Here, you can find some links to tutorials for Frequency Synthesizers and Frequency Synthesis Techniques.

By Frequency Synthesizer we usually mean a stable signal generator which can be switced to put out any one of a discrete set of frequencies and whose frequency stability is derived from a standard oscillator. At least three techniques are used for Frequency Synthesis:

• Direct Synthesis is a popular technique which uses frequency multipliers, frequency dividers and mixers.
• Indirect Synthesis is based upon phase locked loops (PLLs).
• Direct Digital Synthesis uses a digital accumulator to produce a staircase sawtooth. A lookup table then changes the sawtooth to a staircase sinusoid and a D-to-A converter provides the analog output.