Basic Oscillator Theory

Abstract

In undertaking the design of a crystal oscillator, an understanding of basic oscillator principles is not only desirable but essential. Therefore, a brief explanation of the operation of a crystal oscillator is given here. Basically, a crystal oscillator can be thought of as a closed loop system composed of an amplifier and a feedback network containing the crystal. Amplitude of oscillation builds up to the point where nonlinearities decrease the loop gain to unity. The frequency adjusts itself so that the total phase shift around the loop is 0 or 360 degrees. The crystal, which has a large reactance-frequency slope, is located in the feedback network at a point where it has the maximum influence on the frequency of oscillation. A crystal oscillator is unique in that the impedance of the crystal changes so rapidly with frequency that all other circuit components can be considered to be of constant reactance, this reactance being calculated at the nominal frequency of the crystal. The frequency of oscillation will adjust itself so that the crystal presents a reactance to the circuit which will satisfy the phase requirement. If the circuit is such that a loop gain greater than unity does not exist at a frequency where the phase requirement can be met, oscillation will not occur.