Frequency-change signaling in which the change from one significant condition to another is accompanied by decay in amplitude of one or more frequencies and by buildup in amplitude of one or more other frequencies.
Note: Frequency-exchange signaling applies to supervisory signaling and user-information transmission. Synonym two-source frequency keying.

A signal structuring technique employing automatic switching of the transmitted frequency. Selection of the frequency to be transmitted is typically made in a pseudo-random manner from a set of frequencies covering a band wider than the information bandwidth. The intended receiver would frequency-hop in synchronization with the code of the transmitter in order to retrieve the desired information. [NTIA] [RR]… (Read More →)

Frequency modulation in which the modulating signal shifts the output frequency between predetermined values.
Note 1: Usually, the instantaneous frequency is shifted between two discrete values termed the "mark" and "yyspace" frequencies. This is a noncoherent form of FSK.
Note 2: Coherent forms of FSK exist in which there is no phase discontinuity in the output signal. Synonyms frequency-shift modulation,… (Read More →)

Synonym frequency-shift keying.

Synonym frequency-shift keying.

Telegraphy by frequency modulation in which the telegraph signal shifts the frequency of the carrier between predetermined values. [NTIA] [RR]

See FAQ file.

Synonym near-field diffraction pattern.

In optics, the reflection of a portion of incident light at a discrete interface between two media having different refractive indices.
Note 1: Fresnel reflection occurs at the air-glass interfaces at the entrance and exit ends of an optical fiber. Resultant transmission losses, on the order of 4% per interface, can be reduced considerably by the use of index-matching materials.… (Read More →)

In radio communications, one of a (theoretically infinite) number of a concentric ellipsoids of revolution which define volumes in the radiation pattern of a (usually) circular aperture.
Note 1: The cross section of the first Fresnel zone is circular. Subsequent Fresnel zones are annular in cross section, and concentric with the first.
Note 2: Odd-numbered Fresnel zones have relatively intense… (Read More →)