  Laser frequency microcombs provide equidistant coherent frequency markers
over a broad spectrum, enabling new frontiers in chip-scale frequency
metrology, laser spectroscopy, dense optical communications, precision distance
metrology and astronomy. Here we demonstrate thermally stabilized frequency
microcomb formation in dispersion-managed microresonators at the different
mode-locking states featured with the negligible center frequency shift and
broad frequency bandwidth. Subsequently, femtosecond timing jitter in the
microcombs are characterized, supported by precision metrology on the timing
phase, relative intensity noise and instantaneous linewidth. We contrast the
fundamental noise for a range of 89 GHz microcomb states, from soliton crystals
to multiple solitons and single-soliton regimes, determined by pump-resonance
detuning. For the single-soliton state, we report a close-to-shot-noise-limited
relative intensity noise of -153.2 dB/Hz and a quantum-noise-limited timing
jitter power spectral density of 0.4 as2/Hz, at 100 kHz offset frequency. This
is enabled by a self-heterodyne linear interferometer with 94.2 zs/Hz1/2 timing
resolution, 50.6 mHz/Hz1/2 RF frequency resolution, and 6.7 uV/Hz frequency
discrimination sensitivity. We achieve an integrated timing jitter at 1.7 fs,
integrated from 10 kHz to 1 MHz. Measuring and understanding the fundamental
noise parameters in these high-clock-rate frequency microcombs are essential to
advance soliton physics and precision microwave-optical clockwork.
