An optical ranging system includes: a first phase-locked loop (PLL) configured to generate a first frequency signal and a second frequency signal; a second PLL configured to generate a third frequency signal based on a control signal that is formed using the first frequency signal; an optical source coupled to the second PLL, where an intensity of an optical signal emitted by the optical source is configured to be modulated in accordance with the third frequency signal; a first single-photon avalanche diode (SPAD) configured to receive a reflected optical signal; a time-to-digital converter (TDC) coupled to the first SPAD, where the TDC is configured to generate digital samples by sampling an output signal of the first SPAD under control of the second frequency signal; a reference signal generator configured to generate a reference signal; and a mixer configured to mix the reference signal and the digital samples from the TDC.