Frequency synthesizer

A frequency synthesizer is an electronic circuit that generates a range of frequencies from a single reference frequency. Frequency synthesizers are used in many modern devices such as radio receivers, televisions, mobile telephones, radiotelephones, walkie-talkies, CB radios, cable television converter boxes satellite receivers, and GPS systems. A frequency synthesizer may use the techniques of frequency multiplication, frequency division, direct digital synthesis, frequency mixing, and phase-locked loops to generate its frequencies. The stability and accuracy of the frequency synthesizer's output are related to the stability and accuracy of its reference frequency input. Consequently, synthesizers use stable and accurate reference frequencies, such as those provided by crystal oscillators. A frequency synthesizer is an electronic circuit that generates a range of frequencies from a single reference frequency. Frequency synthesizers are used in many modern devices such as radio receivers, televisions, mobile telephones, radiotelephones, walkie-talkies, CB radios, cable television converter boxes satellite receivers, and GPS systems. A frequency synthesizer may use the techniques of frequency multiplication, frequency division, direct digital synthesis, frequency mixing, and phase-locked loops to generate its frequencies. The stability and accuracy of the frequency synthesizer's output are related to the stability and accuracy of its reference frequency input. Consequently, synthesizers use stable and accurate reference frequencies, such as those provided by crystal oscillators. Three types of synthesizer can be distinguished. The first and second type are routinely found as stand-alone architecture: direct analog synthesis (also called a mix-filter-divide architecture as found in the 1960s HP 5100A) and the more modern direct digital synthesizer (DDS) (table-look-up). The third type are routinely used as communication system IC building-blocks: indirect digital (PLL) synthesizers including integer-N and fractional-N. It is in some ways similar to a DDS, but it has architectural differences. One of its big advantages is to allow a much finer resolution than other types of synthesizers with a given reference frequency. Prior to widespread use of synthesizers, in order to pick up stations on different frequencies, radio and television receivers relied on manual tuning of a local oscillator, which used a resonant circuit composed of an inductor and capacitor, or sometimes resonant transmission lines; to determine the frequency. The receiver was adjusted to different frequencies by either a variable capacitor, or a switch which chose the proper tuned circuit for the desired channel, such as with the turret tuner commonly used in television receivers prior to the 1980s. However the resonant frequency of a tuned circuit is not very stable; variations in temperature and aging of components caused frequency drift, causing the receiver to drift off the station frequency. Automatic frequency control (AFC) solves some of the drift problem, but manual retuning was often necessary. Since transmitter frequencies are stabilized, an accurate source of fixed, stable frequencies in the receiver would solve the problem. Quartz crystal resonators are many orders of magnitude more stable than LC circuits and when used to control the frequency of the local oscillator offer adequate stability to keep a receiver in tune. However the resonant frequency of a crystal is determined by its dimensions and cannot be varied to tune the receiver to different frequencies. One solution is to employ many crystals, one for each frequency desired, and switch the correct one into the circuit. 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 channel frequencies from about 88 MHz to 108 MHz; the ability to tune in each channel would require 100 crystals. 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.