A new approach in low noise direct digital synthesis based on a fractional-N frequency divider

Frequency synthesisers in subsampling systems are often a crucial point. In order to achieve a small frequency offset between two signals a fine frequency resolution is required. Therefore a direct digital synthesiser with a frequency resolution of a few Hz would be fine but these systems suffer from unwanted spurious content resulting in a bad phase noise performance. This paper presents a new approach in direct digital synthesis for signals with small frequency offset based on a fractional-N frequency divider. The basic concept as well as a realization of the system are presented. Measurements of this first prototype show a good spurious and phase noise performance.