Noise-like transducers for ultra wide band SAW correlators

This paper presents the development of sampled Gaussian noise transducers for surface acoustic wave (SAW) correlation filters for use in ultra-wideband (UWB) communications and tagging. The orthogonal frequency coded (OFC) SAW correlator concept was previously demonstrated using a uniformly weighted OFC coded dispersive transducer in conjunction with a wideband apodized transducer. The interdigital SAW transducer can accurately represent the noise-signal by electrode apodization. For an ideal-Gaussian white noise signal, there are no correlation sidelobes and cross-correlation of other codes is extremely small. For a finite truncated white-noise Gaussian signal, that is also frequency band-limited, the auto-correlation sidelobes are produced, a function of the signal time bandwidth product. The implementation of noise-like generators and coding can be very useful in UWB systems. The UWB noise transducer used as a generator, can provide greater resistance to jamming and allows an UWB communication system to coexist with other systems. Code generation is performed in the transmitter using the noise weighted SAW filter and is correlated at the receiver using a matching filter device; eliminating the need for complex signal processing. This can lead to a very robust and simple short-range UWB communication system. The theoretical foundation for the signal analysis and transducer implementation will be presented. Coupling of mode (COM) theory will be used to show the important transducer design parameters for transducer evaluation. SAW correlators with fractional bandwidth of greater than 25% are fabricated on lithium niobate (LiNbO3) having a center frequency of 250 MHz. Discussion of the transducer design, analysis and measurements are presented. Results are shown for operation in a matched filter correlator for use in an UWB communication system and compared to predictions, showing good results.