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Nonlinear distortion of quadrature modulated signals in polar transmitter

Keywords:

L.I. Averina – Dr. Sc. (Phys.-Math.), Associate Professor, Department of Electronics of Faculty of Physics, Voronezh State University E-mail: averina@phys.vsu.ru A.V. Avdeev – Post-graduate Student, Department of Electronics of Faculty of Physics, Voronezh State University E-mail: avdeev@phys.vsu.ru V.A. Stepkin – Ph. D. (Phys.-Math.), Associate Professor, Department of Electronics of Faculty of Physics, Voronezh State University E-mail: stepkin@phys.vsu.ru


The features of the structure and operation of the polar transmitter are described. It is shown that in such a transmitter the main sources of nonlinear distortion are: restoring the filter, a digital modulator and a power amplifier. The analytical relations for the calculation of intermodulation products in the transmitter output signal are reviewed. The relative levels of intermodulation products in depend of the time delay of the amplitude and phase components of the input signal envelope are calculated. The behavioral model of polar transmitter that takes into account the identified sources of nonlinear distortions is developed. The paper proposed to use the adjacent channel power ratio (ACPR) and error vector magnitude (EVM) as parameters for estimate of signal distortions. These parameters are calculated in depend of the frequency of the pulse width modulator, the type and order of the restoring filter, the nonlinearity of the power amplifier output characteristic. Analysis of simulation results shows that the nonlinearity of the amplifier output characteristic has a significant influence on the signal distortion only at small time delay between amplitude and phase components of the input signal envelope. Practical recommendations for the choice of parameters of the polar transmitter structural parts are done.
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