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Phase
Before understanding phase modulation, it is important to understand what phase is. A radio frequency can be represented as a dot moving around the edge of a circle, as the circle moves along a straight line. The path traced by the dot forms a wave-like shape, called a sine wave. The phase is the angle within the circle between the dot's start point and its current point on the edge of the circle, and therefore its position on the waveform.
Phase Modulation
Phase modulation alters the phase of the signal being transmitted. It does this by changing the speed with which the dot moves around the edge of the circle. If the signal had not been modulated in this way, the dot would be at a different point -- therefore, the phase has been altered. Because frequency and phase are linked to each other, this also changes the frequency of the signal.
Digital Data Transmission
The basic unit of digital information is the bit, represented by either a zero or a one. This is called binary information -- all digital information can be traced to strings of zeros and ones, therefore any signal that can be modulated to send binary data can send any data. Phase modulation through binary phase shift keying uses two phases separated by 180 degrees on the hypothetical circle described in the previous steps. One of these phases represents zero, the other one, so by phase modulation binary data can be transmitted.
Analog Data Transmission
In principle, phase modulation to encoded data onto an analog system functions in a similar way to frequency modulation (FM) most commonly used in radio. If two signals are sent where the frequency, amplitude and phase of the first signal are known, but in the second, one of these is modulated, the difference between the signals is used to encode the data being transferred. By ̶0;tuning̶1; to a particular signal, and checking the difference between that signal and the carrier signal, data can be decoded.