In laboratory and astronomical polarimetry it is often still customary to employ sinusoidal voltages to control electro-optical modulators. In this case, signals on the main and double frequencies are usually recorded. Control voltages are reasonably high, up to 2 divided by 5 thousand volts, when using crystals like ADP, KDP, KD*P, and of some other types. Obviously, with so high values, nonlinear distortions of control voltages of up to 5 divided by 7% are almost unavoidable. In this paper, in a rather concise form, it is shown that nonlinear distortions of control voltage provide the source for spurious signals. A simple analytical expression is obtained, which relates the spurious signal amplitude to the amount of nonlinear distortions of modulating voltage. By examining a numerical example, it follows in particular that in some cases this factor is able to restrict the sensitivity of circular polarization measurements to 10-3 divided by 10-4. The aforesaid applies also for many objective polarimeters designed for measuring the rotation of the linear polarization plane.