The Mode-Locked SAW Oscillator

Abstract

The mode-locked SAW oscillator is a new type of signal source, one which generates a stable and highly coherent pulsed rf output. The new device is analogous to the self mode-locked microwave oscillator described by Cutler in 1955 and the mode-locked lasersintroduced by Hargrove, et al. in 1964 and by DeMaria, et al. in 1967. In operation, the multiple modes of oscillation, spaced by the reciprocal of the decay time and usually deemed undesirable, are synchronized by inserting a nonlinear element, a signal e~ander, into the feedback loop. The synchronized spectral lines, whose number are governed by the bandwidth of the SAW transducers, combine to form a precise train of short rf pulses~with a repetition rate equal to the reciprocal of the delay time. Furthermore, the rf frequency is a precise harmonic of the pulse repetition rate. The precision, highly synchronized, comb spectrum is a feature which could prove important for frequency synthesis in programmable frequency hopping systems. Experiments with mode-locked oscillators operating near 100 MHz with 50 ns pulse widths, 0.56 MHz repetition rates and comb spectrums spanning more than 20 MHz are described.