Digital synthesizer with phase memory

Abstract

In direct digital synthesizers in the prior art, the output signal maintains a phase continuity whenever it switches, or hops, frequency. This phase continuity shows up as a smooth change in phase from one frequency to the next; the phase of the last frequency transitions into the phase of the new frequency without any discernable disruption. Thus, whenever the output signal returns to a switched frequency that it previously has, the output signal at the newly returned switched frequency has a new phase relative to its previous one at that frequency. For some applications, like simulating continuous different frequency sources, this phase continuity is not desirable. To overcome this disadvantage, the preferred embodiment of the present invention provides phase memory to a direct digital synthesizer so that regardless of the frequency that the output signal switches to, the output signal at that frequency is able to maintain a constant phase relative to a reference system clock pulse. In other words, switching and returning the synthesizer output signal to a selected frequency does not change the phase of the output signal at that frequency; the output signal at each frequency has the same constant phase as it has previously at that frequency. As a result, the synthesizer in accordance with the present invention appears to have a source of numerous continuous oscillators to which it can switch for a number of different frequencies, each having a constant phase.