Asymmetric spectra of phase modulated transitions of square waves

Square waves have instantaneous transitions between fixed upper and lower levels. The timing of the rising and falling transitions or `edges´ may be modulated in time and phase separately and independently. The resulting spectrum sidebands are a combination of the two transition modulations. In general the modulation spectrum is asymmetric with different upper and lower sideband spectra. This paper reports an investigation into why this should be so. New methods of single sideband modulation and demodulation are proposed.