Non-linear effects in varactor tuned resonators

This paper describes the effects of RF power level on the performance of varactor-tuned resonator circuits. A variety of topologies are considered including series and parallel resonators operating in both unbalanced and balanced modes. As these resonators were being designed to produce oscillators with minimum phase noise the insertion loss was set to 6 dB and hence Q_I/Q₀=1/2. To enable accurate analysis and simulation, accurate S parameter and PSPICE models for the varactors were optimised and developed. It is shown that these resonators start to demonstrate non-linear operation at very low power levels demonstrating saturation and lowering of the resonant frequency. On occasion ´squegging´ is observed for modified biasing conditions. The non-linear effects are dependent on the Q, bias voltage and circuit configurations with typical non linear effects occurring at -6 dBm in a circuit with a loaded Q of 63 and a varactor bias voltage 3 volts. Analysis, simulation and measurements are presented which show close correlation.