A 10-Bit 10 GSPS Optical ADC for Radar Signal Processing

Abstract : Next generation radar systems for tactical aircraft will need to improve aircraft survivability, allow for cooperative multistatic mode of operation, provide efficient anti-jamming features, and compensate for motion-induced clutter. Faster digital processors are required, in particular analog-to-digital converters (ADCs) for digital processing of X-band radar systems. This effort investigated and demonstrated an optoelectronic scheme for a high-speed optical ADC. The system was designed with the following objectives in mind: a) maintain the RF signal in the electrical domain throughout the conversion process and eliminate use of bulky optical components; b) low optical power budget (e.g. basic sampling circuit requires 50-60 m optical power); c) direct interface to electronic quantization circuits; and d) compact design. The effort achieved sampling rates of 4 GSPS. The limitation in conversion speed was dictated by the photodiode which had a 3dB-bandwidth of 5GHz. The following tasks were completed: 1) Pspice simulation of sampling circuit; 2) Design implementation and synchronization of the optical clocks needed for a) sampling of incoming RF signals, b) demultiplexing the sampled signals to lower data rate for electronic quantization; 3) Interface of optical sampling circuit with National Instrument LABVIEW data acquisition system for quantization of sampled signals to produce an 8-bit gray code digital output; and 4) Optical sampling circuit optimization resulting in a 50% reduction in optical power consumption.