The estimation of the influence of the multiplier characteristics nonlinearity on the output signals of the quadrature modulator QPSK

In the paper, the analysis of QPSK modulator has been conducted. In this modulator, the function of multiplier is implemented by differential amplifying cascade, which has the transmission characteristic approximated by the polynomial of the third degree. It is shown that considering non-linear characteristics of multiplier, it is possible to evaluate the impact of parasitic component on the form of incoming signal at the modulator, and to determine inaccuracy in finding points in constellation. This can create a basis for selection of the optimal amplitude of carrier frequency. During the calculations, the normalized values were used that allowed to obtain results of more general character and to make the transition to values of current and voltage of specific types of the modulators. The methodic calculation can be applied even in case of presence of other nonlinearities in the multipliers. It has been shown, that deviation of the existing voltages in common mode and in quadrature mode channel modulators under the nonlinear multiplication from the precise values and under the linear multiplication leads to transformation of the point of constellation into the square of ambiguity. The higher is the amplitude of the carrier frequency, the bigger are the sides of the square. Decoding error becomes possible when the squares of the ambiguity of the neighboring points of constellation will intersect.