  The activity of the repeating FRB 20180916B is periodically modulated with a
period of 16.3 days, and FRB 121102 may be similarly modulated with a period of
about 160 days. In some models of this modulation the period derivative is
insensitive to the uncertain parameters; these models can be tested by
measurement of or bounds on the derivative. In other models values of the
uncertain parameters can be constrained. Periodic modulation of aperiodic
bursting activity may result from emission by a narrow beam wandering within a
cone or funnel along the axis of a precessing disc, such as the accretion discs
in X-ray binaries. The production of FRB 200428 by a neutron star that is
neither accreting nor in a binary then shows universality: coherent emission
may occur in a wide range of environments.
