Frequency synthesiser

A frequency synthesiser is an electronic system for generating any of a range of frequencies from a single fixed timebase or oscillator. They are found in many modern devices, including radio receivers, mobile telephones, radiotelephones, walkie-talkies, CB radios, satellite receivers, GPS systems, etc.

Related Topics:
Electronic - Frequencies - Timebase - Oscillator - Radio - Receiver - Mobile telephone - Radiotelephone - Walkie-talkie - CB radio - Satellite receiver - GPS

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Prior to widespread use of synthesizers, radio and television receivers relied on manual tuning of a local oscillator. Some might remember the classic turret tuner commonly used in television receivers prior to the 1980s. Variations in temperature and aging of components caused frequency drift. Automatic frequency control (AFC) solves some of the drift problem, but manual retuning was often necessary. Since transmitter frequencies are well known and very stable, an accurate means of generating fixed, stable frequencies would solve the problem.

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A simple and effective solutions employs the use of many stable resonators or oscillators for each tuning frequency. Quartz crystals offer good stability and are often used for this purpose. This "brute force" technique is practical when only a handful of frequencies are required, but quickly becomes costly and impractical in many applications. For example, the FM radio band in many countries supports 100 individual frequencies from about 88 MHz to 108 MHz. Cable television can support even more frequencies or channels over a much wider band. A large number of crystals increases cost and requires greater space.

Related Topics:
Quartz - Crystals - FM radio - MHz - Channel

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Many coherent and incoherent techniques have been devised over the years. Some approaches include phase locked loops, double mix, triple mix, harmonic, double mix divide, and Direct Digital Synthesis (DDS). The choice of approach depends on several factors, such as cost, complexity, frequency step size, switching rate, phase noise, and spurious output.

Related Topics:
Coherent - Phase locked loop - Direct Digital Synthesis - Phase noise

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Coherent techniques generate frequencies derived from a single, stable master oscillator. In most applications, crystal oscillator are common, but other resonators and frequency sources can be used. Incoherent techniques derive frequencies from a set of several stable oscillators, typically through frequency multiplication, division, and summing/differencing (mixing). The vast majority of synthesizers in commercial applications use coherent techniques due to simplicity and low cost.

Related Topics:
Crystal oscillator - Mixing

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Synthesizers used in commercial radio receivers are largely based on phase-locked loops or PLLs. Many types of frequency synthesiser are available as integrated circuits, reducing cost and size. High end receivers and electronic test equipment use more sophisticated techniques, often in combination.

Related Topics:
Phase-locked loop - Integrated circuit - Electronic

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