Phase coincidence

When two sine waves have the same frequency, they can behave much differently if their cycles begin at different times. Whether or not the phase difference, often called the phase angle and specified in degrees, matters depends on the nature of the circuit. Phase angle can have meaning only when two waves have identical frequencies. If the frequencies differ, even by just a little bit, the relative phase constantly changes, and you can’t specify a single number. In the following discussions of phase angle, assume that the two waves always have identical frequencies. Phase coincidence means that two waves begin at exactly the same moment.

They are “lined up.” This is shown in Fig. 1 for two waves having different amplitudes. (If the amplitudes were the same, you would see only one wave.) The phase difference in this case is 0 degrees. You might say it’s any multiple of 360 degrees, too, but engineers and technicians almost never speak of any phase angle of less than 0 or more than 360 degrees.

Fig. 1 Two sine waves in phase coincidence.

If two sine waves are in phase coincidence, the peak amplitude of the resultant wave, which will also be a sine wave, is equal to the sum of the peak amplitudes of the two composite waves. The phase of the resultant is the same as that of the composite waves.