April 30, 2007 -- Engineers targeting DSP to FPGAs have traditionally used fixed-point arithmetic, mainly because of the high cost associated with implementing floating-point arithmetic. That cost comes in the form of increased circuit complexity and often degraded maximum clock performance. Certain applications demand the dynamic range offered by floating-point hardware but require speeds and circuit sizes usually associated with fixed-point hardware. The fast Fourier transform (FFT) is one DSP building block that frequently requires floating-point dynamic range and high speed. A textbook construction of a pipelined floating-point FFT engine capable of continuous input entails dozens of floating-point adders and multipliers. The complexity of these circuits quickly exceeds the resources available on a single FPGA. We fit the FFT design into a single FPGA without sacrificing speed or floating-point performance by using an alternative FFT algorithm and a hybrid of fixed- and floating-point hardware.

By Ray Andraka. (Andraka is President of Andraka Consulting Group, Inc.)