1. A mathematical statement that describes the transfer characteristics of a system , subsystem, or equipment. 2. The relationship between the input and the output of a system, subsystem, or equipment in terms of the transfer characteristics. Note 1: When the transfer function operates on the input, the output is obtained. Given any two of these three entities, the third can be obtained. Note 2: Examples of simple transfer functions are voltage gains, reflection coefficients, transmission coefficients, and efficiency ratios. An example of a complex transfer function is envelope delay distortion . Note 3: For a negative feedback circuit , the transfer function, T , is given by
where e o is the output, e i is the input, G is the forward gain , and H is the backward gain , i.e., the fraction of the output that is fed back and combined with the input in a subtracter. 3 . Of an optical fiber , the complex mathematical function that expresses the ratio of the variation, as a function of modulation frequency , of the instantaneous power of the optical signal at the output of the fiber, to the instantaneous power of the optical signal that is launched into the fiber. Note: The optical detectors used in communication applications are square-law devices. Their output current is proportional to the input optical power. Because electrical power is proportional to current, when the optical power input drops by one-half (3 dB ), the electrical power at the output of the detector drops by three-quarters (6 dB). [ FAA ]