Spread-spectrum clocks are signal-generating techniques that modulate a specific spectrum of electromagnetic energy in such a manner as to broaden the frequency of the signal spectrum. The resultant bandwidth is much larger than is necessary for sending information, and this larger frequency bandwidth safeguards against jamming and interference; it also makes possible access to multiple users. Inherent in spread spectrum techniques is the use of noise-like signals, which makes such signals hard to detect or block.
Codes
Spread spectrum signals use codes that quickly modulate the data rate and are therefore referred to as "pseudo-random" signals. These signals are wide enough that their resultant lower spectral power density allows them to utilize the same bandwidth as narrow band signals.
Methods
The three methods used for spreading the frequency spectrum are frequency hopping, which uses a rapidly altered signal within the hopping bandwidth; time hopping, which uses signals transmitted in short bursts; and direct sequence, which uses high frequencies to encode digital data. All three methods transmit signals in a pseudo-random fashion, which means that the receiver knows the code that translates the signal.
History
Nikola Tesla, the inventor of alternating current, made the first public reference to frequency hopping in 1900, when filing a patent for an information transmitting device. The US Army Signal Corps used frequency hopping during World War II to create a top-secret secure speech system designed to facilitate safe communication between President Franklin Roosevelt and English Prime Minister Winston Churchill. Widespread public use of spread spectrum technology began in the 1980s with the creation of the OmniTRACS truck communication system. During the same decade, spread spectrum technology was used to create a navigation system for crop dusting aircraft and a newswire satellite system.
Commercial Use
Spread-spectrum technology is used in telecommunications, where the widened frequency bandwidth reduces interference and increases privacy. Such technology includes WiFi, Bluetooth and cordless telephones. Spread-spectrum clock generation (SSCG) is also used to reduce electromagnetic interference (EMI) and signal-integrity in digital circuits with parts that are synchronized by a coordinated clock signal. This EMI-reduction method is much more efficient than shielding or the use of low pass filters, which is why it is commonly integrated into small portable electronic devices with high-resolution LCD displays.
Advantages and Disadvantages
Key advantages of spread-spectrum technology include its ability to allow multiple users, prevent interference and be difficult to recognize or intercept; disadvantages include an inordinately expensive manufacturing process and convoluted circuitry.
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