Development of an Eight Channel Waveform Generator for Beam-forming Applications

Abstract

An eight-channel direct-digital waveform synthesizer has been developed to enable digital beam steering of the transmitted waveform. Built around the Analog Devices AD9910 DDS chip, this eight-channel waveform generator, when used with an eight element linear antenna array, enables the illuminating radiation pattern to be digitally modified on a pulse-to-pulse basis if desired. Developed in support of airborne radar depth-sounding of the polar ice sheets and outlet glaciers, two key benefits of this capability provides include improved surface clutter suppression and more efficient off-nadir illumination for side-looking imaging of the ice-bed interface. Adjusting the starting frequency and phase of the waveform produced by each DDS is analogous to introducing an incremental time delay between otherwise identical chirp waveforms, thus providing the required beam-steering control. Additionally, the AD9910, with a 1-GHz maximum clock frequency, provides amplitude control, both intra-waveform and inter-waveform, for time-sidelobe management and radiation-sidelobe management. An FPGA is used for the management of up to 16 waveforms, zero-pi phase modulation on a per waveform basis, system communication over a serial port, and loading the DDS configuration settings on each system trigger. The board provides matched clock and sync inputs in order to guarantee phase alignment across the multiple DDS chips.