Frequency synthesizers are electronic devices that generate frequencies by using a combination of other frequencies. They are used in cell phones, global positioning systems (GPS), radio receivers, satellite receivers, and mobile telephones. There are three basic types of products: direct analog synthesis (DAS), direct digital synthesis (DDS), and indirect digital. DAS frequency synthesizers feature a mix-filter-divide architecture and are used in some older electronic equipment. DDS frequency synthesizers create arbitrary waveforms digitally, from a single fixed-source frequency. Indirect or phase-locked loop (PLL) frequency synthesizers fall into two subcategories: integer-N and fractional-N.
DDS frequency synthesizers include an electronic controller, random-access memory (RAM), a frequency reference such as crystal oscillator, a digital counter, and a digital-to-analog converter (DAC). These devices operate with programming and running steps. During programming, the electronic controller supplies the RAM with data, typically a series of binary words that represent signal amplitudes at different points in time. The data array forms a table which, in turn, represents a specific wave or shape. During the running step, DDS frequency synthesizers use a digital counter or phase accumulator to advance incrementally each pulse from the frequency reference. The output or phase selects the data-table item, and the DAC converts the data sequence to an analog waveform.
PLL frequency synthesizers use a phase-locked loop to compare the frequencies of two signals in order to produce an error signal that is proportional to the difference between the input frequencies. After this error signal is filtered, a voltage-controlled oscillator (VCO) is used to produce an output frequency. In turn, this frequency is sent through a divider to generate a negative feedback loop. As a rule, the error signal increases when the output frequency drifts. Error-correction drives the signal in the opposite direction, locking the frequency at the other input or reference. With PLL frequency synthesizers, the reference is derived from an oscillator that is very stable in terms of frequency.
Selecting frequency synthesizers requires an analysis of production specifications and application requirements. For example, because PLL frequency synthesizers cannot operate over a relatively wide range of frequencies, these devices may not be suitable for use in some electronic devices.