An improvement in the temperature-compensated DDS

This paper shows an extension of the temperature-compensated DDS; now an independent (free-running) crystal oscillator generates a clock of the direct digital frequency synthesizer (DDS) instead of a dual-mode crystal oscillator (DMXO). An entirely digital compensation of the clock frequency versus temperature (f-T) dependency is used. Temperature sensing is done by the use of a DMXO employing fundamental and 3^rd overtone c-modes of an SC-cut crystal. The modes are combined to produce a temperature-sensitive difference frequency. A long-term frequency stability (i.e. aging) of the SC-cut crystal determines the long-term frequency stability of the whole synthesizer. Over a temperature range of -35°C to +85°C the frequency instability of generated signals is about ±0.15 ppm.