Tunable Subnanosecond Gaussian Pulse Radar Transmitter: Theory and Analysis

This article presents a design methodology and steps to fabricate a tunable step recovery diode (SRD)-based Gaussian pulse transmitter for ultra-wideband (UWB) radar systems. The method exploits a parallel and series combination of two SRDs which are excited using a transistor-based square pulse generator with an amplitude of 9 V. A turning point control circuit is used to control the forward-to-reverse current ratio of the diodes and tune the pulsewidth and magnitude of the Gaussian output pulse. The ringing level of the pulse is reduced using an output diode network. The interconnect between the diodes is designed and optimized to improve the amplitude and pulsewidth. A Gaussian pulse of variable pulsewidth from 60 ps with an amplitude of 1 V to 220 ps with a 6.8 V amplitude and ringing level of less than 11% is achieved. A 90° coupler with broad bandwidth is fabricated to differentiate the Gaussian pulse and extract a dc-free monopulse. Vivaldi antennas with suitable bandwidth are used as transmitting antennas to radiate the monopulses with variable pulse widths. The measured second derivative Gaussian pulses at the receiver end demonstrated pulse widths of 41 and 173 ps for the Gaussian input pulses of 60 and 220 ps, respectively.