Design of crystal oscillators by using simulators

The design of crystal oscillators in cell phone applications intended for high volume production is a challenge. In the past, the development was based on empirical formulas and the designer's experience. During development a good prediction of the oscillator performance is desired. This resulted in a trial and error process. A deficiency of this method is that a lot of impacts could not be studied easily. Today, it is additionally possible to use modern design tools to ensure a so-called "robust design". To enhance the accuracy precise models of various circuit parts have to be available. In this paper models for the circuit components in use and the layout influences will be presented. Also, models for the used resistors and the varactor diode will be described. The models for the resistors and the layout influence will be derived from measured data. The main focus of this paper will be the behaviour model of the varactor diode under large signal conditions: Since, in the crystal oscillator in use, the varactor works under large signal conditions, the simulation has to include capacitance variation due to high RF voltage swing. However, in commonly available SPICE-models only a description of the small signal behaviour is obtained and no large signal model is available.