Three-frequency pseudo-random varying spread spectrum method for filterless digital class-D audio amplifiers

Abstract

This paper presents a novel spread spectrum (SS) method with a pseudo-natural sampling technique for filterless digital class-D audio amplifiers. This method is proposed to reduce the electromagnetic interference (EMI) emissions of amplifiers by lowering the output out-of-band spectral amplitudes around the pulse width modulator carrier frequency and its multiples. The main idea of this method is that the sampling frequency of the modulating signal and the carrier frequency are varied synchronously and randomly among three different values. Since the carrier frequency is variable, the polynomial interpolation Newton–Raphson (PI-NR) algorithm is modified to correct the harmonic distortion caused by uniform-sampling pulse width modulation (UPWM). Based on a field programmable gate array (FPGA), an experimental system is established to analyze the performances of the proposed method and other six reported SS methods. Experimental results show that the proposed method features the lowest total harmonic distortion (THD) of 0.009% with a 6.6 kHz, 0 dBFS (full scale) input, a relatively high signal-to-noise ratio (SNR) of 52.68 dB and a relatively high output out-of-band peak spectral amplitude reduction of 17.75 dB.