Compensation of Imperfections Caused by Digital Pulse Width Modulation Using Frequency Dependent Baseband Injection

All-digital transmitter is a hot topic since a few years. One of the key elements of all-digital transmitters is the modulator, which modulates the baseband IQ signal onto a carrier frequency in the form of digital binary signals. The combination of delta-sigma modulation and pulse width modulation is a very strong candidate. However, the regular sampled pulse width modulator introduces non-linear distortions and images also known as aliasing effects, which depend strongly on the frequency position of the input signal. Due to these non-linear distortions and images, the signal integrity is violated especially when the power spectral density of the baseband input signal is not symmetrical around DC. In this paper, we explore the causes of the frequency dependency in terms of DSM distortions and focus on its compensation approach. Based on the baseband injection method we introduce a filter function to compensate the frequency dependent distortions. In simulations and hardware tests we show effectiveness of the proposed method.