A clock synthesizer is an electronic circuit that converts a single clock signal frequency into a range of output frequencies. Frequency values are given in MHz. Clock synthesizers can generate frequencies in multiples of the original, also known as a conversion rate. They can work in a phase-locked look, also known as a PLL.
Clock synthesizers are semiconductor devices which come in a standard package, such has a QFN, TSSOP or SOIC.
PLL synthesizers are analogue devices. A PLL circuit features a Voltage-Controlled Oscillator (VCO). The VCO output is constantly adjusted to match the phase and frequency of an input signal, using feedback from a Phase Comparator. A PLL circuit is often used to synchronise (lock) RF receiver circuits to an incoming carrier signal. If a frequency divider integer-n is introduced between the VCO output and the phase comparator, the PLL circuit output frequency will be fin/n, locked in phase to fin. If fin comes from a highly-stable fixed frequency oscillator and n is programmable, then the circuit forms the basis of an accurate Digital Frequency Synthesizer.
Direct digital synthesizers (DDS) are digital devices. They are made up of a crystal oscillator, an NCO (numerically controlled oscillator) and a DAC (digital-to-analogue converter). DDS, in comparison to an analogue PLL synthesizer, offer improved phase noise and better control over the output where there are transitions between frequencies. They also offer better jitter performance.