  Microwave photonic technologies, which upshift the carrier into the optical
domain to facilitate the generation and processing of ultrawide-band electronic
signals at vastly reduced fractional bandwidths, have the potential to achieve
superior performance compared to conventional electronics for targeted
functions. For microwave photonic applications such as filters, coherent
radars, subnoise detection, optical communications and low-noise microwave
generation, frequency combs are key building blocks. By virtue of soliton
microcombs, frequency combs can now be built using CMOS compatible photonic
integrated circuits, operated with low power and noise, and have already been
employed in system-level demonstrations. Yet, currently developed photonic
integrated microcombs all operate with repetition rates significantly beyond
those that conventional electronics can detect and process, compounding their
use in microwave photonics. Here we demonstrate integrated soliton microcombs
operating in two widely employed microwave bands, X- and K-band. These devices
can produce more than 300 comb lines within the 3-dB-bandwidth, and generate
microwave signals featuring phase noise levels below 105 dBc/Hz (140 dBc/Hz) at
10 kHz (1 MHz) offset frequency, comparable to modern electronic microwave
synthesizers. In addition, the soliton pulse stream can be injection-locked to
a microwave signal, enabling actuator-free repetition rate stabilization,
tuning and microwave spectral purification, at power levels compatible with
silicon-based lasers (<150 mW). Our results establish photonic integrated
soliton microcombs as viable integrated low-noise microwave synthesizers.
Further, the low repetition rates are critical for future dense WDM channel
generation schemes, and can significantly reduce the system complexity of
photonic integrated frequency synthesizers and atomic clocks.
