Editing Phase (waves) (section)

==Phase comparison==
''Phase comparison'' is a comparison of the phase of two waveforms, usually of the same nominal frequency. In time and frequency, the purpose of a phase comparison is generally to determine the frequency offset (difference between signal cycles) with respect to a reference.<ref name=phnist>
{{cite journal
 | url =https://www.nist.gov/pml/div688/grp40/enc-p.cfm
 | title =Phase
 | last =Time and Frequency from A to Z
 | journal =NIST
 | date =2010-05-12
 | publisher =[[National Institute of Standards and Technology]] (NIST)
 | access-date =12 June 2016
}} This content has been copied and pasted from an NIST web page ''and is in the public domain''.</ref>

A phase comparison can be made by connecting two signals to a [[oscilloscope|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 [[frequency|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.<ref name=phnist/>