Optimum design of low time-bandwidth product SAW filters

Abstract

For years linear optimization algorithms have been used successfully in bandpass filter design. This method has been adopted for the design of pulse compression filters with low time-bandwidth products. In the bandpass filter design, the sidelobe level in the frequency domain is optimized for a fixed impulse response length. In case of the design of a low time-bandwidth product filters, we optimize the sidelobe suppression in the time domain for a fixed overall bandwidth. We get the optimum time response for a given frequency bandwidth and width of the compressed pulse. The spurious time response is minimized. The corresponding optimum frequency response is split up onto expander and compressor, each consisting of two chirped and apodized IDTs. As an example, an expander-compressor pair at 350 MHz with a 3 dB bandwidth of approximately 11 MHz, an overall bandwidth of approximately 33 MHz, and a chirp time of 1.0 μs was designed. The sidelobe suppression in the design is better than 40 dB. The measured filters exhibit a sidelobe level close to -40 dB. Design and measurement are in excellent agreement