SAW/FET Programmable Filter with Varistor Taps for Improved Performance

摘要

The Surface acoustic wave(SAW)/Field effect transistor(FET) programmable transversal filter utilizes a LiNbO3 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 caristor-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 microsec and delay with a programmable bandwidth of 100 MHz at a center frequency of 175 MHz. Reprints. (rh)