Practical Industrial Data Networks:Design, Installation and ...

322 Practical Industrial Data Networks: Design, Installation and TroubleshootingB.3 Modulation techniquesB.3.1B.3.2Modulation can involve either analog or digital signals, or both. It refers to the process ofsuperimposing the information (modulating) signal on to a carrier signal. In itsunmodulated state, the carrier is a constant amplitude, constant frequency signal.Analog modulationIn this case the analog carrier signal is modulated with an analog information signal.There are basically three methods:• Amplitude modulation (AM)The amplitude of the carrier is varied by the amplitude of the informationsignal.• Frequency modulation (FM)The frequency of the carrier is varied by the amplitude of the informationsignal.• Phase modulation (PM)The phase (time displacement) of the carrier signal is varied by the amplitudeof the information signal.RF modulationRF modulation is similar to analog modulation, with the exception that the input(modulating) signal is digital.• Amplitude shift keying (ASK): The amplitude of the carrier signal is variedbetween two values. This is also known as on–off keying (OOK).• Frequency shift keying (FSK): The frequency of the carrier signal is variedbetween two values by the modulating signal.Phase shift keying (PSK) a.k.a. binary phase shift keying (BPSK): The phase of thecarrier signal is changed by the modulating signal. Depending on the number of discretedisplacements, several bits of data can be transmitted simultaneously. For example, withfour shift amounts (0°, 90°, 180° and 270°), two bits e.g. 00, 01,10 and 11 can be sent at atime.• Quadrature amplitude modulation (QAM): Both the phase AND theamplitude of the carrier are changed, and makes it possible to encode asmany as 4 bits at a time.• Trellis coded modulation (TCM): This is similar to QAM, but includesextra bits for error correction.The shift keying modulation methods come in plain and differential forms. Thedifferential versions encode values as changes in a parameter, not in a specific value for aparameter. The differential techniques are easier to implement, and more robust than thenon-differential ones.• Differential amplitude shift keying (DASK): similar to ASK, but encodingdifferent digital values as changes in signal amplitude.• Differential frequency shift keying (DFSK): similar to FSK, but encodingdifferent values as changes in signal frequency.• Differential phase shift keying: similar to PSK, but encoding differentdigital values as changes in signal phase.