Just as high power sources such as amplifiers and lasers have
non-linear amplitude response with input power, they also usually have a non-linear phase
response with input power. The phase response with frequency, however, is in practice
quite linear and does not pose a problem unless very wide modulation bandwidths are being
employed.
The effect of phase change with power level is to cause an amplitude-dependent phase
distortion of the data signal, often termed AM-PM distortion,
(Amplitude Modulation to Phase Modulation). This has a particularly detrimental effect on
phase based modulation formats such as M-ary PSK or M-ary QAM which suffer unwanted rotation of the phase
states of each symbol.
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