Fundamental limitations of oscillator performance

The main characteristics of all low noise oscillators can be described by the same simple feedback circuit model. The model is essentially a positive circuit feedback Q multiplier circuit. The model shows how phase noise is fundamentally limited by the oscillator feedback power P relative to the noise N at the input of the feedback sustaining amplifier, and the effective open circuit mode Q which can be defined in simple terms for all oscillators. The figure of merit is PQ/sup 2//N, where N=1/2 FkT with F as the noise factor of the feedback amplifier. For all but delay line oscillators PQ/sup 2/ can be regarded as the stored energy of the oscillator. The stability of an oscillator is ultimately only determined by the physical stability of the values of the components which define the oscillator frequency, whether it be LC, delay line, or atomic resonance. Low noise oscillators can be designed conceptually by splitting the loop amplifier between input and output and then for the same phase shift designing the circuitry for best signal to noise match at the amplifier input. The paper concentrates on general rather than specific factors affecting oscillator performance, although some examples are given to indicate what factors are more important in practice. A conclusion of the paper is cryogenic operation of oscillators may be one of the only remaining ways forward offering substantial further improvement in oscillator performance.<>