Imperfaction of Pulse Width Modulation Compensation by Using Baseband Injection

All-digital transmitter is a hot topic since a few years. One of the key elements of all-digital transmitter is the modulator, which modulates the baseband IQ signal onto carrier frequency in the form of digital binary signal. The combination of delta-sigma modulation and pulse with modulation is a very strong candidate. However, the regular sampled pulse width modulator introduces non-linear distortions and images also known as aliasing effects for modulated input signal. Due to these non-linear distortions and images, the signal integrity would be violated especially when the power spectral density of the baseband input signal is not exactly symmetrical around DC. In this paper, we propose a simple and straightforward baseband injection method to pre-compensate these non-linear distortions and images. This novel method must not run at the operating frequency of pulse width modulator but can run at a much lower frequency, and moreover, each term of the non-linear distortions and images can be individually compensated without conspicuous mutual influence. In Matlab simulations we observe, that all the compensated non-linear distortions and images would be suppressed over 20 of dB and disappear under the noise floor of delta-sigma modulator.