The differential reference frequency synthesizer

Indirect digital frequency synthesizers cannot achieve fast frequency switching with closely spaced frequencies because of limitations imposed by the requisite narrow loop bandwidth. A novel dual-loop digital frequency synthesizer is presented which satisfies these conflicting requirements and, in most cases, exhibits improved phase noise performance. Two realizations of the novel frequency synthesizer are presented and compared to a conventional dual-loop digital synthesizer using an example. From this example, it is evident that the use of the differential reference frequency synthesizer allows synthesis of closely spaced frequencies with phase-locked loops that use arbitrarily high reference frequencies and, therefore, can have considerably wider bandwidths and commensurately shorter frequency switching times than can be achieved with conventional dual-loop synthesizers.<>