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The Kahn envelope elimination and restoration (EER) technique allows for linear RF power amplification by combining nonlinear, but efficient, radio frequency (RF) and audio frequency (AF) power amplifiers. The EER technique requires a coordinate transform of the Cartesian digital in-phase and quadrature (I & Q) signals to polar amplitude (A) and phase-modulated RF (RF-P) signals. An ideal recombination of the A and RF-P signals in the output stage of the transmitter yields perfect amplified I & Q signals in the output of the transmitter. However, any small mismatch of the delay between A and RF-P paths or bandwidth restrictions within the A path reveal the dominant nonlinearity inherent with the coordinate transform, resulting in out-of-band (OOB) radiation. In this paper, the EER technique is investigated for single-carrier digital modulations. It is shown that the characteristics of the resulting OOB radiation merely depends on the size of the hole in the vector diagram of the digital modulation and is practically independent of other parameters of a digital modulation. Universal curves are given, which can be useful for developing and aligning EER digital transmitters.