respect to a reference. A phase comparison can be made by connecting two signals to a two-channel oscilloscope. The oscilloscope will display two sine signals, as shown in the graphic to the right. In the adjacent image, the top sine signal is the test frequency, and the bottom sine signal represents a signal from the reference. If the two frequencies were exactly the same, their phase relationship would not change and both would appear to be stationary on the oscilloscope display. Since the two frequencies are not exactly the same, the reference appears to be stationary and the test signal moves. By measuring the rate of motion of the test signal the offset between frequencies can be determined. Vertical lines have been drawn through the points where each sine signal passes through zero. The bottom of the figure shows bars whose width represents the phase difference between the signals. In this case the phase difference is increasing, indicating that the test signal is lower in frequency than the reference. Formula for phase of an oscillation or a periodic signal The phase of an oscillation or signal refers to a sinusoidal function such as the following: where , , and are constant parameters called the amplitude, frequency, and phase of the sinusoid. These signals are periodic with period , and they are identical except for a displacement of along the axis. The term phase can refer to several different things: It can refer to a specified reference, such as , in which case we would say the phase of is , and the phase of is . It can refer to , in which case we would say and have the same phase but are relative to their own specific references. In the context of communication waveforms, the time-variant angle , or its principal value, is referred to as instantaneous phase, often just phase. See also Absolute phase AC phase In-phase and quadrature components Instantaneous phase Lissajous curve Phase cancellation Phase problem Phase spectrum Phase velocity Phasor Polarization Coherence, the quality of a wave to display a well defined phase relationship in different regions of its domain of definition Hilbert Transform, A method of changing phase by 90° Reflection phase shift, A phase change that happens when a wave is reflected off of a boundary from fast medium to slow medium References External links "What is a phase?". Prof. Jeffrey Hass.