DesignRFCircuit“How to Design RF Circuits” - OSCILLATORS Andrew Dearn * Introduction Oscillators can generally be categorised as either amplifiers with positive feedback satisfying the wellknown Barkhausen Criteria (Ref. 1), or as negative resistance circuits (Ref. 2). Both concepts are illustrated in Figure 1. At RF and Microwave frequencies the negative resistance design technique is generally favoured. +ve Feedback L C -ve R R Amplifier (a) Negative Resistance Oscillator (d) Positive Feedback Oscillator Figure 1: General Equivalent Circuits for RF Oscillators The procedure is to design an active negative resistance circuit which, under large-signal steady-state conditions, exactly cancels out the load and any other positive resistance in the closed loop circuit. This leaves the equivalent c……