  The repetition rate of a Kerr comb comprising a single soliton in an
anomalous dispersion silicon nitride microcavity is measured as a function of
pump frequency tuning. The contributions from the Raman soliton self-frequency
shift (SSFS) and from thermal effects are evaluated both experimentally and
theoretically; the SSFS is found to dominate the changes in repetition rate.
The relationship between the changes in repetition rate and pump frequency
detuning is found to be independent of the nonlinearity coefficient and
dispersion of the cavity. Modeling of the repetition rate change by using the
generalized Lugiato-Lefever equation is discussed; the Kerr shock is found to
have only a minor effect on repetition rate for cavity solitons with duration
down to ~50 fs.
