Third harmonic measurements are used to assess the nonlinearity level of resistors, as recommended by the IEC/TR 60440 standard. Availability of a signal generator with extremely small (zero in limiting case) content of the third harmonic plays a key role in getting successful linearity measurements. Traditionally, this is achieved with high-purity sinusoidal analog generators. This paper sets forth Fourier analysis for three types of periodic stepped waveforms to choose that which provides minimum content of the third harmonic for the real waveforms with finite values of the slew rate. Such waveforms can be implemented digitally. Numerical calculations demonstrate that there is a six-step waveform with theoretically zero third-harmonic content.