Compensation of imperfections caused by digital pulse width modulation using 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. 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 propose a baseband injection method with low complexity to pre-compensate these unwanted emissions. This novel method does not have to run at the operating frequency of pulse width modulator but can run at a much lower frequency. Moreover, each term of the non-linear distortions and images can be individually compensated without mutual influence. In Matlab simulations we show that all the compensated non-linear distortions and images are suppressed by over 20 dB and disappear below the noise floor of delta-sigma modulator.