Design modifications for increased frequency response in impulse-based ultra-wideband radars

The U.S. Army Research Laboratory (ARL) is currently using an experimental radar with an impulse-based transmitter to gather data on subterranean targets. This synthetic aperture radar (SAR) has an instantaneous bandwidth of 50-1100 MHz, with the upper frequency limit set by the bandwidth of the analog-to-digital (A/D) converter that acts as the baseband receiver for the impulse transmitter. The A/Ds are the biggest limiting factor for most impulse-based radars, as their analog bandwidth and high sampling rates push the state of the art, but effects of transmitter waveforms, antenna response, and preamplifier bandwidth must be considered as well. We discuss a number of design changes that will allow the use of existing A/D converters while expanding the frequency coverage by approximately a factor of two, with major changes to other hardware. The design models chosen are evaluated in a PV-WAVE framework that allows one to chooseg the transmit waveshape and assess the influences of phase stability, signal-to-noise ratio, and system and filter frequency response. A comparison of the model results is presented, along with tradeoffs and recommendations.