  Measuring microcombs in amplitude and phase provides unique insight into the
nonlinear cavity dynamics but spectral phase measurements are experimentally
challenging. Here, we report a linear heterodyne technique assisted by
electro-optic downconversion that enables differential phase measurement of
such spectra with unprecedented sensitivity (-50 dBm) and bandwidth coverage (>
110 nm in the telecommunications range). We validate the technique with a
series of measurements, including single cavity and photonic molecule
microcombs.
