SAW/FET programmable filter with varistor taps for improved performance

Abstract

The SAW/FET programmable transversal filter utilizes a L m SAW delay line that is coupled to a silicon integrated circuit across a 300 nm air gap. We have recently designed and built a SAW/FET with an integrated circuit that incorporates a varistor-controlled tap-weighting circuit. The varistor is a MOSFET whose resistance determines the tap weight and is controlled by the voltage stored on the capacitance of the MOSFET gate. This new design results in higher yields of integrated circuits, better uniformity of tap weights, and better filter performance than in previously reported SAW/FETs in which the tap-weighting circuit comprised programmable varactors. Tap-weight linearity has also been improved. As in earlier devices, the integrated circuit contains the taps, tap weighting and RF summing circuitry to accomplish the transversal filter function. A digital shift register operates as a commutator that allows serial programming of the individual analog tap weights. There are 350 analog taps that yield a 1.5-ps delay with a programmable bandwidth of 100 MHz at a center frequency of 175 MHz.The programmable tap on/off ratio is greater than 30 dB with a k 1.0-dB uniformity. When programmed as a narrow-band filter, the out-of-band rejection is greater than 35 dB across the entire 100-MHz programmable bandwidth. Insertion loss as a programmable bandpass filter is about 40 dB. These values are, we believe, the best reported performance for a wide-bandwidth programmable filter implemented in a compact integrated structure.

The junction FET used as a SAW detector in ZnO-SiO/sub 2/-Si structures

“…For the considered ring-gate structure the drain length approaches 2X when the gate length is reduced, provided the intergate distance remains 2X. Another means of decreasing the off value is the application of a dual structure [2] in which JFET detectors are connected to separate busbars with opposite signs through which common mode rejection is obtained.…”

“…In the past several configurations were investigated, for example the hybrid structure [ I ] , the air-gap structure [2] and the monolithic structure [3]. In the monolithic configuration the acoustic device and the electronic circuits can be fully integrated on a single chip, which results in a compact and reliable signal-processing unit.…”