In order to evaluate the spectral purity degradation produced by varactor doublers, the transfer properties of amplitude and phase fluctuations have been determined with reference to a theoretical analysis developed in a previous work. Quasi-stationary conditions have been assumed and the input and output tuning circuits have been approximated with a linear relation between frequency and reactance. The effect of the bias circuit has been taken into account both for fixed-bias and self-bias conditions. Then the additive-noise contribution of the doubler circuit elements have been obtained. Numerical examples and curves are given for all the evaluated quantities.