Phase and amplitude disturbances in direct digital frequency synthesizers

In this paper the author discusses the problem of the spurious "phase and amplitude" disturbances in direct digital frequency synthesizers (DDFS). This phenomenon has not yet been discussed, to the author´s knowledge, in any of the literature dealing with DDFS. In the literature dedicated to the principles of DDFS, the authors show the change of the stored phase information in the accumulator, via ROM lookup tables, into nice staircase sine waves. However, for normalized frequencies X/Y set approximately, e.g., to 1/3 the output wave is defined only for three phase points and its form is more rectangular (or triangular) than sinusoidal. In addition, in these cases large "quasi-amplitude" modulations are encounted. Here, the author pays attention to the origin of these particular problems which are more widespread that one might expect.