A nonlinear distortions analysis method for diode frequency converter circuits is presented. Volterra series method is used to analyze nonlinear distortions. The analysis was carried out for three types of diode frequency converters: balanced, double balanced and triple balanced. Calculation of the 3rd order nonlinear distortion coefficient is presented. The dependences of the 3rd order nonlinear distortion coefficient on the load resistance and on the local oscillator (LO) voltage amplitude were obtained for two LO operation modes: harmonic and pulse. The error between the calculation and simulation results does not exceed 3 dB. It is shown that the dependences of the 3rd order nonlinear distortion coefficient on the load resistance and on the LO voltage amplitude have several maximums and minimums. By varying the values of the load resistance and the LO voltage amplitude it is possible to calculate the minimum achievable value of the 3rd order nonlinear distortion coefficient.