Phase Dependent Distortion in Direct Conversion Transmitters

The impairments and nonlinear (NL) distortion generated by the analog quadrature modulator in a direct conversion transmitter affect the performance of a transmission system. These effects cause input signal dependent gain variations at the low-pass equivalent I/Q-modulator output. The aim of this paper is to analytically derive the relationship between the mean transmitter gain and the tone phases of single- or two-tone excitation signals. This relationship is separately evaluated for the different distortion sources in the direct conversion transmitter. It will be shown that only two-tone signals with frequencies symmetrical to DC are able to excite these phase-dependent gain variations. Additionally, the conditions for the presence of the distortion mechanisms are highlighted. Using measurements of an experimental transmitter setup, the validity of the derived results is shown. The presence of phase-dependent gain changes were also measured when applying a DC-symmetric multi-tone signal.