A digitally controlled oscillator, or DCO, is an oscillator circuit that generates an analog signal, but whose frequency is controlled by a digital control input (as opposed to a voltage controlled oscillator, whose frequency is set by a control voltage). The DCO first appeared in the 1980s as a sort of intermediate step between the all-analog VCO, and the all-digital implementations that came later. Several DCO circuit designs exist, but most work by synchronizing an analog sawtooth core to a digital countdown timer driven by a high-frequency, highly precise clock. The value loaded into the countdown timer determines the output frequency.