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Saturation of ferrite frequency doublers due to the reaction of the generated second-harmonic field back on the sample, the excitation of unstable spin waves by transverse pumping, and the excitation of unstable spin waves by longitudinal pumping is investigated experimentally. An expression for the second-harmonic power as a function of the power absorbed by the ferrite, which holds for any degree of processional motion ellipticity of the magnetization, is derived including the effect of the reaction field. An experimental method of observing the three saturation effects through the introduction of an externally impressed second-harmonic field is described. Experimental observations of all three saturation mechanisms are reported, and the results are compared with theory. The upper bound on conversion efficiency of a ferrite doubler, assuming adequate output coupling circuits can be devised, is set by longitudinally-pumped spin waves. This upper bound on efficiency is measured experimentally for a typical doubler.