On the Myth of Pulse Width Modulated Spectrum in Theory and Practice

Switch-mode audio power amplifiers are commonly used in sound reproduction. Their well known drawback is the radiation of high frequent energy, which can disturb radio and TV receivers. The designer of switch-mode audio equipment therefore needs to make arrangements to prevent this coupling which would otherwise result in bad audio performance. A deep understanding of the pulse width modulated (PWM) signal is therefore essential, which resulted in different mythic models as pulse, trapezoidal or Double Fourier Series (DFS) representations in the past. This paper will clarify these theoretical approaches by comparing them with reality from both the time and the frequency domain perspective. For validation a switch-mode audio power amplifier was built, delivering the contents material with less than 0.06 % distortion across the audio band at 50 W. The switch-mode signals have been evaluated very precisely in time and spectral domain to enlighten the assumptions about the PWM spectra and decrypt this myth.