A highly stable timebase for subsampling systems using a fractional concept with short periodicity

Concerning UWB applications a highly stable timebase is of great importance. Within this timebase a finely resoluted frequency synthesizer is used to generate two signals with a slight offset in frequency. In order to avoid the poor spectral performance of commercial direct digital synthesizers and to decrease the effort and costs in circuit design compared to a phase locked loop concept a new approach is presented within this paper. This concept utilizes two fractional-N frequency dividers with a short periodicity in division factor sequence. The resulting spurious signals are suppressed using a ceramic bandpass filter. The basic concept as well as a realization of the system are presented. Measurement results show a phase noise performance of -157 dBc/Hz @ 20kHz at a frequency of 5.35 MHz while having a frequency resolution well below 100 Hz.