  The Adler equation with time-periodic frequency modulation is studied. A
series of resonances between the period of the frequency modulation and the
time scale for the generation of a phase slip is identified. The resulting
parameter space structure is determined using a combination of numerical
continuation, time simulations and asymptotic methods. Regions with an integer
number of phase slips per period are separated by regions with noninteger
numbers of phase slips, and include canard trajectories that drift along
unstable equilibria. Both high and low frequency modulation is considered. An
adiabatic description of the low frequency modulation regime is found to be
accurate over a large range of modulation periods.
