Ultra wide band saw correlators using dual orthogonal frequency coded transducers

This paper presents the development of ultra-wideband (UWB) surface acoustic wave (SAW) correlators using dual dispersive orthogonal frequency coded transducers. Orthogonal frequency coding (OFC) is a spread spectrum coding technique with a variety of applications. The OFC SAW correlator spectrally spreads the UWB data beyond that of CDMA due to an increased signal bandwidth. The OFC SAW correlator concept was previously demonstrated using a coded dispersive transducer in conjunction with an unweighted transducer. The performance of the UWB SAW correlator device is distinctly enhanced by using a dual OFC transducer architecture. The dual OFC correlator consists of two seven chip dispersive transducers. Each of the two dispersive transducers implements a dissimilar code; the resulting time domain signal is spread to twice the length of the device compared with a single dispersive transducer. The frequency response resembles a signal using orthogonal frequency division multiplexing (OFDM). The properties of the double dispersive device provides decreased chip distortion, a significant lower insertion loss due to improved transducer chip acoustic matching as well as an increased level of code diversity beyond what is offered by OFC alone. Experimental results of a SAW filter designed with dual OFC transducers are presented. Each SAW correlation filter was designed using seven dissimilar contiguous chip frequencies within the two OFC transducers. SAW correlators with fractional bandwidth of approximately 29% were fabricated on lithium niobate (LiNbO₃) having a center frequency of 250 MHz. A coupling of modes (COM) model is used to predict the SAW filter response and is compared to the experimentally measured data. Good correlation between the predicted COM responses and the measured device data is obtained. Discussion of the design, analysis and measurements are presented. Results are shown for operation in a matched filter correlator for use in an UWB commun- ication system and compared to predictions, showing good results.