A Pseudorandom Sampled High Speed Photonic Analog-to-Digital Converter Architecture M. Airola, M. L. Dennis, D. Novak * and T. R. Clark JHU Applied Physics Laboratory, Laurel, MD 20723 * Pharad, LLC, Glen Burnie, MD 21061 Email: Marc.Airola@jhuapl.edu Abstract: We present an analog-to-digital converter architecture utilizing non-uniform photonic sampling, synchronously clocked digitizers and digital signal processing to accomplish unambiguous digitization of input signals covering multiple Nyquist zones. 1. Introduction High speed analog-to-digital converter (ADC) architectures capable of direct digitization of microwave frequencies would be enabling for many applications [1], provided sufficient resolution could be obtained. Photonic implementations have been of recent research interest with many of the most promising architectures utilizing high speed and wide bandwidth photonic sampling [2, 3], generally consisting of a mode-locked laser (MLL) pulse train and a broadband electro-optic modulator (EOM) to implement the front-end sample and hold operation. A significant limitation on real-time architectures has been the need for highly parallel back-end electronic digitization, due to the limited resolution of high speed electronic digitizers, to convert the photonically sampled signal to the digital electronic domain. Drawbacks of this approach include accurately setting the gain and timing of many interleaved digitizers and practical concerns regarding size, weight, power and cost. For many applications, speed requirements are driven by a desire to directly digitize the microwave carrier with an instantaneous bandwidth greater than the carrier frequency for signal identification and analysis, and/or to avoid the multiple stages of mixing and filtering required for downconversion to within a high resolution electronic ADC operating bandwidth. In this paper, we propose an approach for handling multi-octave, high frequency input microwave signals that leverages the high speed and broadband advantages as well as the precision timing control of photonics, without requiring highly parallel back-end electronic digitization. We provide an analysis of our proposed architecture, utilizing high resolution pseudorandom photonic sampling and novel digital signal processing, in the context of generally available photonic and electronic hardware and discuss our preliminary numerical results demonstrating the system concept. 2. Photonic Analog-to-Digital Converter Concept and Architecture The primary elements of the architecture are shown in Figure 1. The sampling pulse generation hardware forms a fiber-coupled optical pulse train with pseudorandom temporal spacing. This is directed to an EOM, of bandwidth greater than the maximum input signal frequency, which encodes the signal to be digitized onto the pulse train as an amplitude modulation. The encoded pulse train is directed to a photodiode (O/E) for detection and conversion to the electrical domain, and thence to a synchronously clocked electronic digitizer. Timing is preserved between the sampling and clocking paths to ensure digitization of each intended sample time. The bandwidth of the photodiode and subsequent digital electronics need only be large enough to process the information carried on the microwave signal (generally <10% of the microwave carrier frequency). Key for any ADC architecture is a sample and hold operation with aperture time, aperture timing jitter and amplitude resolution suitable for the design bandwidth and resolution. Our approach utilizes an optical sample and hold based on a MLL pulse train and a broadband EOM. The MLL serves as the system master clock and sets the ultimate ADC performance limits through its noise and pulse properties. The amplitude and timing jitter of mode- locked fiber lasers are sufficient [4] to achieve >10 bit resolution to microwave frequencies >20 GHz. The aperture Figure 1: Pseudorandom sampled photonic analog-to-digital converter architecture. ML Laser O/E Electr. Digitizer CLK DSP Digital Electronics HW EOM O/E Signal Sampling Pulse Generation HW PRBS EOM ML Laser O/E Electr. Digitizer CLK DSP Digital Electronics HW EOM O/E Signal Sampling Pulse Generation HW PRBS EOM 108 MM1 3:30 PM – 3:45 PM 1-4244-0925-X/07/$25.00 ©2007 IEEE