Teaching the principles of microwave oscillator design

Theory of oscillators usually starts with an oscillator fundamental circuit containing two components: a negative resistance and a frequency stabilising circuit. Conditions for the oscillation start-up and for the steady state follow then, leading to so-called amplitude and phase conditions expressed in the case of microwave oscillators either in the immitance form or written with the use of the oscillator components reflection coefficients. From the mathematical point of view both approaches should be equivalent. However, it has been shown in the past that only the immitance-based approach guarantees a successful oscillator design. In the paper this question has been reconsidered from the didactic point of view. It has been shown that both approaches are actually equivalent, provided that over-simplifications appearing sometimes in the literature are avoided. The extended Smith chart has been also used, yielding a clear graphical interpretation of the involved relationships.