A low-frequency high resolution digital synthesizer

A high-resolution digital frequency synthesizer was designed to provide modulation and fine frequency interpolation in a new cesium frequency standard. Requirements include resolution, spectral purity, phase-continuous frequency modulation, simplicity, and low power. The authors describe how these requirements were satisfied with a direct digital frequency synthesizer (DDFS). The DDFS consists of a 34-b phase accumulator feeding a 14-b wide address map and a 12-b digital-to-analog converter (DAC). For output frequencies in the range 131.0 to 132.5 kHz, the level of spurious sidebands, is less than -85 dBc. This is compatible with the expected performance of the DAC.< >