  Microwave signal generation with fast frequency tuning underlies many
applications including sensing, imaging, ranging, time keeping, wireless
communication, and high-speed electronics. Soliton microcombs are a promising
new approach for photonic-based microwave signal synthesis. To date, however,
tuning rate has been limited in microcombs (and in frequency combs generally).
Here, we demonstrate the first microwave-rate soliton microcomb whose
repetition rate can be tuned at a high speed. By integrating an electro-optic
tuning/modulation element into a lithium niobate comb microresonator, a
modulation bandwidth up to 75 MHz and a continuous frequency modulation rate up
to 5.0 * 10^14 Hz/s are achieved, several orders-of-magnitude faster than
existing microcomb technology. These features are especially useful for
disciplining an optical VCO to a long-term reference such as an optical clock
or for tight phase lock in dual-microcomb clocks or synthesizers. Moreover, the
microwave signal rate can be set to any X - W band rate. Besides its positive
impact on microwave photonics, fast repetition rate control is generally
important in all applications of frequency combs.
