Digital dispersive delay line replacement for legacy radars

Some legacy phased array radars use analog dispersive delay lines to expand and compress their FM chirp waveforms. These acoustic-based devices are often no longer available. This paper describes the design of a functionally equivalent digital replacement for one of these analog delay lines. Various design options were considered including IF versus baseband processing, time domain versus frequency domain filtering, and COTS boards versus custom ASICs. The system design issues of dynamic range, chirp impulse response design, delay line processing latency, and the effect of mismatch between the transmitter and receiver were addressed. The main challenge of the board level design was to meet the digital signal processing computational demands within the inherently short analog delay line latency. This ultimately required developing a custom VLSI signal processing chip. The 1.3 million transistor ASIC chip performs a 1024 tap finite-impulse response (FIR) filter running at an estimated 2 Msps. The configurable architecture of the ASIC allows the number of FIR filter taps to be traded for sampling rate and could be useful in other radar or communication systems.