SAW/FET programmable filter with varistor taps for improved performance

The SAW (surface acoustic wave)/FET (field-effect transistor) programmable transversal filter utilizes a LiNbO₃ SAW delay line that is coupled to a silicon integrated circuit across a 300 nm air gap. A SAW/FET with an integrated circuit that incorporates a varistor-controlled tap-weighting circuit has been built. The varistor is a MOSFET (metal-oxide-silicon FET) whose resistance determines the tap weight and is controlled by the voltage stored on the capacitance of the MOSFET gate. 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-μs delay with a programmable bandwidth of 100 MHz at a center frequency of 175 MHz. The programmable tap on/off is greater than 30 dB with a ±1.0-dB uniformity. When programmed as a narrowband filter, the out-of-band rejection is greater than 35 dB across the entire 100-MHz programmable bandwidth