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]
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