Monolithic surface wave transversal filter

A monolithic surface wave transversal filter was designed, fabricated, and evaluated. The semiconductor and acoustic elements were developed on (001) p-type silicon oriented for acoustic transmission along the (100) direction. Surface acoustic waves were generated by zinc oxide film layer transducers into an array of 31 metal-gate NMOS piezoresistive FET taps. The drain lines of all taps were connected to serve as the rf output. The phase and amplitude of each tap was controlled by information read into a digital static shift register and a bucket brigade device respectively. The die size for a single 31 bit filter was .810 inch by .160 inch. The conversion loss of the 9.5 finger pair zinc oxide film layer transducers was 15 db. The piezoresistive tap coupling efficiency was in the 35 to 40 db region under normal operating conditions. The overall device efficiency in developing a spread waveform from a short (100 ns) rf input pulse was 75 db which include losses due to signal expansion, channel division, matching and parasitics. The direct coupled signal level was 90 db below the input signal level and the signal to noise ratio of the spread pulse was approximately 20 db. The bucket brigade device was effective in controlling tap amplitude over a 10 db range and the digital shift register provided for random biphase coding of the taps. The yield of working transversal filter chips was greater than 40 percent from 3 inch silicon wafers.