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 →)

A measure of the thermal noise generated in the first stage of a receiver.