The multiloop RF frequency synthesizer is a very useful technique that can be employed when designing high performance synthesizers.
As with any electronic circuit there are trade-offs to be made. The same is true for frequency synthesizers. For example small step sizes result in high division ratios in most instances and in turn this impairs the phase noise performance as well as a number of other performance criteria.
One way of maintaining the levels of performance whilst retaining aspects like small step size, fast switching, good phase noise performance and other aspects is to use more than one loop.
Using a multiloop approach to RF frequency synthesizer design enables much greater levels of performance to be achieved, although at the cost of additional design and more electronic circuitry.
PLL frequency synthesizers are particularly flexible to use and to be controlled by digital commands. In addition to this they offer many distinct advantages over other forms of local oscillator for many applications both in radio communications equipment as well as test equipment, etc. As a result PLL frequency synthesizers are widely used for many RF applications. Even though other forms of generator including Direct Digital Synthesizers are available, the PLL based synthesizer is has many advantages, and can often be used in conjunction with other generators including DDS circuits.