  Modern communication, navigation, and radar systems rely on low noise and
frequency-agile microwave sources. In this application space, photonic systems
provide an attractive alternative to conventional microwave synthesis by
leveraging high spectral purity lasers and optical frequency combs to generate
microwaves with exceedingly low phase noise. However, these photonic techniques
suffer from a lack of frequency tunability, and also have substantial size,
weight, and power requirements that largely limit their use to laboratory
settings. In this work, we address these shortcomings with a hybrid
opto-electronic approach that combines simplified optical frequency division
with direct digital synthesis to produce tunable low-phase-noise microwaves
across the entire X-band. This results in exceptional phase noise at 10 GHz of
-156 dBc/Hz at 10 kHz offset and fractional frequency instability of 1x10^-13
at 0.1 s. Spot tuning away from 10 GHz by 500 MHz, 1 GHz, and 2 GHz, yields
phase noise at 10 kHz offset of -150 dBc/Hz, -146 dBc/Hz, and -140 dBc/Hz,
respectively. The synthesizer architecture is fully compatible with integrated
photonic implementations that will enable a versatile microwave source in a
chip-scale package. Together, these advances illustrate an impactful and
practical synthesis technique that shares the combined benefits of low timing
noise provided by photonics and the frequency agility of established digital
synthesis.
