Frequency synthesizer is a part of clock and timer circuit module. It is a device that generates a large number of discrete frequency signals through various technical channels using one or more standard signals. There are three ways to implement frequency synthesizer: direct analog frequency synthesizer, indirect frequency synthesizer and direct digital frequency synthesizer. They can be divided into three generations according to the chronological order in which they occur.

Frequency synthesizers usually use different techniques, such as frequency doubling/frequency dividing, mixing or phase-locked loops, to generate signals or modify the oscillator. One of the latest technologies, called direct digital synthesis, uses digital programming methods such as lookup tables to generate the desired output frequency.

The PLL frequency synthesizer is similar to a PLL oscillator in that the feedback control circuit of the PLL frequency synthesizer can further adjust the output frequency in a predictable way.

Integer N Phase-Locked Loop Frequency Synthesizer uses the doubling function with negative feedback to generate an output frequency that is several times the reference frequency. Generally, this control function is achieved by a digital circuit that controls the analog voltage by an output voltage-controlled oscillator.

The improvement of fractional N PLL frequency synthesizer over integer N synthesizer is that it allows fractional channels with an operand output frequency to be the ratio of fractional modulus. Therefore, the resulting frequency resolution is part of the frequency of the phase discriminator, which effectively reduces the phase noise compared with the integer N synthesizer.

Direct Digital Synthesizer (DDS) Frequency Synthesizer, the latest digital synthetic radio frequency signal generation technology has been developed to generate radio frequency signals directly from digital reference signals. This process usually involves creating a digital waveform and then converting it to a radio frequency signal through a digital-to-analog (D/A) converter.