phase noise. In addition, the original concept of delayline switching to control the frequency of an oscillator has been extended to yield a further new circuit, based on a three-port ring with a switchedfeed mechanism; results are presented which verify the operation of this new circuit both theoretically and through practical measurement. New techniques for controlling the frequency of microstrip oscillators have thus been established, both theoretically and through practical measurement, which combine simple methods of frequency selection with inherent low-noise performance.