Spectral properties of DDFS: computer simulations and experimental verifications

In the present paper we shall discuss generation of spurious signals in direct digital frequency synthesizers (DDFS) with different output waves with the assistance of computer simulations. First, we shall deal with simple square wave and triangular wave outputs. However, our main attention will be paid to the systems generating sine signals since they exhibit the lowest level below one cannot get. The major sources of spurious signals are phase truncation, the limited length in sine look-up tables, the finite resolution of digital to analog convertors DAC, and intermodulation problems. The latter are not easy to appreciate.