The Tektronix S51 Trigger Countdown Head is a free-running tunnel-diode oscillator designed to provide stable sampling displays of sinewaves from 1GHz to 18GHz. The unit has a front panel sync control that synchronizes the oscillator frequency to a subharmonic of the input signal. The output of the unit is available at a front-panel trigger output connector and through a rear-output connector for internal triggering. The output signal is a direct countdown of the input and permits triggering by a standard sapling time-base unit.