A high performance VHF oscillator with optimized crystal drive power

The common method for predicting the phase noise for crystal-controlled oscillators is to use Leeson´s model. However, many oscillator circuits do not give results consistent with this simple model because of the interaction between the active devices and the frequency control element. A model has been formulated that takes into account the effects of the resonator impedance on the noise levels occurring in an emitter coupled oscillator and its associated buffer circuit. This model uses the standard transistor noise sources to predict the output noise level of a circuit with the resonator connected to the transistor emitter. The model was also used to analyze the performance of a high-performance crystal oscillator that exhibits a low phase noise floor and minimal frequency chirp after power is applied. The drive level needs to be kept as low as possible to prevent frequency pulling caused by heating of the crystal. A transformer is used in the oscillating transistor emitter to control the drive power to the resonator while optimizing the loaded Q of the circuit. A wideband noise floor below -172 dBc/Hz has been achieved while dissipating less than one-half milliwatt of power in the resonator. Higher crystal drive levels provide correspondingly lower noise floors