BRAGG CELL RF SPECTRUM ANALYSER

Abstract

The Bragg cell acousto-optic spectrum analyser (AOSA) is an inherently parallel processing system that offers gigahertz instantaneous bandwidth and high information throughput for RF signals. These unique properties arise from the independent and simultaneous frequency dispersive nature of the acousto-optic interaction process within the Bragg cell which permits the simultaneous wideband acquisition of multiple time-coincident signals. In addition, the high interception rate make the AOSA suitable for the capture and analysis of pulsed signals in an ever widening electromagnetic environment such as those for radar warning or ESM applications. The underlying research for this thesis was initially undertaken to explore the feasibility of constructing an instantaneous wide bandwidth RF receiver capable of detecting multiple signal sources simultaneously. Clearly, such a receiver would be superior to conventional ones and a Bragg cell architecture for the receiver was proposed for these two principal reasons. The Bragg cell R receiver in essence demonstrates an effective way to harness the speed of light to perform signal processing. The thesis starts by closely examining a theoretical discussion on the acousto-optic interaction process within the Bragg cell and extends to cover the instantaneous Fourier Transform properties of the overall system. It reports on the process of evaluating and selecting the appropriate hardware to construct a laboratory AOSA model and also discusses upon the range of experimental results achieved. Since a Bragg cell device operates on the principle of acousto-optic interactions, it is necessary to understand the fundamentals of these interactions before one can proceed with the hardware to construct a receiver system, albeit a laboratory model. These interactions have been studied in detail ever since Brillouin in a pioneering effort attributed the scattering of light in transparent crystals to their thermal density changes and modeled these thermal agitations as equivalent to a superposition of acoustic waves. Since then, two different approaches were popularly taken to model acousto-optic interactions so as to arrive at a quantifiable outcome of the diffraction orders. These two approaches, the vigorous classical scalar diffraction theory and the later more simplistic quantum particle collision approach, are discussed thoroughly in the earlier portion of the thesis. The Fresnel and Fraunhofer diffraction equations are used in conjunction with the phase transformation properties of a converging lens to examine the instantaneous Fourier transform property of the Bragg cell receiver architecture. As with any receiver system, the Bragg cell receiver does have its share of performance limitations. The factors causing these limitations are identified and are constantly reviewed in a bid to achieve better performance with the hardware. In essence , though the bandwidth of the system is quite dependent on the chosen acousto-optic device itself, the weighting effect of its aperture and the truncation effect on the Gaussian-profile intensity laser source determine two other important performance parameters – frequency resolution and dynamic range. The function of the individual components making up the Bragg cell receiver architecture is discussed and the device physics explained. The performance and limitations of each component contributing to the overall system specifications are also noted. In the design section, the details that include evaluating, selecting and even designing to arrive at the appropriate specifications of the individual components needed to construct the Bragg cell receiver were covered at length. Later, to optimize the physical size of the system, it is shown that the optics configuration should be custom-made. Finally, the experimental results of the Bragg cell receiver are presented and discussed. From the experience gathered, recommendations are put forward to further improve on the performance of the Bragg cell receiver. In summary, the constructed receiver model employs a power Bragg cell receiver architecture with integrating photodetectors Over an operating bandwidth of 100 MHz centered at 200 MHz, the frequency resolution of two equal power signals is measured to be 200 kHz. The dynamic range of the system is 20 dB with a sensitivity level of -20 dBm. The system is capable of detecting pulsed RF signals with repetitive frequency of 20 kHz. Weak signals hidden 5 dB below broadband white noise can also be recovered.