Electrically tunable narrow-bandwidth SAW filter

SAW voltage-controlled oscillators (VCOs) are very easily implemented using a conventional hybrid junction and varactors. Taking advantage of this, a similar device may be designed for use as an electrically tunable filter. The device consists of a SAW delay line with moderate insertion loss and closed through a low-noise amplifier (LNA) in order to maintain the loop losses to a few dB. In addition, a phase-shifter is inserted in the loop to match the overall phase. The input is typically the input of the amplifier, while the output may be similarly the output of the amplifier, for low loss, or an auxiliary SAW transducer designed to compensate for the frequency response of the delay line. This is a very agile narrow bandwidth filter with the capability of meeting the requirements of the future vehicular communication system with frequency around 900 MHZ. This type of tunable SAW filter has been first implemented at about 200 MHZ on lithium tantalate and experimental results are reported.