Reflective array compressors using 180° reflecting metal dot arrays

Abstract

Recently, a reflective array compressor using 180° grooved reflectors and a 3-dB multistrip coupler has been described. This has the advantage over conventional 90° reflective devices that the surface waves undergo only one reflection instead of two, and so the insertion loss can be low. This paper reports a new device, using surface acoustic waves propagating on YZ LiNbO3, in which the grooves are replaced by metal dot arrays, removing the need for ion-beam milling and—through variation of dot density—providing a simple method of weighting. The devices designed had a time-bandwidth product of 1000, dispersion time of 20 μs, bandwidth of 50 MHz centered at 150 MHz, and a 40-dB sidelobe six-term Taylor amplitude weighting. Compared with dot array devices using 90° reflectors the design is simplified since the velocity and scattering anisotropy, important for 90° reflectors, need not be considered. The surface wave velocity change under the metal dot array is potentially a source of phase error, but it is shown that a simple empirical correction in the mask design is sufficient to reduce this to around 5° rms. Deviation of the weighting characteristic from the ideal is typically of the order ±1 dB.