The most commonly found single-frequency components of phase jitter on transmitted data signals are those of ringing current, commercial a.c. power and the second to fifth harmonics of these. Since the peak phase deviation caused by such components rarely exceeds 25° peak-to-peak (i.e. low index phase modulation) only one pair of significant sidebands is produced for each sinusoidal component. Hence the main phase jitter modulation usually exists within ±300 Hz of a voice-frequency tone acting as a carrier.