Cross-spectrum Phase Noise Measurements of $10^{-15}$-level Stability Photonic Microwave Oscillators

We present in this paper the characterization of state-of-the-art photonic microwave oscillators by using a commercial cross-correlation-based phase noise analyzer and two auxiliary commercial oscillators of the same type. These X-band frequency sources exhibit a fractional frequency stability of $<4\times10^{-15}$ at 1 s, with phase noise as low as <-90 dBc/Hz at 1 Hz and <-150 dBc/Hz at 1 kHz offset from a 10 GHz carrier. Conventional instruments typically do not permit to reveal this level of fidelities. We show the use of a digital system and two auxiliary independent ultrastable sources to enable cross-correlation measurements within minutes for this level of spectral purity and frequency stability.