An oscillator subsystem included in a phase-locked loop circuit of a computer system may include coarse and fine-tuning circuits. The coarse-tuning circuit may generate a coarse-tuning current based on a reference voltage, and the fine-tuning circuit may generate a fine-tuning current by combining respective currents generated by first and second complement current mirror circuits using a voltage level of a control signal. An oscillator circuit may generate a clock signal whose frequency is based on a combination of the coarse and fine-tuning circuits.