Design Methodology and CAD Tools for Prototyping Delta-Sigma Fractional-N Frequency Synthesizers

In this paper we present the design flow for a fractional-N (frac-N) frequency synthesizer proto type from the system level conceptualization of the PLL to its hardware implementation. The prototyping hardware consists of a of frac-N PLL IC designed in TSMC 0.18 mum mixed signal/RF CMOS process mounted on a RF prototype impedance controlled board. The RF board is interfaced to a digital delta sigma modulator (DSM) realized in a Xilinx FPGA on a commercial evaluation board. Matlab was employed for the system level PLL design. The TSMC 0.18 mum CMOS Process Design Kit was used for the frac-N IC realization. Verilog HDL was used for the implementation of the DSM in the FPGA. With an FPGA based DSM platform, prototypes for different communication modulation formats, and for different DSM topologies, different order integrators, and different dithering schemes can be evaluated quickly and economically. We elaborate on our software selection for the design flow in the University environment. In addition to discussing the prototype design methodology, we present the simulated and measured results for the MASH-11 and MASH- 12 frac-N synthesizers. The prototype encompassed all possible aspects of a complex hardware system design: an IC design, RF board design, board-to-board interface issues and the Verilog coding of DSMs in an FPGA. This process served as a useful tool for learning the various aspects of RF hardware design.