  The newly discovered second repeating fast radio burst (FRB) source, FRB
180814.J0422+73, was reported to exhibit a time-frequency downward drifting
pattern, which is also seen in the first repeater FRB 121102. We propose a
generic geometrical model to account for the observed downward drifting of
sub-pulse frequency, within the framework of coherent curvature radiation by
bunches of electrons or positrons in the magnetosphere of a neutron star. A
sudden trigger event excites these coherent bunches of charged particles, which
stream outwards along open field lines. As the field lines sweep across the
line of sight, the bunches seen later have traveled farther into the less
curved part of the magnetic field lines, thus emitting at lower frequencies. We
use this model to explain the time-frequency downward drifting in two FRB
generation scenarios, the transient pulsar-like sparking from the inner gap
region of a slowly rotating neutron star, and the externally-triggered
magnetosphere reconfiguration known as the "cosmic comb".
