Spectral Analysis of UPWM Signals for Filterless Digital Class D Power Amplifiers

Filterless digital class D power amplifiers usually utilize three-level uniform sampling pulse width modulation (UPWM) technique to convert the digital input signal into a UPWM signal which is used to drive the power stage. Therefore, the spectral estimation expressions for different three-level UPWM signals obtained by modulating the digital signal can be an evaluation tool for signal distortion and electromagnetic interference when filterless digital class D power amplifiers are designed. Based on the time-domain characteristics of three-level UPWM signals, the spectral estimation expressions for six types of three-level UPWM signals are proposed in this paper by using the discrete Fourier transform technique and the frequency response of the zero-order holder. According to these expressions, the relationships among the total harmonic distortion, the pulse repetition frequency and the pulse width resolution of the UPWM signal are studied. A UPWM generator test system based on field-programmable gate array is designed and implemented to measure the spectra of different UPWM signals. The measured results are compared with the calculated results obtained by the proposed expressions to verify the correctness of the proposed expressions.