  We stabilize a chip-scale Si3N4 phase-locked Kerr frequency comb via locking
the pump laser to an independent stable high-Q reference microresonator and
locking the comb spacing to an external microwave oscillator. In this comb, the
pump laser shift induces negligible impact on the comb spacing change. This
scheme is a step towards miniaturization of the stabilized Kerr comb system as
the microresonator reference can potentially be integrated on-chip. Fractional
instability of the optical harmonics of the stabilized comb is limited by the
microwave oscillator used for comb spacing lock below 1 s averaging time and
coincides with the pump laser drift in the long term.
