Design of low-power ROM-less direct digital frequency synthesizer using nonlinear digital-to-analog converter

A design technique that uses nonlinear digital-to-analog converter (DAC) fo r implementing low-power direct digital frequency synthesizer (DDFS) is pro posed. The nonlinear DAC is used in place of the ROM, look-up table for pha se-to-sine amplitude conversion and the linear DAC in a conventional DDFS, Since the proposed design technique for DDFS does not require a ROM, signif icant saving in power dissipation results. The design procedure for impleme nting the nonlinear DAC is presented. To demonstrate the proposed technique , two quadrature DDFS's, one using nonlinear resistor string DAC's and the other using nonlinear current-mode DAC's, were implemented. For a 3,3-V sup ply, the resulted power dissipation for both DDFS's are 4 and 92 mW at a cl ock rate of 25 MHz and 230 MHz, respectively. For both DDFS's, the spurious free dynamic ranges are over 55 dB for low synthesized frequencies.