The equivalent circuit of a negatively biased varactor at X band is investigated in this paper. Three methods for measuring the Qd-values and the equivalent circuit parameters of varactors are suggested. The first method is based on the measurements of the input impedance at various bias voltages and the cartridge impedance of the diode. This method is suitable to a varactor diode with a series resistance rs which is a function of the negatively biased voltage. By the second method

the Qd-value and the varactor parameters are simply calculated from the measurements of the input impedance

provided that equivalent capacitance of the cartridge is known. However

this method is only applicable to the case that the series resistance rs is independent of the negatively biased voltage. In the third method the parameters of a varactor diode are determined by the measurements of the input impedance only at negative biases and the junction capacitance at low frequency; consequently

a method is given for measuring some varactors

such as GaAs

which is inhibited to be biased positively. Experimental results of the measurements of varactor parameters show that they are well checked with each other for all three methods.