Using parallel FFT for multi-gigahertz FPGA signal processing

Chris Eddington, Baijayanta Ray, Synopsys Inc.
3/28/2013 9:37 AM EDT

High-speed fast Fourier transform (FFT) cores are an essential requirement for any real-time spectral-monitoring system. As the demand for monitoring bandwidth grows in pace with the proliferation of wireless devices in different parts of the spectrum, these systems must convert time-domain to frequency-domain ever more rapidly, necessitating faster FFT operations. Indeed, in most modern monitoring systems, it is often necessary to use parallel FFTs to run at sample throughputs of multiple times the pace of the highest clock rate achievable in state-of-the-art FPGAs, such as the Xilinx Virtex-7, taking advantage of wideband A/D converters that can easily attain sample rates of 12.5 Gigasamples/second and more. [1]

At the same time, as communications protocols become increasingly packetized, the duty cycles of signals that need to be monitored are decreasing. This phenomenon requires a dramatic decrease in scan repeat time, which necessitates low-latency FFT cores. Parallel FFTs can help in this regard as well, since the latency scales down almost proportionally to the ratio of sample rate to clock speed.

For all of these reasons, let’s delve into the design of a parallel FFT (PFFT) with runtime-configurable transform length, taking note of the throughput and utilization numbers that are achievable when using parallel FFT.

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