Electrode Reflections, Directionality, and Passband Ripple in Wideband SAW Chirp Filters

Abstract

Abstmct-Experimental and theoretical studies have revealed several effects in surface-acoustic-wave chirp filters that arise from electrode reflections. One effect is that down-chirp filters have lower loss than up-chirp filters, which is shown to be caused by asymmetric phasing of reflections by near-synchronous electrodes. The most prominent effect is large amplitude ripple in the passband of broadband down-chirp mters, occurring at frequencies above the second harmonic of the synchronous frequency of the lowest-frequency electrodes, and caused by surface wave reflections by electrodes with a synchronous frequency that is half the operating frequency. The large amplitude ripple is halved in nondispersive chirp filters and does not occur in up-chirp filters. A further effect is enhanced triple-transit ripple, which is also due to “second harmonic” reflections, and occurs in both upand down-chirp filters. A design procedure that suppresses the large amplitude ripple is described and is found experimentally to be effective. Relations are given for the estimation of directionality, and single and triple transit ripple amplitudes due to second harmonic reflections, for chirp filters with arbitrary chirp law, bandwidth, and dispersion time.