SAW noise-like coded reflector structures

This paper will present theory, analysis and experimental work on new and novel SAW noise-like-reflector (NLR) structures for use in SAW coded reflectors. The NLR structures produce a very wideband, noise like reflectivity. This reflectivity is very different than that produced from typical single frequency CDMA coded reflectors or orthogonal frequency coded (OFC) reflectors. CDMA and OFC structures are designed based on a standard communication approach where the time information is divided into chips. These chips then provide both the processing gain and also limit the sidelobe level of the auto-correlated reflected pulse. The processing gain is defined by the number of chips and operation bandwidth, and the correlation peak-to-sidelobe ratio is increased by increasing the chip count. The NLR structure has no chips; it is designed using a random sequence of varying width and pitch electrodes. The very wideband nature of the NLR yields very different auto- and cross-correlation properties, compared to CDMA and OFC approaches. Work presented will discuss basic theory that was used to predict the performance of the NLR structures and devices. A simple analytic mathematical model was used as a first order approach, which allows both insight and synthesis. The unit cell for each reflector is determined and an ideal time and frequency response is predicted. A random code generator is used to pick the phase and unit cell design. The composite reflector bank is synthesized and the overall performance evaluated. This model provides both the reflector characteristics as well as the auto-correlation properties. Next, a coupling of modes (COM) model was developed to simulate the complex structure, and predict the overall frequency dependent reflectance or transmittance of the structure. Two types of structures: random and random with pulse position modulation (PPM) were examined. The PPM provides another level of possible coding to the NLR. The analytic model and COM model re- - sults are compared and show remarkably good correlation, which confirms that the analytic model provides a good synthesis tool. Both models confirmed the performance and expected advantages of NLR structures. Experimental SAW NLR devices were built at approximately 250 MHz on YZ LiNbO₃. Several different device designs were fabricated and the measured reflector and correlator results agreed well with both the analytic and COM models. The paper will present the design and layout principles, the basic theory, and several designed NLR structures. It will be shown that this new NLR structure will provide unique methods for device coding, as well as other applications.