6.1 Introduction

The oscillator is an essential component for microwave systems such as communications, radars and instrumentation. An ideal oscillator produces a pure sinusoidal carrier with fixed amplitude, frequency and phase. Practical oscillators, however, produce carrier waveforms with parameters (oscillation frequency, output power) that may vary in time, due to temperature changes and component ageing. Oscillator frequency instabilities are characterised primarily by frequency drift (long term variations) and random noise (short term fluctuations). The latter appears as phase and amplitude fluctuations at the oscillator output and will always be of major concern in modulated systems. Phase noise generated by any local oscillator in the transmit/receive chain can significantly affect the performance of digital communication systems.

The early development of microwave solid-state oscillators was mainly based on Gunn and IMPATT diodes. Since the 1970s, the advent of GaAs MESFETs at microwave and millimetre-wave frequencies has given greater freedom to engineers in designing microwave oscillators. GaAs MESFET devices, compared to Gunn and IMPATT diodes, offer several advantages such as suitability for monolithic integration, higher power efficiency and lower phase noise, and they have no threshold current requirements. Free-running oscillators employing GaAs MESFETs, or similar devices, have been reported [1 9] for frequencies up to W-band. Broadband tuneable oscillators using YIG resonators and varactor diodes have also been published [2, 8, 9] in the literature. Recent investigations have been directed towards very low phase-noise designs in MMIC form, especially those using HEMT and HBT devices. Low phase-noise oscillators based on...