RF (radio frequency) : A circuit design that operates in a range of electromagnetic frequencies above the audio range and below visible light. All broadcast transmission, from AM radio to satellites, falls into this range, which is between 30KHz and 300GHz.
Radio Frequency (RF) refers specifically to the electromagnetic field, or radio wave, that is generated when an alternating current is input to an antenna. This field can be used for wireless broadcasting and communications over a significant portion of the electromagnetic radiation spectrum — from about 9 kilohertz (kHz) to thousands of gigahertz (GHz) — referred to as the RF spectrum. As the frequency is increased beyond the RF spectrum, electromagnetic energy takes the form of infrared, visible light, ultraviolet, X rays and gamma rays.
Many types of wireless devices make use of RF fields: radio, television, cordless phones, cell phones, satellite comm systems, and many measuring and instrumentation systems used in manufacturing. Some wireless devices, such as remote control boxes and cordless mice, operate at infrared or visible light frequencies.
The RF spectrum is divided into several ranges, or bands. Each of these bands, other than the lowest frequency segment, represents an increase of frequency corresponding to an order of magnitude (power of ten). FCC rules, combined with the continuing evolution of digital technology, sparked the development of spread spectrum data communication radios. These radios offer significant performance and operation benefits to end-users.
The conventional radio signal which these devices use is referred to as narrow-band, which means that it contains all of its power in a very narrow portion of the radio frequency bandwidth. Due to the relatively small portion of the radio band that an individual radio transmission occupies, the FCC has traditionally favored these conventional radios. However, as a result of the very narrow frequency, these radios are often prone to interference (a single interfering signal at or near their frequency can easily render the radio inoperable).
Spread spectrum is a technique that takes a narrow band signal and spreads it over a broader portion of the radio frequency band, offering the operational advantage of being resistant it interference. Spread spectrum radios are inherently more noise-immune than conventional radios. Thus they will operate with higher efficiency than conventional technology.
In performing spread spectrum, the transmitter takes the input data and spreads it in a predefined method. Each receiver must understand this predefined method and despread the signal before the data can be interpreted. There are two basic methods to performing the spreading: frequency hopping (FHSS) and direct sequence (DSSS). FHSS spreads its signals by “hopping” the narrow band signal as a function of time. DSSS its signal by expanding the signal over a broad portion of the radio band.
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