| For frequency agile communication systems, fast frequency switching in fine frequency steps with good spectral purity is crucial. Direct Digital Frequency Synthesizer (DDFS) is best suitable for these applications, but is not widely employed in wireless communication systems due to its high power consumption. In general, low power and high integration design are two challenges for mixed signal-circuits and communication systems designers. In this dissertation, new design techniques for DDFS at both architecture and circuit levels are proposed and investigated in order to minimize power consumption and optimize performance. A ROM-less low power wide band DDFS prototype using segmented sine wave Digital-to-Analog Converter (DAC) were designed, fabricated and tested to demonstrate the new design techniques.; First, to further reduce power consumption and save chip area, two new phase interpolation ROM less DDFS architectures are proposed. Segmentation technique is applied to the design of sine wave DAC for DDFS: (1) based upon trigonometric identities, a segmented sine wave DAC with fine nonlinear interpolation DAC's is proposed; (2) based upon first order Taylor series and simple linear interpolation, a segmented sine wave DAC with a fine linear interpolation DAC is proposed. Second, a figure of merit (FM) is defined to find the optimal sine wave DAC segmentations for various resolutions of the segmented sine wave DAC's. The device mismatch effects on the performance of segmented sine wave were also discussed. Third, For DDFS using current-steering segmented sine wave DAC with 12-b phase resolution and 11-b amplitude resolution, a behavioral model in Verilog was used to verify the functionality and validate the architecture. Finally, a DDFS prototype was designed and fabricated in a standard 0.25μm CMOS process. The measured SFDR is better than 50 dB with output frequencies up to 3/8 of the 300 MHz clock frequency. The prototype occupies an active area of 1.4 mm2 and consumes 240 mW for 300 MHz clock frequency. The new techniques reduce the power dissipation and die area substantially when compared to conventional ROM based DDFS designs with on-chip DAC. |
