This guide describes several methods of measurement for the small signal characterization of varactors. For the purpose of this guide, a varactor is defined as a two terminal device used as a voltage-dependent capacitance. Such devices c a n be used from d c to microwave frequencies. Examples include low noise parametric amplifiers from audio to microwave frequencies, tuning elements, choppers, and phase shifters. A small-signal equivalent circuit is presented and its validity discussed. Methods for determining the elements of this circuit by means of ac measurements are described in general terms, and restrictions are discussed. The use of dc measurement techniques is discuss in Test Procedure for Semiconductor Diodes IEEE 256. For a consideration of noise measurements, see Part 9: Noise in Linear Two ports , Methods of Testing Electron Tubes - IEEE 158.