Varactor Tuning Diodes as a Source of Intermodulation in RF Amplifiers

In RF amplifiers used in the front ends of VHF receivers which are to be tuned over a wide frequency range, varactor diodes are often used to accomplish the necessary change in tuning capacitance. nce. The variation of capacitance with dc voltage which enables the tuning function to be accomplished results in a nonlinear characteristic stic which, in turn, introduces harmonic and intermodulation dtion.storon. This paper is concerned with the determination of tef12f 1-J2) intermodulation distortion product generated by the varactor diodes and modified by the characteristics of the input tuned circuit. Both third-order and "distortion-on-distortion" second-order intermodulation mechanisms are included in the calculation. An expression is presented for the magnitude of the (2f1-f2) intermodulation intercept which includes the effects of the characteristics of the tuned circuit and the various parameters used to characterize the varactor diode. Design considerations combining both the linear and the intermodulation ation performance of the input circuit are discussed, including design tradeoffs and interrelationships among the circuit parameters and performance variables. Illustrative calculations are given based on the characteristics of the MV3102 varactor diode.