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The Army Research Laboratory is evaluating ultra-wide-band radar imaging techniques for subsurface target detection. Of importance in this effort is the generation of an appropriate waveform and the development of an ultra-wide- band exciter (UWBE). A critical requirement for ground penetrating (GPEN) radar is to identify near-surface or subsurface targets in sufficient detail to allow unambiguous identification. For example, a subsurface mine must be distinguishable from benign subsurface soil strata or other man-made objects (e.g., decoys). To optimize the measured signal-to-clutter ratio requires high cross-range and down- range resolution. High cross-range resolution is achieved by collecting radar returns while the radar is in motion and using synthetic aperture techniques to process those returns. High down-range resolution is achieved by the transmission of wide-bandwidth waveforms. The UWBE design uses a wide-bandwidth (approximately 3 GHz) linear frequency modulated (LFM) waveform. Another critical requirement for GPEN radar is the need for efficient propagation of the radar waveform into the soil, which enhances the detection and recognition of subsurface objects. Since low frequencies (approximately 10 MHz) propagate better into soils than do high frequencies, a low chirp start frequency is desired. The use of a LFM waveform that spans from HF to S Band presents another problem of co-site interference with commercial communication equipment (FM, TV, and cellular radio). Since broadcasting in these bands is restricted, a method has been developed to arbitrarily notch out portions of the transmitting bandwidth. This paper will discuss the use of an arbitrary waveform generator (AWG) from Tektronix with a switching local oscillator (LO) architecture to generate the low start frequency wideband LFM waveform required. The AWG with its 1 GHz clock is bandwidth limited to approximately 400 MHz by Nyquist sampling and filter design constraints. The longer LFM waveform is generated by frequency offsetting and concatenating multiple LFM waveform packets from the AWG. The frequency offset is controlled by the switching LO architecture, where the switching time is on the order of a few nanoseconds. Each AWG output can be pre-programmed with notches in the band for interference suppression, as well as a phase offset to maximize the phase continuity of the desired LFM waveform.
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