Phase and amplitude disturbances in direct digital frequency synthesizers.

In the present paper we shall discuss the problem of the spurious ''phase and amplitude'' disturbances in direct digital frequency synthesizers (DDFS) [1, 2]. This phenomenon has not yet been discussed, to the author knowledge, in any of the papers or books dealing with DDFS. In all articles or papers dedicated to the principles of DDFS the authors show the change of the stored phase information in the accumulator, via ROM look up 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 we encounter a large ''quasi-amplitude'' modulations. Here, we will pay attention to the origin of these particular problems which are more widespread that one might expect.